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Dichroic filters are created using optically transparent materials.

In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through a material; translucency (also called translucence or translucidity) only allows light to pass through diffusely. The opposite property is opacity. Transparent materials are clear, while translucent ones cannot be seen through clearly.

When light encounters a material, it can interact with it in several different ways. These interactions depend on the nature of the light (its wavelength, frequency, energy, etc.) and the nature of the material. Light waves interact with an object by some combination of reflection, and transmittance with refraction.

Some materials, such as plate glass and clean water, allow much of the light that falls on them to be transmitted, with little being reflected; such materials are called optically transparent. Many liquids and aqueous solutions are highly transparent. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are mostly responsible for excellent optical transmission.

Materials which do not allow the transmission of light are called opaque. Many such substances have a chemical composition which includes what are referred to as absorption centers. Many substances are selective in their absorption of white light frequencies. They absorb certain portions of the visible spectrum, while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. This is what gives rise to color. The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering.[1]

Comparisons of 1. opacity, 2. translucency, and 3. transparency; behind each panel is a star

Contents

Introduction

With regards to the absorption of light, primary material considerations include:

  • At the electronic level, absorption in the ultraviolet and visible (UV-Vis) portions of the spectrum depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific frequency, and does not violate selection rules. For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules. Thus, there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
  • At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and on selection rules. Nitrogen and oxygen are not greenhouse gases because the absorption is forbidden by the lack of a molecular dipole moment.

With regards to the scattering of light, the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include:

  • Crystalline structure: How close-packed its atoms or molecules are, and whether or not the atoms or molecules exhibit the long-range order evidenced in crystalline solids.
  • Glassy structure: Scattering centers include fluctuations in density and/or composition.
  • Microstructure: Scattering centers include internal surfaces such as grains, grain boundaries, and microscopic pores.

Nature of light

Complete spectrum of electromagnetic radiation with the visible portion highlighted
The spectrum of colors which collectively constitute white (or visible) light, as seen in their dispersion through a triangular dispersive prism

Radiant energy is energy which is propagated in the form of Electromagnetic waves. The type of light which we perceive through our optical sensors (eyes) is referred to as white light, and it is composed of a range of colors (ROYGB: red, orange, yellow, green, blue) over a range of wavelengths, or frequencies. Visible (white) light is only a small fraction of the entire spectrum of electromagnetic radiation. At the short end of that wavelength scale is invisible ultraviolet (UV) light. At even shorter wavelengths than UV are X-rays and gamma-rays. At the longer end of that spectrum is infrared (IR) light, which is used for night vision and other heat-seeking devices. At longer wavelengths than infrared are microwaves (radar), and radio / television waves.

Electromagnetic radiation is classified according to the frequency (or wavelength, which is inversely proportional to the frequency) of the light. This includes (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet (UV) radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. A small window of frequencies, called the visible (or white light) portion of the spectrum, is sensed by the naked eye of various organisms.[2]

The simplest representation of a beam of light is through the use of the light ray. The most important properties of the light ray are that it contains no mass and that it travels along a straight line. Light rays interact with the materials (liquids and solids) in several different ways; it is absorbed, reflected or transmitted by the object. In the case of reflection, the interaction depends on the physical and chemical properties of the substance. If the materials surface is perfectly smooth (e.g. a mirror), rays of light collectively undergo total reflection (or specular reflection), leaving the surface of the glass at a particular angle and all in a parallel line with each other.

Light scattering

Diffuse reflection

Rough and irregular surfaces cause light rays to be reflected in many random directions. This type of reflection is called “diffuse reflection”, and is typically characterized by wide variety of reflection angles. Most of the objects visible to the naked eye are identified via diffuse reflection. Another term commonly used for this type of reflection is “light scattering”. Light scattering from the surfaces of objects is our primary mechanism of physical observation.[3][4]

Light scattering in liquids and solids therefore depends on the wavelength of the light being scattered. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension (or spatial scale) of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometer (one millionth of a meter) scattering centers (or particles) as small as one micrometer have been observed directly in the light microscope (e.g. Brownian motion).[5][6]

Absorption of light in solids

When light strikes an object, it usually has not just a single frequency (or wavelength) but many. Objects have a tendency to selectively absorb, reflect or transmit light of certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. The manner in which visible light interacts with an object is dependent upon the frequency of the light, the nature of the atoms in the object, and often the nature of the electrons in the atoms of the object.

Some materials allow much of the light that falls on them to be transmitted through the material without being reflected. Materials that allow the transmission of light waves through them are called optically transparent. Chemically pure (undoped) window glass and clean river or spring water are prime examples of this.

Materials which do not allow the transmission of any light wave frequencies are called opaque. Such substances have a chemical composition which includes what are referred to as absorption centers. Most materials are composed of materials which are selective in their absorption of white light frequencies. Thus they absorb certain portions of the visible spectrum, while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. In the visible portion of the spectrum, this is what gives rise to color.[7][8]

Meiningen Catholic Church, 20th century glass

Color centers are largely responsible for the appearance of specific wavelengths of visible light all around us. Moving from longer (0.7 micrometer) to shorter (0.4 micrometer) wavelengths: red, orange, yellow, green and blue (ROYGB) can all be identified by our senses in the appearance of color by the selective absorption of specific light wave frequencies (or wavelengths). Mechanisms of selective light wave absorption include:

  • Electronic: Transitions in electron energy levels within the atom (e.g. pigments). These transitions are typically in the ultraviolet (UV) and/or visible portions of the spectrum.
  • Vibrational: Resonance in atomic/molecular vibrational modes. These transitions are typically in the infrared portion of the spectrum.
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UV-Vis: Electronic transitions

In electronic absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons within the atoms which compose the substance. In this case, the electrons will absorb the energy of the light wave and increase their energy state, often moving outward from the nucleus of the atom into an outer shell or orbital.

The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the atomic number Z in the periodic chart). Recall that all light waves are electromagnetic in origin. Thus they are affected strongly when coming into contact with negatively charged electrons in matter. When photons (individual packets of light energy) come in contact with the valence electrons of atom, one of several things can and will occur:

  • An electron absorbs the energy of the photon and sends it back out the way it came in. This results in reflection or scattering.
  • An electron cannot absorb the energy of the photon and the photon continues on its path. This results in transmission (provided no other absorption mechanisms are active).
  • An electron selectively absorbs a portion of the photon, and the remaining frequencies are transmitted in the form of spectral color.

Most of the time, it is a combination of the above that happens to the light that hits an object. The electrons in different materials vary in the range of energy that they can absorb. Most glasses, for example, blocks ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum.

Thus, when a material is illuminated, individual photons of light can make the valence electrons of an atom transition to a higher electronic energy level. The photon is destroyed in the process and the absorbed radiant energy is transformed to electric potential energy. Several things can happen then to the absorbed energy. as it may be re-emitted by the electron as radiant energy (in this case the overall effect is in fact a scattering of light), dissipated to the rest of the material (i.e. transformed into heat), or the electron can be freed from the atom (as in the photoelectric and Compton effects).

Infrared: Bond stretching

Normal modes of vibration in a crystalline solid.

The primary physical mechanism for storing mechanical energy of motion in condensed matter is through heat, or thermal energy. Thermal energy manifests itself as energy of motion. Thus, heat is motion at the atomic and molecular levels. The primary mode of motion in crystalline substances is vibration. Any given atom will vibrate around some mean or average position within a crystalline structure, surrounded by its nearest neighbors. This vibration in 2-dimensions is equivalent to the oscillation of a clock’s pendulum. It swings back and forth symmetrically about some mean or average (vertical) position. Atomic and molecular vibrational frequencies may average on the order of 1012 cycles per second (hertz).

When a light wave of a given frequency strikes a material with particles having the same or (resonant) vibrational frequencies, then those particles will absorb the energy of the light wave and transform it into thermal energy of vibrational motion. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When infrared light of these frequencies strikes an object, the energy is reflected or transmitted.

If the object is transparent, then the light waves are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted.[9][10]

Transparency in insulators

An object may be not transparent either because it reflects the incoming light or because it absorbs the incoming light. Almost all solids reflect a part and absorb a part of the incoming light.

When light falls onto a block of metal, it encounters atoms that are tightly packed in a regular lattice and a "sea of electrons" moving randomly between the atoms.[11] In metals, most of these are non-bonding electrons (or free electrons) as opposed to the bonding electrons typically found in covalently bonded or ionically bonded non-metallic (insulating) solids. In a metallic bond, any potential bonding electrons can easily be lost by the atoms in a crystalline structure. The effect of this delocalization is simply to exaggerate the effect of the "sea of elctrons". As a result of these electrons, most of the incoming light in metals is reflected back, which is why we see a shiny metal surface.

Most insulators (or dielectric materials) are held together by ionic bonds. Thus, these materials do not have free conduction electrons, and the bonding electrons reflect only a small fraction of the incident wave. The remaining frequencies (or wavelengths) are free to propagate (or be transmitted). This class of materials includes all ceramics and glasses.

If a dielectric material does not include light-absorbent additive molecules (pigments, dyes, colorants), it is usually transparent to the spectrum of visible light. Color centers (or dye molecules, or "dopants") in a dielectric absorb a portion of the incoming light wave. The remaining frequencies (or wavelengths) are free to be reflected or transmitted. This is how colored glass is produced.

Most liquids and aqueous solutions are highly transparent. For example, water, cooking oil, rubbing alcohol, air, natural gas, are all clear. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are chiefly responsible for their excellent optical transmission. The ability of liquids to "heal" internal defects via viscous flow is one of the reasons why some fibrous materials (e.g. paper or fabric) increase their apparent transparency when wetted. The liquid fills up numerous voids making the material more structurally homogeneous.

Light scattering in an ideal defect-free crystalline (non-metallic) solid which provides no scattering centers for incoming lightwaves will be due primarily to any effects of anharmonicity within the ordered lattice. Lightwave transmission will be highly directional due to the typical anisotropy of crystalline substances, which includes their symmetry group and Bravais lattice. For example, the seven different crystalline forms of quartz silica (silicon dioxide, SiO2) are all clear, transparent materials.[12]

Optical waveguides

Propagation of light through a multi-mode optical fiber.
A laser beam bouncing down an acrylic rod, illustrating the total internal reflection of light in a multimode optical fiber.

Optically transparent materials focus on the response of a material to incoming light waves of a range of wavelengths. Guided light wave transmission via frequency selective waveguides involves the emerging field of fiber optics and the ability of certain glassy compositions as a transmission medium for a range of frequencies simultaneously (multimode optical fiber) with little or no interference between competing wavelengths or frequencies. This resonant mode of energy and data transmission via electromagnetic (light) wave propagation is relatively lossless.

An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The refractive index is the parameter reflecting the speed of light in a material. (Refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given medium. The refractive index of a vacuum is therefore 1). The larger the refractive index, the more slowly light travels in that medium. Typical values for core and cladding of an optical fiber are 1.48 and 1.46, respectively.

When light traveling in a dense medium hits a boundary at a steep angle, the light will be completely reflected. This effect, called total internal reflection, is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles will be propagated. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding. Optical waveguides are used as components in integrated optical circuits (e.g. combined with lasers or light-emitting diodes, LEDs) or as the transmission medium in local and long haul optical communication systems.

Mechanisms of attenuation

Light attenuation by ZBLAN and silica fibers

Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal.

Attenuation is caused primarily by scattering and absorption. The scattering of light is caused by molecular level irregularities (compositional fluctuations) in the glass structure. This same phenomenon is seen as one of the limiting factors in the transparency of infrared missile domes. Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. In optical fiber, light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.[13][14]

Multi-phonon absorption

Longitudinal (acoustic) compression wave in a 2-dimensional lattice.
Transverse (optical) plane wave

The design of any optically transparent device requires the selection of materials based upon knowledge of their properties and limitations. The lattice absorption characteristics observed at the lower frequency regions (mid-infrared to far-infrared wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching)in the far-infrared spectral region (>10 µm).

The concepts of temperature and thermal equilibrium associated with ionic solids are based on individual atoms and molecules in the system possessing vibrational motion. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies. These thermal vibrational modes are associated with atomic and molecular displacements, producing both longitudinal and transverse waves of atomic and molecular displacement.

In the longitudinal (or acoustic) mode, the displacement of particles from their positions of equilibrium coincides with the propagation direction of the wave. Mechanical longitudinal waves have been also referred to as compression waves. For transverse (or optical) modes, individual particles move perpendicular to the propagation of the wave.

As the rules of quantum mechanics apply to all the different vibrational modes in the solid, the lattice pulsates as a complete assembly in discrete energy steps, or thermal phonons. A phonon is a quantized mode of vibration occurring in a rigid crystal lattice. The study of phonons is an important part of solid state physics, because phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivity.

The phonon is related to both the frequency of vibration and the temperature. If the temperature is raised, the amplitude of vibration increases. The concept of the phonon is therefore considered as the quantum of lattice vibrational energy onto which is superimposed a complex pattern of standing and/or traveling waves that represent changes in temperature. If the solid is at a uniform temperature, the standing wave concept is adequate as the phonon vibrations are uniformly distributed.

Multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation, reaching a maximum coupling with the radiation when the frequency is equal to the vibrational mode of the molecular dipole (e.g. Si-O bond in quartz) in the far-infrared spectral region.

All of the resonant absorption processes involved in an optically transparent material can be explained by the same common principle. At particular frequencies, the incident radiation is allowed to propagate through the lattice producing the observed transparency. Other frequencies however, are forbidden when the incident radiation is at resonance with any of the properties of the lattice material (e.g. molecular vibrational frequencies), and as such are transferred as thermal energy, exciting the atoms or electrons.

In order that a mode of vibration can absorb, a mechanism for coupling the vibrational motion to the electromagnetic radiation must exist. Transfer of electromagnetic radiation from the incident medium to the material is in the form of a couple, where the lattice vibration produces an oscillating dipole moment which can be driven by the oscillating electric field of the light wave, or radiation. Thus, the energy absorbed from the light wave will be converted into vibrational motion of the molecules.

Light scattering in ceramics

Optical transparency in polycrystalline materials is limited by the amount of light which is scattered by their microstructural features. Light scattering depends on the wavelength of the light. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometer, scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materials include microstructural defects such as pores and grain boundaries. The volume fraction of microscopic pores has to be less than 1% for high-quality optical transmission, that is the material density should be 99.99% of the theoretical crystalline density. In addition to pores, most of the interfaces in a typical metal or ceramic object are in the form of grain boundaries which separate tiny regions of crystalline order. When the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.

In the formation of polycrystalline materials (metals and ceramics) the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during formation (or pressing) of the object. Moreover, the size of the grain boundaries scales directly with particle size. Thus a reduction of the original particle size well below the wavelength of visible light (about 1/15 of the light wavelength or roughly 600/15 = 40 nm) eliminates much of light scattering, resulting in a translucent or even transparent material.

Computer modeling of light transmission through translucent ceramic alumina has shown that microscopic pores trapped near grain boundaries cause act as primary scattering centers. The volume fraction of porosity had to be reduced below 1% for high-quality optical transmission (99.99 percent of theoretical density). This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol-gel chemistry and nanotechnology.[15][16][17][18][19][20]

Applications

Transparent ceramics have recently acquired a high degree of interest and notoriety, the basic applications being high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications.

The development of transparent panel products will have other potential advanced applications including high strength, impact-resistant materials that can be used for domestic windows and skylights. Perhaps more important is that walls and other applications will have improved overall strength, especially for high-shear conditions found in high seismic and wind exposures. If the expected improvements in mechanical properties bear out, the traditional limits seen on glazing areas in today's building codes could quickly become outdated if the window area actually contributes to the shear resistance of the wall.

Currently available infrared transparent materials typically exhibit a trade-off between optical performance, mechanical strength and price. For example, sapphire (crystalline alumina) is very strong, but it is expensive and lacks full transparency throughout the 3-5 micrometer mid-infrared range. Yttria is fully transparent from 3-5 micrometers, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. Not surprisingly, a combination of these two materials in the form of the yttrium aluminium garnet (YAG) is one of the top performers in the field

See also

References

  1. ^ Fox, M. (2002). Optical Properties of Solids. Oxford University Press. 
  2. ^ Giancoli, D.C. (1988). Physics for Scientists and Engineers. Prentice Hall. 
  3. ^ Kerker, M. (1969). The Scattering of Light. Academic, New York. 
  4. ^ Mandelstam, L.I. (1926). "Light Scattering by Inhomogeneous Media". Zh. Russ. Fiz-Khim. Ova. 58: 381. 
  5. ^ van de Hulst, H.C. (1981). Light scattering by small particles. New York: Dover. ISBN 0486642283. 
  6. ^ Bohren, C.F. and Huffmann, D.R. (1983). Absorption and scattering of light by small particles. New York: Wiley. 
  7. ^ Simmons, J. and Potter, K.S. (2000). Optical Materials. Academic Press. 
  8. ^ Uhlmann, D.R., et al. (1991). Optical Properties of Glass. Amer. Ceram. Soc.. 
  9. ^ Gunzler, H. and Gremlich, H. (2002). IR Spectroscopy: An Introduction. Wiley. 
  10. ^ Stuart, B. (2004). Infrared Spectroscopy: Fundamentals and Applications. Wiley. 
  11. ^ Mott, N.F. and Jones, H.. Theory of the Properties of Metals and Alloys. Clarendon Press, Oxford (1936) Dover Publications (1958). 
  12. ^ Griffin, A. (1968). "Brillouin Light Scattering from Crystals in the Hydrodynamic Region". Rev. Mod. Phys. 40: 167. doi:10.1103/RevModPhys.40.167. 
  13. ^ Smith, R.G. (1972). "Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering". Appl. Opt. 11: 2489. doi:10.1364/AO.11.002489. 
  14. ^ Archibald, P.S. and Bennett, H.E. (1978). "Scattering from infrared missile domes". Opt. Eng. 17: 647. http://adsabs.harvard.edu/abs/1978SPIE..133...71A. 
  15. ^ Yoldas, B.E. (1979). "Monolithic glass formation by chemical polymerization". J. Mat. Sci. 14: 1843. doi:10.1007/BF00551023. 
  16. ^ Prochazka, S. and Klug, S.J. (1983). "Infrared-Transparent Mullite Ceramic". J. Am. Ceram. Soc. 66: 874. doi:10.1111/j.1151-2916.1983.tb11004.x. 
  17. ^ Ikesue, A., et al. (1995). "Fabrication and Optical Properties of High Performance Polycrystalline Ceramics of Solid State Lasers". J. Am. Ceram. Soc 78: 1033. doi:10.1111/j.1151-2916.1995.tb08433.x. 
  18. ^ Ikesue, A (2002). "Polycrystalline Lasers". Optical Materials 19: 183. doi:10.1016/S0925-3467(01)00217-8. 
  19. ^ Barnakov, Y.A., et al. (2007). "The Progress Towards Transparent Ceramics Fabrication". Proc. SPIE 6552: 111. doi:10.1117/12.721328. 
  20. ^ Yamashita, I., et al. (2008). "Transparent Ceramics". J. Am. Ceram. Soc. 91: 813. doi:10.1111/j.1551-2916.2007.02202.x. 

Further reading

  • Electrodynamics of continuous media, Landau, L. D., Lifshits. E.M. and Pitaevskii, L.P., (Pergamon Press, Oxford, 1984)
  • Laser Light Scattering: Basic Principles and Practice Chu, B., 2nd Edn. (Academic Press, New York 1992)
  • Solid State Laser Engineering, W. Koechner (Springer-Verlag, New York, 1999)
  • Introduction to Chemical Physics, J.C. Slater (McGraw-Hill, New York, 1939)
  • Modern Theory of Solids, F. Seitz, (McGraw-Hill, New York, 1940)
  • Modern Aspects of the Vitreous State, J.D.MacKenzie, Ed. (Butterworths, London, 1960)

External links


s are created using optically transparent materials.]]

In the field of optics, transparency is the material property of allowing light to pass through a substance. The opposite property is opacity. Transparent materials are clear (i.e. they can be seen through). Translucent materials allow light to pass through them only diffusely (i.e. they cannot be seen through clearly).

Thus, when light encounters a material, it can interact with it in several different ways. These interactions depend on the nature of the light (its wavelength, frequency, energy, etc.) and the nature of the material. Light waves interact with the surface of an object primarily by either reflection or transmission. A secondary response is refraction.

Some materials allow much of the light that falls on them to be transmitted through the material without being reflected or refracted. Materials that allow the transmission of light waves through them are called optically transparent. Chemically pure window glass and clean river or spring water are prime examples of this.

Most aqueous solutions are highly transparent. In fact, the vast majority of pure liquids are highly transparent. The random nature of the molecular structure of pure liquids is one of the physical properties which provides its capacity for excellent optical transmission. Since most glasses possess the same molecular stucture as their liquid precursors ("frozen in" liquid structure) the corresponding optical transparency of glass is no mystery.

Materials which do not allow the transmission of any light wave frequencies are called opaque. Many such substances have a chemical composition which includes what are referred to as absorption centers. Thus, many substances are selective in their absorption of white light frequencies. They absorb certain portions of the visible spectrum, while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. This is what gives rise to color.

The attenutation of lightwaves of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering. These are the critical issues which need to be addressed in order to develop a rigorous understanding of the mechanisms responsible for the propagation of lightwaves in condensed matter (liquids and solids).

, 2. translucency, and 3. transparency; behind each panel is a star]]

Contents

Introduction

With regards to the absorption of light, primary material considerations include:

  • At the electronic level, chemical absorption in the UV-Vis portion of the spectrum depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific frequency.
  • At the atomic or molecular level, physical absorption in the IR portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds.

For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible lightwaves. Thus, there is no absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.

With regards to the scattering of light, primary material considerations include:

  • Crystalline stucture: How close-packed its atoms or molecules are, and whether or not the atoms or molecules exhibit the long-range order evidenced in crystalline solids.
  • Glassy structure: Scattering centers include fluctuations in density and/or composition.
  • Microstructure: Scattering centers include internal surfaces such as grains, grain boundaries, and microscopic pores.

In light scattering, the length scale of any or all of these structural features – relative to the wavelength of the light being scattered – is critical.

Nature of light

through a triangular glass prism]]

Radiant energy is energy which is propagated in the form of electromagnetic waves. Most people think of natural sunlight or electrical light, when considering this form of energy. The type of light which we perceive through our optical sensors (eyes) is classified as white light, and is composed of a range of colors (red, orange, yellow, green, blue, indigo, violet) over a range of wavelengths, or frequencies.

Visible (white) light is only a small fraction of the entire spectrum of electromagnetic radiation. At the short end of that wavelength scale is invisible ultraviolet (UV) light from the sun. At the longer end of that spectrum is infrared (IR) light, which is used for night vision and other heat-seeking devices. At even shorter wavelengths than UV are X-rays and Gamma-rays. At longer wavelengths than IR are microwaves (radar), and radio / television waves.

Electromagnetic radiation is classified according to the frequency (or wavelength) of the light wave. This includes (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared (IR) radiation, visible light, ultraviolet (UV) radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and Gamma rays have the shortest. A small window of frequencies, called the visible (or white light) portion of the spectrum, is sensed by the naked eye of various organisms.

The simplest representation of a beam of light is through the use of the “light ray”. The most important property of the light ray is that it contains no mass, and that it travels in a perfectly straight line. Light rays interact with the surfaces of condensed matter (liquids and solids) in several different ways. Visible light waves consist of a continuous range of wavelengths or frequencies. When a light wave (or ray) with a single frequency strikes an object, a number of things could happen.

  1. The light wave could be absorbed by the object.
  2. The light wave could be reflected by the object.
  3. The light wave could be transmitted by the object.

In the case of reflection, the interaction of a light ray with the surface of an object depends on the physical and chemical properties of the substance. If a surface is perfectly smooth (e.g. a glass mirror), rays of light collectively undergo total reflection (or specular reflection), leaving the surface of the glass at a particular angle and all in a parallel line with each other. The angle of reflection can be measured with respect to the normal direction of travel for the light ray.

Light scattering

Rough and irregular surfaces cause light rays to be reflected in many random directions. We refer to this type of reflection as “diffuse reflection”, and is typically characterized by wide variety of reflection angles. Most of the objects that you see with the naked eye are visible due to diffuse reflection. Another term commonly used for this type of reflection is “light scattering”. Light scattering from the surfaces of objects is our primary mechanism of physical observation.[1][2]

Thus light scattering depends on the wavelength of the light being scattered. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension (or spatial scale) of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometre (one millionth of a meter) scattering centers (or particles) as small as one micrometre have been observed directly in the light microscope (e.g. Brownian motion). The indirect imaging of anything smaller (e.g. atoms and molecules) requires the use of scattered electrons or transmitted electrons in a high vacuum Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), or a combination of the two (STEM).

Absorption of light in solids

Rarely however does just a single frequency (or wavelength) of light strike an object. While it does happen, it is more usual that light of many frequencies or even all frequencies are incident towards the surface of objects. When this occurs, objects have a tendency to selectively absorb, reflect or transmit light of certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. The manner in which visible light interacts with an object is dependent upon the frequency of the light, the nature of the atoms in the object, and often the nature of the electrons in the atoms of the object.

Some materials allow much of the light that falls on them to be transmitted through the material without being reflected. Materials that allow the transmission of light waves through them all called optically transparent. Chemically pure (undoped) window glass and clean river or spring water are prime examples of this.

Materials which do not allow the transmission of any light wave frequencies are called opaque. Such substances have a chemical composition which includes what are referred to as absorption centers. Most materials are composed of materials which are selective in their absorption of white light frequencies. Thus they absorb certain portions of the visible spectrum, while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. In the visible portion of the spectrum, this is what gives rise to color.[3][4]

Color centers are largely responsible for the appearance of specific wavelengths of visible light all around us. Moving from longer (0.8 micrometre) to shorter (0.1 micrometre) wavelengths: red, orange, yellow, green, blue, indigo, violet (ROYGBIV) can all be identified by our senses in the appearance of color by the selective absorption of specific light wave frequencies (or wavelengths).

Spectrum of visible colors

Red =     , orange =     , yellow =     , green =     , blue =     , indigo =     , violet =     .

Mechanisms of selective light wave absorption include:

a) Chemical: Transitions in electron energy levels within the atom (e.g. pigments). These transitions are typically in the ultraviolet (UV) and/or visible portions of the spectrum.
b) Physical: Resonance in atomic/molecular vibrational modes. These transitions are typically in the infrared (IR) portion of the spectrum.

UV-Vis: Electronic transitions

In chemical absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons within the atoms which compose the chemical substance. In this case, the electrons will absorb the energy of the light wave and increase their energy state, often moving outward from the nucleus of the atom into an outer shell or orbital.

The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the atomic number Z in the periodic chart). Recall that all light waves are electromagnetic in origin. Thus they are affected strongly when coming into contact with negatively charged electrons in matter. When photons (individual packets of light energy) come in contact with the valence electrons of atom, one of several things can and will occur:

1) An electron absorbs all of the energy of the photon and stores it. This gives rise to luminescence, fluorescence and phosphorescence.

2) An electron absorbs the energy of the photon and sends it back out the way it came in. This results in total reflection.

3) An electron cannot absorb the energy of the photon and the photon continues on its path. This results in total transmission.

4) An electron selectively absorbs a portion of the photon, and the remaining frequencies are transmitted in the form of spectral color.

has been added to produce a bluish color]]


Most of the time, it is a combination of the above that happens to the light that hits an object. The electrons in different materials vary in the range of energy that they can absorb. A lot of glass, for example, blocks out ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum.

Thus, when a material is illuminated, individual photons of light can make the valence electrons of an atom transition to a higher electronic energy level. The photon is destroyed in the process and the absorbed radiant energy is transformed to electric potential energy. Several things can happen then to the absorbed energy. as it may be re-emitted by the electron as radiant energy (in this case the overall effect is in fact a scattering of light), dissipated to the rest of the material (i.e. transformed into heat), or the electron can be freed from the atom (as in the photoelectric and Compton effects).

Infrared (IR): Bond stretching

The primary physical mechanism for storing mechanical energy of motion in condensed matter is through heat, or thermal energy. Thermal energy manifests itself as energy of motion. Thus, heat is motion at the atomic and molecular levels. The primary mode of motion in crystalline substances is vibration. [Since amorphous or glassy substances are actually supercooled liquids, molecular vibrations are accompanied by short-range displacement, which is associated with plastic deformation and viscous flow on various timescales]. Any given atom will vibrate around some mean or average position within a crystalline structure, surrounded by its nearest neighbors. This vibration in 2-dimensions is equivalent to the oscillation of a clock’s pendulum. It swings back and forth symmetrically about some mean or average (vertical) position. Atomic and molecular vibrational frequencies may average on the order of 1012 cycles per second (Hertz).

Vibrational frequencies respond sympathetically not only to other oscillations of identical frequency, but also to other oscillations of resonant frequencies (see Harmonic series). Such frequencies are integer multiples of the fundamental (or natural) frequency. Any light wave having a frequency with a wavelength that is ½, ⅓, ¼, 1/5, 1/6 etc. of the atomic vibrational frequency’s wavelength will be in resonance, and will therefore satisfy the resonant condition. When a light wave with the same natural frequency (or a resonant one) impinges upon an object (liquid or solid), the natural vibrational frequency of the atoms in the object will be reinforced, and the amplitude (intensity) of the vibrations will be increased. The energy of the light wave will be absorbed by the object. That is, if a light wave of a given frequency strikes a material with particles having the same or (resonant) vibrational frequencies, then those particles will absorb the energy of the light wave and transform it into thermal energy of vibrational motion.

The selective absorption of infrared light by a particular material occurs because the selected frequency of the light wave matches the frequency at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strike an object, the energy is reflected or transmitted.

If the object is transparent, then the light waves are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted.

Transparency in insulators

A solid object may be not transparent either because it reflects the incoming light or because it absorbs the incoming light. Of course, almost all solids reflect a part and absorb a part of the incoming light.

E.G. When light falls onto a block of metal, it encounters atoms that are tightly packed in a regular lattice and a "sea of electrons" moving randomly between the atoms[5]

Most of the light is scattered back from this kind of material, which is why we see a shiny metal surface. Metals reflect most of the light because they have free electrons. These electrons are shaken by the electric field of the light which is an electromagnetic wave, and emit two waves. One wave is in the direction of the incoming wave, which is visible as the reflected wave. The other wave is similar in amplitude and in the same direction as the incoming wave which. But since they are not traveling in phase, the deconstructive interference gives rise to a zero amplitude wave. Thus no light is transmitted through liquid or solid metals.

Most insulators (or dielectric materials) are held together by ionic bonds. This class of materials includes all ceramics and glasses. Such substances are not electrical conductors, and thus do not have free conduction electrons. So the electrons bound to atoms (bonding electrons) reflect only a small fraction of the incident wave. The remaining frequencies (or wavelengths) are free to be transmitted.

A dielectric (even a black one) always reflects a part of the incoming light. If it doesn't contain light-absorbent molecules (pigments, dyes, colorants), it is usually transparent to visible light. If you put colorant (dye) molecules in the dielectric, they will absorb a portion of the incoming light wave. The remaining frequencies (or wavelengths) are free to be reflected or transmitted.

Glassy solids are often referred to as supercooled liquids, but possess the mechanical properties of both a solid and a liquid, depending on the time scale under consideration. In their molecular structure, their molecules do not exhibit the long-range order exhibited by crystalline substances. In addition, while a glassy solid does exhibit some viscous fluid flow and plastic deformation, this only occurs on geologic timescales. Thus, it behaves mechanically as a solid for all practical intents and purposes.

Most aqueous solutions are highly transparent. In fact, the vast majority of pure liquids are highly transparent. The random nature of the molecular structure of pure liquids – accompanied by ceaseless mobility and constant rearrangement at the molecular level – is the physical property which provides its capacity for excellent optical transmission.

Since most glasses possess the same molecular stucture as their liquid precursors ("frozen in" liquid structure) their corresponding optical transparency is similar. Structural irregularites on the same length scale as the wavelength of visible light provide gaps and holes which allow portions of the light waves to pass through.[6] It should be noted, however, that this does not imply that they are 100% transparent.

Alternatively, the molecules of any crystalline substance are highly organized in relation to one another. They are typically arranged in a close-packed geometry, and possess a high degree of long-range molecular order and alignment. In a perfect, defect-free single crystal, extensive arrays of long-range symmetry may provide crystal planes whose interplanar spacing provides ideal pathways for the transmission of light waves. This is why many crystalline substances (such as crystalline silica, or quartz) appear to be transparent.

As the substance changes from a solid to a liquid, however, that high degree of order at the molecular level is lost. Thus, the molecules in glass are not packed into a tight lattice. Unless the glass is tinted (or “doped”), it does not contain molecules that capture light with a particular energy. So when light encounters glass, most of it passes straight through, or is transmitted. Hence, many glasses (such as amorphous silica, SiO2) are transparent.

Most liquids and gases are transparent. For example, water, cooking oil, rubbing alcohol, air, natural gas, are all clear. This is due to a fundamental structural difference between solids and liquids (or glasses, "supercooled liquids") at the atomic or molecular level. This transition form order to disorder is the primary reason that most liquids and glasses are transparent.

The standard definition of a glass (or vitreous solid) requires the solid phase to be formed by rapid melt quenching. Glass is therefore formed via a supercooled liquid, and cooled sufficiently rapidly (relative to the characteristic crystallization time) that the disordered atomic or molecular configuration is frozen into the solid state. Thus, the structure of a glass exists in a metastable state with respect to its crystalline form. By definition as an amorphous solid, the atomic structure of a glass lacks any long range translational periodicity. However, by virtue of the local chemical bonding constraints glasses do possess a high degree of short-range order with respect to local atomic and/or molecular geometries in the form of polyhedra.

This notion might be compared to a pile of bricks stacked neatly on top of one another. But as the substance changes to a liquid or glass, the molecules are not stacked neatly anymore. Randomly spaced crystalline defects (e.g. point or line vacancies, interfacial boundaries, etc.) may exist which allow portions of the light waves to pass through. That is, the greater the randomness of the molecular organization of the substance, the easier it is for the light to pass through. A more formal treatment of this subject relies upon the identification of microstructural features in glasses at the molecular level which act as scattering centers for a range of EM wavelengths.

Scattering in glasses and liquids

Thermal motion in liquids can be decomposed into elementary longitudinal vibrations (or acoustic phonons) while transverse vibrations (or shear waves) were originally described only in elastic solids exhibiting the highly ordered crystalline state of matter. This is the fundamental reason why simple liquids cannot support a shearing stress, but rather yield via macroscopic plastic deformation (or viscous flow). Thus, the fact that a solid deforms while retaining its rigidity while a liquid yields to macroscopic viscous flow in response to the application of a shearing force is accepted by many as the mechanical distinction between the two. [7][8][9]

The inadequacies of this conclusion, however, were pointed out by Frenkel in his revision of the theory of elasticity in liquids. This revision follows directly from the continuous characteristic of the structural transition from the liquid state into the solid one when this transition is not accompanied by crystallization – ergo the supercooled liquid. Thus we see the intimate correlation between transverse acoustic phonons (or shear waves) and the onset of rigidity upon vitrification, as described by Bartenev in his mechanical description of the vitrification process. [10]

The relationship between these transverse waves and the mechanism of vitrification has been described by one author who proposed that the onset of correlations between such phonons results in an orientational ordering or "freezing" of local shear stresses in glass-forming liquids, thus yielding the glass transition. Molecular motion in condensed matter can therefore be represented by a Fourier series whose physical interpretation consists of a superposition of supersonic longitudinal and transverse waves of atomic displacement with varying directions and wavelengths. In monatomic systems, we call these waves: density fluctuations. [Note: In polyatomic systems, they may also include compositional fluctuations. [11] [12] [13]

The velocities of longitudinal acoustic phonons in condensed matter are directly responsible for the thermal conductivity which levels out temperature differentials between compressed and expanded volume elements. Kittel proposed that the behavior of glasses is interpreted in terms of an approximately constant "mean free path" for lattice phonons, and that the value of the mean free path is of the order of magnitude of the scale of disorder in the molecular structure of a liquid or solid. Klemens subsequently emphasized that heat transport in dielectric solids occurs through elastic vibrations of the lattice, and that this transport is limited by elastic scattering of acoustic phonons by lattice defects (e.g. randomly spaced vacancies). These predictions were confirmed by experiments on commercial glasses and glass ceramics, where mean free paths were apparently limited by "internal boundary scattering" to length scales of 10 - 100 micrometres. [14] [15][16] [17]

The first theoretical study of the light scattering by thermal phonons was done by Mandelstam in 1918 (see Fabelinskii, 1968; Landau, et al.,1984) and published in 1926 (Mandelstam, 1926). Brillouin predicted independently the scattering of light from thermally excited acoustic waves. Gross gave the experimental confirmation of such a prediction in liquids and crystals. With the development of laser technology, the original experiments using the technique of Brillouin scattering on fused silica glass confirmed the existence of structural interfaces and defects on spatial scales of 10 - 100 micrometres. The mechanism of the absorption of sound in solids – which is responsible for the damping of elastic waves of atomic and molecular displacement (or density and compositional fluctuations) – was considered by Akheiser, who regarded the absorption as arising partly from heat flow and partly from viscous damping. In this interpretation, modulated phonons "relax" towards local thermal equilibrium via anharmonic phonon-phonon collisions. This relaxation is an entropy producing process which removes energy from the sound wave driving it, and thus damps it. These conclusions would appear to be consistent with Zener's interpretation of internal friction in crystalline solids being due to intergranular thermal currents. [18] [19] [20]

Improvements in the theory have made it possible to reasonably predict the acoustic loss of non-crystalline solids from the known thermal and elastic properties. Results indicate that infrared optical vibrational modes can contribute to such phenomena. This is not surprising in light of the notion that optic phonons can indeed carry heat in crystalline solids if the acousto-optic energy gap is small enough, and if the optic phonon group velocity is large enough. [21][22]

Mechanisms of attenuation of high-frequency shear modes and longitudinal waves were considered by Mason, et al. at Bell Labs with viscous liquids, polymers and glasses. The subsequent work in the Physics Department of the Catholic University of America led to an entirely new interpretation of the glass transition in viscous liquids in terms of a spectrum of structural relaxation phenomena occurring over a range of length and time scales. Experimentally, the use of light scattering experiments makes possible the study of molecular processes from time intervals as short as 10−11 sec. This is equivalent to extending the available frequency range from 109 Hz or greater than 109 Hz. [23] [24] [25] [26] [27] [28] [29]

According to both Brillouin and Fabelinskii, the scattering is caused by the diffraction of incident planar monochromatic light waves by spontaneous, sinusoidal density fluctuations (i.e. standing thermal sound waves, or acoustic phonons). The light wave is considered to be scattered by the density maximum or amplitude of the acoustic phonon, in the same manner that X-rays are scattered by the crystal planes in a solid. The role of the crystal planes in this process is analogous to the planes of the sound waves or density fluctuations.

Density fluctuations are responsible for the phenomenon of critical opalescence, which arises in the region of a continuous, or second-order, phase transition. The phenomenon is most commonly demonstrated in binary fluid mixtures, such as methanol and cyclohexane. As the critical point is approached the sizes of the gas and liquid region begin to fluctuate over increasingly large length scales. As the length scale of the density fluctuations approaches the wavelength of light, the light is scattered and causes the normally transparent fluid to appear cloudy.

The study of light scattering by thermally driven density fluctuations (or Brillouin scattering) has been utilized successfully for the measurement of structural relaxation and viscoelasticity in liquids, as well as phase separation, vitrification and compressibilities in glasses. In addition, the introduction of Dynamic Light Scattering (or Photon Correlation Spectroscopy)has made possible the measurement of the time dependence of spatial correlations in liquids and glasses in the relaxation time gap between 10−6  sec and 10−2  sec in addition to even shorter time scales – or faster relaxation events. [30] [31] [32] [33] [34] [35]

It has therefore become quite clear that light scattering is an extremely useful tool for monitoring the dynamics of structural relaxation in glasses on various temporal and spatial scales and therefore provides an ideal tool for quantifying the capacity of various glass compositions for guided light wave transmission well into the far infrared portions of EM spectrum.[36][37][38][39]

Optical waveguides

Optically transparent materials focus on the response of a material to incoming light waves of a range of wavelengths. Frequency selective optical filters can be utilized to alter or enhance the brightness and contrast of a digital image. Guided light wave transmission via frequency selective waveguides involves the emerging field of fiber optics and the ability of certain glassy compositions as a transmission medium for a range of frequencies simultaneously (multimode optical fiber) with little or no interference between competing wavelengths or frequencies. This resonant mode of energy and data transmission via electromagnetic (light) wave propagation, though low powered, is relatively lossless.

An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The index of refraction is a way of measuring the speed of light in a material. (Note: The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in a given medium. (The index of refraction of a vacuum is therefore equal to 1, by definition). The larger the index of refraction, the more slowly light travels in that medium. Typical values for core and cladding of an optical fiber are 1.48 and 1.46, respectively.

rod, illustrating the total internal reflection of light in a multimode optical fiber.]]

When light traveling in a dense medium hits a boundary at a steep angle, the light will be completely reflected. This effect is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles will be propagated. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding.

Optical waveguides are used as components in integrated optical circuits (e.g. light-emitting diodes, LEDs) or as the transmission medium in local and long haul optical communication systems. Also of value to the emerging materials scientist is the sensitivity of materials to thermal radiation in the infrared (IR) portion of the EM spectrum. This infrared homing (or "heat-seeking") capability is responsible for such diverse optical phenomena as "night vision" and IR luminescence.

Mechanisms of attenuation

Attenuation in fibre optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance travelled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is typically usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal.

Attenuation is caused primarily by scattering and absorption. The scattering of light is caused by molecular level irregularities (compositional fluctuations) in the glass structure. This same phenomenon is seen as one of the limiting factors in the tranparency of IR missile domes. Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. In optical fiber, light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.[40]

Multi-phonon absorption

The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The lattice absorption characteristics observed at the lower frequency regions (mid IR to far-infrared wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching)in the far-infrared (>10 µm).

The concepts of temperature and thermal equilibrium associated with ionic solids are based on individual atoms and molecules in the system possessing vibrational motion. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies. These thermal vibrational modes are associated with atomic and molecular displacements, producing both longitudinal and transverse waves of atomic and molecular displacement.

In the longitudinal (or acoustic) mode, the displacement of particles from their positions of equilibrium coincides with the propagation direction of the wave. Mechanical longitudinal waves have been also referred to as compression waves. For transverse (or optical) modes, individual particles move perpendicular to the propagation of the wave.

As the rules of quantum mechanics apply to all the different vibrational modes in the solid, the lattice pulsates as a complete assembly in discrete energy steps, or thermal phonons. A phonon is a quantized mode of vibration occurring in a rigid crystal lattice. The study of phonons is an important part of solid state physics, because phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivity.

The phonon is related to both the frequency of vibration and the temperature. If the temperature is raised, the amplitude of vibration increases. The concept of the phonon is therefore considered as the quantum of lattice vibrational energy onto which is superimposed a complex pattern of standing and/or travelling waves that represent changes in temperature. If the solid is at a uniform temperature, the standing wave concept is adequate as the phonon vibrations are uniformly distributed.

Multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation, reaching a maximum coupling with the radiation when the frequency is equal to the vibrational mode of the molecular dipole (e.g. Si-O bond) in the far-infrared.

All of the resonant absorption processes involved in an optically transparent material can be explained by the same common principal. At particular frequencies, the incident radiation is allowed to propagate through the lattice producing the observed transparency. Other frequencies however, are forbidden when the incident radiation is at resonance with any of the properties of the lattice material (e.g. molecular vibrational frequencies), and as such are transferred as thermal energy, exciting the atoms or electrons.

In order that a mode of vibration can absorb, a mechanism for coupling the vibrational motion to the electromagnetic radiation must exist. Transfer of electromagnetic radiation from the incident medium to the material is in the form of a couple, where the lattice vibration produces an oscillating dipole moment which can be driven by the oscillating electric field (E) of the light wave, or radiation. Thus, the energy absorbed from the light wave will be converted into vibrational motion of the molecules.

Light scattering in ceramics

Optical transparency in polycrystalline materials is limited by the amount of light which is scattered by their microstructural features.

Light scattering depends on the wavelength of the light being scattered. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometre (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materials include microstructural defects such as pores and grain boundaries.

Thus, opacity results from the incoherent scattering of light at surfaces and interfaces. In addition to pores, most of the interfaces in a typical metal or ceramic object are in the form of grain boundaries which separate tiny regions of crystalline order. When the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.

In the formation of polycrystalline materials (metals and ceramics) the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during formation (or pressing) of the object. Moreover, the size of the grain boundaries scales directly with particle size. Thus a reduction of the original particle size below the wavelength of visible light (~ 0.3 micrometres for shortwave violet) eliminates any light scattering, resulting in a transparent material.

Furthermore, in the early 1970s, a GE chemist pioneered the computer modeling of light transmission through translucent ceramic alumina. His model showed that microscopic pores in ceramic, mainly trapped at the junctions of microcrystalline grains, caused light to scatter and prevented true transparency. The volume fraction of these microscopic pores had to be less than 1% for high-quality optical transmission. I.E. the density had to be 99.99 percent of the theoretical crystalline density.

This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol-gel chemistry and nanotechnology.[41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]

Applications

Transparent ceramics have recently acquired a high degree of interest and notoriety, the basic applications being high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications.

The development of transparent panel products will have other potential advanced applications including high strength, impact-resistant materials that can be used for domestic windows and skylights. Perhaps more important is that walls and other applications will have improved overall strength, especially for high-shear conditions found in high seismic and wind exposures. If the expected improvements in mechanical properties bear out, the traditional limits seen on glazing areas in today's building codes could quickly become outdated if the window area actually contributes to the shear resistance of the wall.

Currently available IR transparent materials typically exhibit a trade-off between optical performance and mechanical strength. E.G. sapphire (crystalline alumina) is very strong, but lacks full transparency throughout the 3-5 micrometre mid-IR range. Yttria is fully transparent from 3-5 micrometres, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. Not surprisingly, a combination of these two materials in the form of the yttria-alumina garnet (YAG) has proved to be one of the top performers in the field.

High-powered lasers

Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) has proven to be one of the best solid-state laser materials in the history of quantum mechanics. Its indisputable dominance in a broad variety of laser applications is determined by a combination of high emission cross section with long spontaneous emission lifetime, high damage threshold, mechanical strength, thermal conductivity, and low thermal beam distortion. The fact that the Czochralski crystal growth of Nd:YAG is a matured, highly reproducible and relatively simple technological procedure adds significantly to the value of the material.

Large ceramic laser elements can be produced at a relatively low cost. These components are free of internal stress or intrinsic birefringence, and allow relatively large doping levels or optimized custom-designed doping profiles. This makes ceramic laser elements particularly important for high-energy laser applications. Thus, a 1.46 kW Nd:YAG laser has been demonstrated Konoshima Chemical Co. in Japan.

Livermore researchers realized that these ceramics might greatly benefit high-powered lasers used in the National Ignition Facility (NIF) Programs Directorate. In particular, a Livermore research team began to acquire advanced transparent ceramics from Konoshima to determine if they could meet the optical requirements needed for Livermore's Solid-State Heat Capacity Laser (SSHCL). Livermore researchers have also been testing applications of these materials for applications such as advanced drivers for laser-driven fusion power plants.

Armor and IR windows

There is an increasing need in the military sector for high-strength, robust materials which have the capability to transmit light (electromagnetic waves) in the visible (0.2 – 0.8 micrometre) and mid-Infrared (1 – 5 micrometres) regions of the spectrum. These materials are needed for applications requiring transparent armor.

seen through Night Vision Goggles]]

Transparent armor is a material or system of materials designed to be optically transparent, yet protect from fragmentation or ballistic impacts. The primary requirement for a transparent armor system is to not only defeat the designated threat but also provide a multi-hit capability with minimized distortion of surrounding areas. Transparent armor windows must also be compatible with night vision equipment. New materials that are thinner, lightweight, and offer better ballistic performance are being sought.

Existing transparent armor systems typically have many layers, separated by polymer (e.g. polycarbonate) interlayers. The polymer interlayer is used to mitigate the stresses from thermal expansion mismatches, as well as to stop crack propagation from ceramic to polymer. The polycarbonate is also currently used in applications such as visors, face shields and laser protection goggles. The search for lighter materials has also led to investigations into other polymeric materials such as transparent nylons, polyurethane, and acrylics. The optical properties and durability of transparent plastics limit their use in armor applications. E.G. Investigations carried out in the 1970’s had shown promise for the use of polyurethane as armor material, but the optical properties were not adequate for transparent armor applications.

Several glasses are utilized in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glasses, and fused silica. Plate glass has been the most common glass used due to its low cost, but greater requirements for the optical properties and ballistic performance have generated the need for new materials. Chemical or thermal treatments can increase the strength of glasses, and the controlled crystallization of certain glass systems can produce transparent glass-ceramics. The AREVA T&D Technology Centre (Stafford, UK), currently produces a lithium disilicate based glass-ceramic known as TransArm, for use in transparent armor systems. The inherent advantages of glasses and glass-ceramics include having lower cost than most other ceramic materials, the ability to be produced in curved shapes, and the ability to be formed into large sheets.

Transparent crystalline ceramics are also used to defeat advanced threats. Three major transparent candidates currently exist: aluminum oxynitride (AlON), magnesium aluminate spinel (spinel), and single crystal aluminum oxide (sapphire). Aluminum oxynitride spinel (Al23O27N5), one of the leading candidates for transparent armor, is produced by Raytheon Corporation as AlON and marketed under the trade name Raytran.

The incorporation of nitrogen into an aluminum oxide stabilizes a spinel phase, which due to its cubic crystal structure, is an isotropic material that can be produced as a transparent polycrystalline material. Polycrystalline materials can be produced in complex geometries using conventional ceramic forming techniques such as pressing, (hot) isostatic pressing, and slip casting.

Raytheon has produced an 11-inch (Template:Convert/LoffAonSon) x 11-inch curved AlON window, and is currently investigating the scale-up and cost reduction of aluminum oxynitride. The Air Force Research Laboratory (AFRL) is currently funding Raytheon to investigate cost reduction of AlON to produce larger windows, which will allow Raytheon to scale-up AlON such that it can be produced in large sizes at reasonable costs. The Army Research Laboratory is simultaneously investigating transient liquid phase sintering of aluminum oxynitride to reduce processing costs. A reaction sintering technique using a reactive liquid is the focus of the investigation, producing small samples with transmission of 85% and haze of 14% as seen on Figure 3.

Magnesium aluminate spinel (MgAl2O4) is a ceramic with a cubic crystal structure and is transparent in its polycrystalline form. It has been shown that the use of a hot isostatic press can improve its optical and physical properties. Spinel offers some processing advantages over AlON, such as the fact that spinel powder is available from commercial manufacturers while AlON powders are proprietary to Raytheon.

It is also capable of being processed at much lower temperatures than AlON, and has been shown to possess superior optical properties within the IR region. Spinel shows promise for many applications, but is currently not available in bulk form from any manufacturer. Efforts to commercialize spinel are underway.

Single crystal aluminum oxide (Sapphire - Al2O3) is a transparent ceramic. Sapphire’s crystal structure is rhombohedral and its anisotropic properties vary with crystallographic orientation. Transparent alumina is currently one of the most mature transparent ceramics, and is available from several manufacturers. But the cost is high due to the processing temperature involved, as well as machining costs to cut parts out of single crystal boules. It also has a very high mechanical strength – but that is dependent on the surface finish.

There are current programs to scale-up sapphire grown by the heat exchanger method or edge defined film-fed growth processes. Its maturity stems from its use in the EM windows and semiconductor industries. Crystal Systems Inc., which uses single crystal growth techniques, is currently scaling their sapphire boules to 13-inch (Template:Convert/LoffAonSon) diameter and larger.

Saphikon, Inc. produces transparent sapphire using an edge, defined growth technique. Sapphire grown by this technique produces an optically inferior material to that which is grown via single crystal techniques, but is much less expensive. Saphikon is currently capable of producing ¼" thick sapphire, in 12" x 15" sheets. ARL is currently investigating use of this material in a laminate design for transparent armor systems.

IR Missile Guidance

AIM-9 Sidewinder
Place of origin United States

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles (electrons and protons) within atoms is converted to electromagnetic radiation.

Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as “heat-seekers”, since infrared (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infra-red wavelengths of light compared to objects in the background.

The current material of choice for high speed IR-guided missile domes is single crystal sapphire. The material is not used because it is optimal for the application. It is used because it is the best material currently available. The optical transmission of sapphire does not actually extend to cover the entire mid-wave IR band (3-5 µm), but starts to drop off at wavelengths greater than approximately 4.5 µm at room temperature. While the strength of sapphire is better than that of other available mid-range IR dome materials at room temperature, it weakens above approximately 600°C.

Alternate materials, such as yttrium oxide, offer better optical performance, but inferior mechanical durability. Future high speed IR-guided missiles will require new domes that are substantially more durable than those in use today, while still retaining maximum transparency across the entire operational waveband. A long standing trade-off exists between optical bandpass and mechanical durability within the current collection of single-phase IR transmitting materials, forcing missile designers to compromise on system performance. Optical nanocomposites may present the opportunity to engineer new materials that overcome this traditional compromise.

In optical nanocomposites, two or more interpenetrating phases are mixed in a sub-micrometre grain sized, fully dense body. IR light scattering can be minimized (or even eliminated) in the material as long as the grain size of the individual phases is significantly smaller than IR wavelengths. Experimental data suggests that limiting the grain size of the nanocomposite to approximately 1/15th of the wavelength of light is sufficient to limit scattering.

Nanocomposites of yttria and magnesia have been produced with a grain size of approximately 200 nm. These materials have yielded near theoretical transmission in the 3-5 µm IR band. Additionally, such composites have yielded higher strengths than those observed for single phase individual constituents. It is anticipated that further development will result in high strength, high transparency materials suitable for application as next generation high speed missile domes.

Enhancement of mechanical properties in nanocomposite ceramic materials has been extensively studied. Significant increases in strength (2-5 times), toughness (1-4 times), and creep resistance have been observed in systems including SiC/Al2O3, SiC/Si3N4, SiC/MgO, and Al2O3/ZrO2. In addition, excellent optical transparency in the 3-5 µm mid-wave IR band has been demonstrated in yttria-magnesia nanocomposite ceramics. Uniform microstructures have been obtained experimentally, with a characteristic grain size of approximately 200 nm. Increases in strength and hardness were observed over that of pure transparent yttria or magnesia.

These results were obtained using a non-optimized processing scheme. Further improvements in performance are expected as processing is optimized. These preliminary results for optical nanocomposite materials offer much promise as high speed missile dome materials.

See also

References

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Further reading

  • M. Kerker: The scattering of light, New York, Academic, 1969.
  • H. C. van de Hulst: Light scattering by small particles, New York, Dover, 1981.
  • C. F. Bohren, D. R. Huffmann: Absorption and scattering of light by small particles. New York, Wiley-Interscience, 1983.
  • P. W. Barber, S. S. Hill: Light scattering by particles: Computational methods. Singapore, World Scientific, 1990.
  • G. Mie, “Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen,” Leipzig, Ann. Phys. 330, 377–445 (1908)[1]
  • M. Mishchenko, L. Travis, A. Lacis: Scattering, Absorption, and Emission of Light by Small Particles, Cambridge University Press, 2002.
  • Molecular Scattering of Light, Fabelinskii, I. L., (PLenum Press, New York 1968)
  • The Structure and Mechanical Properties of Inorganic Glasses, Bartenev, G.M. (Wolters-Noordhoof, 1970)
  • Electrodynamics of continuous media, Landau, L. D., Lifshits. E.M. and Pitaevskii, L.P., (Pergamon Press, Oxford, 1984)
  • Brillouin Scattering, Grimsditch, M., Handbook of Elastic Properties of Solids, Liquids, and Gases. Volume I: Dynamic Methods for Measuring the Elastic Properties of Solids, M. Levy, H. Bass, R. Stern and V. Keppens, Eds. (Academic Press, New York, 2001)
  • Laser Light Scattering: Basic Principles and Practice Chu, B., 2nd Edn. (Academic Press, New York 1992)
  • Introduction to Chemical Physics, J.C. Slater (McGraw-Hill, New York, 1939)
  • Modern Theory of Solids, F. Seitz, (McGraw-Hill, New York, 1940)
  • Modern Aspects of the Vitreous State, J.D.MacKenzie, Ed. (Butterworths, London, 1960)
  • Low Temperature Solid State Physics, Rosenburg, H.M. (Clarendon Press, Oxford, 1963)
  • Introduction to Solid State Physics, Kittel, C. (Wiley Interscience, 8th Edn., 2004)
  • Introduction to Ceramics, Kingery, W.D., Bowen, H.K. and Uhlmann, D.R., (John Wiley & Sons, New York, 1976)
  • Elements of Materials Science and Engineering, Van Vlack, L.H. (Prentice-Hall 1989)
  • Physics for Scientists and Engineers, Giancoli, D.C., (Prentice Hall, 1988)
  • Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, C.J. Brinker and G.W. Scherer (Academic Press, 1990)
  • Sol-Gel Materials: Chemistry and Applications, J.D. Wright and N. Sommerdijk
  • Sol-Gel Technologies for Glass Producers and Users, M.A. Aegerter and M. Mennig
  • Sol-Gel Optics: Processing and Applications, L. Klein, (Springer Verlag 1994)
  • Sol-Gel: A Low temperature Process for the Materials of the New Millenium, Jean Phalippou (2000) www.solgel.com/articles
  • IR Spectroscopy: An Introduction, H. Gunzler and H. Gremlich (Wiley-VCH, Verlag, Germany, 2002)
  • Infrared Spectroscopy: Fundamentals and Applications, B. Stuart (John Wiley & Sons, Ltd., England, 2004)
  • Solid State Laser Engineering, W. Koechner (Springer-Verlag, New York, 1999)
  • Zarzycki, J., Glasses and the Vitreous State, (Cambridge University Press, 1991)

External Sources


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