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Absorption

Definition: a process where light energy is converted to another form of energy

More specific terms: infrared absorption, excited-state absorption, pump absorption, light-induced absorption, multiphonon absorption, multiphoton absorption, two-photon absorption, pump absorption

German: Absorption

Category: general optics

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Through different kinds of processes, which are explained in the following, light can be absorbed in various media. This implies that the optical energy is converted into some other form of energy (but sometimes back again to optical energy). In most cases, the energy is eventually transformed into heat (thermal energy).

The term absorption is not only used for absorption processes, but also often for related quantities, e.g. instead of absorption coefficient.

Strong enough absorption can make an object opaque, or at least introduce some level of opacity.

Absorption Processes

A general distinction is between intrinsic and extrinsic absorption. Extrinsic absorption (also sometimes called parasitic absorption) results from things which could in principle be avoided – for example from impurities and structural defects which could be absent in pure high quality material. Intrinsic absorption results from basic properties of the pure material.

Intrinsic Absorption Processes

The following kind of physical processes can be involved in the intrinsic linear absorption of light:

  • In an insulating material (dielectric) or a semiconductor, having a certain band gap energy, strong linear light absorption occurs when the photon energy of the light exceeds the bandgap energy (which is often possible only in the ultraviolet spectral region). Each absorption of a photon causes the excitation of one electric carrier across the bandgap, so that one obtains one additional carrier in the conduction band and one hole in the valence band. The electronic excitation energy can subsequently be converted to heat (i.e., to multiple phonons) or partially to fluorescence light, often within nanoseconds or even faster.
  • Massive linear absorption in ionic crystals and in glasses is possible at long (infrared) wavelengths through coupling to optical phonons. At somewhat shorter wavelengths, substantial multiphonon absorption is normally obtained. For example, in fused silica that happens for wavelengths beyond the infrared absorption edge at approximately 2 μm.
  • The situation is different in metals, where the conduction band is not completely filled. Due to strong reflection, one can have light absorption only within a thin surface layer, which can nevertheless be substantial in certain wavelength regions. The absorption properties can also depend on the roughness of the surface, and are sometimes intentionally modified by nano-structuring.

There are also many cases where a material contains some absorbing dopant while the host material itself exhibits only negligible absorption. This is the case for solid-state (doped-insulator) gain media.

Extrinsic Absorption Processes

Various types of processes, which would in principle be avoidable, lead to extrinsic absorption for example in optical glasses, in nonlinear crystal materials and in laser crystals:

  • There can be ionic impurities which introduce absorption based on electronic transitions. These are usually either rare earth or transition metal materials. Depending on the type of elements (Cr, Cu or Fe), their charge state and their chemical environment (ligands), their absorption spectra can be quite different. Absorption lines in the visible spectral range lead to a color appearance if the absorption is strong enough.
  • Similarly, semiconductor nanocrystals can lead to absorption, in that case related to interband transitions for photon energies above the bandgap energy. Such semiconductor nano crystals are often formed only in a secondary heat treatment, the details of which can substantially influence the absorption features.
  • Metallic nanoparticles also introduce absorption, apart from scattering.
  • Various types of lattice defects can also introduce absorption. See the article on color centers.

Impurities can also modify intrinsic absorption features – for example, shift the band gap energy and the corresponding absorption edge when a semiconductor compound is formed.

Linear and Nonlinear Absorption

Linear absorption means that the absorption coefficient is independent of the optical intensity. There are also nonlinear absorption processes, where the absorption coefficient is a linear or higher-order function of the intensity. For example, two-photon absorption is a process where two photons are absorbed simultaneously, and the absorption coefficient rises linearly with the intensity. Multiphoton absorption processes of higher order are often involved in laser-induced damage caused by intense laser pulses.

Saturable absorption can also be considered as a kind of nonlinear absorption. Here, however, the absorption coefficient is reduced under the influence of intense light, e.g. because the starting electronic level for the light absorption is depleted.

Effects on the Absorbing Medium and its Optical Properties

As light carries energy, the absorption of light is associated with the deposition of energy in the absorbing medium. In most cases, that energy is mostly converted into heat, although sometimes a substantial amount of the received energy is radiated away as fluorescence.

Light absorption processes e.g. in solid materials generally arise from the interaction of the electromagnetic wave with electrons, exciting those to excited energy levels. Thereafter, it takes some time (the electron–lattice thermalization time) for that energy to be transferred to the atomic nuclei, i.e., to vibration energy. That typically happens within a couple of picoseconds, and thereafter it takes far longer times to distribute that heat over some volume of the medium. That means that the thermalization, let alone the heat conduction, can take far more time than the pulse duration of a femtosecond laser. That has important implications for laser material processing with ultrafast lasers, where the involved processes cannot be understood as simply heating up the material. Instead, one is dealing with highly non-equilibrium states of matter, which can lead to rapid application of material while very nearby other material, not directly hit by the laser radiation, is not even significantly heated.

Further, the modified population of electronic states can substantially modify the absorption at the wavelength of the absorbed light and also at other wavelengths. It has already been mentioned above that absorption may be saturated. In other cases, light absorption is strongly increased by the light-induced changes of the state of matter. That is often exploited in laser material processing, where the initial absorption e.g. by a metal is weak, but strongly increases once the material is strongly excited (anomalous absorption). In various materials, one may obtain excited-state absorption at wavelengths where the material would normally not be absorbing. In semiconductors, at high intensities one obtains free carrier absorption.

If absorption of light causes heating of the absorbing medium, that will subsequently lead to thermal expansion. The heating is often strongly inhomogeneous; for example, it may occur within a focused laser beam. The local thermal expansion then leads to mechanical stress in the medium, which can even result in fracture when the deposited thermal power or energy is sufficiently high. Further, the temperature causes a slight local modification of the refractive index, which (together with stress-related effects) can cause thermal lensing effects.

Absorption of light can also have electrical effects. For example, there are photoresistors, where the electrical resistance is reduced by absorbed light. In photodiodes and phototransistors, one exploits the internal photoelectric effect, related to the excitation of electric carriers by light absorption.

In some special cases, nearly all of the absorbed light causes fluorescence rather than heat, and there can be even a net cooling effect (→ laser cooling). It may even happen that at some (typically longer) wavelengths one obtains laser amplification for strong enough excitation of the medium, usually involving a population inversion. The medium may then generate laser radiation which may remove a substantial fraction of the deposited energy.

If light is absorbed by atoms or molecules of a gas, light forces associated with the absorption may become relevant. They can be used for Doppler cooling, for example.

Quantifying Absorption

Absorption in a semi-transparent medium is usually quantified with an absorption coefficient, telling which fraction of the optical power is lost per unit length. The inverse of an absorption coefficient is called an absorption length. The absorption of a given length of material (e.g. of a plate with a certain thickness) can be quantified with an absorbance.

If absorption is caused by some absorbing dopant, the contribution to the absorption per dopant atom or ion is often quantified with an absorption cross section.

As absorption coefficients are wavelength-dependent, one often produces absorption spectra, showing an absorption coefficient as a function of wavelength or optical frequency.

Non-transparent objects can be attributed an absorptance, which is the fraction of incident light which is absorbed rather than transmitted, reflected or scattered.

Generation of Quantum Noise via Absorption

Even simple linear absorption processes introduce some amount of quantum noise. This can be intuitively understood by considering that some of the incident photon are randomly removed, while other photons remain in the light beam. An initially perfectly regular stream of photons (→ amplitude-squeezed light) would thus be converted into a random stream of photons, exhibiting some intensity noise.

If the incident light is in a coherent state, exhibiting the standard shot noise level, the extra noise added through linear absorption is just enough to keep the residual light at the shot noise level (which is relatively stronger for weaker light).

Nonlinear absorption processes can modify quantum noise properties in more complicated ways.

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See also: absorption coefficient, absorption length, absorbance, absorptance, nonlinear absorption, two-photon absorption, quantum noise
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