Photon effects

Although the list of photon effects in Table 2.1 is extensive, only the photoconductive, photovoltaic, and photoemissive ones have been widely 

The most widely employed effect is photoconductivity, in which the radiation changes the electrical conductivity of the material upon which it is incident. In nearly all cases the change in conductivity is measured by means of electrodes attached to the sample. It is possible, however, to observe the effect by inserting the sample in a microwave cavity, such as Sommers [2.30] has done. Because the microwave method of detecting photoconductivity is very seldom used, it will not be discussed here. 

Intrinsic and Extrinsic. Photoconductivity can be observed in virtually all semiconductors. Intrinsic photoconductivity requires the excitation of a free 

No intrinsic photoconductivity occurs for radiation of wavelength greater than Aq. a convenient expression to determine the long wavelength limit in micrometers of an intrinsic photoeffect for a semiconductor whose energy gap is expressed in electron volts is 

T able 2.2 lists energy gaps of selected semiconductors which have been employed as photodetectors at the temperatures indicated. The corresponding long wavelength limits are shown. 

1 Excitation of additional carriers Photoconductivity Electrically biased Intrinsic Extrinsic 

Microwave biased Photovoltaic effect p — M junction Avalanche p — i — n 

3 Localized interactions Infrared quantum counter Phosphor Photographic film 

Dynode multiplication (photomultipliers) Channel electron multiplication 

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Finally, some liquid-crystalline elastomers exhibit interesting photonic effects [200,201]. Of particular importance are non-linear optical properties. These involve interactions of light with the elastomer in a way that some of the characteristics of the incident light change, specifically its phase or frequency (including frequency doubling or frequency mixing) [202,203]. 

Molecular features responsible for the enhancement of three photon effects were originally identified in a rather empirical way, by scanning hundreds of organic compounds (5, 6) using the now standard second-harmonic generation (SHG) powder test (7). The... 

S. Esposito, Classical vs=fi c Solutions of Maxwell s Equations and the Tunneling Photon Effect, preprint, 1997. 

Two-photon effect on bulk material 78 Average diffusion length [44]... 

Tricyclic anti-depressive drugs 520 Trimeprazine base e29 Trimethylamine, ornithine, amines, histamine, hypoxanthine 257 Trinitrocompounds 877 TSM 815 Tumour cell 102 Turnover number 919 Tween 20 808, 814 Two-photon effect 100... 

Spectral (photon) effectiveness The reciprocal of the photon ftuence rate, [J, at wavelength X, causing identical photoresponse. Ay, per unit time Ay I At). The effectiveness spectrum is directly proportional to the conversion spectrum of the sensory pigment, if spectral attenuance is negligible. 

Most experiments do not depend on order parameters of higher rank, e.g. the influence of orientational order on an absorption band is completely described in terms of S and D (Luckhurst, 1993). On the other hand, Raman-spectra being based on a two-photon effect are influenced additionally by the order parameters of the next level, such as the Legendre polynomial P4 (Pershan, 1979). This is of considerable theoretical interest, however, up to now of less importance for practical applications. There are some further experimental techniques for gathering information on the orientational order, among these are fluorescence, neutron and electron scattering. Probably the most reliable method is NMR (Emsley, 1985), however this usually means deuteration of all hydrogen atoms but one. 

Figure 3 is a plot of the energy per photon versus wavelength. As one can see, a photon at 300 nm is about 2.8 times more energetic as one at 800 nm. Thus, if one uses a constant energy at each wavelength to obtain an action spectrum, one will underestimate the per photon effect at 300 nm by about 66%. 

Direct photon effect on a substrate is photoablation of a solid surface. After the substrate absorbs strong laser light, the material at the irradiation site is spontaneously etched away to a depth of 1 This... 

The intensity of the excitation pulse is not critical, yet it is advisable to use low-energy density. High-energy flux enhances the probability of undesired two-photon effects. 

K. Ueno, N. Hirano, O. Tsutsumi, T. Shiono and T. Ikeda, Liquid-crystalline materials for photonics Effect of glass transition on stability of optical image stored in polymer azobenzene liquid crystals, Polym. Prepr., Jpn., 1995, 44, 1820. 

This has two main advantages here. Firstly, since very high light intensities are required for two-photon effects, emission will occur predominantly from the focal point of the laser beam where it is most... 

Here, P Vs ( )Ps is the single-photon effective interaction, which is identical to the second-order effective interaction (37) with the Fock-space wave operator. 

Spectroscopic measurements (intensity, phase of incident, and emitted photons), which yield chemical, electronic, structural, and thickness information, for example, by pho-ton/photon effects (e.g. ellipsometry) or photon absorption (e.g. reflectome-try) or structural information (e.g. from Extended X-ray absorption fine structure (EXAFS) or Electron Back Scatter Diffraction EBSD). 

There are a number of excellent references which will enable the reader to obtain an overview of infrared and optical detector research and development. Among the books published in the 1960s which contain much basic information still of value are those by Smith et al. [2.1], Holier et al. [2.2], Kruse et al. [2.3], Jamieson et al. [2.4], Wolfe [2.5], Conn and Avery [2.6], and Hudson [2.7], The theoretical basis for photoconductivity and the other photon effects can be found in Ryvkin [2.8], Bube [2.9], Pell [2.10], and an issue of Applied Optics devoted to photon effects [2.11]. Imaging devices are well covered in the two volumes by Biherman and Nudelman [2.12,13] and in six volumes in the series Advances in Electronics and Electron Physics [2.14]. Of more recent interest is Moss et al. [2.15], Willardson and Beer [2.16], Materials for Radiation Detection [2.17], Dimmock [2.18], Seib and Aukerman [2.19], Emmons et al. [2.20], Anderson and McMurtry [2.21], Melchior et al. [2.22], an issue of the Proceedings of the IEEE devoted to remote sensing [2.23], and a symposium on unconventional infrared detectors [2.24]. 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

The third of the principal photon effects is the photoemissive one, also known as the external photoeffect. As the name implies, the action of the incident radiation is to cause the emission of an electron from the surface of the photocathode into the surrounding space, there to be collected by an anode. The spectral responses of selected photocathodes are illustrated in Fig. 27. 

In addition to the three principal photon effects, there exist many others of lesser importance. These are discussed below. 

Hot Electron Bolometer, Putley Detector. Whether it is more appropriate to include the hot electron bolometer and Putley detector in a list of detectors employing photon effects, or to instead list them with thermal effects, is somewhat arbitrary. Both employ photon effects in that incident photons interact with free electrons in a semiconductor. However, they are thermal (as the name bolometer implies) in that the effects are explainable in terms of a change in the effective temperature of the free electrons. Because the interpretation is mainly from the viewpoint of a photon-electron interaction, they are included here in the list of photon effects. 

Whether optical heterodyne detection should be classed as a wave interaction effect or a photon effect is not obvious. Because it depends upon the interaction of the electric field vector of the signal radiation with that from a reference source, it is listed here as a wave interaction effect. 

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