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Transmission, Reflection, and Absorption of Light
Published in Solomon Musikant, Optical Materials, 2020
The optical region of Fig. 1.3 is shown in an expanded plot for a typical dielectric in Fig. 1.5. In this case, the data are plotted as absorption coefficient versus wavelength. The optical absorption edge on the short-wavelength side of the diagram is a consequence of the band structure of the material and is an intrinsic characteristic of the structure of the material, as is the vibrational edge on the long-wavelength side. Here the absorption is due to the interaction of the low-energy photons with the lattice structure elasticity. However, in the effective bandpass region there generally exist extrinsic absorptions due to imperfections. These imperfections may be impurity atoms, unintentional second phases, voids, nonhomogeneous composition, and, in multiphase and polycrystalline materials, boundary effects.
Optical Properties of Quantum Nanostructures
Published in Jyoti Prasad Banerjee, Suranjana Banerjee, Physics of Semiconductors and Nanostructures, 2019
Jyoti Prasad Banerjee, Suranjana Banerjee
The schematic diagram (Figure 9.3) shows the absorption coefficient α versus photon energy ħɷ for different semiconductors like Ge, Si, GaAs, and InP. It is observed that the absorption coefficient increases sharply from the bandgap absorption edge in the short-wavelength region. The absorption edge is determined from the bandgap energy of the particular semiconductor. The shape of the curve depends on the band structure. For example, in case of Direct Bandgap (DBG) semiconductor like GaAs and InP, the absorption curve shows a steeper rise as compared to Indirect Bandgap Semiconductors like Si and Ge. The interband photo-transition in GaAs and InP gives rise to the optical spectrum covering a wide range of wavelength from IR to visible region. These semiconductors therefore find useful applications in optoelectronic or photonic devices.
Band Structure Details and Photoconductivity
Published in N.V. Joshi, Photoconductivity, 2017
The form of the absorption edge and the structure near the band gap can be understood by examining the variation in the absorption coefficient as a function of wavelength (or energy). Photon absorption depends on the transition probability under a perturbing electromagnetic field of incident radiation, which is generally expressed with operator O^. The matrix element of the transition probability between initial state i and final state f can be written as M=∫ψiO^ψf
Relation Between the Characteristic X-Ray Intensity and Incident Electron Energy Using the Monte Carlo Method and Measurements
Published in Nuclear Technology, 2022
Runqiu Gu, Jianfeng Cheng, Wanchang Lai, Xianli Liao, Guangxi Wang, Juan Zhai, Chenhao Zeng, Jinfei Wu, Xiaochuan Sun
The intensities of the K-series characteristic X-rays of molybdenum, rhodium, and silver and the L-series characteristic X-rays of W and Pt have maximum values at 500, 600, 650, 250, and 300 keV. The peak of the bremsstrahlung spectrum produced by the interaction between the incident electron beam and the target substance varies with increasing incident electron energy. The characteristic X-ray of the target material will have multiple absorption edges, such as the K-edge, L-edge, and M-edge, as the atomic number increases. The greater the atomic number is, the higher the fluorescence yield of the absorption edge is at a lower energy, so the intensity of the characteristic X-ray energy is stronger, and the transition near the absorption edge is more pronounced. Thus, the bremsstrahlung spectrum will exhibit multiple peaks. This probably occurs for the reasons described previously. The efficiency of direct excitation of characteristic X-rays by electrons will gradually decrease with increasing incident electron energy, and the indirect excitation of the bremsstrahlung peak will dominate in a particular energy range. Thus, the characteristic X-rays of several target materials will decrease slowly in this energy range. The various trends of the K-series characteristic X-rays of W and Pt occur for the same reasons.
Annealing induced transformations in structural and optical properties of Ge30Se70−xBix thin films
Published in Phase Transitions, 2019
Adyasha Aparimita, R. Naik, C. Sripan, R. Ganesan
The exponential nature of the tail of the absorption edge indicates the presence of localized states in the energy band gap. The origin of this band tailing arises from the random fluctuations of the internal fields associated with structural disorders which are observed in many amorphous solids [67]. The subgap absorption involves transitions from localized states and has an exponential-like dependence on photon energy. The amount of tailing can be estimated to a first approximation by plotting the absorption edge data in terms of an equation originally given by Urbach [68], which has been applied to many glassy materials [69,70].where ν is the frequency of the radiation, α0 is a constant, h is Planck’s constant and Ee is Urbach’s energy that measures the width of the tails of localized states in the band gap and represents the degree of disorder in the amorphous materials [71]. By plotting the dependence of log (α) vs hν will give a straight line and the inverse of the slope of the straight line measures the width of the tails of the localized states into the gap at band edges. The absorption process in this range depends upon the transition between extended states in one band and localized states in the exponential tail of the band. The Urbach energy is increased for the annealed Ge30Se70−xBix films than as-prepared thin film as presented in Table 4. This shows the increase in chemical disorderness that brings down the band gap like other studies [27,72].
Influence of Ge addition on the optical properties of As40Se50Ge10 thin film probed by spectroscopy techniques
Published in Phase Transitions, 2018
Ramakanta Naik, Jagnaseni Pradhan, Chinnaiyah Sripan, R. Ganesan
The absorption coefficient (α) is calculated from the transmission data by using the following equation:where d is the thickness of the film, R is the reflectivity and T is the transmission of the films. Figure 7 shows the dependence of absorption coefficient with wavelength, in which the value of α is found to be increased with Ge addition into As40Se60. The value of k(λ) can be calculated by taking the α values by using the following equation:where k is the extinction coefficient, λ is the wavelength and α is the absorption coefficient. The variation of k (Figure 7 inset) shows the increase of k due to Ge addition into As40Se60. Optical band gap:The fundamental edge in crystalline material is directly related to the transitions of electrons between the conduction and valence band that leads to direct and indirect band gaps. Due to the absence of electronic band structure in k-space in amorphous materials, the transitions are purely non-direct. A photon of known energy excites an electron from a lower to a higher energy state which is called as absorption edge in the absorption process. The absorption edge is divided into three types such as residual below-gap absorption, Urbach tails and inter-band absorption [31].