Calculation Of Optical Absorption Research Articles

Calculation of optical absorption associated with indirect transitions in silicon n-i-p-i structures

Indirect transitions in silicon n-i-p-i structures are investigated theoretically. A method is used in which these structures are characterized by their internal electric fields, which change the absorption in a Franz–Keldysh-like manner. Our calculations, in which the influence of excitons is included, are based upon the effective-mass approximation. The conclusions are that the absorption only increases in an energy region for which the absorption is very small in the first place. It is further shown that excitonic effects are very important in this geometry, in spite of the strong internal electric fields, or, equivalently, in spite of the large spatial separation of the carriers.

Dynamic effects of the impurity potential and electron interactions on a soliton in trans-polyacetylene.

We calculate the potential energy of a charged soliton on a chain of polyacetylene in the presence of an (interstitial) impurity atom and Hubbard on-site interactions. Using this potential, we find the lowest two bound states of the soliton which determine the ``Goldstone-mode'' contribution to infrared absorption. The wave functions are then used to incorporate soliton dynamics into the calculation of optical absorption by the chain. Also included is a study of interaction effects on neutral solitons in pure polyacetylene.

Proposal for strained type II superlattice infrared detectors

We show that strained type II superlattices made of InAs-Ga1−xInxSb x∼0.4 have favorable optical properties for infrared detection. By adjusting the layer thicknesses and the alloy composition, a wide range of wavelengths can be reached. Optical absorption calculations for a case where λc∼10 μm show that near threshold the absorption is as good as for the HgCdTe alloy with the same band gap. The electron effective mass is nearly isotropic and equal to 0.04 m. This effective mass should give favorable electrical properties, such as small diode tunneling currents and good mobilities and diffusion lengths.

Eigenstate-free, Green function, calculation of molecular absorption and fluorescence line shapes

Green function techniques are used to develop a simple and efficient method towards the calculation of optical absorption, excitation, and dispersed fluorescence spectra of large harmonic polyatomic molecules. The molecular line shapes are expressed in terms of Fourier transforms of appropriate correlation functions which may be explicitly evaluated. Closed expressions are derived for a general harmonic molecule with two electronic states including equilibrium displacements, frequency changes, and Dushinsky rotation, within the Condon approximation. A simple method for extracting the complete set of parameters characterizing the ground and the electronically excited states from supersonic beam absorption and emission spectra is presented. Detailed calculations are performed for a model system with ten vibrational modes, and the sensitivity of the various experimental observables to Dushinsky rotation is analyzed.

Effect of spatial averaging on the compositional analysis of crystals by absorption spectroscopy

Calculations of optical absorption based on a model of a single crystal containing spatially periodic compositional variations are presented. These variations can contribute a significant soure of systematic error in the analysis of composition by optical or surface techniques. The model is most appropriate for melt-grown crystals and in particular for striated semiconductor crystals, and the surface concentration profile which it predicts is confined by comparison with a published x-ray topographic study of silicon. Implications of the results for optical absorption studies of impurities in silicon crystals are discussed and it is shown that significant measurement errors may occur.

Optical absorption in semiconductors within the framework of the configurational-coordinate model

Abstract A calculation of optical absorption in amorphous semiconductors in a wide spectral region has been made on the basis of a ‘molecular’ approach. The theoretical spectral dependence of the absorption coefficient is in a good agreement with experiment. An expression giving the relationship between the slope of the Urbach tail and the parameters describing the behaviour of the optical absorption coefficient at higher photon energies, has been obtained.

Boundary conditions and optical absorption in the soliton model of polyacetylene

In the present paper the boundary conditions and the optical absorption are studied for both the discrete and continuum models of solitions in polyacetylene. The topological property of the soliton is incorporated appropriately to derive the correct boundary conditions for the electronic wave functions which are essential for a proper calculation of optical absorption and other properties of the system. A general sum rule for the absorption coefficient is derived for the continuum model which is exactly satisfied by the result of the present calculation. The errors in the previous calculations are indicated. Finally, the theoretical results are compared with the experimental data.

Calculation of optical absorption and magnetic circular dichroism lineshapes of the B-band in KBr: In +

Lineshapes of the optical absorption and magnetic circular dichroism spectra of KBr:In + in the B-band region have been calculated using the semiclassical approximation. Integration over the interaction mode coordinates was carried out by the method of gaussian quadrature. The calculated absorption lineshape is in good agreement with that obtained from the observed spectrum. The very weak absorption peak on the high energy side of the B-band, which was earlier attributed to a dimer center, is now seen to be a part of the B-band itself. The calculated MCD lineshape reveals only two prominent peaks — one negative and the other positive — in contrast to the experimental spectrum which comprises three peaks. Possible reasons for this discrepancy are discussed.

Theoretical optical spectra for large aggregates of chromophores

The problem of calculating optical absorption and circular dichroism spectra for large aggregates of chromophores interacting through electronic transition dipoles is studied. The emphasis throughout is on relating band shapes of such aggregate spectra to band shapes of the consituent chromophores for all interchromophore coupling strengths. We argue that because of its close connections to an exact formulation, an approximation developed earlier (DGS model) predicts spectral band shapes much more accurately than other commonly used methods of calculating aggregate spectra. This is particularly true for intermediate or strong coupling, i.e., for interchromophore coupling strengths comparable to or greater than the bandwidths of the individual chromophores. A number of calculations of optical absorption and circular dichroism spectra for simple helical polymers have been carried out using the DGS model. These calculations exploit the formal identity of the DGS model to the ’’cellular disorder’’ problem of solid state physics and the existence of well established methods for solving this problem. The theoretical spectra found in this way differ considerably from those found using other methods, especially in intermediate and strong coupling. Consequences of these results for several polymers of biological interest are discussed.

Simple calculation of optical absorption of Ni in the near infrared

The authors present a calculation of optical absorption in Ni metal by electronic interband transitions between filled and empty states within the Ni d bands. The matrix elements of the momentum operator required for the calculation are evaluated by an analytical method based on the tight-binding model of the d electrons. The method yields results in good accord with earlier work by Wang and Callaway (1974) and is readily extended to discuss optical absorption in Ni-based alloys.

Acceptor-to-Band Transitions in Semiconductors: Photoluminescence, Exponential Absorption Edges, and Final-State Interactions

The effects of disorder and of final-state electron-hole interactions on acceptor-to-band absorption and photoluminescence spectra of semiconductors are treated theoretically within the effective-mass approximation. The results of the calculations are used to analyze photoluminescence spectra from lightly doped GaAs: Cd measured by Williams and Bebb. It is shown that although a proper one-electron calculation of optical absorption ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ should include the effects of electron-hole spatial correlations, the excitonic Coulomb final-state interaction exactly cancels the spatial correlations in the case of infinite hole mass. Therefore the usual one-electron approximation, which neglects both spatial and Coulombic correlations, is valid for small electron-hole mass ratios (${m}_{e}\ensuremath{\ll}{m}_{h}$); for GaAs: Cd, the finite-mass-ratio corrections are nearly independent of energy and can be neglected. The major portion of the discrepancies between the Williams-Bebb data and Eagles's one-electron theory cannot be attributed to final-state interactions, but can be attributed to small internal microfields of strength ${10}^{3}$ to ${10}^{4}$ V/cm generated by disorder. The sources of these microfields are most likely to be piezoelectric phonons, but ionized impurities or excitons could possibly generate such fields. It is shown that photoluminescence lines are particularly sensitive to disorder and that the band-to-acceptor luminescence edge $L(\ensuremath{\omega})$ should not be a precisely exponential function of photon energy $\ensuremath{\hbar}\ensuremath{\omega}$. Instead the microfield model predicts that the edge shape should be approximately given by ln $L(\ensuremath{\omega})\ensuremath{\propto}{(\ensuremath{\hbar}{\ensuremath{\omega}}_{0}\ensuremath{-}\ensuremath{\hbar}\ensuremath{\omega})}^{\ensuremath{\alpha}}$, with $1<\ensuremath{\alpha}<\frac{3}{2}$. The existing data are compatible with $\ensuremath{\alpha}=\frac{3}{2}$, but the possibility of $\ensuremath{\alpha}=1$ cannot be ruled out without additional photoluminescence measurements further down the edge. If such measurements indicate that $\ensuremath{\alpha}$ is precisely equal to unity, the electric-microfield theory of exponential absorption edges may require revision.

A Model Calculation of Optical Absorption in Transition Metals

AbstractUsing a model Hamiltonian containing a s‐band, d‐band, and s‐d hybridization the optical absorption in the Hartree‐Fock approximation is calculated. It is also shown that RPA corrections to optical absorptions are negligible as long as the s‐band is free‐electron like.

Kinematic Resonances in Crystalline Xenon

A resolvent formalism is applied to the calculation of optical absorption in solid Xe. For realistic models of the energy bands and the electron-hole interaction, the spectrum contains narrow resonances (poles of the $t$ matrix) in the continuum. Although the form of these kinematic resonances is strongly dependent on the truncation radius of the interaction, the results support Phillips's explanation of resonant structure in the optical data on Xe.

Optical absorption of the impurity ag and models for the E‐Band in an alkali halide

AbstractA method used in more recent work is applied to the calculation of optical absorption of a neutral silver atom (AgI) in KCl. The predicted results show that the E‐band in KCl: Ag is not due to the impurity AgI located at a substitutional site of the host crystal. From present theoretical results we then propose two speculative models for the origin of the E‐band: 1. The E‐band is due to the impurity AgI located at an interstitial site tetrahedrally surrounded by four alkali ions as well as by four halide ions. 2. The E‐band is due to the interstitial impurity AgI perturbed by some complex whose perturbation is very small.

Potential Barrier Parameters in Thin-Film Al–Al2O3-Metal Diodes

Internal photoelectric thresholds for collection of electrons through a thin Al2O3 layer formed by anodization in an aqueous nonsolvent electrolyte were measured as a function of photon energy, applied bias, oxide thickness, temperature, and overlayer metal. These data were analyzed in terms of photoexcitation mechanisms and optical absorption calculations to determine the potential barrier profile presented by anodized Al2O3 between metal films. The barrier profile was found to be given by the work function of the metal minus the electron affinity of the oxide, modified by the image effect for an electron between conductors and by the applied bias and the work function difference between the metals. For this agreement the electron affinity of the oxide was chosen to be approximately 2.0 eV and the high-frequency dielectric constant was used. A slight asymmetry was observed for Al–Al2O3–Al devices, possibly resulting from the metal-oxide transition regions. The potential barrier was found to increase with decreasing temperature, consistent with I-V characteristics.