Linear Optical Absorption Coefficients Research Articles

Polaron Effects on the Intersubband Optical Absorption Coefficient and Refractive Index Changes of an Infinite Confining Potential Quantum Box

In this work, The effects of electron-phonon interaction on the linear and nonlinear optical absorption coefficients and refractive index changes are investigated theoretically. In this regard the compact-density matrix approach and iterative methods are employed and the optical absorption coefficients and refractive index changes of a GaAs quantum box are obtained. The results show that the magnitude of these quantities are enhanced, the peaks are shifted to the higher energies and the width of the peaks becomes larger as the influence of electron-phonon interaction is considered.

Linear and nonlinear optical absorption coefficients and refractive index changes in spherical quantum dots: Effects of impurities, electric field, size, and optical intensity

In the present work, the case of a spherical quantum dot with parabolic confinement subjected to an external electric field with the presence of an impurity, the linear and third-order nonlinear optical absorption coefficients as well as refractive index changes have been calculated. The numerical method we are using for the calculation of the energy levels and the corresponding wave functions is the potential morphing method in the effective mass approximation. As our results indicate an increase of the electric field and/or the position of the impurity and/or the quantum dot radius redshifts the peak positions of the total absorption coefficient and total refractive index changes. Additionally, an increase of the position of the impurity and/or the quantum dot radius decreases the total absorption coefficient and increases the total refractive index changes. An increase also of the electric field decreases the total absorption coefficient but does not significantly affect the peak values of the total refractive index changes. Finally, an increase of the optical intensity considerably changes the total absorption coefficient as well as the total refractive index changes.

Studies on Linear and Nonlinear Intersubband Optical Absorptions in a Wurtzite AlGaN/GaN Coupling Quantum Well

Based on the density matrix approach and iterative procedure, a detailed procedure for the calculation of the linear and nonlinear intersubband optical absorption coefficients is given in wurtzite GaN-based coupling quantum wells (CQWs). The simple analytical formulas for electronic eigenstates and the linear and nonlinear optical absorption coefficients in the systems are also deduced. Numerical result on a typical AlGaN/GaN CQW shows that, the linear and nonlinear optical absorption coefficients sensitively depend on the structural parameters of the CQW system as well as the incident optics beam intensity.

Optical and nonlinear optical properties of an octasubstituted liquid crystalline copper phthalocyanine

The optical and nonlinear optical properties of an octasubstituted, liquid crystalline copper phthalocyanine, which was synthesized by the reaction of 1,2-dibromo-4,5-bis[dodecanoxymethyl]-benzene with a small excess of CuCN in dilute anhydrous N, N′-dimethylacetamide solution, are described. The ratio of the excited to ground state absorption cross-section ( κ = σ ex/ σ 0) and the linear optical absorption coefficient of the compound were measured in dry toluene solution; their values were about 2.4 ± 0.5, and 1.4 cm −1, respectively. In the solid state the compound shows signs of irreversible thermal damage when illuminated by laser at the lowest energy per pulse setting used. In solution, the values of both the nonlinear absorption coefficient and the third-order nonlinear susceptibility decreased with increasing incident intensity, this being indicative of saturation effects arising from higher triplet excitations.

Linear and nonlinear intersubband optical absorption coefficients and refractive index changes in a quantum box with finite confining potential

In this work, both the intersubband optical absorption coefficients and the refractive index changes are calculated exactly in a quantum box. Analytical expressions for the linear and nonlinear intersubband absorption coefficients and refractive index changes are obtained by using the compact-density matrix approach. Numerical results are presented for typical GaAs/Al x Ga 1− x As quantum box system. The linear, third-order nonlinear and total absorption and refractive index changes are investigated as a function of the incident optical intensity and structure parameters such as box-edge length and stoichiometric ratio. Our results show that both the incident optical intensity and the structure parameters have a great effect on the total absorption and refractive index changes.

Electron–phonon interaction effect on optical absorption in cylindrical quantum wires

Electron–phonon interaction effects on linear and nonlinear optical absorption in cylindrical quantum wires are investigated. The linear and nonlinear optical absorption coefficients are obtained by using compact-density-matrix approach and iterative method, and the numerical results are presented for GaAs/AlAs cylindrical quantum-well wires. The results show that electron–phonon interaction not only influences the relaxation rate but also distinctly influences the wave functions and energies of the electron. The correction of electron–phonon interaction effect on the wave functions of the electron dominates the values of absorption coefficients. Moreover, the correction of electron–phonon interaction effect on the energies of the electron makes the absorption peaks blue shift and become wider.

Optical absorption of a quantum well with an adjustable asymmetry

The effects of asymmetry and the electric field on the electronic subbands and the nonlinear intersubband optical absorption of GaAs quantum wells represented by a Poschl-Teller confining potential are studied. The potential itself can be made asymmetric through a correct choice of its parameter set and this adjustable asymmetry is important for optimizing the absorption. In that way optimal cases can be created. We calculate the modified wave functions and electronic subbands variationally. The linear and the nonlinear optical intersubband absorption coefficients are calculated. Numerical results for a typical GaAs quantum well are presented. The nonlinear part of the absorption coefficient is strongly modified by the asymmetry parameters while the electric field affects it at smaller values of the parameters. - 78.67.De Quantum wells

Interband optical absorptions in a parabolic quantum dot

The linear and nonlinear optical absorptions considering excitonic effect in a parabolic quantum dot are studied. Analytic forms of the linear and third-order nonlinear optical absorption coefficients are obtained for a symmetric parabolic quantum dot using the compact density matrix formalism. Based on this model, numerical results are presented for a typical CdS parabolic quantum dot. The calculated results show that the factors of the parabolic potential confinement and the incident optical intensity have great influence on the optical absorption coefficients. Furthermore, the optical absorption saturation intensity can be controlled by adopting a proper parabolic confinement potential.

Dynamic localization in finite quantum-dot superlattices: A pure ac field effect

We propose that dynamic localization can be unambiguously observed through the linear optical absorption coefficient of a finite quantum dot superlattice. It is shown that the miniband collapse is almost exact in a pure ac field driven system. In the presence of Coulomb interaction, the dynamic localization is partially preserved, showing clear fingerprints in the modulation of the excitonic spectra. The spectra are obtained within an extended semiconductor Bloch equation framework, taking into account the center-of-mass motion for a finite system.

Calculation of Linear and Nonlinear Intersubband Optical Absorptions in Electric-Field-Biased Semiparabolic Quantum Wells

By using the compact density matrix approach and iterative procedure, a detailed procedure for the calculation of the linear and nonlinear intersubband optical absorption coefficients is given in the electric-field-biased semi-parabolic quantum wells (QWs). The simple analytical formulas for the linear and nonlinear optical absorption coefficients in the systems are also deduced. Numerical result on typical GaAs materials shows that, the linear and nonlinear optical absorption coefficients sensitively depend on the applied electric field and the confined potential frequency of the semiparabolic QW systems as well as the incident optics beam intensity.

Far-infrared absorption in GaAs:Te liquid phase epitaxial films

The far-infrared absorption spectrum of n-type GaAs is of interest for applications such as GaAs photoconductors and blocked impurity band detectors. The linear optical absorption coefficients α for three n-type GaAs films of varying doping concentrations have been measured in the range of 10 to 100 cm−1 using Fourier transform infrared spectrometry. These results show α having maximum values of between 46 cm−1 at 1×1015 cm−3 and approximately 800 cm−1 at 2.1×1016 cm−3. The formation and widening of a donor impurity band with increasing impurity concentration is clearly demonstrated.

Far-infrared absorption in Sb-doped Ge epilayers near the metal–insulator transition

Epitaxial germanium layers doped near the metal–insulator transition were grown by liquid phase epitaxy from a Pb melt. Far-infrared absorption was measured between ∼20 cm−1 and 150 cm−1. Linear optical absorption coefficients were determined for Ge:Sb in the doping range of 9.0×1014 cm−3<ND<6.7×1016 cm−3. The peak absorption was found to increase linearly with dopant concentration as expected. The absorption at ∼50 cm−1 increased superlinearly with Sb concentration as a result of impurity banding. The optimal Sb concentration for extended far-infrared photoconductive response (λ>200 μm) in blocked impurity band detectors is discussed.

Two-photon absorption in potassium niobate

We report measurements of thermal self-locking of a Fabry–Perot cavity containing a potassium niobate (KNbO3) crystal. We develop a method to determine linear and nonlinear optical absorption coefficients in intracavity crystals by detailed analysis of the transmission line shapes. These line shapes are typical of optical bistability in thermally loaded cavities. For our crystal we determine the one-photon absorption coefficient at 846 nm to be α=(0.0034±0.0022) m-1, the two-photon absorption coefficient at 846 nm to be β=(3.2±0.5)×10-11 m/W, and the one-photon absorption coefficient at 423 nm to be (13±2) m-1. We also address the issue of blue-light-induced infrared-absorption and determine a coefficient for this excited-state absorption process. Our method is particularly well suited to bulk absorption measurements where absorption is small compared with scattering. We also report new measurements of the temperature dependence of the index of refraction at 846 nm and compare with values in the literature.

Microcavity effect in the optical intersubband absorption of a multiple-quantum-well structure

In a nonlocal semiclassical local-field approach, the linear optical intersubband absorption coefficient of a multiple-quantum-well (MQW) structure inside a microcavity is derived. It is shown that intersubband absorption spectra of the MQW structure are strongly dependent of the effective cavity length. When the cavity length is comparable to the wavelength of light, it is demonstrated that a notable splitting of the intersubband absorption line may occur for the MQW structure having a sufficient number of quantum wells.

Stimulated thermal scattering in isotropic media

A rigorous theoretical treatment of stimulated thermal scattering for an isotropic medium is presented. A phenomenological damping term is introduced into the electromagnetic field equation and then this equation is simultaneously solved with the equations describing the medium. As the set of equations is nonlinear, the solutions have been obtained under suitable approximations. The implications of these approximations, as far as performing the experiments is concerned, are thoroughly discussed. The nonlinear nature of the problem is responsible for saturation, and the optical-loss term gives rise to depletion. Both these phenomena have been included in our treatment. Whereas the 'saturation' is always present in a real system, 'depletion' can be an important effect in a medium involving a large linear optical absorption coefficient α. The experimental relevance of both these effects is discussed. Furthermore, we have derived the threshold condition on the laser field intensity. In the appropriate limits, our results reduce to those already reported in the literature.

Lattice Anharmonicity and Optical Absorption in Polar Crystals. II. Classical Treatment in the Linear Approximation

An expression for the elements of the dielectric susceptibility tensor for an ionic crystal has been derived in a manner analogous to that employed by Kubo in his treatment of magnetic susceptibility. In the high-temperature (classical) limit, this expression reduces to the Laplace transform of the autocorrelation function of the single normal coordinate which interacts directly with the external radiation field. We have applied this formalism to the calculation of the high-temperature linear optical absorption coefficient of an anharmonic ionic crystal for which we have retained only cubic anharmonic terms in the lattice Hamiltonian. We have solved directly for the Laplace transform of the autocorrelation function to lowest order in the anharmonic coupling constant by solving the equations of motion for the normal coordinates after they have been linearized. The linear absorption coefficient obtained from the susceptibility tensor is of Lorentzian form with a frequency-dependent damping constant which varies linearly with temperature. The absorption coefficient for a diatomic linear chain with nearest neighbor interactions has been evaluated.