Free-carrier absorption in a parabolic quantum well with consideration of scattering on ionized impurities

Intra-subband transitions caused by light absorption in a parabolic quantum well is considered taking into account the scattering by ionized impurity centers. To calculate the scattering matrix element, the Born approximation is used and the interaction with the impurity is described by the Coulomb potential. An analytical expression for the absorption coefficient of processes with the initial absorption of photon and further scattering by an ionized impurity center is obtained. For absorption coefficient the frequency characteristics and dependence on the width of quantum well are examined.     

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Dipole-allowed optical absorption in a parabolic quantum dot with two electrons

Dipole-allowed optical absorption in a parabolic quantum dot with two electrons are studied by using the exact diagonalization techniques and the compact density-matrix approach. Numerical results are presented for typical GaAs parabolic quantum dots. The results show that the total optical absorption coefficient of two electrons in quantum dot is about five times smaller than that of one electron in quantum dot.     

Resonant light scattering by a gradient quantum well

Resonant scattering of monochromatic light by a quantum well with the frequency of its excitonic resonance varying along a certain direction in the plane of the well is studied experimentally and theoretically. It is shown that the simplest model of a thin inhomogeneous layer that yields an exact solution of the direct and inverse scattering problems allows one to successfully describe the experimental observation of resonant scattering by a GaAs/AlGaAs quantum well with the frequency of the exciton resonance linearly varying in the plane of the well.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Inelastic light scattering by acoustic phonons in quantum dots and quantum films

The Raman scattering spectra were analyzed in order to find and explain similarities and differences in the interaction between electrons and acoustic phonons in quantum dots and quantum films.     

Light absorption involving longitudinal optical phonons in semiconductor quantum dots

A theory of single-photon interband transitions involving optical phonons in semiconductor quantum dots (QDs) has been developed. This theory assumes that the electron subsystem of QDs with infinite potential walls is in strong confinement, and its energy spectrum can be described according to the two-band semiconductor model. Longitudinal optical phonons are considered to be related to the QD electron subsystem via polar (Fröhlich) electronphonon interaction. It is shown that, in these approximations, only the off-diagonal part of electron-phonon interaction leads to the generation of electron-hole pairs with the participation of phonons; the selection rules for these transitions differ from those for zero-phonon transitions. Analytical expressions for the light-absorption coefficients of ensembles of identical and size-distributed QDs have been obtained.     

Two-electron linear intersubband light absorption in a biased quantum well

We point out a novel manifestation of many-body correlations in the linear optical response of electrons confined in a quantum well. Namely, we demonstrate that along with the conventional absorption peak at a frequency omega close to the intersubband energy delta, there exists an additional peak at frequency h omega approximately = 2delta. This new peak is solely due to electron-electron interactions, and can be understood as excitation of two electrons by a single photon. The actual peak line shape is comprised of a sharp feature, due to excitation of pairs of intersubband plasmons, on top of a broader band due to absorption by two single-particle excitations. The two-plasmon contribution allows us to infer intersubband plasmon dispersion from linear absorption experiments.     

Diploe-allowed optical absorption of an exciton in a spherical parabolic quantum dot

A investigation of the linear and nonlinear optical properties of an exciton in a spherical parabolic quantum dot has been performed by using the matrix diagonalization method. The optical absorption coefficients between the ground state (L=0,π=+1) and the first excited state (L=1,π=-1) have been examined based on the computed energies and wave functions. The results are presented as a function of the incident photon energy for the different values of the incident optical intensity and the confinement strength. We found the optical absorption coefficient is strongly affected by the incident optical intensity and the confinement strength.     

Electric-field dependence of the intersubband optical absorption in a semiconductor quantum well

We present theoretical results of intersubband linear optical absorption in the conduction band of a GaAsAlGaAs quantum well with an applied electric field taking into account the field dependent linewidth. Our analysis is based on the one electron density matrix formulation with intrasubband relaxation processes due to polar optical phonon scattering and tunneling of electrons. We show that (a) for an increasing electric field the absorption peak corresponding to the transition of states 1 → 2 is shifted higher in energy and (b) the peak amplitude increases if the Fermi level is fixed and decreases if the electron density in the well is fixed when an increasing electric field is applied. The linewidth broadening also reduces the peak absorption amplitude.     

Optical absorption caused by the transitions between donor-acceptor pairs in a parabolic quantum well

The interimpurity optical absorption in a parabolic quantum well is studied theoretically. Under the assumption of a lightly doped semiconductor, the probabilities of acceptor-to-donor transitions are determined and the corresponding coefficient of light absorption is calculated. Within the framework of the nearest neighbor model, the broadening of impurity levels connected with the spatial distribution of donor-acceptor pairs is taken into account. The dependence of the absorption coefficient on the impurity concentration is determined and the blueshift in the absorption spectrum is studied.     

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E _2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.     

The quantum acoustomagnetoelectric field in a quantum well with a parabolic potential

The acoustomagnetoelectric (AME) field in a quantum well with a parabolic potential (QWPP) has been studied in the presence of an external magnetic field. The analytic expression for the AME field in the QWPP is obtained by using the quantum kinetic equation for the distribution function of electrons interacting with external phonons. The dependence of the AME field on the temperature T of the system, the wavenumber q of the acoustic wave and external magnetic field B for the specific AlAs/GaAs/AlAs is achieved by using a numerical method. The problem is considered for both cases: The weak magnetic field region and the quantized magnetic field region. The results are compared with those for normal bulk semiconductor and superlattices to show the differences, and we use the quantum theory to calculate the AME field in the QWPP.     

Intersubband scattering as a tool to study the symmetry properties of a parabolic quantum well

The electron mobility in parabolic quantum wells is measured as a function of carrier density at low temperatures. The curves display pronounced maxima that indicate the onset of intersubband scattering, i.e. the population of another subband. We find that the population of a given subband only depends on the total carrier density almost independent of the position of the wavefuntion within the well which can be tuned via a front and back gate electrode. This is in agreement with predictions for a perfect parabolic potential. If a potential spike is inserted in the center of the PQW we find a complicated behavior for the mobility versus carrier density maxima which reflects the changed symmetry of the potential.     

Resonance elastic scattering of light by a quantum well with statistically uneven boundaries

A theory is formulated for the elastic scattering of light through quasi-two-dimensional exciton states in a quantum well with randomly uneven walls. The nonlocal exciton susceptibility is expressed in terms of random functions describing the shape of the quantum well boundaries up to and including linear terms in the unevenness height. The resonance elastic scattering cross sections in the presence of arbitrary statistical unevenness are calculated in the Born approximation for all channels in which the initial and final states are represented by an electromagnetic TM or TE mode. The spectral and angular dependences of the scattering probability are calculated with the unevenness characterized by Gaussian correlation functions. It follows from numerical estimates that elastic scattering in quantum wells should be observed for unevenness having an rms height of the order of the thickness of an atomic monolayer. Fiz. Tverd. Tela (St. Petersburg) 41, 330–336 (February 1999)     

Linear circular dichroism in nonlinear absorption of light in a quantum well

The far infrared absorption spectrum caused by optical transitions of holes between size-quantization subbands is calculated for p-GaAs/AlGaAs(001) quantum-well structures. The selection rules for optical transitions at the center of the two-dimensional Brillouin zone are determined. Allowance is made for resonant saturation of one-photon electronic transitions between size-quantized subbands of light and heavy holes. The linear circular dichroism in one-photon nonlinear (resonant) and two-photon absorption of light in a size-quantized well are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 1347–1349 (July 1998)     

Intersubband optical absorption in a quantum well under a tilted magnetic field

By using an appropriate coordinate transform we have calculated the intersubband optical absorption in the single square well under a tilted magnetic field. In this study the dependence of the intersubband transitions on the magnetic field strength and the direction of the magnetic field (tilt angle) is discussed. We show that intersubband optical absorption is sensitive to the tilt angle. This behaviour in the intersubband optical absorption gives a new degree of freedom in regions of interest device applications.