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Excitons and free charge carriers with large wave vectors form a nonradiative reservoir, which can strongly affect properties of bright excitons due to the exciton-exciton and exciton-carrier interactions. In the present work, the dynamics of quasiparticles in the reservoir at different areal densities is experimentally studied in a GaAs/AlGaAs quantum well using a time-resolved reflectance spectroscopy of nonradiative broadening of exciton resonances. The population of the reservoir is controlled either by the excitation power or by the temperature of the structure under study. The dynamics is quantitatively analyzed in the framework of the model developed earlier [Kurdyubov et al., Phys. Rev. B {\bf 104}, 035414 (2021)]. The model considers several dynamic processes, such as scattering of photoexcited excitons into the reservoir, dissociation of excitons into free charge carriers and the reverse process, carrier-induced exciton scattering into the light cone depleting the reservoir, thermally activated nonradiative losses of charge carriers. We have found that competition of these processes leads to highly nonlinear dynamics of reservoir excitons, although their density is far below the exciton Mott transition densities. Characteristic rates of the processes and their dependencies on the excitation power and the temperature are determined., Submitted to PRB

The energy spectrum of the exciton and carrier states in a shallow GaAs/AlGaAs quantum well is experimentally studied by means of the spectroscopy of the nonradiative broadening of exciton resonances and the spectroscopy of the photoluminescence excitation. The observed peculiarities of the spectra are treated using the numerical solution of the one-dimensional Schrodinger equation for free carriers and three-dimensional equation for excitons in the quantum well. The conduction and valence band offsets in the shallow GaAs quantum well are determined.

Optically inactive or dark excitons play an important role in exciton and polariton devices. On one hand, they supply excitons to the light cone and feed the photoluminescence signal. On the other hand, they repel radiatively active excitons due to the exchange interaction and contribute to the formation of lateral potentials for exciton and polariton condensates. On top of this, they play an important role in scattering and energy relaxation dynamics of quasiparticles in semiconductors. So far, because of optical inaccessibility, studies were focused typically on one experimental technique, giving information about one quantity of dark excitons. Here we present a comprehensive study of the dark exciton reservoir in a high-quality 14-nm GaAs/AlGaAs quantum well using several experimental techniques. We develop a new method of nonradiative broadening spectroscopy of exciton resonances and combine it with nondegenerate pump-probe spectroscopy. The exciton and carrier dynamics in the reservoir is monitored via dynamic broadening of exciton resonances induced by exciton-exciton and exciton-carrier scattering. The dynamics is found to be strongly dependent on the optical excitation conditions. Based on the experimental results, we develop a model of dynamics in a reservoir of excitons and free carriers. The model allows us to describe the experimentally measured photoluminescence kinetics with no fitting parameters. We also demonstrate the optical control of the dark exciton density by means of an additional excitation that creates imbalance of free carriers depleting the reservoir. These results shed light onto the dynamics of the excitonic ``dark matter'' and pave the way to the high-precision engineering of optically induced potentials in exciton-polariton and integrated photonic devices. We expect that the observed results can be transferred also to other semiconductors so that the current quantum well serves as a high-quality model system.

Mechanisms of the suppression of the electron-hole exchange interaction in nonradiative excitons with a large in-plane wave vector in high-quality heterostructures with quantum wells are analyzed theoretically. It is shown that the dominant suppression mechanism is exciton-exciton scattering accompanied by the mutual spin flips of like carriers (either two electrons or two holes), comprising the excitons. As a result, the electron spin polarization in nonradiative excitons may be retained for a long time. The analysis of experimental data shows that this relaxation time can exceed one nanosecond. This long-term and optically controllable spin memory in an exciton reservoir may be of interest for future information technologies.

Reflectance spectroscopy is used to study exciton scattering caused by phonons, nonradiative excitons, and free carriers in a heterostructure with a wide (In,Ga)As/GaAs quantum well. Nonradiative excitons and free carriers are created via additional monochromatic illumination by a tunable laser. Constants of exciton–acoustic phonon and exciton–LO phonon scattering are determined from the temperature variations in the nonradiative broadening of exciton resonances. The excitation spectra of nonradiative broadening reveal sharp resonances associated with exciton–exciton scattering and a smooth background caused by exciton–free carrier scattering.

Excitons as a physical phenomenon have been crying out for application since their discovery, but their subtle nature is a drawback here. Small binding energies restrict the existence of excitons at room temperature, and these quasiparticles are far from stable in time, too. Nevertheless, these shortcomings can be viewed instead as $s\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$, and so this study explores excitons as a probe for characterizing semiconductor nanostructures and optimizing their growth. Advanced numerical modeling, leading-edge growth techniques, and precise optical measurements are combined to obtain a fresh perspective on these important optoelectronic systems.

Resonance dielectric response of excitons is studied for the high-quality GaAs/InGaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances with various profiles. A standard approach for the phenomenological analysis of the profiles is generalized by introducing of different phase shifts for the light waves reflected from the QWs at different exciton resonances. Perfect agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to obtain reliable parameters of the exciton resonances including the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of Schr\"{o}dinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation during the heterostructure growth is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

authoren We show experimentally that the uniaxial stress applied along the twofold symmetry axis of a high-quality heterostructure containing a wide GaAs/AlGaAs quantum well leads to considerable modification of the exciton-polariton reflectivity spectra. We observe: (i) the energy splitting of the light-hole and heavy-hole exciton resonances to appear and (ii) the increase of the quasiperiod (divergence) of spectral oscillations related to the optically allowed exciton-polariton transitions. A theoretical analysis shows that the first effect is due to the strain-induced reduction of crystal symmetry. The divergence of spectral oscillations is found to be due to the strain-induced decrease of the heavy-hole exciton mass. The effective mass is reduced by 5% at the pressure of GPa. This effect shows the potentiality of spectroscopic ways of strain detection in semiconductors.

Exciton-polaritons in planar waveguides are of great interest for application in polariton circuits due to the large polariton group velocity in the plane of the waveguide. We demonstrate the ability to control the exciton-polariton coupling by light in an AlGaAs-based planar waveguide with GaAs/AlGaAs quantum well. The transition between strong and weak coupling regimes observed with increasing light intensity is explained by the increase in exciton mode losses due to the quantum well charging. This assumption is confirmed by the reflection spectroscopy with resonant illumination.

In this work we have investigated the exciton reflectance spectra of a high quality heterostructure with a GaAs/AlGaAs quantum well in a transverse magnetic field (Voigt geometry). It has been shown that application of the magnetic field leads to a decrease of energy distance between spectral features related to the excitonlike polariton modes. This effect has been treated as the magneto-induced increase of the exciton mass. We have shown that the hydrogenlike and diamagnetic exciton models are insufficient to describe the exciton behavior in the intermediate magnetic fields studied. Considering the symmetry of the problem, we have developed a phenomenological model which adequately describes the experimental data.

Exciton dynamics is experimentally studied for a heterostructure with a wide InGaAs/GaAs quantum well. The exciton-phonon, exciton-exciton, and excitons-carrier scattering is studied by means of reflectance spectroscopy in different experimental conditions. The non-radiative broadening of exciton resonances is extracted from the experimental data and used as a measure of the scattering efficiency. The exciton-phonon scattering is studied at different sample temperatures. The exciton-exciton and exciton-carrier scattering is studied at an additional illumination of the sample with radiation of a CW tunable laser with different photon energies. Direct information about the characteristic time of relaxation processes is obtained in the spectrally resolved pump-probe experiments.

In this paper we consider two methods for the theoretical modelling of the reflection and diffraction spectra from an excitonic diffraction grating made from a quantum well with spatial modulation of the exciton resonance. The modelling was performed in the single-scattering approximation, and by the step-by-step approximations of the exact solution of Maxwell’s equations. The expressions are obtained for the modulation of the inhomogeneous broadening of the exciton resonance. Theoretical results are compared with the experimental data for the quantum well grown on an ion irradiated substrate, and for a quantum well irradiated with ions after growth.

This article reports the optical properties of visible–NIR light absorbing titania powders fabricated by thermochemical oxidation in air of technically pure titanium (99T) and of three titanium alloys (Ti-4Al-2Mn, Ti-6Al-3Sn, and Ti-7Al-2Mo-1Cr) that contained aluminum as the major impurity (from 0.21 to 6.5 at. %) and other metals indicated in the notations. The resulting titania specimens were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy. The titania samples obtained from the alloys are considered to be heavily doped and codoped materials. The TiO2 specimens displayed a wide color range: from yellow for the sample obtained from the 99T substrate to dark brown for the sample obtained from the Ti-4Al-2Mn substrate. After numerical analysis in the region 3.2–0.6 eV, the absorption spectra of titania obtained from titanium alloys were found to consist of a superposition of single absorption bands originating from intrinsic defect stat...

Zeeman splitting of quantum-confined states of excitons in InGaAs quantum wells (QWs) is experimentally found to depend strongly on quantization energy. Moreover, it changes sign when the quantization energy increases with a decrease in the QW width. In the 87-nm QW, the sign change is observed for the excited quantum-confined states, which are above the ground state only by a few meV. A two-step approach for the numerical solution of the two-particle Schr\"odinger equation, taking into account the Coulomb interaction and valence-band coupling, is used for a theoretical justification of the observed phenomenon. The calculated variation of the $g$-factor convincingly follows the dependencies obtained in the experiments.

A theoretical model of reflectance spectra is applied to the analysis of spectra of the two high-quality structures with asymmetric InGaAs/GaAs quantum wells. The analysis allows to quantitatively describe the exciton resonances related to the lowest quantum-confined exciton states. Main parameters of the quantum well potential profiles are obtained in the modeling.

Multiple frequency quantum beats of a system of the coherently excited quantum confined exciton states in a high-quality semiconductor structure containing a wide InGaAs/GaAs quantum well are experimentally detected by the spectrally resolved pump-probe method. The beat signal is observed both at positive and at negative delays between the pump and probe pulses. Several quantum beat (QB) frequencies are observed in the experiments, which coincide with the interlevel spacings in the exciton system. A theoretical model is developed, which allows one to attribute the QBs at negative delay to the four-wave mixing (FWM) signal detected at the nonstandard direction. The beat signal is strongly enhanced by the interference of the FWM signal with the secondary emission induced by the probe pulse. At positive delays, the QBs are due to the interference of the quantum confined exciton states. The decay time for QBs is of the order of several picoseconds both at positive and negative delays. This is close to the relaxation time of the exciton population that allows one to consider the exciton depopulation as the main mechanism of the coherence relaxation in the system under study.

The binding energy and the corresponding wave function of excitons in GaAs-based finite square quantum wells (QWs) are calculated by the direct numerical solution of the three-dimensional Schrodinger equation. The precise results for the lowest exciton state are obtained by the Hamiltonian discretization using the high-order finite-difference scheme. The microscopic calculations are compared with the results obtained by the standard variational approach. The exciton binding energies found by two methods coincide within 0.1 meV for the wide range of QW widths. The radiative decay rate is calculated for QWs of various widths using the exciton wave functions obtained by direct and variational methods. The radiative decay rates are confronted with the experimental data measured for high-quality GaAs/AlGaAs and InGaAs/GaAs QW heterostructures grown by molecular beam epitaxy. The calculated and measured values are in good agreement, though slight differences with earlier calculations of the radiative decay rate...