R. P. Seisyan

Tamm plasmon polaritons: Slow and spatially compact light

We report on the first experimental observation of Tamm plasmon polaritons (TPPs) formed at the interface between a metal and a dielectric Bragg reflector (DBR). In contrast to conventional surface plasmons, TPPs have an in-plane wavevector less than the wavevector of light in vacuum, which allows for their direct optical excitation. The angular resolved reflectivity and transmission spectra of a GaAs∕AlAs DBR covered by Au films of various thicknesses show the resonances associated with the TPP at low temperatures and room temperature. The in-plane dispersion of TTPs is parabolic with an effective mass of 4×10−5 of the free electron mass.

Exciton luminescence of quasi-two-dimensional solid solutions

Absorption and luminescence of the quantum wells formed by the (Zn-Cd)Se and (Ga-In)As solid solutions are studied in the range of exciton size-quantization ground state. The spectra observed are described by a model assuming the two-dimensional character of fluctuation states in quantum wells and the presence of a percolation threshold within the absorption contour.

Diamagnetic Exciton Polariton in the Interband Magnetooptics of Semiconductors

The experimentally observed magnetic-field dependence of the integrated absorption coefficient in Al0.15Ga0.85As samples at 1.7 K is interpreted. It is established that the dependence results from the competition of two mechanisms: an increase in integrated absorption due to an increase in the oscillator strength as a result of magnetic-field-induced compression of the exciton wave function and a decrease associated with the magnetic freezing-out of charged scattering centers. An analysis of the integrated absorption shows that diamagnetic exciton polaritons are formed in the samples in a magnetic field.

Diamagnetic excitons in semiconductors (Review)

Optical properties of semiconductor crystals in the presence of a high magnetic field have been considered. The field turn-on gives rise to oscillations of the optical-absorption edge or, more specifically, the formation of a complex absorption spectrum with a periodic structure, referred to as the spectrum of “diamagnetic excitons.” Such spectra appear a source of the most accurate knowledge about the band structure of semiconductors. Moreover, these spectra can be used for simulating the low-dimensional state in semiconductors and possible interpretation of the emission spectra of neutron stars. The proposed analytical review is based on extensive experimental and theoretical data contained mostly in cited original works of the author with colleagues.

Diffusive Propagation of Exciton-Polaritons through Thin Crystal Slabs.

If light beam propagates through matter containing point impurity centers, the amount of energy absorbed by the media is expected to be either independent of the impurity concentration N or proportional to N, corresponding to the intrinsic absorption or impurity absorption, respectively. Comparative studies of the resonant transmission of light in the vicinity of exciton resonances measured for 15 few-micron GaAs crystal slabs with different values of N, reveal a surprising tendency. While N spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as N1/6. We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in a semiconductor.

Width of the excitonic absorption line in AlxGa1 − xas alloys

Experimental data on the width of the absorption line corresponding to the exciton ground state in high-quality samples of AlxGa1 − xAs (x = 0.15) quasi-binary alloy, obtained by Seisyan et al. (2005), are analyzed. The line corresponding to the 1s state is divided into separate components taking into account strain-induced splitting; Lorentzian- and Gaussian-shape contributions to the absorption curve; and overlap with the continuum absorption band, which broadens with increasing temperature. Analysis of the integrated absorption, relying on the fact that the temperature dependence of the absorption is typical of exciton polaritons in media possessing spatial dispersion, was carried out with the following parameters: critical value of the dissipation damping of the exciton Γc = 0.32 meV and maximum value of the absorption coefficient Kmax = 89.5 eV/cm. This analysis makes it possible to determine the homogeneous component of the line broadening, which, up to the critical temperature Tc = 155 K does not exceed 0.2 meV. It is found that the “natural” width of the 1s-exciton line does not exceed 2.6 meV at T = 1.7 K, which is in agreement with theoretical estimates. At T = 1.7–60 K, the inhomogeneous broadening of the exciton peak related to the exciton scattering and localization by fluctuations of the alloy composition exceeds contributions related to phonons and ionized impurities by more than an order of magnitude and is dominant, but does not affect the integrated absorption by excitonic polaritons.

Optical transmittance of thin GaAs wafers upon laser pumping in the region of exciton resonances and the continuum of states: Exciton-exciton interaction

The transmittance spectra of thin “pure” GaAs bulk wafers upon optical pumping corresponding to the band of the ground state of the exciton series are recorded at a temperature of T = 1.7 K. The wafers were grown by molecular-beam epitaxy and vapor-phase epitaxy. An increase in the line amplitude and width upon pumping is observed, with no noticeable changes in the spectral position of the line peak. The increase is similar to that observed upon pumping in the continuum of states, but occurs at a somewhat lower rate. Estimation of the concentration of excitons created by pumping provides a means for determining the exciton-exciton interaction constant and comparing the result with known data. The integrated absorption method makes it possible to refine the exciton polariton-free charge carrier and exciton polariton-impurity interaction constants. The differential photoabsorption of the samples at the pumping modulation frequency is measured. The resultant modulated absorption spectra demonstrate the connection between induced absorption and the formation of differential spectra.

Temperature-dependent excitonic absorption in long-period multiple InxGa1 − xAs/GaAs quantum well structures

Temperature variations in the fundamental absorption edge of long-period InxGa1 − xAs/GaAs structures are studied for samples with different numbers of quantum wells and similar periods. The quantum wells were close in composition and width. Experimental data are interpreted in the model of exciton-polariton light transfer involving localized excitons in confined structures with a finite number of quantum wells. The experimentally observed low-temperature anomaly of the integrated absorption coefficient is attributed to reemission of resonance localized excitons along a finite chain of quantum wells, with no excitonic transfer. The radiative decay time of an exciton in a single quantum well is estimated from the experimental data. It is demonstrated that, at low temperatures, the major contribution to the width of the experimentally observed absorption line corresponding to the ground heavy-hole exciton state is made by inhomogeneous broadening of the line by the field of potential fluctuations associated with the compositional disorder of the alloy. At low temperatures, the inhomogeneous broadening is much more pronounced than the broadening governed by the true radiative and nonradiative dissipative decay.

Fano effect in the magnetoabsorption spectra of gallium arsenide

The magnetooptic absorption spectra of high-quality homoepitaxial GaAs layers in a magnetic field B up to 7.5 T at T=1.7 K are investigated. It is shown that the Fano effect is involved in the formation of certain lines of the magnetooptic spectrum. The parameters of the phenomenological Fano function are determined. It is shown that polariton effects of the diamagnetic exciton play a significant role in the investigated processes.

Elastic scattering of exciton polaritons

The probability of elastic scattering by impurities of exciton polaritons in thin GaAs samples has been calculated theoretically. It has been shown that thin samples of commercially pure GaAs satisfy the conditions for the existence of exciton polaritons. It has also been demonstrated that, in the case of GaAs, the inclusion of the polariton effect in the analysis leads only to a slight decrease in the scattering probability calculated for a “bare” exciton. The role of elastic scattering of exciton polaritons in the absorption of light by a semiconductor has been discussed.