Excitons In Semiconductor Quantum Wells Research Articles

All-optical control of excitons in semiconductor quantum wells

Applying the Floquet theory, we developed the method to control excitonic properties of semiconductor quantum wells (QWs) by a high-frequency electromagnetic field. It is demonstrated, particularly, that the field induces the blue shift of exciton emission from the QWs and narrows width of the corresponding spectral line. As a consequence, the field strongly modifies optical properties of the QWs and, therefore, can be used to tune characteristics of the optoelectronic devices based on them.

Polarimetry of photon echo on charged and neutral excitons in semiconductor quantum wells

Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver rich information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle φ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a CdTe/(Cd, Mg)Te quantum well. The echoes of the trion and donor-bound exciton are linearly polarized at the angle 2φ with respect to the first pulse polarization and their amplitudes are weakly dependent on φ. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as cosφ.

Hanle model of a spin-orbit coupled Bose-Einstein condensate of excitons in semiconductor quantum wells

We present a theoretical model of a driven-dissipative spin-orbit coupled Bose-Einstein condensate of indirect excitons in semiconductor quantum wells (QW's). Our steady-state solution of the problem shares analogies with the Hanle effect in an optical orientation experiment. The role of the spin pump in our case is played by Bose-stimulated scattering into a linearly-polarized ground state and the depolarization occurs as a result of exchange interaction between electrons and holes. Our theory agrees with the recent experiment [A. A. High et al., Phys. Rev. Lett. 110, 246403 (2013)], where spontaneous emergence of spatial coherence and polarization textures have been observed. As a complementary test, we discuss a configuration where an external magnetic field is applied in the structure plane.

Direct measurement of polariton–polariton interaction strength

Exciton-polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions, typically using excitons in semiconductor quantum wells (QWs) placed at the antinodes of the same cavity. Quantum phenomena such as Bose-Einstein condensation (BEC), quantized vortices, and macroscopic quantum states have been reported at temperatures from tens of Kelvin up to room temperatures, and polaritonic devices such as spin switches \cite{Amo2010} and optical transistors have also been reported. Many of these effects of exciton-polaritons depend crucially on the polariton-polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty of determining the absolute densities of polaritons and bare excitons. Here we report the direct measurement of the polariton-polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 40 $\mu$m to the center of a laser-generated annular trap, we are able to separate the polariton-polariton interactions from polariton-exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons squarely into the strongly-interacting regime. When there is a condensate, we see a sharp transition to a different dependence of the renormalization on the density, which is evidence of many-body effects.

Quantifying spectral diffusion by the direct measurement of the correlation function for excitons in semiconductor quantum wells

The phenomenon of spectral diffusion is common to a variety of inhomogeneously broadened systems. Spectral diffusion can be quantified through the frequency–frequency correlation function (FFCF), which is often approximated using observables from a variety of experimental techniques. We present a direct measurement of the temperature-dependent FFCF for excitons in semiconductor quantum wells using two-dimensional coherent spectroscopy. This technique enables the FFCF to be quantified without making any assumptions of the FFCF dynamics. Our results show that the Gauss–Markov approximation, which assumes exponential decay dynamics of the FFCF, is only valid for sample temperatures above 50 K. We compare our results with those obtained by the ellipticity and center-line slope measurements.

Superfluorescence spectra of excitons in quantum wells

We study the fluorescence light emitted from GaAs excitons in semiconductor quantum wells. The excitons are modeled as interacting bosons. By combining quantum optical methods for the excitonic emission spectrum with many particle descriptions of the transmission through the medium, we can evaluate the spectra outside the well. Comparing with experimental spectra, we get a very good agreement. The method helps to explain the main features of the observed spectra. It is demonstrated that the observed spectra show clear evidence of superfluorescent emission.

Interference Effects on Optical Spectra of Mahan Exciton in Semiconductor Quantum Wells

Optical absorption spectra of modulation-doped n-type semiconductor quantum-well structures are studied theoretically. It is shown that effective Coulomb interaction of a photo-excited hole with the sea of electrons in n=1 conduction-subband via 2D hydrogenic exciton states associated with the unoccupied higher subbands yields an appearance of new subband interference structure on the continuous tail of the Mahan exciton. Numerical results with use of material parameters for GaAs exhibit clearly such effects on the calculated absorption spectra.

Nonlinear laser cavities as nonclassical light sources

Light-emitting atoms or material in a cavity are well known to generate thermal or coherent fields depending on, e.g., the quality of the cavity and the strength of the excitation of the emitting material. We investigate the possibilities of generating antibunched photons and nonclassical light by adding nonlinear elements in the cavity and show that it is indeed feasible. In particular, we focus on cavities with emitters formed by macroscopic many-body systems like gas of excitons in semiconductor quantum wells where different nonlinearities in the emitters and absorbers will be shown to lead to bunched, Poissonian, and antibunched photon statistics. We show that, while the single two-state system with suitable coupling parameters can produce antibunched photons, macroscopic systems need an additional nonlinearity to generate antibunching phenomenon. Our results demonstrate that a nonclassical cavity field can be generated by a conventional laser when a suitable nonlinear absorbing element is added to the cavity structure. Thus, the proposed device configuration can be manufactured using standard optoelectronic materials and processing techniques.

Quantum-correlated two-photon transitions to excitons in semiconductor quantum wells

The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers.

Optical absorption by excitons in semiconductor quantum wells in tilted magnetic and electric fields

An analytical approach to the problem of the fundamental and exciton magnetoelectroabsorption in a narrow quantum well (QW) is developed. The external magnetic and electric fields are parallel and both tilted with respect to the QW growth axis. The width of the QW is taken to be much less than the magnetic length and the exciton Bohr radius. The effect of the electric field on size quantized states reduces to the size-quantized Stark shift of the well subbands. Analytical dependencies of the coefficient of the optical absorption and the exciton binding energy on the strengths of the external fields, width of the QW, exciton parameters and tilt angle are obtained and discussed. Novel effects forbidden in bulk material are found to occur. These are based on the interplay between the parallel magnetic and electric fields which in turn is caused by the splitting of the tilted fields into transverse and longitudinal components. In particular, an inversion effect is revealed. Estimates of the expected experimental values are provided for GaAs/AlGaAs QW.

Optical nonlinearities and Rabi flopping of an exciton population in a semiconductor interacting with strong terahertz fields

A fully microscopic theory is developed to study the nonlinear terahertz dynamics of incoherent excitons in semiconductor quantum wells. The theory is evaluated for strong-field excitation conditions leading to terahertz-induced population transfer, Rabi oscillations, and extreme-nonlinear dynamics. The characteristic signatures of the different effects in typical terahertz transmission-reflection experiments are identified.

Two-dimensional optical spectroscopy of excitons in semiconductor quantum wells: Liouville-space pathway analysis

We demonstrate how dynamic correlations of heavy-hole and light-hole excitons in semiconductor quantum wells may be investigated by two-dimensional correlation spectroscopy. The coherent response to three femtosecond optical pulses is predicted to yield cross (off-diagonal) peaks that contain direct signatures of many-body two-exciton correlations. Signals generated at various phase-matching directions are compared.

Tunable giant Faraday rotation of exciton in semiconductor quantum wells embedded in a microcavity

The Faraday rotation of an exciton in a GaAs quantum well (QW) embedded in a microcavity is investigated theoretically. The authors find that the Faraday rotation is enhanced remarkably by the microcavity, with a magnitude about two orders of magnitude larger than that of a single QW without microcavity. The Faraday rotation can be tuned by changing the incident angle of the pump and probe lights, or by varying the temperature or an external electric field. With an appropriate detuning between the cavity mode of the pump and probe lights, the Faraday rotation spectrum displays a strongly asymmetric line shape, which can easily be detected experimentally.

Direct measurement of acoustic-phonon scattering of hot quantum-well excitons

Due to their high momentum, hot excitons do not couple to photons directly. Thus, the interaction of excitons with phonons is typically studied only for zero-momentum excitons. We show here that the excitonic recombination process assisted by optical phonons provides a direct method to study the acoustic-phonon scattering of hot excitons in semiconductor quantum wells. From the thermal broadening of the hot-exciton luminescence, we obtain the broadening coefficient as a measure of the coupling strength. The coefficient is found to increase with exciton kinetic energy. This finding is in contrast to theoretical predictions taking into account only the direct exciton-phonon scattering process.

Spectral speckle analysis of resonant secondary emission from solids

A linear optical method to measure coherence and dephasing of excitations in solids is presented. The spectrally resolved degree of coherence of resonantly scattered light is deduced from the intensity fluctuations over its emission directions (speckles). The spectral intensity correlation gives a direct measure of the dephasing rate within the inhomogeneously broadened ensemble. For localized excitons in semiconductor quantum wells, the combination of static disorder, phonon scattering, and radiative decay leads to a spectral dependence of the emission coherence and the dephasing rate, which is well described by model calculations.

Spin dynamics driven by the electron-hole exchange interaction in quantum wells

We present a theoretical study of the spin dynamics of site-localized excitons in semiconductor quantum wells driven by the electron-hole interaction. Using a simple model for the distribution of sites dephasing leading to a strong spin polarization decay.

Binding energy of charged excitons in semiconductor quantum wells in the presence of longitudinal electric fields

We present variational calculations of the binding energy for positively and negatively charged excitons (trions) in idealized ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum wells with parabolic electrons and holes energy dispersions. The configuration interaction method is used with a physically meaningful single-particle basis set. We have shown that the inclusion of more than one electron quantum-well solution in the basis is important to obtain accurate values for the binding energies. The effects of longitudinal electric-field and quantum-well confinement on the charged excitons bound states are studied in the absence of magnetic field and the conditions for the trion ionization are discussed.

Erratum: Excitons and charged excitons in semiconductor quantum wells [Phys. Rev. B 61, 13 873 (2000)

Erratum: Excitons and charged excitons in semiconductor quantum wells [Phys. Rev. B 61, 13 873 (2000)

Faraday rotation in a study of charged excitons inCd1−xMnxTe

Interband Faraday rotation is usually interpreted as resulting from a difference in refractive indices caused by the Zeeman effect in a magnetic field. We show that, in the case of transitions due to the creation of excitons in semiconductor quantum wells containing charge carriers, a strong dependence of the line amplitude on the circular polarization in the presence of a magnetic field can also generate Faraday rotation. The spectral dependence of this Faraday rotation is completely different from that in the usual case. We demonstrate that the amplitude-related Faraday effect can dominate the rotation spectrum of charged excitons. We show that the Faraday rotation is particularly useful in studies of excitons in quantum wells with two-dimensional carrier gas where line intensities vary strongly with magnetic field.

Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells

A variational calculation of the spin-singlet and spin-triplet state of a negatively charged exciton (trion) confined to a single quantum well and in the presence of a perpendicular magnetic field is presented. We calculated the probability density and the pair correlation function of the singlet and triplet trion states. The dependence of the energy levels and of the binding energy on the well width and on the magnetic field strength was investigated. We compared our results with the available experimental data on GaAs/AlGaAs quantum wells and find that in the low magnetic field region (B<18 T) the observed transition are those of the singlet and the dark triplet trion (with angular momentum $L_z=-1$), while for high magnetic fields (B>25 T) the dark trion becomes optically inactive and possibly a transition to a bright triplet trion (angular momentum $L_z=0$) state is observed.