Excitons In Quantum Wells Research Articles

Whispering-Gallery Quantum-Well Exciton Polaritons in an Indium Gallium Arsenide Microdisk Cavity.

Despite appealing high-symmetry properties that enable strong spatial confinement and ultrahigh-Q, optical whispering-gallery modes of spherical and circular resonators have been absent from the field of quantum-well exciton polaritons. Here we observe whispering-gallery exciton polaritons in a gallium arsenide microdisk cavity filled with indium gallium arsenide quantum wells, the test bed materials of polaritonics. Strong coupling is evidenced in photoluminescence and resonant spectroscopy accessed through concomitant confocal microscopy and near-field optical techniques. Excitonic and optical resonances are tuned by varying temperature and disk radius, revealing Rabi splittings between 5 and 10meV. A dedicated analytical quantum model for such circular whispering-gallery polaritons is developed, which reproduces the measured values. At high power, lasing is observed and accompanied by a blueshift of the emission consistent with the regime of polariton lasing. With experimental methods and theory now established, whispering-gallery-mode polaritons in round dielectric resonators appear as a new viable platform toward low loss polaritonics.

Polarized photoluminescence and g-factor in an inhomogeneous ensemble of quantum dots in magnetic fields

In this work, polarized photoluminescence caused by quasi-equilibrium spin orientation of excitons at Zeeman sublevels in an ensemble of quantum dots of different sizes is studied theoretically. The following unexpected results are found: (i) the splitting of photoluminescence bands in a magnetic field in an ensemble of quantum dots is much larger than the Zeeman splitting of exciton levels in a single dot, (ii) contrary to the Boltzmann distribution the low-energy luminescence band has a lower intensity than the high-energy band, (iii) the sign of the circular polarization of photoluminescence changes along the contour of the bands, (iv) a universal formula for the dependence of the exciton g-factor on the quantum dot size is obtained. All of the above features of the polarized luminescence spectra are explained by variations of the exciton g-factor when its quantization energy changes. The results obtained are applicable not only to ensembles of quantum dots, but also to any ensemble of localized states: excitons in quantum wells under conditions of well-width fluctuations, impurity states in quantum wells, etc.

Manipulation of optical coherence of quantum-well excitons by transverse magnetic field

We have studied experimentally spin-dependent photon echoes from excitons in an InGaAs/GaAs quantum well subject to a transverse magnetic field (Voigt geometry). Larmor precession of the spins of the electron and heavy hole in an exciton leads to a periodic transfer of coherence between bright and dark exciton states. The increase in dephasing time due to the transition to the dark states could be useful for coherent control development. A comprehensive analysis shows a good agreement between a four-wave mixing experiment and the predictions of a theoretical treatment based on a five-level exciton model comprising a ground state and two pairs of bright and dark states. The extracted optical dephasing time of bright and dark excitons is equal to 30 and 130 ps, respectively. Exploiting the photon echo reveals evidence of electron Larmor precession with in-plane $g$ factor $|{g}_{e,\ensuremath{\perp}}|=0.44\ifmmode\pm\else\textpm\fi{}0.05$ and heavy hole precession as well. The precession frequency of the latter depends nonlinearly on the applied magnetic field, and the corresponding $g$ factor reaches a value of $|{g}_{h,\ensuremath{\perp}}|\ensuremath{\approx}0.3$ at $B=6$ T. Estimates for the heavy hole $g$-factor spreading as well as the isotropic exchange interaction constant are provided.

Quantum correlations of exciton–polaritons induced by magnetic field

From the canonical and rotating-wave transformations, we obtained the effective lower-polariton Hamiltonian depending on the applied magnetic field from the coupling of one of the quantum-well excitons and photons in a semiconductor microcavity. When the pump field is circularly polarized, the nonlinear interaction can induce a strong polariton blockade near the resonant peaks, which can be suppressed by a large pump in weak magnetic field situations. Thus, the platform behavior in the variation of the polariton occupation proves the polariton quantum blockade. On the other hand, when the pump field is linearly polarized, we achieve a single-photon emitter whose polariton polarization can be adjusted by an applied magnetic field. Polaritons with opposite spins, on the other hand, are anti-bunching in the lower excitation regime and become bunching only in weak magnetic field situations and a high excitation regime. Our findings constitute a potential step toward the realization of strongly coupled polaritonic systems.

Non-trivial dependence of spectral characteristics of excitons in quantum wells on the resonant optical excitation power

Basic exciton parameters, such as the energy of exciton transition and the radiative and nonradiative broadenings, are experimentally studied by means of reflectance spectroscopy for a heterostructure with a 14-nm GaAs/AlGaAs quantum well. Particular attention is paid to the nonradiative broadening which is sensitive to densities of free carriers and long-lived nonradiative excitons. A sublinear increase of the broadening of the heavy-hole and light-hole exciton resonances is observed when the light-hole exciton resonance is excited with increasing power. A simple model is developed, which allows one to well reproduce the observed dependence. Keywords: exciton, quantum well, nonradiative broadening.

Резонансное отражение света оптической решеткой экситонов, сформированной 100 квантовыми ямами InGaN

Optical properties of a structure with a periodic system of 100 InGaN quantum wells (QWs) separated by non-tunneling GaN barriers have been investigated at room temperature. The structure periodicity corresponded to the Bragg diffraction condition at the frequency of the QW excitons. Numerical modeling using transfer matrices gave a quantitatively accurate fit of the experimental results. The model included the resonance response of A, B, and C excitons in QWs and an optical absorption tail in the barriers and buffer layer. We have determined the radiative and non-radiative broadening of the excitons in the InGaN QWs.

Coupling of Quantum-Well Excitons to Plasmons in One-Dimensional Metal Nanocylinder Gratings

Theory of plasmon-exciton coupling is developed for a metal nanocylinder grating located in the vicinity of a semiconductor quantum well. In dipole approximation, the effective polarizability of cylinders is derived, taking into account generation of quantum-well excitons in the near field of surface plasmons. Given the polarizability, the spectra of reflection and absorption are calculated for metal-semiconductor systems to study the effect of plasmon-exciton interaction. The excitonic response is shown to be enhanced by high-quality plasmons of Ag cylinders, although strong coupling is not attained for realistic nanostructure parameters.

Magnetic control over the zitterbewegung of exciton–polaritons

The effect of the zitterbewegung consisting in trembling of trajectory of propagating particles may, in principle, be found in a variety of physical systems characterized by split kinetic energy dispersion branches. However, in a majority of material systems the effect is too weak to be observable. Specially designed semiconductor heterostructures representing optical microcavities with embedded quantum wells allow observing the zitterbewegung of exciton–polaritons that are optical cavity modes strongly hybridized with excitons in quantum wells. Here we show that external magnetic fields applied in the plane of the microcavity amplify this effect and allow for tuning the amplitude and the period of oscillations of polariton trajectories, thus being a convenient tool of control. These results pave the way towards realization of ballistic polariton transistors based on the spin–orbit effect, conceptually similar to Datta-and-Das transistors.

Optically active energy states of the exciton in quantum wells of various widths

The optically active energy states of the exciton in GaAs/Al0.3Ga0.7As quantum wells of various widths are calculated by the direct solution of the three-dimensional Schrodinger equation. A dependence of energies on the quantum well width as a parameter is obtained for widths up to 200 nm. The results are compared with the model of quantization of the exciton as a whole in wide quantum wells. The radiative decay rates for several lowest exciton states are also found.

Quantum confined Rydberg excitons in reduced dimensions

In this paper we propose first steps towards calculating the energy shifts of confined Rydberg excitons in quantum wells, wires, and dots. The macroscopic size of Rydberg excitons with high quantum numbers implies that already μm sized lamellar, wire-like, or box-like structures lead to quantum size effects, which depend on the principal Rydberg quantum number Such structures can be fabricated using focused ion beam milling of cuprite crystals. Quantum confinement causes an energy shift of the confined object, which is interesting for quantum technology. We find in our calculations that the Rydberg excitons gain a potential energy in the μeV to meV range due to the quantum confinement. This effect is dependent on the Rydberg exciton size and, thus, the principal quantum number The calculated energy shifts in the μeV to meV energy range should be experimentally accessible and detectable.

Energy spectrum of excitons in square quantum wells

Energies of the ground and excited states of excitons in GaAs/AlGaAs and InGaAs/GaAs finite square quantum wells (QWs) of various widths are calculated. This is achieved by studying the three-dimensional Schrödinger equation for the exciton in a QW and, in particular, by determining the lower energy boundary of the continuous spectrum of the corresponding differential operator. The eigenvalue problem for the Schrödinger equation is solved numerically by the finite-difference method properly taking into account discontinuities of the material parameters at the interfaces of the QW. The calculated bound states of electron-hole pairs are classified based on the types of their dominant in-plane and quantum-confinement one-dimensional functions of the wave function factorized form. A dependence of energy levels on a QW width as a parameter is thoroughly studied for widths up to 100 nm. The accurate radiative decay rates for calculated s-like exciton states are also obtained. Calculated energy spectra are confronted with the experimental reflectance spectra measured for high-quality InGaAs/GaAs heterostructures with QWs. The ground and, at least, a few excited states of the heavy-hole exciton in QW are identified in the experimental spectra.

Yukawa model of screening in low-dimensional excitons: diagonalization, perturbation, variation, and resummation analysis

The Yukawa interaction describes Coulomb screening in several physical systems. We investigate bound quantum states of the Yukawa potential in low-dimensional structures as a model of e.g. excitons in semiconductor nanostructures. Diagonalization, perturbation, variation, and resummation methods are all applied to the problem and their accuracy is compared. For moderate positive screening, all methods are found to be applicable and, in particular, the variational approach is highly accurate except near the threshold for bound states. In contrast, only hypergeometric resummation captures the correct behaviour in the negative (antiscreened) regime. We also determine the dimensional dependence of the critical screening, above which bound states cease to exist. As an application, the critical screening is used to compute critical doping levels for the existence of bound excitons in quantum wells.

Purcell Effect and Nonlinear Behavior of the Emission in a Periodic Structure Composed of InAs Monolayers Embedded in a GaAs Matrix

AbstractEnhancement of spontaneous emission in a resonant Bragg quantum well (QW) structure with 60 periods of triple InAs monolayers embedded in a GaAs matrix is studied experimentally and theoretically. From measurements of the time‐resolved photoluminescence, besides the QW exciton at 1.47 eV, a specific super‐radiant (SR) emission demonstrating nonlinear properties is found. The SR mode shows a near‐quadratic dependence of intensity on excitation power, while its energy position follows the Bragg condition. It is revealed that the SR mode shows a peculiar non‐monotonic dependence of intensity on direction, with a maximum observed at approximately 40°. The enhancement in the SR emission at a specific direction is correlated well with suggested theoretical consideration of the modal Purcell factor for periodic quantum well structures.

Optical studies of InAs/GaAs monolayer Bragg superlattices

Periodic sequences of InAs monolayer in GaAs have been grown by molecular beam epitaxy. The properties of the structure have been modelled by transfer matrix calculation and experimentally studied by time-resolved photoluminescence. The structure obtained provides good localisation of excitons in quantum wells and opens possibilities for various optical effects. Coupling of excitons in quantum wells with electromagnetic field confined by a Bragg arrangement of layers produces a peculiar emission mode – a superradiant mode. Measurements and calculations show that for specific values of frequency of light and angle of emission, enhancement of spontaneous emission rate occurs.

A parallel magnetic field driven confinement versus separation of charges in GaAs quantum well investigated by magneto-photovoltage and magneto-photoluminescence spectroscopy

Opto-electronic properties of excitons under parallel and perpendicular magnetic field are investigated on various thickness of GaAs/AlGaAs quantum well. An analytical expression of diamagnetic energy under a parallel magnetic field is derived by solving the Schrödinger equation of exciton in quantum wells. The transformation underlying the solution of Schrödinger equation suggests that an in-plane separation between electron and hole may increase with an increasing parallel magnetic field, leading to a reduced coulomb interaction and slow radiative recombination rate. As a consequence of this, even in the absence of an external electrical bias, parallel magnetic field induces a chiral charge transportation across the hetero-interfaces of a quantum well, which gives rise to an increasing in-plane magneto-photovoltage signal. The present investigation provides a unique method to control the recombination and transportation of charges by the strength and orientation of a magnetic field.

Frequency comb generation in a resonantly pumped exciton-polariton microring resonator.

We develop a scheme for generation of a regular sequence of narrow spectral lines (optical frequency comb) in semiconductor micro-ring resonators operating in the strong-coupling regime. A strong optical nonlinearity of exciton-polaritons, forming as mixed states between the microcavity photons and quantum-well excitons, allows for a low-threshold operation. This work demonstrates visibility of using the exciton-polaritons for the purposes of generation of GHz combs and trains of picoseconds pulses for future all-polariton information processing schemes.

Anisotropic superfluidity of two-dimensional excitons in a periodic potential

We study anisotropies of helicity modulus, excitation spectrum, sound velocity and angle-resolved luminescence spectrum in a two-dimensional system of interacting excitons in a periodic potential. Analytical expressions for anisotropic corrections to the quantities characterizing superfluidity are obtained. We consider particularly the case of dipolar excitons in quantum wells. For GaAs/AlGaAs heterostructures as well as MoS$_2$/hBN/MoS$_2$ and MoSe$_2$/hBN/WSe$_2$ transition metal dichalcogenide bilayers estimates of the magnitude of the predicted effects are given. We also present a method to control superfluid motion and to determine the helicity modulus in generic dipolar systems.

Plasmon-excitonic polaritons in superlattices

A theory for propagation of polaritons in superlattices with resonant plasmon-exciton coupling is presented. A periodical superlattice consists of a finite number of cells with closely located a quantum well and a monolayer of metal nanoparticles. Under study is the spectrum of hybrid modes formed of the quasitwo- dimensional excitons of quantum wells and the dipole plasmons of metal particles. The problem of electrodynamics is solved by the method of Green’s functions with taking account of the resonant polarization of quantum wells and nanoparticles in a self-consistent approximation. The effective polarizability of spheroidal particles occupying a square lattice is calculated with taking into consideration the local-field effect of dipole plasmons of the layer and their images caused by the excitonic polarization of nearest quantum well. Optical reflection spectra of superlattices with GaAs/AlGaAs quantum wells and silver particles are numerically analyzed. Special attention is paid to the superradiant regime originated in the Bragg diffraction of polaritons in superlattice. Superradiance is investigated separately for plasmons and excitons, and then for hybrid plasmonexcitonic polaritons. It is demonstrated that the broad spectrum of reflectance associated with plasmons depends on the number of cells in superlattice, and it has a narrow spectral dip in the range of plasmon-excitonic Rabi splitting.

Excitons in tunnel coupled CdTe and (Cd,Mn)Te quantum wells

The photoluminescence (PL) from structures containing Cd0.95Mn0.05Te and CdTe quantum wells (QWs) separated by a narrow (1.94 nm) barrier was studied. The PL lines of comparable intensities from several possible exciton states were observed simultaneously at energy distances substantially exceeding kT. This means that the energy transfer in the studied systems is slower than the radiative recombination of the confined excitons. For the CdTe QW width of about 8.7–9 nm, indirect excitons with the electron and heavy hole chiefly localized in the CdTe and Cd1−xMnxTe QWs, respectively, were detected in the magnetic field. These indirect excitons have PL energy of about 10–20 meV above the PL line of the direct excitons in the CdTe QW. The observation of the PL from the indirect excitons which are not the lowest excitations in the structure is a distinctive feature of the system.Photoluminescence intensity dependence on the energy and the magnetic field.

Photoluminescence Properties of β-Ga2O3 Thin Films Produced by Ion-Plasma Sputtering

Photoluminescence and photoexcitation spectra of β-Ga2O3 thin films prepared by high-frequency ion-plasma sputtering in an Ar atmosphere were investigated. Photoluminescence spectra were deconvoluted by the Alentsev—Fock method into ultimate constituents. The nature of two strong bands with maxima at 2.95 and 3.14 eV and two weak bands with maxima at 3.90 and 4.25 eV was discussed. The two strong bands were attributed to an associate originating from the interaction of oxygen and gallium vacancies; the weak ones, recombination of excitons in quantum wells formed by acceptor clusters. It was found that the damping time constant for the 3.14-eV band was 105 μs; for the 2.95-eV band, 114 μs. The similarity of the decay time constants for these bands confirmed their relationship to a common associate.