Excitons In Quantum Wells Research Articles

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si0.85Ge0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.

Optical spectroscopy of a resonant Bragg structure with InGaN/GaN quantum wells

The optical reflectance and transmittance spectra of a periodic InGaN/GaN semiconductor heterostructure with 60 quantum wells are studied at room temperature. The period of the structure was chosen such that, at some angles of incidence of light, the energy of a photon resonantly reflected from the Bragg structure coincides with the excitation energy for quantum-well excitons in quantum wells. The parameters of these excitons are determined by fitting the spectra measured at two different angles of incidence, 30° and 60°. We take into account the resonant exciton transitions in quantum wells as well as the transitions into the continuous spectrum. The radiative decay parameter is determined to be (0.20 ± 0.02) meV.

Excitons in asymmetric quantum wells

Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs 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. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger 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 is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

Resonantly enhanced spin-lattice relaxation ofMn2+ions in diluted magnetic (Zn,Mn)Se/(Zn,Be)Se quantum wells

The dynamics of spin-lattice relaxation in the magnetic ${\mathrm{Mn}}^{2+}$ ion system of (Zn,Mn)Se/(Zn,Be)Se quantum-well structures are studied using optical methods. Pronounced cusps are found in the giant Zeeman shift of the quantum-well exciton photoluminescence at specific magnetic fields below 10 T, when the Mn spin system is heated by photogenerated carriers. The spin-lattice relaxation time of the Mn ions is resonantly accelerated at the cusp magnetic fields. Our theoretical analysis demonstrates that a cusp occurs at a spin-level mixing of single ${\mathrm{Mn}}^{2+}$ ions and a quick-relaxing cluster of nearest-neighbor Mn ions, which can be described as intrinsic cross-relaxation resonance within the Mn spin system.

Exciton recombination in spontaneously formed and artificial quantum wells AlxGa1‐xN/AlyGa1‐yN (x&lt;y∼0.8)

AbstractWe report on photoluminescence (PL) spectroscopy and electron microscopy studies of an AlGaN quantum well (QW) structure grown by molecular beam epitaxy under metal‐rich conditions with substrate rotation. Both techniques reveal unintentional formation within the AlGaN barriers of a quasiperiodic structure of thin Ga‐rich layers, whose period is controlled by both the substrate rotation rate and the AlGaN growth rate. These compositional modulations act as 1‐3 monolayer thick QWs emitting below 250 nm with an internal quantum efficiency (IQE) as high as ∼30% at room temperature under weak excitation. Variational calculations of the QW exciton properties indicate that the observed high IQE can result from strong three‐dimensional localization of the excitons confined in the narrow QWs. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ultra-low-power hybrid light-matter solitons.

New functionalities in nonlinear optics will require systems with giant optical nonlinearity as well as compatibility with photonic circuit fabrication techniques. Here we introduce a platform based on strong light–matter coupling between waveguide photons and quantum-well excitons. On a sub-millimetre length scale we generate picosecond bright temporal solitons at a pulse energy of only 0.5 pJ. From this we deduce a nonlinear refractive index three orders of magnitude larger than in any other ultrafast system. We study both temporal and spatio-temporal nonlinear effects and observe dark–bright spatio-temporal polariton solitons. Theoretical modelling of soliton formation in the strongly coupled system confirms the experimental observations. These results show the promise of our system as a high speed, low power, integrated platform for physics and devices based on strong interactions between photons.

Time-resolved photoluminescence properties of hybrids based on inorganic AlGaN/GaN quantum wells and colloidal ZnO nanocrystals

Dynamic properties are studied for the AlGaN/GaN quantum well (QW) structures with and without the coating of colloidal ZnO nanocrystals (NCs). The QW exciton recombination rate was reduced in such hybrids compared to the bare QW structure only in the sample with the thinnest cap layer of 3nm. Assuming that one of the recombination mechanisms in this hybrid is non-radiative resonant energy transfer (NRET) between the QW and the energy acceptor material i.e. ZnO NCs, the maximum pumping efficiency was estimated to be ∼42% at 60K. The NRET effect is, however, vanished after several months despite that the hybrid structures are composed of chemically stable components.

Plasmon-excitonic scattering of light from a nanoparticle located near a quantum well

A solution is presented for the problem of resonant elastic scattering of polarized light from a nanoparticle and a quantum well located near semiconductor surface. Coupling between surface plasmons of the metal particle and quasi-2D excitons of the quantum well is taken into account. The problem is solved by the Green’s functions technique treating the resonant polarization response of particle and quantum well in a self-consistent approximation. The effective polarizability is found for a metal nanoparticle of ellipsoidal shape with account of dynamical effect of “image” charges caused by semiconductor surface and quantum well. Spectra are numerically calculated for a model structure “metal-semiconductor” including a silver nanoparticle and a quantum well AlGaAs/GaAs. Appearance of exciton-plasmon interaction in the resonant scattering of light is interpreted as an enhancement by surface plasmons of the optical response due to quantum-well quasi-2D excitons.

MEMS-based air-gap cavity for entangled photon generation via polariton-polariton scattering

A structure for entangled photon generation via polariton-polariton scattering is proposed. We have designed an air-gap planar cavity with embedded quantum wells, in which the photonic modes are in strong coupling regime with excitons of quantum wells. An electrical tuning mechanism is designed by introducing an air-gap layer and an integrated electrical actuator to the distributed Bragg reflector (DBR) mirror structure. Our structure consists of 40 (17) bottom (top) DBRs with 12 GaAs/AlAs (7/3 nm) quantum wells at the antinodes of the electromagnetic field working at 767-nm wavelength. Initial air-gap thickness is 1500 nm. The Q factor and Rabi splitting of the structure are 1742 and 14 meV, respectively. Reflectivity of cavity/air interface is 0.32, and suspended DBR reflectivity is 0.97. Average indices of suspended mirror and cavity are 3.2062 and 3.1901, respectively. According to our simulation results, by changing air-gap thickness from 1000 to 1350 nm, the wavelength tuning range is extended to 30 nm via applying a maximum voltage of 12 V.

Resonant Bragg structures based on III-nitrides

Abstract

Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures

Hybrid samples based on ZnO colloidal nanocrystals (NCs) deposited on AlGaN/GaN quantum well (QW) structures with different top barrier thickness d = 3, 6 and 9 nm are studied by time-resolved photoluminescence. Thermal behavior of the QW exciton lifetime in the hybrids and in the bare QW structures has been compared and it has been found that the QW exciton recombination rate increases in the hybrid having d = 3 nm and decreases in the hybrid with d = 6 nm, while no change has been observed for the structure with d = 9 nm. It is suggested that non-radiative resonance energy transfer from the QW excitons to the ZnO NCs and a variation of the surface potential can both influence the QW exciton lifetime in the hybrids.

Exciton spin noise in quantum wells

A theory of spin fluctuations of excitons in quantum wells in the presence of nonresonant excitation has been developed. Both bright and dark excitonic states have been taken into account. The effect of a magnetic field applied in a quantum-well plane has been analyzed in detail. We demonstrate that in relatively small fields, the spin-noise spectrum consists of a single peak centered at a zero frequency, while an increase of magnetic field results in the formation of the second peak in the spectrum, owing to an interplay of the Larmor effect of the magnetic field and the exchange interaction between electrons and holes forming excitons. Experimental possibilities to observe the exciton spin noise are discussed, particularly by means of ultrafast spin-noise spectroscopy. We show that the fluctuation spectra contain, in addition to individual contributions of electrons and holes, information about the correlation of their spins.

Optics of spin-noise-induced gyrotropy of an asymmetric microcavity

The optical gyrotropy noise of a high-finesse semiconductor Bragg microcavity with an embedded quantum well (QW) is studied at different detunings of the photon mode with respect to the QW exciton resonances. A strong suppression of the noise magnitude for the photon mode frequencies lying above exciton resonances is found. We show that such a critical behavior of the observed optical noise power is specific to asymmetric Fabry-Perot resonators. As follows from our analysis, at a certain level of intracavity loss, the reflectivity of the asymmetric resonator vanishes, while the polarimetric sensitivity to the gyrotropy changes dramatically when moving across the critical point. The results of model calculations are in a good agreement with our experimental data on the spin noise in a single-quantum-well microcavity and are confirmed also by the spectra of the photoinduced Kerr rotation in the pump-probe experiments.

Strong exciton-photon coupling in open semiconductor microcavities

We present a method to implement 3-dimensional polariton confinement with in-situ spectral tuning of the cavity mode. Our tunable microcavity is a hybrid system consisting of a bottom semiconductor distributed Bragg reflector (DBR) with a cavity containing quantum wells (QWs) grown on top and a dielectric concave DBR separated by a micrometer sized gap. Nanopositioners allow independent positioning of the two mirrors and the cavity mode energy can be tuned by controlling the distance between them. When close to resonance, we observe a characteristic anticrossing between the cavity modes and the QW exciton demonstrating strong coupling. For the smallest radii of curvature concave mirrors of 5.6 μm and 7.5 μm, real-space polariton imaging reveals submicron polariton confinement due to the hemispherical cavity geometry.

Two-photon injection of polaritons in semiconductor microstructures

We experimentally demonstrate that two-photon pumping of "dark" excitons in quantum wells embedded in semiconductor microcavities can result in exciton-polariton injection and photon lasing. In the case of a semiconductor micropillar pumped at half of the exciton frequency, we observe a clear threshold behavior, characteristic of the vertical cavity surface emitting laser transition. These results are interpreted in terms of stimulated emission of terahertz photons, which allows for conversion of "dark" excitons into exciton-polaritons.

Quantum Breathing Mode of Trapped Particles: From Nanoplasmas to Ultracold Gases

AbstractParticles spatially confined in trapping potentials have attracted increasing interest over the recent decade. Of particular importance are systems of charged particles, such as non‐neutral plasmas, nanoplasmas, electrons in metal clusters, electrons in quantum‐confined semiconductor structures (“artificial atoms”), electrons on the surface of liquid helium, ions in traps or highly charged particles (grains) in dusty plasmas. A second example of recent interest are systems with other kinds of pair interactions, including dipole interaction, which is important for excitons in quantum wells or ultracold Fermi and Bose gases in traps and optical lattices.Trapped systems are fundamentally different from macroscopic systems since they are dominated by strong spatial inhomogeneity and finite size effects (the properties depend on the exact particle number). Furthermore, by changing the strength of the confinement potential, the many‐particle state of the system can be externally controlled—from weak coupling (gas‐like) to strong coupling (crystal‐like). While trapped classical particles are meanwhile well understood and accessible to first‐principle computer simulations, their quantum counterparts still pose big challenges, both for experiment and theory. Therefore, collective properties that can be easily measured or computed and allow to diagnose the many‐particle state of the system are of prime importance. It has been found that the quantum breathing mode (monopole oscillation) is one of the most important such properties. In recent years a number of theoretical studies has demonstrated that the quantum breathing mode is ideally suited to measure the coupling strength (the degree of nonideality) of a trapped system, its kinetic and interaction energy and other key observables. This give rise to a novel kind of “spectroscopy” of trapped systems.In this review these developments are summarized. The quantum breathing mode is studied for trapped fermions and bosons with Coulomb and dipole interaction, respectively. A systematic description of collective oscillations and especially the breathing mode is provided. Making use of time‐dependent perturbation theory, it is shown how the corresponding breathing frequencies are connected to the properties of the initial equilibrium system. This gives rise to the application of the quantum mechanical sum rules. It is demonstrated how an improved version of the conventional sum rule formulas is suitable for an accurate description of the breathing mode in small systems. Finally, the dependence of the breathing mode on the particle number N is analyzed and the limit of large N is studied for one‐dimensional and two‐dimensional systems. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Optical properties of semiconductor in planar plasmonic structures: strong coupling and lasing

In this paper we describe the modifications in the GaAs quantum well (QW) optical properties induced by the proximity of a metallic film. The formation of hybrid plasmon/exciton states composed of a mix of heavy-hole exciton/light-hole exciton mixed states is demonstrated with reflectometry experiments. Alternative surface modes, namely Tamm plasmons, can also induce hybridization with the QW excitons with Rabi splitting energy of 11.5 meV. Plasmonic Tamm states are interface modes formed at the boundary between a photonic structure and a metallic layer. These modes present both the advantages of surface plasmons and of microcavities photonic modes. A conventional lasing effect will also be described in Tamm plasmon structure containing QWs. Tamm plasmons can be spatially confined by structuring the metallic part of the system, thus reducing the size of the mode and allowing various geometries. Due to the relatively low damping and the versatility of the Tamm geometries, these modes are good candidates for a new type of lasers.

Surface potential effect on excitons in AlGaN/GaN quantum well structures

AlGaN/GaN quantum well (QW) heterostructures grown by metalorganic chemical vapor deposition on sapphire and on free-standing GaN substrates have been studied by temperature dependent time-resolved photoluminescence. A dominant contribution of the exciton radiative lifetime is observed in homoepitaxial samples even at enhanced temperatures up to 100 K. The QW-related emission is found to be more sensitive to the near surface built-in electric field in the homoepitaxial samples, revealed as a red shift of the QW exciton energy with decreasing the cap layer thickness. Absence of such shift in the heteroepitaxial samples suggests, assuming a surface potential of 0.5 eV, an increased polarization field due to residual compressive stress.

Luminescence in ZnMnTe/ZnMgTe and CdMnTe/CdMgTe structures with different parameters of quantum wells

The low-temperature luminescence of ZnMnTe/ZnMgTe and CdMnTe/CdMgTe quantum well structures with different quantum well widths and different Mn proportions is studied at optical-excitation power densities ranging from 104 to 106 W cm−2. Because of saturation of the lowest excited state 4 T 1 of the 3d shell of Mn2+ ions, transitions to higher states start to play an important role. As a result, the intracenter luminescence of Mn2+ ions deteriorates at high excitation levels. Simultaneously, the temperature-dependent saturation of the main exciton-emission band e1hh1 of the quantum wells occurs, and the band e2hh2 emerges. As the optical excitation is increased, the intracenter luminescence band of Mn2+ ions changes its shape. This effect is attributed to the faster saturation of the excited states of interface ions. For CdMnTe/CdMgTe structures, the effect of the quantum well width and Mn content on the relation between the emission intensities corresponding to excitons in quantum wells, excitons in barriers, and the 3d shell of Mn2+ ions is established.