Session J39: Exciton, Polariton, and Electron Dynamics in two-dimensional semiconductor structures

Valence-band Mixing in Strain-based Traps for Indirect Excitons in Coupled Quantum Wells

Excitons in a coupled quantum well system under an applied bias are spatially separated, with electrons and holes in opposite wells. The pair can interact via the Coulomb interaction, but recombination is suppressed. Indirect excitons exhibit a long (up to tens of microseconds), tunable lifetime and are of interest to those studying high density excitonic phase transitions in equilibrium. Recent experiments have revealed sharply temperature- and density-dependent transitions in the luminescence pattern of strain-trapped indirect excitons [1]. Localized strain shifts the valence and conduction bands, creating a trap for the case of hydrostatic stretching [2]. Two factors differentiate this technique from electrostatic traps, the splitting between the light- and heavy-hole exciton states varies with stress and the presence of shear strain modifies the symmetry of the ground state. Due to strong dependence of the radiative lifetime on the tunneling rate, the indirect heavy- and light-hole excitons have drastically different recombination rates; meaning that mixing sharply alters the luminescence pattern. We will discuss the modeling of the structure of the single-particle potential and the role that interband mixing plays in the appearance of high density phase transitions.   

Dark excitons and Bose-Einstein condensation in strain-trapped excitons in coupled quantum wells

It is possible to make excitons in coupled quantum wells with very long lifetime compared to their thermalization, allowing us to study equilibrium behavior. Using a localized stress to create traps for excitons in coupled quantum wells, we have demonstrated that at low temperature, high density, and large stress, the spatial pattern of photoluminescence (PL) from interwell excitons transitions to one with a dimmed center. This pattern emerges despite the center remaining the region of highest exciton density. This darkening is related to a strain-induced interaction between the light hole and heavy hole states. However, while this explanation provides a mechanism to explain many of the features, a few important predictions of this theory are not borne out by experiments. An alternate explanation is possible, utilizing an increasing population of dark (J=2) excitons and a separation of the dark and bright species. It has been proposed that a Bose-Einstein Condensate in this system would occur in a dark state, and this transition is consistent with the onset criteria of the pattern formation and explains how a slight bright/dark energy difference could lead to spatial separation of the species. Experiments employing a magnetic field to turn `dark' excitons slightly `bright' should allow the disambiguation of the role of dark excitons in this system. I will review this pattern formation and discuss data from experiments employing a magnetic field.   

Trapping Long-Lifetime Superfluid Polaritons Using a Laser-Generated Barrier

We present results with new microcavity structures that have polaritons that live as long as 100 ps, about two orders of magnitude longer than in previous structures. We show that an exciton reservoir can be used to create a potential barrier for a superfluid polariton gas. In these experiments, we use a non-resonant laser to create a superfluid polariton gas at the same location as a population of excitons. The excitonic component of the polariton can coherently scatter with the excitons. Since the excitons have mass four orders of magnitude larger than the polaritons, they form essentially a static barrier for the polaritons. By resolving the polaritons both in real space and momentum space, we determine that the polaritons roll down the potential barrier created by the excitons. These results are consistent with numerical solutions of the Gross-Pitaevskii equation for the measured parameters of this system. The potential to use the exciton reservoir as a barrier/trap will allow exciting new ways to study macroscopic coherence phenomena such as Josephson oscillations and superfluid vortices.   

All Optical Switching Using Exciton-Polariton Renormalization in Microcavities

We report on a method of all-optical switching based on resonantly pumped exciton-polariton microcavities. Depending on geometry, each optical transistor could be used to make an AND or an AND NOT gate. Optical switching in these samples may be limited by the lifetime of the polaritons, allowing for near THz frequency switching. These gates could be used for fast signal processing or optical computing. Where optical computing calls for signals and gates to be at the same wavelength, we can use a gate beam at an angle, since the dispersion relation of polaritons allows for the absorption of a gate higher in energy than the k=0 state. High exciton and free carrier densities lead to a renormalization of the lower polariton (LP) and increase its energy. If the k=0 LP becomes resonant with the signal beam, then the gate may be used to turn on transmission (or consequently turn off reflectivity) of that signal. We will present achievable on/off ratios and switching speeds as well as discuss modulating an intense signal with a weaker gate.   

Two-Dimensional Mott-Hubbard Electrons in an Artificial Honeycomb Lattice

Artificial crystal lattices can be used to tune repulsive Coulomb interactions between electrons. We trapped electrons, confined as a two-dimensional gas in a gallium arsenide quantum well, in a nanofabricated lattice with honeycomb geometry [1,2]. In our most recent studies [3] we probed the excitation spectrum of electrons in the honeycomb lattice with lattice spacing ranging down to 90nm in a magnetic field identifying collective modes that emerged from the Coulomb interaction in the artificial lattice, as predicted by the Mott-Hubbard model. These observations allow us to determine the Hubbard gap and suggest the existence of a Coulomb-driven ground state [3]. The proposed research promises to further expand current realms of study of quantum simulators. While the experiments are challenging, studies of electrons confined to artificial lattices should provide key perspectives on strong electron correlation in condensed matter science.\\[4pt] [1] M. Gibertini et al. Phys. Rev. B RC 79, 241406 (2009)\\[0pt] [2] G. De Simoni et al. Appl. Phys. Lett. 97, 132113 (2010)\\[0pt] [3] A. Singha et al. Science 332, 1176 (2011)   

Cooperative carrier dynamics in InGaAs/GaAs quantum wells in high magnetic fields

Ultrafast spectroscopy in strong magnetic fields provides a powerful means for studying quantum coherence in many-body systems. A high magnetic field leads to tunable energy quantization, which in turn results in a substantial enhancement of densities of states and suppression of scattering. Here, we study superfluorescence (SF), i.e., cooperative spontaneous emission of ultradense electron-hole plasmas in InGaAs quantum wells in a perpendicular magnetic field up to 17.5 T. We observe SF both through time-resolved photoluminescence and differential transmission (DT) measurements. We create an ultradense electron-hole plasma with an intense femtosecond laser pulse, and after a certain delay, an ultrashort burst of coherent radiation emerges. At the same time, an abrupt decrease in population from full inversion to zero was observed through DT measurements. Furthermore, the DT signals strongly depended on the probe energy, showing an anomalous negative DT signal (i.e., induced absorption) under certain circumstances.   

High-Order Sideband Generation in Quantum Wells Driven by Intense THz Radiation: Electron-Hole Recollisions

Non-linear mixing of optical beams with intense terahertz beams has been observed in semiconductor heterostructures both as changes to the absorption spectrum and as sideband generation. Sidebands are generated when a material has a non-linear dielectric response that mixes the electric field of the two beams and produces radiation at the optical frequency plus or minus multiples of the THz frequency. Perturbatively generated sidebands have been observed up to fourth order when the optical beam resonantly excites excitons in GaAs quantum wells. We present here our observation of high-order sideband generation (HSG) from excitons in InGaAs QWs. Sidebands of up to 18$^{th}$ order are observed. From the THz intensity dependence and THz polarization dependence (linear-circular) of the sideband generation, we show that the observed phenomenon is non-perturbative. We note that the exciton system driven by an intense THz field is analogous to an atomic system driven by intense optical fields, as is the case for high-order harmonic generation (HHG). The recollision model used to describe HHG in atoms is then extended to describe HSG in excitons. Experimental results indicate that the recollision model is a valid description of the nature of HSG in excitons.   

Intersubband electrons coupled to zone-folded coherent acoustic phonons in a GaN/AlN superlattice

We present spectrally-resolved resonant pump-probe measurements on a strongly polar GaN/AlN superlattice. The transmitted and reference probe spectra are detected at the same read out rate. Analysis of the normalized probe spectra reveals electronic contributions as well as the first three folded longitudinal acoustic phonons, matching calculated frequency values predicted by the elastic continuum model. The observed modes couple strongly to intersubband electrons and modulate the spectral width and position of the intersuband absorption. We conclude that this electron-phonon coupling takes place via piezoelectric effects and a phonon-induced modulation of the subband effective mass.   

Intersubband polariton bleaching observed in ultrafast mid-infrared spectroscopy

Strong coupling between intersubband transition in quantum wells and microcavity field leads to the concept of intersubband (ISB) polaritons. Their linear properties have been explored for several years, mainly focusing on optical spectra and electroluminescence. Among the implemented microcavity geometries, the one based on a photonic crystal (Zanotto et. al., Appl. Phys. Lett. 2010) is paricularly interesting as it allows simultaneous access to both polariton branches at anticrossing. Here we report on the nonlinear response of ISB polaritons, investigating their bleaching, an effect already studied in the excitonic framework. When ISB polaritons are probed by an intense mid-infrared laser pulse, the typical double-peaked transmission spectrum is converted in a single-peaked one. By tuning the intensity of the femtosecond pulse that covers both polariton branches, the whole range between weak and strong coupling regimes has been swept. This study reveals that there is a threshold for pumping above which polariton states are destroyed, and restricts the pump intensity range that can be employed in an optically-pumped ISB polariton laser. Moreover, as a consequence of nonlinear optical properties, saturable absorbers and optically bistable devices could be implemented.   

Pump-probe experiment in LaMnO3/SrTiO3 superlattices and thin film

We present the time dependent transmittance of LaMnO3 thin film and [(LaMnO3)n/(SrTiO3)8]20 (n=2 and 8) superlattices grown on SrTiO3 substrate. We used the laser pulse pump-probe technique. We observed two phonon oscillation in the LaMnO3 film at 8 THz and at 15 THz. In the superlattices, 8 THz mode seemed obliterated. The phonon oscillation damping time constant was also different. In LaMnO3 thin film, we could observe the oscillation until $\sim$ 1.5 ps. In the superlattices, the damping time constant was smaller: $\sim$ 0.7 ps for n=8 superlattice and $\sim$ 0.3 ps for n=2 superlattice. We will discuss number of phonon mode and the damping time constant in terms of the sample geometry and the electronic struncture.   

Excitonics of Hybrid Nanostructures Arranged with Mixed Dimensionality

We present a complete study of the F\"{o}rster-type nonradiative energy transfer in hybrid nanostructures composed of nanoparticles, nanowires and quantum wells, and investigate the effects of quantum confinement in different dimensions. We systematically consider all possible combinations in terms of dimensionality for exciton-exciton interactions in these hybrid architectures, and analyze the resulting energy transfer rates for item-to-item excitonic coupling as a function of dimensionality. We derive a full set of analytical expressions and show that the exciton transfer strongly depends on the dimensionality and geometry of the hybrid system. Arrangements of such nanostructures with mixed dimensionality ranging from the low dimensionality to the high offer important high-efficiency applications in photovoltaics [1,2], while in the reciprocal case (from the high dimensionality to the low) in light generation [3] and LEDs [4]. [1] J. Sambur, et al., Science 330, 63 (2010). [2] M. D. Kelzenberg, et al., Nature Materials 9, 239--244 (2010). [3] R. Yan, et al., Nature Photonics 3, 569-576 (2009). [4] H.V. Demir, et al., Nano Today (2011) doi:10.1016/j.nantod.2011.10.006   

Many-particle effects in the photoluminescent response of silicon quantum-dot solids

Monodisperse colloidal suspensions of ligand-coated silicon nanocrystals (SiNCs), synthesized through a nonthermal low-pressure plasma reaction, are prepared through density-gradient ultracentrifugation in mixed organic solvents. The SiNC fractions are then self-assembled into close-packed quantum-dot ``solids'' and clusters, and photoluminescent properties of the resulting ordered ensembles are characterized through optical spectroscopy. We find striking manifestations of particle-particle interactions in the measured optical response, and we model these effects using Monte Carlo simulations of the photobleaching kinetics in dense SiNC packings.   

Spectral dependence of the Aharonov Bohm effect in the magneto-photoluminescence of layered ZnTe-ZnSe structures

Aharonov-Bohm (AB) oscillations in the mangeto-photoluminescence (PL) intensity of multilayered ZnTe/ZnSe structures grown via migration enhanced epitaxy (MEE) using three submonolayer deposition cycles of Zn-Te-Zn sandwiched between ZnSe barriers confirmed the presence of type-II ZnTe-based QDs. These co-exist with isoelectronic centers (ICs) as evident from the PL spectra. The spectral dependence of the transition magnetic field and the magnitude of the AB oscillation in intensity are investigated. A qualitative probing of distribution in the ensemble of QDs and ICs was done. The transition magnetic field changed from a lower value at the lower energy side of the PL emission to a higher value at the higher energy side which confirmed the lateral QD size distribution. AB oscillations at spectral positions dominated by emission from ICs were also observed suggesting that the presence of QDs also affects the ICs although the magnitude of the oscillation in the AB peak decreases at such spectral positions.