Showing papers by "Andreas Löffler published in 2012"

Power-law decay of the spatial correlation function in exciton-polariton condensates

Abstract: We create a large exciton-polariton condensate and employ a Michelson interferometer setup to characterize the short- and long-distance behavior of the first order spatial correlation function. Our experimental results show distinct features of both the two-dimensional and nonequilibrium characters of the condensate. We find that the gaussian short-distance decay is followed by a power-law decay at longer distances, as expected for a two-dimensional condensate. The exponent of the power law is measured in the range 0.9–1.2, larger than is possible in equilibrium. We compare the experimental results to a theoretical model to understand the features required to observe a power law and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions. Our results indicate that Berezinskii–Kosterlitz–Thouless-like phase order survives in open-dissipative systems.

Exciton?polariton condensates with flat bands in a two-dimensional kagome lattice

Abstract: Microcavity exciton–polariton condensates, as coherent matter waves, have provided a great opportunity to investigate hydrodynamic vortex properties, superfluidity and low-energy quantum state dynamics. Recently, exciton condensates were trapped in various artificial periodic potential geometries: one-dimensional (1D), 2D square, triangular and hexagonal lattices. The 2D kagome lattice, which has been of interest for many decades, exhibits spin frustration, giving rise to magnetic phase order in real materials. In particular, flat bands in the 2D kagome lattice are physically interesting in that localized states in the real space are formed. Here, we realize exciton–polariton condensates in a 2D kagome lattice potential and examine their photoluminescence properties. Above quantum degeneracy threshold values, we observe meta-stable condensation in high-energy bands; the third band exhibits a signature of weaker dispersive band structures, a flat band. We perform a single-particle band structure calculation to compare measured band structures.

Characterization of two-threshold behavior of the emission from a GaAs microcavity

Abstract: We compare two regimes indicative of polariton lasing and photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the quantum-well exciton. In particular, we investigate the cavity emission subsequent to nonresonant pulsed excitation. For the ground state emission from the lower energy-momentum dispersion branch we find a two-threshold behavior in the input-output curve where each transition is accompanied by characteristic changes of the in-plane mode dispersion. We demonstrate that the thresholds are unambiguously evidenced in the photon statistics of the emission based on the second-order correlation function. Moreover, the distinct two-threshold behavior is confirmed in the evolution of the emission pulse duration. Our findings show that a comprehensive study of spectral and temporal characteristics of the emission from a semiconductor microcavity can be used to characterize the different emission regimes.

All-optical control of quantized momenta on a polariton staircase

Abstract: Here we demonstrate a simple and reconfigurable way to create a polariton condensate in well defined discrete momentum states, allowing us to manipulate the local polariton flow. To this end, we created a spatially varying potential formed in the presence of noncondensed carriers by subjecting a microcavity to spatially modulated nonresonant optical excitation. The choice of the spatial shape of this potential allows us to tailor the properties of the polariton condensate in momentum space. Our results demonstrate a way to prepare a polariton condensate in an adjustable momentum state and provide a first step toward the creation of functional all-optical elements for polaritonic logic circuits on demand by projecting circuits onto an unprocessed planar sample.

Multi-dimensional laser spectroscopy of exciton polaritons with spatial light modulators

Abstract: We describe an experimental system that allows one to easily access the dispersion curve of exciton-polaritons in a microcavity. Our approach is based on two spatial light modulators, one for changing the excitation angles (momenta), and the other for tuning the excitation wavelength. We show that with this setup, an arbitrary number of states can be excited accurately and that re-configuration of the excitation scheme can be done at high speed.

Relaxation dynamics of optically imprinted polariton wires

Abstract: We create an optically imprinted gain-trapped polariton wire with a length of 15 μm and a width of 2μm by modulating the shape of a non-resonant excitation spot using a high-resolution spatial light modulator (SLM). We study the spatially, spectrally and temporally resolved emission from the wire and find that the system passes several regimes, starting with an intense emission peak originating from the wire center and develops towards longer living emission from its side. The temporal development of the emission wavelengths corresponding to these peaks allow us to characterize these different emission regimes in terms of photon lasing and polariton condensation.

Direct photoluminescence observation of the negative Bogoliubov branch in an exciton-polariton condensate

Abstract: Bose-Einstein condensates exhibit fascinating behavior such as superfluidity and vortex formation, effects owing their presence to self-interactions between the underlying bosons [1]. In exciton-polariton condensates another peculiar effect has been theoretically predicted, where the negative energy Bogoliubov dispersion becomes observable in the photoluminescence (PL) spectrum [2–5]. The effect originates from the anomalous mixing of creation and destruction operators of polaritons, which results in the creation of a Bogoliubov particle when the PL is observed. Here we directly observe the PL of the negative Bogoliubov dispersion branch for the first time in a high density regime far above the exciton-polariton condensation threshold. The observation is made possible by strong coupling being maintained at densities corresponding to ∼ 100 times the threshold density for condensation. The presence of strong coupling at such densities is evidenced by second-order coherence and PL measurements showing clear differences to photon lasing, a weak coupling phenomenon. Comparison of the negative dispersion PL spectrum shows good agreement to theoretical predictions.

Multiexcitonic emission from single elongated InGaAs/GaAs quantum dots

Abstract: In this work, we present both experimental data and simulations of multiexcitonic emission spectra of single self-assembled elongated In0.3Ga0.7As/GaAs quantum dots. The emission spectra reveal an unusual evolution with the increased excitation power density. First, a biexciton line appears simultaneously with its low energy sideband, the origin of which has already been postulated previously and related to the interaction of a quantum dot biexciton with excitons generated in the surrounding wetting layer. A further increase of the excitation causes a disappearance of the exciton line accompanied with a transformation of the biexciton sharp line and its sideband into a redshifting broad emission band. The latter recalls a typical feature of the transition from excitonic emission into electron-hole plasma called Mott transition, which is possible to occur in wire-like structures under the conditions of very high carrier densities. However, we propose an alternative explanation and show that this behavior can be well explained based on a multilevel rate equation model, indicating that such a dependence of the emission spectra is a fingerprint of a formation of multiexcitonic states. Further, we discuss the importance of various quantum system parameters as the radiative lifetimes or spectral linewidths.