Crossover from exciton-polariton condensation to photon lasing in an optical trap.

Optical trapping has been proven to be an effective method of separating exciton-polariton condensates from the incoherent high-energy excitonic reservoir located at the pumping laser position. This technique has significantly improved the coherent properties of exciton-polariton condensates, when compared to a quasi-homogeneous spot excitation scheme. Here, we compare two experimental methods on a sample, where a single spot excitation experiment allowed us only to observe photonic lasing in the weak coupling regime. In contrast, the ring-shaped excitation resulted in the two-threshold behavior, where an exciton-polariton condensate manifests itself at the first and photon lasing at the second threshold. Both lasing regimes are trapped in an optical potential created by the pump. We interpret the origin of this confining potential in terms of repulsive interactions of polaritons with the reservoir at the first threshold and as a result of the excessive free-carrier induced refractive index change of the microcavity at the second threshold. This observation offers a way to achieve multiple phases of photonic condensates in samples, e.g., containing novel materials as an active layer, where two-threshold behavior is impossible to achieve with a single excitation spot.

Carrier dynamics in (Ga,In)(Sb,Bi)/GaSb quantum wells for laser applications in the mid-infrared spectral range.

We present experimental studies on low-temperature ([Formula: see text]) carrier dynamics in (Ga,In)(Sb,Bi)/GaSb quantum wells (QWs) with the nominal In content of 3.7% and the Bi ranging from 6 to 8%. The photoreflectance experiment revealed the QW bandgap evolution with [Formula: see text] % Bi, which resulted in the bandgap tunability roughly between 629 and [Formula: see text], setting up the photon emission wavelength between 1.97 and [Formula: see text]. The photoluminescence experiment showed a relatively small 3-10[Formula: see text] Stokes shift regarding the fundamental QW absorption edge, indicating the exciton localisation beneath the QW mobility edge. The localised state's distribution, being the origin of the PL, determined carrier dynamics in the QWs probed directly by the time-resolved photoluminescence and transient reflectivity. The intraband carrier relaxation time to the QW ground state, following the non-resonant excitation, occurred within 3-25[Formula: see text] and was nearly independent of the Bi content. However, the interband relaxation showed a strong time dispersion across the PL emission band and ranging nearly between 150 and [Formula: see text], indicating the carrier transfer among the localised state's distribution. Furthermore, the estimated linear dispersion variation parameter significantly decreased from [Formula: see text] to [Formula: see text] with increasing the Bi content, manifested the increasing role of the non-radiative recombination processes with Bi in the QWs.

Collective Excitations of Exciton-Polariton Condensates in a Synthetic Gauge Field.

Collective (elementary) excitations of quantum bosonic condensates, including condensates of exciton polaritons in semiconductor microcavities, are a sensitive probe of interparticle interactions. In anisotropic microcavities with momentum-dependent transverse-electric-transverse-magnetic splitting of the optical modes, the excitations' dispersions are predicted to be strongly anisotropic, which is a consequence of the synthetic magnetic gauge field of the cavity, as well as the interplay between different interaction strengths for polaritons in the singlet and triplet spin configurations. Here, by directly measuring the dispersion of the collective excitations in a high-density optically trapped exciton-polariton condensate, we observe excellent agreement with the theoretical predictions for spinor polariton excitations. We extract the interaction constants for polaritons of the same and opposite spin and map out the characteristic spin textures in an interacting spinor condensate of exciton polaritons.

Enhancement of photoluminescence from GaInNAsSb quantum wells upon annealing: improvement of material quality and carrier collection by the quantum well.

In this study we apply time resolved photoluminescence and contactless electroreflectance to study the carrier collection efficiency of a GaInNAsSb/GaAs quantum well (QW). We show that the enhancement of photoluminescence from GaInNAsSb quantum wells annealed at different temperatures originates not only from (i) the improvement of the optical quality of the GaInNAsSb material (i.e., removal of point defects, which are the source of nonradiative recombination) but it is also affected by (ii) the improvement of carrier collection by the QW region. The total PL efficiency is the product of these two factors, for which the optimal annealing temperatures are found to be ~700 °C and ~760 °C, respectively, whereas the optimal annealing temperature for the integrated PL intensity is found to be between the two temperatures and equals ~720 °C. We connect the variation of the carrier collection efficiency with the modification of the band bending conditions in the investigated structure due to the Fermi level shift in the GaInNAsSb layer after annealing.

Carrier dynamics in type-II GaAsSb/GaAs quantum wells.

Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.

Spin coherence of holes in GaAs/(Al,Ga)As quantum wells.

Carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a diluted hole gas is studied by picosecond pump-probe Kerr rotation. For resonant optical excitation of the positively charged exciton the spin precession shows two types of oscillations: Electron spin beats decaying with the charged exciton radiative lifetime of 50 ps, and long-lived hole spin beats with dephasing times up to 650 ps, which decrease with increasing temperature, underlining the importance of hole localization. The mechanism of hole spin coherence generation is discussed.