Elastic strain engineering is an important way to reversibly tune the properties of micro/nanoscale semiconductors for promising applications in the emerging advanced nanotechnologies, such as the strain enhanced high-mobility transistors, nanogenerators, etc. However, direct observation and control of energetic carrier dynamics with precise strain gradient fields remains a challenge. Here, observation of temperature-dependent exciton funnel dynamics is reported in a precisely controlled strain gradient field by time-resolved photoluminescence. An efficient exciton hopping process is revealed at picosecond time scale as the donor-bound excitons in ZnO microwires funnel along the strain gradient, which strongly depends on the temperature. Combined experiments and simulations unravel that, in addition to the exciton funnel by the strain gradient, both the excitation efficiency and emission lifetime of the excitons increase gradually from the compressive side to the tensile side due to the role of local strain regulation. The results give a clear physical picture of the energetic carrier dynamics in the strain gradient field in semiconductors, which provides a promising paradigm for the design of high-performance optoelectronic devices.

We study the influence of local inhomogeneities on carrier recombination dynamics in single InGaN/GaN core-shell microrods (MRs) by means of time-resolved microphotoluminescence (TRPL) at 10 K. At low carrier density ($\ensuremath{\sim}{10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$), the carrier recombination in the $m$-plane quantum well is dominated by radiative processes and the recorded decay times along the MR equally amount to about 400 ps, corresponding to a bimolecular coefficient of $1.1\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. When the excited carrier density exceeds ${10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, both the efficiency and the decay time of the PL in the quantum well drop significantly, which indicates the onset of Auger recombination. Based on a modified ABC model, we estimate a C coefficient varying from $0.5\ifmmode\pm\else\textpm\fi{}0.2$ to $2.2\ifmmode\pm\else\textpm\fi{}0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ from the lower to the upper part of the MR. This increase is accompanied by a rise of PL linewidth in the low excitation regime, indicating an increase of alloy disorder. Relaxation of the $k$-selection rule by alloy disorder is expected to play an important role in the observed increase of Auger coefficient. These results confirm that Auger recombination is sensitive to disorder and can be significantly enhanced in strongly disordered systems. We conclude that it is therefore crucial to minimize the degree of disorder in the active layer for high power LEDs based on core-shell MRs.

The optical properties of InGaN/GaN core-shell microrods are studied by time-resolved cathodoluminescence. Probing the carrier dynamics along the length of the rod from 4 to 300 K enables us to decompose radiative (τr) and non-radiative (τnr) lifetimes. At 300 K, τnr decreases from 500 at the bottom of the rod to 150 ps at its top. This variation results from an increased In-content in the upper part of the rod that causes a higher density of point defects. We further observe that thanks to the use of nonpolar m-plane growth, τr remains below 1.5 ns up to room temperature even with a thick active layer, which is promising for pushing the onset of the efficiency droop to higher current densities.

We demonstrate the existence of the excited state of an exciton-polariton in a semiconductor microcavity. The strong coupling of the quantum well heavy-hole exciton in an excited $2s$ state to the cavity photon is observed in nonzero magnetic field due to surprisingly fast increase of Rabi energy of the $2s$ exciton-polariton in magnetic field. This effect is explained by a strong modification of the wave function of the relative electron-hole motion for the $2s$ exciton state.

We demonstrate the existence of a novel quasiparticle, an exciton in a semiconductor doubly dressed with two photons of different wavelengths: a near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultrastrong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields. The doubly dressed quasiparticles retain the bosonic nature of their constituents, but their internal quantum structure strongly depends on the intensity of the applied terahertz field.

We report on the effect of decoherence on polariton bistability. The polariton hysteresis loop is shown to collapse in a similar way when increasing the temperature or under nonresonant excitation power. The hysteresis upward threshold is pulled to lower excitation power, whereas the downward threshold remains almost constant. This effect is explained by the population of an incoherent reservoir that induces dephasing and repulsive interaction that saturates at large densities. All experimental findings are accurately simulated with the excitonic Bloch equations and indicate that reservoir-induced dephasing can be dominant over the reservoir-induced energy blueshift.

The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.

We determine experimentally the spinor interaction constants of lower polaritons alpha 1 and alpha 2 using a resonant pump-probe spectroscopy with a spectrally narrow pump pulse. Our experimental findings are analyzed with the Bogoliubov-type theory and a mean-field two-channel model based on the lower polariton and biexciton basis. We find an enhancement of the attractive interaction and a dissipative nonlinearity of lower polaritons with antiparallel spins in the vicinity of the biexciton resonance when the energy of two lower polaritons approaches energetically that of the biexciton. These observations are consistent with the existence of a scattering resonance between lower polaritons and biexcitons (polaritonic Feshbach resonance).

The use of a Kerr nonlinearity to generate squeezed light is a well-known way to surpass the quantum noise limit along a given field quadrature. Nevertheless, in the most common regime of weak nonlinearity, a single Kerr resonator is unable to provide the proper interrelation between the field amplitude and squeezing required to induce a sizable deviation from Poissonian statistics. We demonstrate experimentally that weakly coupled bosonic modes allow exploration of the interplay between squeezing and displacement, which can give rise to strong deviations from the Poissonian statistics. In particular, we report on the periodic bunching in a Josephson junction formed by two coupled exciton-polariton modes. Quantum modeling traces the bunching back to the presence of quadrature squeezing. Our results, linking the light statistics to squeezing, are a precursor to the study of nonclassical features in semiconductor microcavities and other weakly nonlinear bosonic systems.

We study the carrier-density-dependent recombination dynamics in $m$-plane InGaN/GaN multiple quantum wells in the presence of $n$-type background doping by time-resolved photoluminescence. Based on Fermi's golden rule and Saha's equation, we decompose the radiative recombination channel into an excitonic and an electron-hole pair contribution, and extract the injected carrier-density-dependent bimolecular recombination coefficients. Contrary to the standard electron-hole picture, our results confirm the strong influence of excitons even at room temperature. Indeed, at 300 K, excitons represent up to 63 \ifmmode\pm\else\textpm\fi{} 6% of the photoexcited carriers. In addition, following the Shockley-Read-Hall model, we extract the electron and hole capture rates by deep levels and demonstrate that the increase in the effective lifetime with injected carrier density is due to asymmetric capture rates in presence of an $n$-type background doping. Thanks to the proper determination of the density-dependent recombination coefficients up to high injection densities, our method provides a way to evaluate the importance of Auger recombination.

In this review, we will try to summarize the results that we have obtained on the measurement of polariton interactions. We will describe here the samples, the experimental systems and some of the important results. We will also give a few highlights on the theoretical description of these results. One of the main topics of this review will be the observation of the Feshbach resonance for polaritons, and its interpretation through the coupling of two lower polaritons into a biexciton.

We investigate the dynamics of donor bound excitons (D°XA) at T = 10 K around an isolated single edge dislocation in homoepitaxial GaN, using a picosecond time-resolved cathodoluminescence (TR-CL) setup with high temporal and spatial resolutions. An ∼ 1.3 meV dipole-like energy shift of D°XA is observed around the dislocation, induced by the local strain fields. By simultaneously recording the variations of both the exciton lifetime and the CL intensity across the dislocation, we directly assess the dynamics of excitons around the defect. Our observations are well reproduced by a diffusion model. It allows us to deduce an exciton diffusion length of ∼24 nm as well as an effective area of the dislocation with a radius of ∼95 nm, where the recombination can be regarded as entirely non-radiative.

In this Comment I challenge the main conclusions of Ref. [1]. The main reason for this Comment is the large number of questions on the details of the experiment that led me to request the raw data. I have obtained two sets of raw data. After a long discussion with the authors and the editors of PRL we decided that I should use the second set of data. Within the limited space of a Comment, I will only discuss here two of the published figures.

The interaction between coherent polaritons and incoherent excitons plays an important role in polariton physics. Using resonant pump-probe spectroscopy with selective excitation of single polariton branches, we investigate the different dephasing mechanisms responsible for generating a long-lived exciton reservoir. As expected, pumping the upper polariton results in a strong dephasing process that leads to the generation of a long lived reservoir. Unexpectedly, we observe an efficient reservoir creation while exciting only the lower polariton branch when the detuning is increased towards positive detuning. We propose a simple theoretical model, the polaritonic Bloch equations, to describe the dynamics of the system.

Received 3 November 2015DOI:https://doi.org/10.1103/PhysRevLett.117.029701© 2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPolaritonsCondensed Matter, Materials & Applied Physics

We demonstrate the occurrence of spatial multistability using laterally confined microcavity exciton-polaritons. By coherently exciting with a blue detuned laser a series of confined polariton modes, we investigate the effects of multistability on the transmitted laser beam as a function of the excitation power. At each threshold of the hysteresis loop, a switching of the mode profile of the laser beam is associated with a significant energy jump of each of the confined polariton modes in the mesa. A simulation of this behavior is achieved with a multimode generalization of the Gross-Pitaevskii equations in the exciton photon basis. The mechanism behind the spatial multistability is identified as a repulsive cross interaction between polaritons in different modes.

Probing local emission properties of organic-inorganic lead halide perovskite material can provide evidence regarding the photovoltaic performance of perovskite solar cells. Herein, cathodoluminescence, which has the potential to resolve emission characteristics in the nanoregime, has been exploited to carry out temperature-dependent studies on individual well-faceted CH3NH3PbI3-xBrx perovskite single crystals. The spatial distribution of emission recorded at 4 and 300 K reveals that the periphery of the perovskite crystals radiates predominantly, which establishes that such an unusual emission characteristic is independent of the crystallographic phase of CH3NH3PbI3-xBrx crystals. Investigation based on scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy deduces that the asymmetric cathodoluminescence is associated with the nonhomogeneous distribution of methylammonium cations in CH3NH3PbI3-xBrx single crystals. These results emphasize the unraveling of a correlation between the composition and spectroscopic properties of perovskite crystals in the nanoregime, which eventually can influence the overall photovoltaic performance of the devices based on them.

This study demonstrates the formation of extremely smooth and uniform formamidinium lead bromide (CH(NH2)2PbBr3 = FAPbBr3) films using an optimum mixture of dimethyl sulfoxide and N,N‐dimethylformamide solvents. Surface morphology and phase purity of the FAPbBr3 films are thoroughly examined by field emission scanning electron microscopy and powder X‐ray diffraction, respectively. To unravel the photophysical properties of these films, systematic investigation based on time‐integrated and time‐dependent photoluminescence studies are carried out which, respectively, bring out relatively lower nonradiative recombination rates and long lasting photogenerated charge carriers in FAPbBr3 perovskite films. The devices based on FTO/TiO2/FAPbBr3/spiro‐OMeTAD/Au show highly reproducible open‐circuit voltage (Voc) of 1.42 V, a record for FAPbBr3‐based perovskite solar cells. Voc as a function of illumination intensity indicates that the contacts are very selective and higher Voc values are expected to be achieved when the quality of the FAPbBr3 film is further improved. Overall, the devices based on these films reveal appreciable power conversion efficiency of 7% under standard illumination conditions with negligible hysteresis. Finally, the amplified spontaneous emission (ASE) behavior explored in a cavity‐free configuration for FAPbBr3 perovskite films shows a sharp ASE threshold at a fluence of 190 μJ cm−2 with high quantum efficiency further confirming the high quality of the films.

Second-order time correlation measurements with a temporal resolution better than 3 ps were performed on a CdTe microcavity where spontaneous Bose-Einstein condensation is observed. After the laser pulse, the nonresonantly excited thermal polariton population relaxes into a coherent polariton condensate. Photon statistics of the light emitted by the microcavity evidences a clear phase transition from the thermal state to a coherent state, which occurs within 3.2 ps after the onset of stimulated scattering. Following this very fast transition, we show that the emission possesses a very high coherence that persists for more than 100 ps after the build-up of the condensate.

Using femtosecond pump-probe spectroscopy, we identify excitation-induced dephasing as a major mechanism responsible for the breakdown of the strong coupling between excitons and photons in a semiconductor microcavity. The effects of dephasing are observed on the transmitted probe-pulse spectrum as a density-dependent broadening of the exciton-polariton resonances and the emergence of a third resonance at high excitation density. A striking asymmetry in the energy shift between the upper and the lower polaritons is also evidenced. Using the excitonic Bloch equations, we quantify the respective contributions to the energy shift of many-body effects associated with Coulomb fermion exchange and photon assisted exchange processes and the contribution to collisional broadening.