Mark Steger

Bose-Einstein Condensation of Long-Lifetime Polaritons in Thermal Equilibrium

The experimental realization of Bose-Einstein condensation (BEC) with atoms and quasiparticles has triggered wide exploration of macroscopic quantum effects. Microcavity polaritons are of particular interest because quantum phenomena such as BEC and superfluidity can be observed at elevated temperatures. However, polariton lifetimes are typically too short to permit thermal equilibration. This has led to debate about whether polariton condensation is intrinsically a nonequilibrium effect. Here we report the first unambiguous observation of BEC of optically trapped polaritons in thermal equilibrium in a high-Q microcavity, evidenced by equilibrium Bose-Einstein distributions over broad ranges of polariton densities and bath temperatures. With thermal equilibrium established, we verify that polariton condensation is a phase transition with a well-defined density-temperature phase diagram. The measured phase boundary agrees well with the predictions of basic quantum gas theory.

Long-range ballistic motion and coherent flow of long-lifetime polaritons

Exciton-polaritons can be created in semiconductor microcavities. These quasiparticles act as weakly interacting bosons with very light mass, of the order of

Direct measurement of polariton–polariton interaction strength

10^{-4}

Single-wavelength, all-optical switching based on exciton-polaritons

times the vacuum electron mass. Many experiments have shown effects which can be viewed as due to a Bose-Einstein condensate, or quasicondensate, of these particles. The lifetime of the particles in most of those experiments has been of the order of a few picoseconds, leading to significant nonequilibrium effects. By increasing the cavity quality, we have made new samples with longer polariton lifetimes. With a photon lifetime on the order of 100-200 ps, polaritons in these new structures can not only come closer to reaching true thermal equilibrium, a desired feature for many researchers working in this field, but they can also travel much longer distances. We observe the polaritons to ballistically travel on the order of one millimeter, and at higher densities we see transport of a coherent condensate, or quasicondensate, over comparable distances. In this paper we report a quantitative analysis of the flow of the polaritons both in a low-density, classical regime, and in the coherent regime at higher density. Our analysis gives us a measure of the intrinsic lifetime for photon decay from the microcavity and a measure of the strength of interactions of the polaritons.

Chiral modes at exceptional points in exciton-polariton quantum fluids

Yongbao Sun,1∗ Yoseob Yoon, Mark Steger, Gangqiang Liu, Loren N. Pfeiffer, Ken West, David W. Snoke,2∗ and Keith A. Nelson Department of Chemistry and Center for Excitonics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Department of Physics, University of Pittsburgh, 3941 O’Hara St., Pittsburgh, PA 15218, USA Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA

The exciton-polariton microcavity as an optical transistor

We present a method for all-optical switching using a microcavity exciton-polariton system. Unlike many other switching methods, we use a single wavelength for both the signal and gate, which means that this method could be used for cascading optical logic gates and amplification within an all-optical circuit. We resonantly pump the sample at an angle with a laser beam and probe the sample with another beam at normal incidence. Upon saturation of the exciton-polariton states, the normal-incidence resonance increases in energy to permit the probe beam to be transmitted through the sample. Experimental results demonstrate switching using a GaAs/AlAs microcavity. Switching times on the order of ten picoseconds and on/off ratios on the order of 10:1 have been observed, and we propose design options to improve upon these.

Enhanced coherence between condensates formed resonantly at different times.

We demonstrate the generation of chiral modes-vortex flows with fixed handedness in exciton-polariton quantum fluids. The chiral modes arise in the vicinity of exceptional points (non-Hermitian spectral degeneracies) in an optically induced resonator for exciton polaritons. In particular, a vortex is generated by driving two dipole modes of the non-Hermitian ring resonator into degeneracy. Transition through the exceptional point in the space of the systems parameters is enabled by precise manipulation of real and imaginary parts of the closed-wall potential forming the resonator. As the system is driven to the vicinity of the exceptional point, we observe the formation of a vortex state with a fixed orbital angular momentum (topological charge). This method can be extended to generate higher-order orbital angular momentum states through coalescence of multiple non-Hermitian spectral degeneracies. Our Letter demonstrates the possibility of exploiting nontrivial and counterintuitive properties of waves near exceptional points in macroscopic quantum systems.

Interaction between stimulated current injection and polariton condensate

Exciton-polariton systems were demonstrated as a medium for all-optical switching independently by several groups in 2012 and 2013. The polariton system is promising for such an application due to its strong nonlinearities and potentially low threshold densities. The photonic component of the polariton enables long-range motion and low losses. We review reports on several different techniques for polariton optical switching, and discuss how these depend on the high density effects in microcavity polaritons.

Publisher Correction: Single-shot condensation of exciton polaritons and the hole burning effect

We demonstrate coherence between exciton-polariton condensates created resonantly at different times. The coherence persists much longer than the individual particle dephasing time, and this persistence increases as the particle density increases. The observed coherence time exceeds that of the injecting laser pulse by more than an order of magnitude. We show that this significant coherence enhancement relies critically on the many-body particle interactions, as verified by its dependence on particle density, interaction strength, and bath temperature, whereas the mass of the particles plays no role in the condensation of resonantly injected polaritons. Furthermore, we observe a large nonlinear phase shift resulting from intra-condensate interaction energy. Our results provide a new approach for probing ultrafast dynamics of resonantly-created condensates and open new directions in the study of coherence in matter.

Time-resolved two-photon excitation of long-lifetime polaritons

In this paper, we see a strong effect of the injected current on the light emission from the polariton condensate in an n-i-n structure, when we monitor the luminescence intensity under applied bias at various pump powers. We present here three thresholds for nonlinear increase of the intensity. We show that small changes of the incoherent injected current lead to stimulated enhancement of the coherent light emission from free carriers. We conclude that the polariton condensatecurrent system is a highly nonlinear electro-optical system.