Konstantinos G. Lagoudakis

Observation of half-quantum vortices in an exciton-polariton condensate

Quantum Division The notion of quantum mechanics is that variables are expressed as integer values. In quantum fluids, for instance, vortices are quantized in terms of the polarization and phase shifts observed as multiples of 2π, that is, full rotations of each variable around the vortex core. Theoretical work has predicted that in some instances there should be half-quantum vortices, where circumventing the vortex core is characterized by just π rotations of the phase and polarization. By observing exciton-polariton condensates Lagoudakis et al. (p. 974) present evidence for the existence of these long-predicted half-quantum vortices. Evidence is presented for the existence of half-quantum vortices in exciton-polariton condensates. Singly quantized vortices have already been observed in many systems, including the superfluid helium, Bose-Einstein condensates of dilute atomic gases, and condensates of exciton-polaritons in the solid state. Two-dimensional superfluids carrying spin are expected to demonstrate a different type of elementary excitations referred to as half-quantum vortices, characterized by a π rotation of the phase and a π rotation of the polarization vector when circumventing the vortex core. We detect half-quantum vortices in an exciton-polariton condensate by means of polarization-resolved interferometry, real-space spectroscopy, and phase imaging. Half-quantum vortices coexist with single-quantum vortices in our sample.

Inverse design and demonstration of a compact and broadband on-chip wavelength demultiplexer

An on-chip integrated wavelength demultiplexer designed using an inverse computational algorithm is experimentally demonstrated. 1,300 and 1,550 nm wavelength light is sorted in a device area of just 2.8 × 2.8 μm2.

Coherent Oscillations in an Exciton-Polariton Josephson Junction

We report on the observation of spontaneous coherent oscillations in a microcavity polariton bosonic Josephson junction. Condensation of exciton polaritons here takes place under incoherent excitation in a double potential well naturally formed in the disorder. Coherent oscillations set on at an excitation power well above the condensation threshold. The time resolved population and phase dynamics reveal the analogy with the ac Josephson effect. A theoretical two-mode model describes the observed effects, explaining how the different realizations of the pulsed experiment can be in phase.

Probing the Dynamics of Spontaneous Quantum Vortices in Polariton Superfluids

The experimental investigation of spontaneously created vortices is of utmost importance for the understanding of quantum phase transitions towards a superfluid phase, especially for two-dimensional systems that are expected to be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time-resolved near-field interferometry we track the path of such vortices, created at random locations in an exciton-polariton condensate under pulsed nonresonant excitation, to their final pinning positions imposed by the stationary disorder. We formulate a theoretical model that successfully reproduces the experimental observations.

Coexisting nonequilibrium condensates with long-range spatial coherence in semiconductor microcavities

Real- and momentum-space spectrally resolved images of microcavity polariton emission in the regime of condensation are investigated under nonresonant excitation using a laser source with reduced intensity fluctuations on the time scale of the exciton lifetime. We observe that the polariton emission consists of many macroscopically occupied modes. Lower-energy modes are strongly localized by the spatial polaritonic potential disorder on a scale of a few microns. Higher-energy modes have finite k vectors and are delocalized over 10– 15 m. All the modes exhibit long-range spatial coherence comparable to their size. We provide a theoretical model describing the behavior of the system with the results of the simulations in good agreement with the experimental observations. We show that the multimode emission of the polariton condensate is a result of its nonequilibrium character, the interaction with the local polaritonic potential, and the reduced intensity fluctuations of the excitation laser.

Synchronized and Desynchronized Phases of Exciton-Polariton Condensates in the Presence of Disorder

Condensation of exciton polaritons in semiconductor microcavities takes place despite in-plane disorder. Below the critical density, the inhomogeneity of the disorder limits the spatial extension of the ground state. Above the critical density, in the presence of weak disorder, this limitation is spontaneously overcome by the nonlinear interaction, resulting in an extended synchronized phase. In the case of strong disorder, several non-phase-locked condensates can be evidenced. The transition from a synchronized phase to a desynchronized phase is addressed by sampling the cavity disorder.

Coherent Generation of Nonclassical Light on Chip via Detuned Photon Blockade.

The on-chip generation of nonclassical states of light is a key requirement for future optical quantum hardware. In solid-state cavity quantum electrodynamics, such nonclassical light can be generated from self-assembled quantum dots strongly coupled to photonic crystal cavities. Their anharmonic strong light-matter interaction results in large optical nonlinearities at the single photon level, where the admission of a single photon into the cavity may enhance (photon tunneling) or diminish (photon blockade) the probability for a second photon to enter the cavity. Here, we demonstrate that detuning the cavity and quantum-dot resonances enables the generation of high-purity nonclassical light from strongly coupled systems. For specific detunings we show that not only the purity but also the efficiency of single-photon generation increases significantly, making high-quality single-photon generation by photon blockade possible with current state-of-the-art samples.

Spontaneous pattern formation in a polariton condensate.

Exciton-polariton condensation can be regarded as a self-organization phenomenon, where phase ordering is established among particles in the system. In such condensed systems, further ordering can occur in the particle density distribution, under particular experimental conditions. In this work we report on spontaneous pattern formation in a polariton condensate under nonresonant optical pumping. The slightly elliptical ring-shaped excitation laser that we employ forces condensation to occur into a single-energy state with periodic boundary conditions, giving rise to a multilobe standing-wave patterned state.

Nonclassical higher-order photon correlations with a quantum dot strongly coupled to a photonic-crystal nanocavity

We use the third- and fourth-order autocorrelation functions

Dynamics of Long-Range Ordering in an Exciton-Polariton Condensate

g^{(3)}(\tau_1,\tau_2)