24 research outputs found
Tilting flat bands in an empty microcavity
Recently microcavities with anisotropic materials are shown to be able to
create novel bands with non-zero local Berry curvature. The anisotropic
refractive index of the cavity layer is believed to be critical in opening an
energy gap at the tilted Dirac points. In this work, we show that an
anticrossing between a cavity mode and a Bragg mode can also form within an
empty microcavity without any birefringent materials. Flat bands are observed
within the energy gap due to the particular refractive index distribution of
the sample. The intrinsic TE-TM splitting and XY splitting induce the squeezing
of the cavity modes in momentum space, so that the flat bands are
spin-dependently tilted. Our results pave the way to investigate the spin orbit
coupling of photons in a simple microcavity without anisotropic cavity layers
Spiraling vortices in exciton-polariton condensates
We introduce the phenomenon of spiraling vortices in driven-dissipative
(non-equilibrium) exciton-polariton condensates excited by a non-resonant pump
beam. At suitable low pump intensities, these vortices are shown to spiral
along circular trajectories whose diameter is inversely proportional to the
effective mass of the polaritons, while the rotation period is mass
independent. Both diameter and rotation period are inversely proportional to
the pump intensity. Stable spiraling patterns in the form of complexes of
multiple mutually-interacting vortices are also found. At elevated pump
intensities, which create a stronger homogeneous background, we observe more
complex vortex trajectories resembling Spirograph patterns
Lasing threshold doubling at the crossover from strong to weak coupling regime in GaAs microcavity
In a polariton, laser coherent monochromatic light is produced by a low-energy state of the system at the bottom of a polariton âtrapâ, where a condensate of polaritons is formed, requiring no conventional population inversion. Following the recent realization of polariton light-emitting diodes (LEDs) based on GaAs microcavities (MCs) operating up to room temperature, efforts have been directed towards the demonstration of an electrically injected polariton laser. However, until now, low-threshold polariton lasing in GaAs MCs under optical pumping has been reported only at low temperatures. Here, we investigate the temperature dependence of lasing threshold across the border of the strong-to-weak coupling regime transition in high-finesse GaAs MCs under non-resonant optical pumping. Remarkably, we find that although lasing in the strong coupling regime is lost when the temperature is raised from 25 to 70 K, the threshold only doubles, in stark contrast with the expected difference of two orders of magnitude. Our results can be explained by considering temperatureinduced thermalization of carriers to high wavevector states, increasing the reservoirâs overall carrier lifetime, resulting in an order of magnitude higher steady-state carrier density at 70 K under similar pumping conditions
Electrically controlling vortices in a neutral exciton polariton condensate at room temperature
Manipulating bosonic condensates with electric fields is very challenging as
the electric fields do not directly interact with the neutral particles of the
condensate. Here we demonstrate a simple electric method to tune the vorticity
of exciton polariton condensates in a strong coupling liquid crystal (LC)
microcavity with CsPbBr microplates as active material at room temperature.
In such a microcavity, the LC molecular director can be electrically modulated
giving control over the polariton condensation in different modes. For
isotropic non-resonant optical pumping we demonstrate the spontaneous formation
of vortices with topological charges of +1, +2, -2, and -1. The topological
vortex charge is controlled by a voltage in the range of 1 to 10 V applied to
the microcavity sample. This control is achieved by the interplay of a built-in
potential gradient, the anisotropy of the optically active perovskite
microplates, and the electrically controllable LC molecular director in our
system with intentionally broken rotational symmetry. Besides the fundamental
interest in the achieved electric polariton vortex control at room temperature,
our work paves the way to micron-sized emitters with electric control over the
emitted light's phase profile and quantized orbital angular momentum for
information processing and integration into photonic circuits
Spin selective filtering of polariton condensate flow
Spin-selective spatial filtering of propagating polariton condensates, using a controllable spin-dependent gating barrier, in a one-dimensional semiconductor microcavity ridge waveguide is reported. A nonresonant laser beam provides the source of propagating polaritons, while a second circularly polarized weak beam imprints a spin dependent potential barrier, which gates the polariton flow and generates polariton spin currents. A complete spin-based control over the blocked and transmitted polaritons is obtained by varying the gate polarization
Single-shot spatial instability and electric control of polariton condensates at room temperature
In planar microcavities, the transverse-electric and transverse-magnetic
(TE-TM) mode splitting of cavity photons arises due to their different
penetration into the Bragg mirrors and can result in optical spin-orbit
coupling (SOC). In this work, we find that in a liquid crystal (LC) microcavity
filled with perovskite microplates, the pronounced TE-TM splitting gives rise
to a strong SOC that leads to the spatial instability of microcavity polariton
condensates under single-shot excitation. Spatially varying hole burning and
mode competition occurs between polarization components leading to different
condensate profiles from shot to shot. The single-shot polariton condensates
become stable when the SOC vanishes as the TE and TM modes are spectrally well
separated from each other, which can be achieved by application of an electric
field to our LC microcavity with electrically tunable anisotropy. Our findings
are well reproduced and traced back to their physical origin by our detailed
numerical simulations. With the electrical manipulation our work reveals how
the shot-to-shot spatial instability of spatial polariton profiles can be
engineered in anisotropic microcavities at room temperature, which will benefit
the development of stable polariton-based optoeletronic and light-emitting
devices
Dynamics of a polariton condensate transistor switch
We present a time-resolved study of the logical operation of a polariton condensate transistor switch.
Creating a polariton condensate (source) in a GaAs ridge-shaped microcavity with a non-resonant pulsed laser beam, the polariton propagation towards a collector, at the ridge edge, is controlled by a
second weak pulse (gate), located between the source and the collector. The experimental results are
interpreted in the light of simulations based on the generalized Gross-Pitaevskii equation, including
incoherent pumping, decay, and energy relaxation within the condensate
Talbot Effect for Exciton Polaritons
e demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter wavesâan exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic âTalbot carpetâ is produced by loading the exciton-polariton condensate into a microstructured one-dimensional periodic array of mesa traps, which creates an array of phase-locked sources for coherent polariton flow in the plane of the quantum wells. The spatial distribution of the Talbot fringes outside the mesas mimics the near-field diffraction of a monochromatic wave on a periodic amplitude and phase grating with the grating period comparable to the wavelength. Despite the lossy nature of the polariton system, the Talbot pattern persists for distances exceeding the size of the mesas by an order of magnitude. Thus, our experiment demonstrates efficient shaping of the two-dimensional flow of coherent exciton polaritons by a one-dimensional âflat lens.
Publisher Correction: Single-shot condensation of exciton polaritons and the hole burning effect
Correction to: Nature Communications https://doi.org/10.1038/s41467-018-05349-4; published online: 9 August 2018
The original PDF version of this Article had an incorrect Published online date of 25 December 2018; it should have been 9 August 2018. This has been corrected in the PDF version of the Article. The HTML version was correct from the time of publication
Chiral Modes at Exceptional Points in Exciton-Polariton Quantum Fluid
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 system's 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