30 research outputs found
Anomalous dispersion and negative-mass dynamics of exciton polaritons in an atomically thin semiconductor
Dispersion engineering is a powerful and versatile tool that can vary the
speed of light signals and induce negative-mass effects in the dynamics of
electrons, quasiparticles, and quantum fluids. Here, we demonstrate that
dissipative coupling between bound electron-hole pairs (excitons) and photons
in an optical microcavity can lead to the formation of exciton polaritons with
an inverted dispersion of the lower polariton branch and hence, a negative
mass. We perform a direct measurement of the anomalous dispersion in an
atomically thin WS crystal embedded in a planar microcavity, and
demonstrate that the propagation direction of the negative-mass polaritons is
opposite to their momentum. Our study introduces a new concept of non-Hermitian
dispersion engineering for exciton polaritons and shows a pathway for realising
new phases of quantum matter in a solid state.Comment: 7 pages, 4 figure
Topological phase transition in an all-optical exciton-polariton lattice
Topological insulators are a class of electronic materials exhibiting robust
edge states immune to perturbations and disorder. This concept has been
successfully adapted in photonics, where topologically nontrivial waveguides
and topological lasers were developed. However, the exploration of topological
properties in a given photonic system is limited to a fabricated sample,
without the flexibility to reconfigure the structure in-situ. Here, we
demonstrate an all-optical realization of the orbital Su-Schrieffer-Heeger
(SSH) model in a microcavity exciton-polariton system, whereby a cavity photon
is hybridized with an exciton in a GaAs quantum well. We induce a zigzag
potential for exciton polaritons all-optically, by shaping the nonresonant
laser excitation, and measure directly the eigenspectrum and topological edge
states of a polariton lattice in a nonlinear regime of bosonic condensation.
Furthermore, taking advantage of the tunability of the optically induced
lattice we modify the intersite tunneling to realize a topological phase
transition to a trivial state. Our results open the way to study topological
phase transitions on-demand in fully reconfigurable hybrid photonic systems
that do not require sophisticated sample engineering.Comment: 7 pages, 4 figure
Effect of optically-induced potential on the energy of trapped exciton-polaritons below the condensation threshold
Exciton-polaritons (polaritons herein) offer a unique nonlinear platform for
studies of collective macroscopic quantum phenomena in a solid state system.
Shaping of polariton flow and polariton confinement via potential landscapes
created by nonresonant optical pumping has gained considerable attention due to
the degree of flexibility and control offered by optically-induced potentials.
Recently, large density-dependent energy shifts (blueshifts) exhibited by
optically trapped polaritons at low densities, below the bosonic condensation
threshold, were interpreted as an evidence of strong polariton-polariton
interactions [Nat. Phys. 13, 870 (2017)]. In this work, we further investigate
the origins of these blueshifts in optically-induced circular traps and present
evidence of significant blueshift of the polariton energy due to reshaping of
the optically-induced potential with laser pump power. Our work demonstrates
strong influence of the effective potential formed by an optically-injected
excitonic reservoir on the energy blueshifts observed below and up to the
polariton condensation threshold and suggests that the observed blueshifts
arise due to interaction of polaritons with the excitonic reservoir, rather
than due to polariton-polariton interaction.Comment: 10 pages, 8 figure
Collective state transitions of exciton-polaritons loaded into a periodic potential
O.A.E. acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG project EG344/2-1) and by the EU project (FP7, PIRSES-GA-2013-612600) LIMACONA. I.G.S. acknowledges support from the Academy of Finland through its Centre of Excellence Programs (Projects No. 250280 and No. 251748); Government of Russian Federation (project MK-5903.2016.2); and Dynasty Foundation. E.E., T.G., I.G.S., and E.A.O. acknowledge support by the Australian Research Council.We study the loading of a nonequilibrium, dissipative system of composite bosons - exciton polaritons - into a one dimensional periodic lattice potential. Utilizing momentum resolved photoluminescence spectroscopy, we observe a transition between an incoherent Bose gas and a polariton condensate, which undergoes further transitions between different energy states in the band-gap spectrum of the periodic potential with increasing pumping power. We demonstrate controlled loading into distinct energy bands by modifying the size and shape of the excitation beam. The observed effects are comprehensively described in the framework of a nonequilibrium model of polariton condensation. In particular, we implement a stochastic treatment of quantum and thermal fluctuations in the system and confirm that polariton-phonon scattering is a key energy relaxation mechanism enabling transitions from the highly nonequilibrium polariton condensate in the gap to the ground band condensation for large pump powers.PostprintPostprintPeer reviewe
Controlled ordering of topological charges in an exciton-polariton chain
We demonstrate, experimentally and theoretically, controlled loading of an exciton-polariton vortex chain into a 1D array of trapping potentials. Switching between two types of vortex chains, with topological charges of the same or alternating sign, is achieved by appropriately shaping an off-resonant pump beam that drives the system to the regime of bosonic condensation. In analogy to spin chains, these vortex sequences realise either a "ferromagnetic" or an "anti-ferromagnetic" order, whereby the role of spin is played by the orbital angular momentum. The ferromagnetic ordering of vortices is associated with the formation of a persistent chiral current. Our results pave the way for the controlled creation of nontrivial distributions of orbital angular momentum and topological order in a periodic exciton-polariton system.PostprintPostprintPeer reviewe