arXiv subjects

E. Estrecho

Publications and source records attributed to E. Estrecho.

16 recordsLinked to original sources

arXiv2024

Coherence of a non-equilibrium polariton condensate across the interaction-mediated phase transition

The emergence of spatial coherence in a confined two-dimensional Bose gas of exciton-polaritons with tuneable interactions offers a unique opportunity to explore the role of interactions in a phase transition in a driven-dissipative quantum system, where both the phase transition and thermalisation are mediated by interactions. We investigate, experimentally and numerically, the phase correlations and steady-state properties of the gas over a wide range of interaction strengths by varying the photonic/excitonic fraction of the polaritons and their density. We find that the first order spatial coherence function exhibits algebraic decay consistent with the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Surprisingly, the exponent of the algebraic decay is inversely proportional to the coherent density of polaritons, in analogy to equilibrium superfluids above the BKT transition, but with a different proportionality constant. Our work paves the way for future investigations of the phenomenon of phase transitions and superfluidity in a driven-dissipative setting

cond-mat.quant-gas↗

arXiv2022

Negative-mass exciton polaritons induced by dissipative light-matter coupling 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 particles and quasiparticles. Here, we show 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 direct measurements of the anomalous dispersion in atomically thin (monolayer) WS$_2$ crystals embedded in planar microcavities 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 opens a pathway for realising new phases of quantum matter in a solid state.

cond-mat.mes-hall↗

arXiv2022

Enhancing ground state population and macroscopic coherence of room-temperature WS$_2$ polaritons through engineered confinement

Exciton-polaritons (polaritons herein) in transition-metal dichalcogenide monolayers have attracted significant attention due to their potential for polariton-based optoelectronics. Many of the proposed applications rely on the ability to trap polaritons and to reach macroscopic occupation of their ground energy state. Here, we engineer a trap for room-temperature polaritons in an all-dielectric optical microcavity by locally increasing the interactions between the WS$_2$ excitons and cavity photons. The resulting confinement enhances the population and the first-order coherence of the polaritons in the ground state, with the latter effect related to dramatic suppression of disorder-induced inhomogeneous dephasing. We also demonstrate efficient population transfer into the trap when optically injecting free polaritons outside of its periphery.

cond-mat.mes-hall↗

arXiv2021

Motional narrowing, ballistic transport, and trapping of room-temperature exciton polaritons in an atomically-thin semiconductor

Atomically-thin transition metal dichalcogenide crystals (TMDCs) hold great promise for future semiconductor optoelectronics due to their unique electronic and optical properties. In particular, electron-hole pairs (excitons) in TMDCs are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, the excitons can hybridise with cavity photons to form exciton polaritons (polaritons herein), which display both ultrafast velocities and strong interactions. The ability to manipulate and trap polaritons on a microchip is critical for future applications. Here, we create a potential landscape for room-temperature polaritons in monolayer WS$_2$, and demonstrate their free propagation and trapping. We show that the effect of dielectric disorder, which restricts the diffusion of WS$_2$ excitons and broadens their spectral resonance, is dramatically reduced in the strong exciton-photon coupling regime leading to motional narrowing. This enables the ballistic transport of WS$_2$ polaritons across tens of micrometers with an extended range of partial first-order coherence. Moreover, the dephasing of trapped polaritons is dramatically suppressed compared to both WS$_2$ excitons and free polaritons. Our results demonstrate the possibility of long-range transport and efficient trapping of TMDC polaritons in ambient conditions.

cond-mat.mes-hall↗

arXiv2021

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.

physics.optics↗

arXiv2021

Low-energy collective oscillations and Bogoliubov sound in an exciton-polariton condensate

We report the observation of low-energy, low-momenta collective oscillations of an excitonpolariton condensate in a round "box" trap. The oscillations are dominated by the dipole and breathing modes, and the ratio of the frequencies of the two modes is consistent with that of a weakly interacting two-dimensional trapped Bose gas. The speed of sound extracted from the dipole oscillation frequency is smaller than the Bogoliubov sound, which can be partly explained by the influence of the incoherent reservoir. These results pave the way for understanding the effects of reservoir, dissipation, energy relaxation, and finite temperature on the superfluid properties of exciton-polariton condensates and other two-dimensional open-dissipative quantum fluids.

cond-mat.quant-gas↗

arXiv2020

Collective excitations of exciton-polariton condensates in a synthetic gauge field

Collective (elementary) excitations of quantum bosonic condensates, including condensates of exciton polaritons in semiconductor microcavities, are a sensitive probe of interparticle interactions. In anisotropic microcavities with momentum-dependent TE-TM splitting of the optical modes, the excitations dispersions are predicted to be strongly anisotropic, which is a consequence of the synthetic magnetic gauge field of the cavity, as well as the interplay between different interaction strengths for polaritons in the singlet and triplet spin configurations. Here, by directly measuring the dispersion of the collective excitations in a high-density optically trapped exciton-polariton condensate, we observe excellent agreement with the theoretical predictions for spinor polariton excitations. We extract the inter- and intra-spin polariton interaction constants and map out the characteristic spin textures in an interacting spinor condensate of exciton polaritons.

cond-mat.quant-gas↗

arXiv2019

Direct measurement of polariton-polariton interaction strength in the Thomas-Fermi regime of exciton-polariton condensation

Bosonic condensates of exciton polaritons (light-matter quasiparticles in a semiconductor) provide a solid-state platform for studies of non-equilibrium quantum systems with a spontaneous macroscopic coherence. These driven, dissipative condensates typically coexist and interact with an incoherent reservoir, which undermines measurements of key parameters of the condensate. Here, we overcome this limitation by creating a high-density exciton-polariton condensate in an optically-induced "box" trap. In this so-called Thomas-Fermi regime, the condensate is fully separated from the reservoir and its behaviour is dominated by interparticle interactions. We use this regime to directly measure the polariton-polariton interaction strength, and reduce the existing uncertainty in its value from four orders of magnitude to within three times the theoretical prediction. The Thomas-Fermi regime has previously been demonstrated only in ultracold atomic gases in thermal equilibrium. In a non-equilibrium exciton-polariton system, this regime offers a novel opportunity to study interaction-driven effects unmasked by an incoherent reservoir.

cond-mat.quant-gas↗

arXiv2019

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.

cond-mat.quant-gas↗

arXiv2018

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 realised by means of appropriate shaping of an incoherent 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 controlled creation of nontrivial distributions of orbital angular momentum and topological order in a periodic exciton-polariton system.

cond-mat.quant-gas↗

arXiv2017

Chiral modes at exceptional points in exciton-polariton quantum fluids

We demonstrate 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). Our method can be extended to generate high-order orbital angular momentum states through coalescence of multiple non-Hermitian spectral degeneracies, which could find application in integrated optoelectronics.

cond-mat.quant-gas↗

arXiv2017

Spontaneous condensation of exciton polaritons in the single-shot regime

Bose-Einstein condensate of exciton polaritons in a semiconductor microcavity is a macroscopically populated coherent quantum state subject to concurrent pumping and decay. Debates about the fundamental nature of the condensed phase in this open quantum system still persist. Here, we gain a new insight into the spontaneous condensation process by imaging long-lifetime exciton polaritons in a high-quality inorganic microcavity in the single-shot optical excitation regime, without averaging over multiple condensate realisations. In this highly non-stationary regime, a condensate is strongly influenced by the `hot' incoherent reservoir, and reservoir depletion is critical for the transition to the ground energy and momentum state. Condensates formed by more photonic exciton polaritons exhibit dramatic reservoir-induced density filamentation and shot-to-shot fluctuations. In contrast, condensates of more excitonic quasiparticles display smooth density and are second-order coherent. Our observations show that the single-shot measurements offer a unique opportunity to study formation of macroscopic phase coherence during a quantum phase transition in a solid state system.

cond-mat.quant-gas↗

arXiv2016

Visualising Berry phase and diabolical points in a quantum exciton-polariton billiard

Diabolical points (degeneracies) can naturally occur in spectra of two-dimensional quantum systems and classical wave resonators due to simple symmetries. Geometric Berry phase is associated with these spectral degeneracies. Here, we demonstrate a diabolical point and the corresponding Berry phase in the spectrum of hybrid light-matter quasiparticles -- exciton-polaritons in semiconductor microcavities. It is well known that sufficiently strong optical pumping can drive exciton-polaritons to quantum degeneracy, whereby they form a macroscopically populated quantum coherent state similar to a Bose-Einstein condensate. By pumping a microcavity with a spatially structured light, we create a two-dimensional quantum billiard for the exciton-polariton condensate and demonstrate a diabolical point in the spectrum of the billiard eigenstates. The fully reconfigurable geometry of the potential walls controlled by the optical pump enables a striking experimental visualisation of the Berry phase associated with the diabolical point. The Berry phase is observed and measured by direct imaging of the macroscopic exciton-polariton wavefunctions.

cond-mat.quant-gas↗

arXiv2016

Talbot effect for exciton-polaritons

We demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter waves -- exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic 'Talbot carpet' is produced by loading the exciton-polariton condensate into a microstructured one dimensional periodic array of mesa traps, which creates an array of 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.

cond-mat.quant-gas↗

arXiv2015

Collective state transitions of exciton-polaritons loaded into a periodic potential

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.

cond-mat.mes-hall↗

arXiv2015

Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard

Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (i.e., non-Hermitian) quantum system: it requires constant pumping of energy and continuously decays releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localisation, and dynamical properties. Using a spatially-structured optical pump, we create a chaotic exciton-polariton billiard. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies -- exceptional points. These are known to cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption, and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the nontrivial topological modal structure exclusive to non-Hermitian systems. We also observe the mode switching and topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way for studies of non-Hermitian quantum dynamics of exciton-polaritons, which can lead to novel functionalities of polariton-based devices.

cond-mat.quant-gas↗