arXiv subjects

K. Baldwin

Publications and source records attributed to K. Baldwin.

8 recordsLinked to original sources

arXiv2025

Supersolid crystals of dipolar excitons in a lattice

In condensed-matter physics, long-range correlations introduce quantum states of matter that challenge intuition. For instance, supersolids combine symmetry-breaking crystalline structure, i.e. density order, and frictionless superfluid flow. Envisioned over fifty years ago, supersolids have proven to only exist under very stringent conditions, with experimental evidence limited to few observations. Many-body phases with supersolid properties in fact reduce to a few recent observations for weakly interacting Bose gases. Here, we demonstrate a new framework to realize supersolid crystals in the strong interaction regime, by confining dipolar bosons in a lattice with long-range hopping. We study dipolar excitons that genuinely realize this lattice model. At fractional lattice fillings - 1/4, 1/3 and 1/2 - we report mesoscopic quantum solids, across over 100 sites, spontaneously breaking translational symmetry. At the same time, we show that off-diagonal long-range order is induced by long-range hopping, such that exciton solids are superfluids. State-of-the-art numerical methods quantitatively confirm that supersolidity builds up in the ground-state of the lattice Hamiltonian. Our studies of strongly-correlated supersolid crystals open new frontiers for exploration in condensed matter physics.

cond-mat.quant-gas↗

arXiv2023

Bose-Einstein Condensation in Gap-Confined Exciton-Polariton States

The development of patterned multi-quantum well heterostructures in GaAs/AlGaAs waveguides has recently allowed to achieve exciton-polariton condensation in a topologically protected bound state in the continuum (BIC). Remarkably, condensation occurred above a saddle point of the polariton dispersion. A rigorous analysis of the condensation phenomenon in these systems, as well as the role of the BIC, is still missing. In the present Letter we theoretically and experimentally fill this gap, by showing that polariton confinement resulting from the negative effective mass and the photonic energy gap in the dispersion play a key role in enhancing the relaxation towards the condensed state. In fact, our results show that low-threshold polariton condensation is achieved within the effective trap created by the exciting laser spot regardless of whether the resulting confined mode is long-lived (polariton BIC) or short-lived (lossy mode). In both cases, the spatial quantization of the polariton condensate and the threshold differences associated to the corresponding state lifetime are measured and characterized. For a given negative mass, a slightly lower condensation threshold from the polariton BIC mode is found and associated to its suppressed radiative losses as compared to the lossy one.

cond-mat.mes-hall↗

arXiv2023

Persistent, controllable circulation of a polariton ring condensate

Persistent circulation is a canonical effect of superfluidity. In previous experiments, quantized circulation has been observed in polariton condensates, usually far from equilibrium, but persistent current in the absence of any stirring has not been seen. We report here the direct observation of persistent circulation of a polariton condensate with no driving force and with no observable change in time. We can cause the condensate to circulate in either direction on demand using a short laser pulse, after which the condensate continues to circulate for dozens to hundreds of rotations around a ring trap without any further stimulation. Our theoretical model successfully shows how the pulse, despite not carrying any angular momentum, causes the circulation.

cond-mat.quant-gas↗

arXiv2022

Extended Bose-Hubbard model with dipolar excitons

The Hubbard model constitutes one of the most celebrated theoretical frameworks of condensed-matter physics. It describes strongly correlated phases of interacting quantum particles confined in lattice potentials. For bosons, the Hubbard Hamiltonian has been deeply scrutinised for short-range on-site interactions. However, accessing longer-range couplings has remained elusive experimentally. This marks the frontier towards the extended Bose-Hubbard Hamiltonian that allows insulating ordered phases at fractional lattice fillings. Here we implement this Hamiltonian by confining semiconductor dipolar excitons in an artificial two-dimensional square lattice. Strong dipolar repulsions between nearest neighbouring lattice sites then stabilise an insulating state at half filling. This characteristic feature of the extended Bose-Hubbard model exhibits signatures theoretically expected for a checkerboard spatial order. Our work thus highlights that dipolar excitons enable controlled implementations of boson-like arrays with strong off-site interactions, in lattices with programmable geometries and over 100 sites.

cond-mat.str-el↗

arXiv2022

Fabrication of Nanostructured GaAs/AlGaAs Waveguide for Low-Density Polariton Condensation from a Bound State in the Continuum

Exciton-polaritons are hybrid light-matter states that arise from strong coupling between an exciton resonance and a photonic cavity mode. As bosonic excitations, they can undergo a phase transition to a condensed state that can emit coherent light without a population inversion. This aspect makes them good candidates for thresholdless lasers, yet short exciton-polariton lifetime has made it difficult to achieve condensation at very low power densities. In this sense, long-lived symmetry-protected states are excellent candidates to overcome the limitations that arise from the finite mirror reflectivity of monolithic microcavities. In this work we use a photonic symmetry protected bound state in the continuum coupled to an excitonic resonance to achieve state-of-the-art polariton condensation threshold in GaAs/AlGaAs waveguide. Most important, we show the influence of fabrication control and how surface passivation via atomic layer deposition provides a way to reduce exciton quenching at the grating sidewalls.

physics.optics↗

arXiv2022

Polariton Bose-Einstein condensate from a Bound State in the Continuum

Optical bound states in the continuum (BIC) are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone, i.e., in the energy-momentum dispersion region for which radiation can propagate out of the lattice plane. These BICs possess an invariant topological charge given by the winding number of the polarization vectors, similarly to vortices in quantum fluids, such as superfluid helium and atomic Bose-Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not been fully exploited, yet. Here we show Bose-Einstein condensation of polaritons, hybrid light-matter excitations, occuring in a BIC thanks to its peculiar non-radiative nature. The combination of the ultra-long BIC lifetime and the tight confinement of the waveguide geometry allow to achieve an extremely low threshold density for condensation, which is not reached in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we unveil new ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light-matter optical circuits

physics.optics↗

arXiv2021

Ultrafast, low-energy, all-optical switch in polariton waveguides

The requirement for optical-electrical-optical conversion of signals in optical technologies is often one of the majors bottleneck in terms of speed and energy consumption. The use of dressed photons (also called polaritons), that allows for intrinsic sizable interactions, could significantly improve the performances of optical integrated elements such as switches or optical gates. In this work we demonstrate the ultrafast switch of a laser coupled into a polaritonic waveguide triggered by an optical pulse resonant with the same dispersion but at a lower energy. Our experiments show two effects capable to interrupt the transmission of the laser in two different time ranges: a sub-picosecond time range due to the optical Stark effect, and a picosecond range governed by the creation of a charge reservoir. In the latter regime we found that at certain power of excitation the activation of dark states allows for a long persistence of the switching much beyond the bright exciton lifetime.

physics.optics↗

arXiv2020

Observation of Algebraic Time Order for Two-Dimensional Dipolar Excitons

Emergence of algebraic quasi-long-range order is a key feature of superfluid phase transitions at two dimensions. For this reduced dimensionality interactions prevent Bose-Einstein condensation with true long range order, at any finite temperature. Here, we report the occurence of algebraic order in a strongly interacting quantum liquid formed by dipolar excitons confined in a bilayer semiconductor heterostructure. We observe a transition from exponential to algebraic decay of the excitons temporal coherence, accompanied by a universal scaling behaviour of the equation of state. Our results provide strong evidence for a Berezinskii-Kosterlitz-Thouless (BKT) transition in a multi-component boson-like system governed by strong dipolar interactions.

cond-mat.quant-gas↗