Key role of the moire potential for the quasi-condensation of interlayer excitons in van der Waals heterostructures

Interlayer excitons confined in bilayer heterostructures of transition metal dichalcogenides (TMDs) offer a promising route to implement two-dimensional dipolar superfluids. Here, we study the experimental conditions necessary for the realisation of such collective state. Particularly, we show that the moire potential inherent to TMD bilayers yields an exponential increase of the excitons effective mass. To allow for exciton superfluidity at sizeable temperatures it is then necessary to intercalate a high-$\kappa$ dielectric between the monolayers confining electrons and holes. Thus the moire lattice depth is sufficiently weak for a superfluid phase to theoretically emerge below a critical temperature of around 10 K. Importantly, for realistic experimental parameters interlayer excitons quasi-condense in a state with finite momentum, so that the superfluid is optically inactive and flows spontaneously.Comment: 6 pages, 4 figure

High spectral resolution of GaAs/AlAs phononic cavities by subharmonic resonant pump-probe excitation

We present here precise measurement of the resonance frequency, lifetime and shape of confined acoustic modes in the tens of GHz regime in GaAs/AlAs superlattice planar and micropillar cavities at low temperature ($\sim 20\,\textrm{K}$). The subharmonic resonant pump-probe technique, where the repetition rate of the pump laser is tuned to a subharmonic of the cavity resonance to maximize the amplitude of the acoustic resonance, in combination with a Sagnac interferometer technique for high sensitivity ($\sim 10 \,\textrm{fm}$) to the surface displacement, has been used. The cavity fundamental mode at $\sim 20\,\textrm{GHz}$ and the higher order cavity harmonics up to $\sim 180\,\textrm{GHz}$ have been clearly resolved. Mechanical Q-values up to $2.7 \times 10^4$ have been measured in a planar superlattice, and direct spatial mapping of confined acoustic modes in a superlattice cavity micropillar has been demonstrated. The Q-frequency product obtained is $\sim 5 \times 10^{14}$ demonstrating the suitability of these superlattice cavities for optomechanical applications.Comment: 5 pages, 4 figure

Quasi-condensation of bilayer excitons in a periodic potential

We study two-dimensional excitons confined in a lattice potential, for high fillings of the lattice sites. We show that a quasi-condensate is possibly formed for small values of the lattice depth, but for larger ones the critical phase-space density for quasi-condensation rapidly exceeds our experimental reach, due to the increase of the excitons effective mass. On the other hand, in the regime of a deep lattice potential where excitons are strongly localised at the lattice sites, we show that an array of phase-independent quasi-condensates, different from a Mott insulating phase, is realised.Comment: 5 pages 4 figure

A Microscopic Lattice for Two-dimensional Dipolar Excitons

We report a two-dimensional artificial lattice for dipolar excitons confined in a GaAs double quantum well. Exploring the regime of large fillings per lattice site, we verify that the lattice depth competes with the magnitude of excitons repulsive dipolar interactions to control the degree of localisation in the lattice potential. Moreover, we show that dipolar excitons radiate a narrow-band photoluminescence, with a spectral width of a few hundreds of micro-eV at 340 mK, in both localised and delocalised regimes. This makes our device suitable for explorations of dipolar excitons quasi-condensation in a periodic potential.Comment: 5 pages, 4 figure

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Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte, 201

Étude expérimentale des propriétés optiques et acoustiques de cavités planaires et en micro-piliers de GaAs/AlAs pour l'optomécanique en micro-cavité

Here, we experimentally characterize acoustic and optical modes of GaAs/AlAs Bragg mirror planar and micropillar cavities. The goal is to study the optomecanical coupling of such sys- tem with optomecanically induced transparency experiments. To characterize the high frequency acoustic modes (20 GHz) of the cavity, we use picosecond acoustic experiments, along with a subharmonic excitation technique, at low temperature. It allows to overcome the frequency resolution limitation of the experiment. Thanks to this technique, we measure acoustic quality factors as high as 3 · 10 ^4 , which gives a quality factor frequency product Q m f m of 5 × 10 ^14 Hz at 20 K. The optical mode characterization is performed with a reflectometric setup, which is also used for the induced transparency measurements. We did not experimentally observed any induced transparency signal. From calculation realized to simulate the result we expected, we were able to estimate an upper bound of the optomecanical coupling of the studied GaAs/AlAs Bragg mirror planar cavity at low temperature, g 0<2π·60 kHz.Dans cette thèse, nous caractérisons expérimentalement les modes acoustiques et op- tiques de cavités à miroirs de Bragg de GaAs/AlAs, planaires et en micro-piliers. L’objectif est d’étudier le couplage optomécanique de ce type de système avec des expériences de transpa- rence induite optomécaniquement. La caractérisation des modes acoustiques de haute fréquence (20 GHz) est effectuée par expériences d’acoustique pico-seconde, à basse température. On utilise la technique d’excitation sous-harmonique résonante afin de contourner la limite de résolution fréquentielle de l’expérience. Grâce à cette technique, nous avons mesuré des facteurs de qua- lité acoustiques jusqu’à 3 · 10 ^4 , ce qui donne un produit facteur de qualité-fréquence Q m f m de 5 × 10 ^14 Hz à 20 K. La caractérisation des modes optiques s’effectue avec un montage de mesure de réflectivité qui est également utilisé pour les expériences de transparence induite optomécaniquement. Nous n’avons pas observé expérimentalement de signal de transparence induite. À partir de calculs réalisés pour simuler les résultats attendus, nous avons pu estimer une borne supérieure du couplage optomécanique à basse température de ces cavités à miroirs de Bragg de GaAs/AlAs planaires, g 0 < 2π·60 kHz

Experimental study of optical and acoustic properties of planar and micropilar GaAs/AlAs cavities for microcavity optomechanics

Dans cette thèse, nous caractérisons expérimentalement les modes acoustiques et op- tiques de cavités à miroirs de Bragg de GaAs/AlAs, planaires et en micro-piliers. L’objectif est d’étudier le couplage optomécanique de ce type de système avec des expériences de transpa- rence induite optomécaniquement. La caractérisation des modes acoustiques de haute fréquence (20 GHz) est effectuée par expériences d’acoustique pico-seconde, à basse température. On utilise la technique d’excitation sous-harmonique résonante afin de contourner la limite de résolution fréquentielle de l’expérience. Grâce à cette technique, nous avons mesuré des facteurs de qua- lité acoustiques jusqu’à 3 · 10 ^4 , ce qui donne un produit facteur de qualité-fréquence Q m f m de 5 × 10 ^14 Hz à 20 K. La caractérisation des modes optiques s’effectue avec un montage de mesure de réflectivité qui est également utilisé pour les expériences de transparence induite optomécaniquement. Nous n’avons pas observé expérimentalement de signal de transparence induite. À partir de calculs réalisés pour simuler les résultats attendus, nous avons pu estimer une borne supérieure du couplage optomécanique à basse température de ces cavités à miroirs de Bragg de GaAs/AlAs planaires, g 0 < 2π·60 kHz.Here, we experimentally characterize acoustic and optical modes of GaAs/AlAs Bragg mirror planar and micropillar cavities. The goal is to study the optomecanical coupling of such sys- tem with optomecanically induced transparency experiments. To characterize the high frequency acoustic modes (20 GHz) of the cavity, we use picosecond acoustic experiments, along with a subharmonic excitation technique, at low temperature. It allows to overcome the frequency resolution limitation of the experiment. Thanks to this technique, we measure acoustic quality factors as high as 3 · 10 ^4 , which gives a quality factor frequency product Q m f m of 5 × 10 ^14 Hz at 20 K. The optical mode characterization is performed with a reflectometric setup, which is also used for the induced transparency measurements. We did not experimentally observed any induced transparency signal. From calculation realized to simulate the result we expected, we were able to estimate an upper bound of the optomecanical coupling of the studied GaAs/AlAs Bragg mirror planar cavity at low temperature, g 0<2π·60 kHz

Quasi-condensation of bilayer excitons in a periodic potential

5 pages 4 figuresWe study two-dimensional excitons confined in a lattice potential, for high fillings of the lattice sites. We show that a quasi-condensate is possibly formed for small values of the lattice depth, but for larger ones the critical phase-space density for quasi-condensation rapidly exceeds our experimental reach, due to the increase of the excitons effective mass. On the other hand, in the regime of a deep lattice potential where excitons are strongly localised at the lattice sites, we show that an array of phase-independent quasi-condensates, different from a Mott insulating phase, is realised