arXiv subjects

Dario Ballarini

Publications and source records attributed to Dario Ballarini.

48 records · Page 3Linked to original sources

arXiv2018

Interacting polariton fluids in a monolayer of tungsten disulfide

Atomically thin transition metal dichalcogenides (TMDs) possess a number of properties that make them attractive for realizing room-temperature polariton devices. An ideal platform for manipulating polariton fluids within monolayer TMDs is that of Bloch surface waves, which confine the electric field to a small volume near the surface of a dielectric mirror. Here we demonstrate that monolayer tungsten disulfide ($\text{WS}_2$) can sustain Bloch surface wave polaritons (BSWPs) with a Rabi splitting of 43 meV and propagation constants reaching 33 $μ$m. In addition, we evidence strong polariton-polariton nonlinearities within BSWPs, which manifest themselves as a reversible blueshift of the lower polariton resonance by up to 12.9$\pm$0.5 meV. Such nonlinearities are at the heart of polariton devices and have not yet been demonstrated in TMD polaritons. As a proof of concept, we use the nonlinearity to implement a nonlinear polariton source. Our results demonstrate that BSWPs using TMDs can support long-range propagation combined with strong nonlinearities, enabling potential applications in integrated optical processing and polaritonic circuits.

cond-mat.quant-gas↗

arXiv2017

Entangling one polariton with a photon: effect of interactions on a single-polariton quantum state

Polaritons are quasi-particles originating from the coupling of light with matter that demonstrated quantum phenomena at the many-particle mesoscopic level, such as BEC and superfluidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behaviour has remained fully accountable for by classical and mean-field theories. In this Article, we report the first experimental demonstration of a genuinely-quantum manifestation of microcavity polaritons, by swapping, in a two-photon entangled state generated by parametric down-conversion, a photon for a polariton. Furthermore, we show how single polaritons are affected by polariton-polariton interactions in a propaedeutic demonstration of their qualities for quantum information applications.

cond-mat.mes-hall↗

arXiv2016

Twist of generalized skyrmions and spin vortices in a polariton superfluid

We study the spin vortices and skyrmions coherently imprinted into an exciton-polariton condensate on a planar semiconductor microcavity. We demonstrate that the presence of a polarization anisotropy can induce a complex dynamics of these structured topologies, leading to the twist of their circuitation on the Poincaré sphere of polarizations. The theoretical description of the results carries the concept of generalized quantum vortices in two-component superfluids, which are conformal with polarization loops around an arbitrary axis in the pseudospin space.

cond-mat.quant-gas↗

arXiv2016

Topological order and equilibrium in a condensate of exciton-polaritons

We report the observation of the Berezinskii-Kosterlitz-Thouless transition for a 2D gas of exciton-polaritons, and through the joint measurement of the first-order coherence both in space and time we bring compelling evidence of a thermodynamic equilibrium phase transition in an otherwise open driven/dissipative system. This is made possible thanks to long polariton lifetimes in high-quality samples with small disorder and in a reservoir-free region far away from the excitation spot, that allow topological ordering to prevail. The observed quasi-ordered phase, characteristic for an equilibrium 2D bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. Finally, measurements in the weak-coupling regime confirm that polariton condensates are fundamentally different from photon lasers and constitute genuine quantum degenerate macroscopic states.

cond-mat.quant-gas↗

arXiv2016

Room-temperature superfluidity in a polariton condensate

Superfluidity---the suppression of scattering in a quantum fluid at velocities below a critical value---is one of the most striking manifestations of the collective behaviour typical of Bose-Einstein condensates. This phenomenon, akin to superconductivity in metals, has until now only been observed at prohibitively low cryogenic temperatures. For atoms, this limit is imposed by the small thermal de Broglie wavelength, which is inversely related to the particle mass. Even in the case of ultralight quasiparticles such as exciton-polaritons, superfluidity has only been demonstrated at liquid helium temperatures. In this case, the limit is not imposed by the mass, but instead by the small exciton binding energy of Wannier-Mott excitons, which places the upper temperature limit. Here we demonstrate a transition from normal to superfluid flow in an organic microcavity supporting stable Frenkel exciton-polaritons at room temperature. This result paves the way not only to table-top studies of quantum hydrodynamics, but also to room-temperature polariton devices that can be robustly protected from scattering.

cond-mat.quant-gas↗

arXiv2016

Ultrafast flow of interacting organic polaritons

The strong-coupling of an excitonic transition with an electromagnetic mode results in composite quasi-particles called exciton-polaritons, which have been shown to combine the best properties of their bare components in semiconductor microcavities. However, the physics and applications of polariton flows in organic materials and at room temperature are still unexplored because of the poor photon confinement in such structures. Here we demonstrate that polaritons formed by the hybridization of organic excitons with a Bloch Surface Wave are able to propagate for hundreds of microns showing remarkable third-order nonlinear interactions upon high injection density. These findings pave the way for the studies of organic nonlinear light-matter fluxes and for a technological promising route of dissipation-less on-chip polariton devices working at room temperature.

physics.optics↗

arXiv2015

Merging of vortices and antivortices in polariton superfluids

Quantised vortices are remarkable manifestations on a macroscopic scale of the coherent nature of quantum fluids, and the study of their properties is of fundamental importance for the understanding of this peculiar state of matter. Cavity-polaritons, due to their double light-matter nature, offer a unique controllable environment to investigate these properties. In this work we theoretically investigate the possibility to deterministically achieve the annihilation of a vortex with an antivortex through the increase of the polariton density in the region surrounding the vortices. Moreover we demonstrate that by means of this mechanism an array of vortex-antivortex pairs can be completely washed out.

cond-mat.other↗

arXiv2014

Polariton Induced Enhanced Emission from an Organic Dye under Strong Coupling Regime

Exciton-polaritons in semiconductors are quasi-particles which have recently shown the capability to undergo phase transition into a coherent hybrid state of light and matter. The observation of such quasi-particles in organic microcavities has attracted increasing attention for their characteristic of reaching condensation at room temperature. In this work we demonstrate that the emission properties of organic polaritons do not depend on the overlap between the absorption and emission states of the molecule and that the emission dynamics are modified in the strong coupling regime, showing a significant enhancement of the photoluminescence intensity as compared to the bare dye. This paves the way to the investigation of molecules with large absorption coefficients but poor emission efficiencies for the realization of polariton condensates and organic electrically injected lasers by exploiting strong exciton-photon coupling regimes.

cond-mat.mes-hall↗

arXiv2014

Relaxation Oscillations in the Formation of a Polariton Condensate

We report observation of oscillations in the dynamics of a microcavity polariton condensate formed under pulsed non resonant excitation. While oscillations in a condensate have always been attributed to Josephson mechanisms due to a chemical potential unbalance, here we show that under some localisation conditions of the condensate, they may arise from relaxation oscillations, a pervasive classical dynamics that repeatedly provokes the sudden decay of a reservoir, shutting off relaxation as the reservoir is replenished. Using non-resonant excitation, it is thus possible to obtain condensate injection pulses with a record frequency of 0:1 THz.

cond-mat.mes-hall↗

arXiv2014

Interaction-Shaped Vortex-Antivortex Lattices in Polariton Fluids

Topological defects such as quantized vortices are one of the most striking manifestations of the superfluid nature of Bose-Einstein condensates and typical examples of quantum mechanical phenomena on a macroscopic scale. Here we demonstrate the formation of a lattice of vortex-antivortex pairs and study, for the first time, its properties in the non-linear regime at high polarion-density where polariton-polariton interactions dominate the behaviour of the system. In this work first we demonstrate that the array of vortex-antivortex pairs can be generated in a controllable way in terms of size of the array and in terms of size and shape of it fundamental unit cell. Then we demonstrate that polariton-polariton repulsion can strongly deform the lattice unit cell and determine the pattern distribution of the vortex-antivortex pairs, reaching a completely new behaviour with respect to geometrically generated vortex lattices whose shape is determined only by the geometry of the system.

cond-mat.quant-gas↗

arXiv2014

Room temperature Bloch surface wave polaritons

Polaritons are hybrid light-matter quasi-particles that have gathered a significant attention for their capability to show room temperature and out-of-equilibrium Bose-Einstein condensation. More recently, a novel class of ultrafast optical devices have been realized by using flows of polariton fluids, such as switches, interferometers and logical gates. However, polariton lifetimes and propagation distance are strongly limited by photon losses and accessible in-plane momenta in usual microcavity samples. In this work, we show experimental evidence of the formation of room temperature propagating polariton states arising from the strong coupling between organic excitons and a Bloch surface wave. This result, which was only recently predicted, paves the way for the realization of polariton devices that could allow lossless propagation up to macroscopic distances.

cond-mat.mes-hall↗

arXiv2013

All-optical polariton transistor

While optical technology provides the best solution for the transmission of information, optical logics still calls for qualitative new concepts to be explored. Exciton-polaritons are composite particles, resulting from the strong coupling between excitons and photons, which have recently demonstrated exceptional properties like huge non-linearities, long range coherence and suppression of scattering. Here we demonstrate a switching scheme for polaritons moving in the plane of a microcavity which satisfy all the requirements for an all-optical transistor. Under resonant excitation, the power threshold for the nonlinear increase of the polariton density is varied by a weak control beam, obtaining up to 19 times amplification with switching energies in the range of attojoule per square micron. Polariton propagation in the plane of the microcavity is then used to control the switching of a second, spatially separated transistor, opening the way to the implementation of polariton integrated circuits.

cond-mat.mes-hall↗