Dario Ballarini

Collective fluid dynamics of a polariton condensate in a semiconductor microcavity

Semiconductor microcavities offer unique systems in which to investigate the physics of weakly interacting bosons. Their elementary excitations, polaritons—mixtures of excitons and photons—can accumulate in macroscopically degenerate states to form various types of condensate in a wide range of experimental configurations, under either incoherent or coherent excitation. Condensates of polaritons have been put forward as candidates for superfluidity, and the formation of vortices as well as elementary excitations with linear dispersion are actively sought as evidence to support this. Here, using a coherent excitation triggered by a short optical pulse, we have created and set in motion a macroscopically degenerate state of polaritons that can be made to collide with a variety of defects present in the microcavity. Our experiments show striking manifestations of a coherent light–matter packet, travelling at high speed (of the order of one per cent of the speed of light) and displaying collective dynamics consistent with superfluidity, although one of a highly unusual character as it involves an out-of-equilibrium dissipative system. Our main results are the observation of a linear polariton dispersion accompanied by diffusionless motion; flow without resistance when crossing an obstacle; suppression of Rayleigh scattering; and splitting into two fluids when the size of the obstacle is comparable to the size of the wave packet. This work opens the way to the investigation of new phenomenology of out-of-equilibrium condensates.eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website.

All-optical control of the quantum flow of a polariton condensate

Vortex–antivortex pairs in a polariton condensate are experimentally trapped and manipulated by a light beam in a semiconductor microcavity. Quantum hydrodynamical effects are observed and corroborated by time-dependent simulations.

Room-temperature superfluidity in a polariton condensate

Superfluidity is a phenomenon usually restricted to cryogenic temperatures, but organic microcavities provide the conditions for a superfluid flow of polaritons at room temperature. 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 condensates1. This phenomenon, akin to superconductivity in metals, has until now been observed only 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 been demonstrated only at liquid helium temperatures2. In this case, the limit is not imposed by the mass, but instead by the small binding energy of Wannier–Mott excitons, which sets the upper temperature limit. Here we demonstrate a transition from supersonic to superfluid flow in a polariton condensate under ambient conditions. This is achieved by using an organic microcavity supporting stable Frenkel exciton-polaritons at room temperature. This result paves the way not only for tabletop studies of quantum hydrodynamics, but also for room-temperature polariton devices that can be robustly protected from scattering.

High-speed 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 individual 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 study of organic nonlinear light–matter fluxes and for a technologically promising route of the realization of dissipation-less on-chip polariton devices operating at room temperature.

Ultrafast Control and Rabi Oscillations of Polaritons

We report the experimental observation and control of space and time-resolved light-matter Rabi oscillations in a microcavity. Our setup precision and the system coherence are so high that coherent control can be implemented with amplification or switching off of the oscillations and even erasing of the polariton density by optical pulses. The data are reproduced by a quantum optical model with excellent accuracy, providing new insights on the key components that rule the polariton dynamics.

Ultrastrong light-matter coupling in electrically doped microcavity organic light emitting diodes

The coupling of the electromagnetic field with an electronic transition gives rise, for strong enough light-matter interactions, to hybrid states called exciton-polaritons. When the energy exchanged between light and matter becomes a significant fraction of the material transition energy an extreme optical regime called ultrastrong coupling (USC) is achieved. We report a microcavity embedded p-i-n monolithic organic light emitting diode working in USC, employing a thin film of squaraine dye as active layer. A normalized coupling ratio of 30% has been achieved at room temperature. These USC devices exhibit a dispersion-less angle-resolved electroluminescence that can be exploited for the realization of innovative optoelectronic devices. Our results may open the way towards electrically pumped polariton lasers.

Real-space collapse of a polariton condensate

Microcavity polaritons are two-dimensional bosonic fluids with strong nonlinearities, composed of coupled photonic and electronic excitations. In their condensed form, they display quantum hydrodynamic features similar to atomic Bose–Einstein condensates, such as long-range coherence, superfluidity and quantized vorticity. Here we report the unique phenomenology that is observed when a pulse of light impacts the polariton vacuum: the fluid which is suddenly created does not splash but instead coheres into a very bright spot. The real-space collapse into a sharp peak is at odd with the repulsive interactions of polaritons and their positive mass, suggesting that an unconventional mechanism is at play. Our modelling devises a possible explanation in the self-trapping due to a local heating of the crystal lattice, that can be described as a collective polaron formed by a polariton condensate. These observations hint at the polariton fluid dynamics in conditions of extreme intensities and ultrafast times.

Exciton–Plasmon Coupling Enhancement via Metal Oxidation

In this paper, we report on the effect of metal oxidation on strong coupling interactions between silver nanostructures and a J-aggregated cyanine dye. We show that metal oxidation can sensibly affect the plexcitonic system, inducing a change in the coupling strength. In particular, we demonstrate that the presence of oxide prevents the appearance of Rabi splitting in the extinction spectra for thick spacers. In contrast, below a threshold percentage, the oxide layer results in an higher coupling strength between the plasmon and the Frenkel exciton. Contrary to common belief, a thin oxide layer seems thus to act, under certain conditions, as a coupling mediator between an emitter and a localized surface plasmon excited in a metallic nanostructure. This suggests that metal oxidation can be exploited as a means to enhance light-matter interactions in strong coupling applications.

Room temperature Bloch surface wave polaritons

A recent theoretical proposal suggested strong coupling between excitons and Bloch Surface Waves, which are photonic modes that exist at the interface between a truncated photonic crystal, and an ideally semi-infinite dielectric medium. In this work we report on the observation of strong coupling between the Bloch surface wave supported by an inorganic multilayer structure and J-aggregate excitons in an organic semiconductor. The dispersion curves of the resulting polariton modes are investigated by means of angle-resolved attenuated total reflection as well as photoluminescence experiments. The measured Rabi splitting is 290 meV.

Control and ultrafast dynamics of a two-fluid polariton switch

We investigate the cross interactions in a two-component polariton quantum fluid coherently driven by two independent pumping lasers tuned at different energies and momenta. We show that both the hysteresis cycles and the on-off threshold of one polariton signal can be entirely controlled by a second polariton fluid. Furthermore, we study the ultrafast switching dynamics of a driven polariton state, demonstrating the ability to control the polariton population with an external laser pulse, in less than a few picoseconds.