Giovanni Lerario

Investigating charge dynamics in halide perovskite-sensitized mesostructured solar cells

Charge generation and transport in (CH3NH3)PbI3−xClx sensitized mesostructured solar cells are investigated. A highly efficient charge generation is directly proven by time correlated single photon counting analysis. Photoinduced absorption and transient photovoltage investigations depict double charge recombination dynamics. To explain the high device performances according to those spectroscopic observations, we suggest the existence of two complementary paths for electron transport, involving either TiO2 or perovskite matrixes.

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.

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.

Interactions and scattering of quantum vortices in a polariton fluid

Quantum vortices, the quantized version of classical vortices, play a prominent role in superfluid and superconductor phase transitions. However, their exploration at a particle level in open quantum systems has gained considerable attention only recently. Here we study vortex pair interactions in a resonant polariton fluid created in a solid-state microcavity. By tracking the vortices on picosecond time scales, we reveal the role of nonlinearity, as well as of density and phase gradients, in driving their rotational dynamics. Such effects are also responsible for the split of composite spin–vortex molecules into elementary half-vortices, when seeding opposite vorticity between the two spinorial components. Remarkably, we also observe that vortices placed in close proximity experience a pull–push scenario leading to unusual scattering-like events that can be described by a tunable effective potential. Understanding vortex interactions can be useful in quantum hydrodynamics and in the development of vortex-based lattices, gyroscopes, and logic devices.Superfluid flow around a vortex is quantized so that vortices become discrete, particle-like defects, with interactions mediated by the surrounding fluid. Here, the authors use a polariton system to experimentally investigate the behavior and scattering of vortices in a two-component superfluid.

Polariton devices and quantum fluids

Exciton-polaritons, composite particles resulting from the strong coupling between excitons and photons, have shown the capability to undergo condensation into a macroscopically coherent quantum state, demonstrating strong non-linearities and unique propagation properties. These strongly-coupled light-matter particles are promising candidates for the realization of semiconductor all-optical devices with fast time response and small energy consumption. Recently, quantum fluids of polaritons have been used to demonstrate the possibility to implement optical functionalities as spin switches, transistors or memories, but also to provide a channel for the transmission of information inside integrated circuits. In this context, the possibility to extend the range of light-matter interaction up to room temperature becomes of crucial importance. One of the most intriguing promises is to use organic Frenkel excitons, which, thanks to their huge oscillator strength, not only sustain the polariton picture at room temperature, but also bring the system into the unexplored regime of ultra-strong coupling. The combination of these materials with ad-hoc designed structures may allow the control of the propagation properties of polaritons, paving the way towards their implementation of the polariton functionalities in actual devices for opto-electronic applications.