Session A52: Invited Session: Coherent Flow and Vortices in Polariton Condensates

Quantum features in the hydrodynamic flow of a superfluid of light

After a number of experiments showing the power of fluids of light in semiconductor microcavity devices for superfluid hydrodynamic studies, a growing activity is being devoted to {\em quantum} hydrodynamic features, where hydrodynamic quantities such as density, current, etc. must be described by quantum operators. As a concrete example, we shall consider the emission of phonon pairs from a sonic horizon via analog Hawking radiation processes. The robustness of entanglement against the driven-dissipative nature of the microcavity photon fluid will be discussed and perspectives to detect it will be sketched. In the last part, I will discuss the potential of a different, propagating architecture in view of studies of the conservative quantum dynamics of a photon fluid. After a brief summary of the general theoretical framework, our attention will be focused to a slab geometry able to exploit the power of quantum fluids of light to study the physics of quantum quenches.   

Half-quantum flow of a polariton spinor condensate in a ring geometry

We have created a macroscopic ring trap for exciton-polaritons with long lifetime, about 200 ps at resonance. In this trap we have obtained Bose condensation of the polaritons, as seen in the strongly peaked energy spectrum and in the phase coherence across the trap, with coherence length of at least 50 microns. Studies of the phase gradient of the ring condensate show that it spontaneously goes into a quantized circulation state, sometimes circulating one way, sometimes the opposite way. Because this is a spinor condensate, states with only a half quantum of angular momentum are possible, accompanied by a 180-degree rotation of the polarization angle between the two spinor states. The circulating states with lowest energy in our experiments have this property, and in addition, the sign of the spin flips from one side of the ring to the other, unlike the case of a standard ``half-quantum vortex.''   

Hydrodynamics and coherence properties of polaritons in lattices

At the frontier between non-linear optics and the physics of Bose Einstein condensation, microcavity polaritons opened a new research field, both for fundamental studies of bosonic quantum fluids in a driven dissipative system, and for the development of new devices for all optical information processing. In this talk, I will review how semiconductor microcavities can be engineered into 1D and 2D lattices, allowing to implement complex hamiltonians and to study the hydrodynamics and coherence properties of polaritons in a novel and controlled environment. I will first show how we could generate polaritons in a 1D quasi-periodic Fibonacci potential and reveal features characteristic for a fractal energy spectrum, opening the way to the investigation of the anomalous propagation (neither ballistic nor diffusive) predicted in such structures. Then I will present a 2D honeycomb lattice for polaritons, which allows direct imaging of Dirac cones, paving the way for studies of the hydrodynamics of massless Dirac polaritons. Finally 1D lattices sustaining a non-dispersive band or ``flat band'' will be presented: here reduced spatial coherence is evidenced as a consequence of phase frustration.   

Pattern formation in interacting exciton-polariton condensates

Strongly coupled semiconductor microcavities support the formation of exciton-polaritons, which can condense into macroscopically occupied quantum states or quantum liquids. The investigation of such systems revealed a number of effects commonly associated with the formation of a macroscopic phase, for instance superfluid-like behavior [1] or the appearance of quantized vortices. One of the focal points of current research regards the possibility of optically manipulating polariton condensates to realize new experiments and potential applications like all-optical polariton circuits. We develop this vision by employing a spatial light modulator to create arbitrary excitation patterns, where nonresonant excitation of polariton condensates allows us to define the potential landscape experienced by the condensates. Novel effects regarding the interaction of multiple polaritonic quantum liquids are revealed, in particular phase-locking between freely-flowing condensates [2], the formation of vortex lattices for multiple pump spots at large separations and the transition to a trapped configuration as the pump spots are moved closer together [3,4]. These results enhance our ability to explore new features in macroscopic coherent systems and bring us closer to practical applications with polariton condensates such as creating all-optical coherent circuits [5]. \\[4pt] [1] A. Amo et al., Nature 457, 291 (2009)\\[0pt] [2] G. Tosi et al.,Nature Phys. 8,190 (2012)\\[0pt] [3] P. Cristofolini et al.,Phys. Rev. Lett. 110, 186403 (2013)\\[0pt] [4] A. Dreismann et al., PNAS 111, 8770 (2014)\\[0pt] [5] T. Gao et al., Phys. Rev. B 85, 235102 (2012)   

Vortex-lattice phase order in a microcavity exciton-polariton lattice system

Microcavity exciton-polaritons are bosonic quasi-particles in microcavity-quantum-well structures, exhibiting spontaneous coherence to form condensates. We have engineered two-dimensional polariton-lattice systems for investigating quantum phase order associated with high-orbital symmetry. In particular, we have observed two-degenerate vortex-antivortex lattice order at the inequivalanet K and K$'$ points in the honeycomb lattice. Under the inversion symmetry, we identify the handedness of the vortex-antivortex phase order via an interferometry technique, which leads the quest for the nature of degenerate condensates at non-zero momentum values. We envision that the polariton-lattice systems will provide exciting opportunities to explore new quantum order arising from the interplay of topology, spin, orbital and various symmetry properties. We embark on a journey to deepen our understandings in quantum nature and to develop its novel applications.