On the Condensation of Polaritons

E. Hanamura

Eliezer Estrecho, Tingge Gao, Nataliya Bobrovska +12 more

Bosonic condensates of exciton polaritons typically coexist and interact with an incoherent reservoir, which undermines measurements of key parameters of the condensate. Here, the authors overcome this limitation by creating a high-density polariton condensate in an optically induced box trap. In this interaction-dominated Thomas-Fermi regime, the condensate is fully separated from the reservoir, which enables a direct measurement of the polariton-polariton interaction strength. The results of the measurement are in agreement with theoretical predictions.

Lucy Pickup

Microcavity-polaritons are quasi-particles formed when light and matter excitations couple strongly in cavities that have confinement lengths similar to the wavelength of light. Polariton’s light e↵ective mass, due to the photonic component, in conjunction with their composite boson nature, allows a transition into a macroscopically occupied quantum degenerate state (polariton condensate) for temperatures ranging from 4K up to room temperature. Due to the exciton reservoirs produced under non-resonant excitation and polariton’s strong interparticle interactions (originating from their matter component) potential landscapes for the polaritons can be sculpted with the non-resonant laser beam(s) pumping the system. The work presented in this thesis is specifically concerned with inorganic polariton condensates in optically imprinted potential landscapes. With annular excitation geometries it is possible to form optically trapped polariton condensates, where the condensate is spatially delocalised from the exciton reservoir. Implementing this optical trap geometry, the first all-optical polariton bistability under non-resonant excitation is demonstrated in the spinor of the condensate. Furthermore, the separation from the exciton reservoir results in significant enhancement of the coherence properties. First order coherence times exceeding 1.5ns are demonstrated for the optical trap excitation geometry; whilst excitation regimes where the condensate overlaps the reservoir demonstrate sub 100ps coherence times. For narrow Gaussian excitation geometries, the spatial overlap of the condensate and the reservoir results in polaritons ballistically propagating away from pump with a well defined in-plane wavevector. Utilising this inherent in-plane propagation, two freely expanding polariton condensates (a polariton dyad) are shown to remain coupled, even for separation distances exceeding 100μm. Through an in-depth experimental and theoretical investigation of the polariton dyad, it is determined that it is critically important to account for the finite propagation time of polaritons from one condensate centre to the other (i.e. the coupling is time-delayed). The experimental work in this thesis is then concluded by introducing all-optical polariton band structure engineering. Utilising optically imprinted polariton condensates and their exciton reservoirs to both create and image the band structures. In addition to the experimental results, experimental procedures and analysis techniques, that have been developed and implemented to facilitate the research, are presented.

Fabrice P. Laussy

Y. Yamamoto, Hui Deng, H. Haug

Valentin Averchenko, A. P. Alodzhants, Sergei M Arakelyan +5 more

A quantum model of Bose—Einstein condensation based on processes involving polaritons excited in an intracavity absorbing cell with resonance atoms, which is manifested in the spectral parameters of the system, is considered. It is shown that the spectral 'condensation' appears which is directly related to the degeneracy of a weakly interacting gas of polaritons resulting in quasi-condensation at room temperature. The possibility of obtaining polariton condensation as a new phase state by using the confinement of polaritons in an atomic optical harmonic trap is discussed.