Exciton-polaritons as an out-of-equilibrium refrigerant gas

Owing to light-matter interaction, the translational or vibrational degrees of freedom of a gas or a solid, at thermal equilibrium with their environment, can be either warmed up or cooled down by shining a beam of light on them. Using this principle, S. Chu, C. Cohen-Tannoudji and W. D. Phillips developed the so-called Doppler cooling method to optically cool down a gas of atoms [1]. Anti-Stokes fluorescence (ASF) is an analogous mechanism currently envisaged to cool down solids. It was first proposed by Peter Pringsheim in 1929 [2]: a laser is tuned to the vibrational ground state of an electronic transition, such that most reemitted photons take away with them the energy of one or several thermal phonons. However, due to the finite probability of non-radiative recombination in solids, the method is less efficient than Doppler cooling for atom gases. So far, successful cooling to T = 155 K has been achieved in Ytterbium-doped crystals [3]. More recently, cooling in semiconductor nanostructures down to T = 260 K has been achieved [4]. In the latter case, a large non-radiative recombination rate was compensated by a high energy ASF, fixed by the excitonic transition. In this work, we propose to replace light by polaritons to cool down a solid-state microcavity (MC) in the strong coupling regime. To do so, a laser injects resonantly a CW polariton field in the ground state, at k|| = 0, ћω0 (see Fig. 1). Owing to polaritons cavity-like dispersion, Stokes fluorescence (phonon emission) is fully inhibited while ASF is permitted and enhanced by the polaritons excitonic fraction. In this context, we investigated a Selenide-based MC, as it provides (i) a polariton radiative bandwidth (27.5 meV at T = 6 K in this work) comparable to the mean thermal energy at room temperature, (ii) stable polaritons up to elevated temperatures. By carrying out angle-resolved Raman spectroscopy, we measured the cooling power generated by polariton ASF. We determine the temperature and laser power range where a net cooling rate is achievable. Our experiments show that the highest cooling power is achieved at an initial temperature of only 50 K [5]. It is furthermore shown that due to the out-of-equilibrium characteristics of the scattered polaritons more heat per scattering event is removed from the phonon bath than in the case at thermal equilibrium. Finally, we find that in high-Q MCs, the cooling rate is limited by two-photon absorption, which helps us to derive a MC design better suited to achieve higher cooling power.