Relaxation Oscillations in the Formation of a Polariton Condensate

Abstract

We report observation of oscillations in the dynamics of a microcavity polariton condensate formed under pulsed non resonant excitation. While oscillations in a condensate have always been attributed to Josephson mechanisms due to a chemical potential unbalance, here we show that under some localisation conditions of the condensate, they may arise from relaxation oscillations, a pervasive classical dynamics that repeatedly provokes the sudden decay of a reservoir, shutting off relaxation as the reservoir is replenished. Using non-resonant excitation, it is thus possible to obtain condensate injection pulses with a record frequency of 0.1 THz. Light-matter particles-that arise from the strong interaction between the photonic field in a semiconductor microcavity and the exciton dipole in quantum wells (QWs)-have recently shown a variety of interesting phenomena related to non-equilibrium condensation. These particles, polaritons, have striking similarities with BoseEinstein condensates (BECs) of atomic gases, and, in other aspects, with photon lasers. Nonetheless, polaritons have demonstrated peculiarities of their own that set them apart from both quantum condensates and lasers. Recently many observation in fluid dynamics have shown such peculiar behaviors, related to the polaritonic dispersion on the one hand and its dissipative character on the other hand. Fundamental phenomena like condensation [1, 2], superfluidity [3][4][5], quantised vorticity [6,7], quantum turbulence [8,9] and phase transitions [10] have shown to deviate from conventional BEC, stimulating new models for the quantum dynamics of polariton condensates. These models consider some of the many parameters that are typical of polaritons, including the upper and lower dispersions, pumping, dissipation, spin, non-linearities, etc. One component that is seldom considered in details is the exciton reservoir. This, however, often plays an important role, even under resonant excitation [11,12], and in particular when it comes to polariton relaxation and condensation.We will show here how a polariton condensate formed by non-resonant excitation, in a confined region of space of a few micron squared, can exhibit marked oscillations of its population. While many oscillatory behaviours have been observed [13,14] or predicted [15] in the polariton literature, relating them to coherent and/or quantum phenomena, we report semi-classical oscillations due to the interplay between reservoir feeding and Bose stimulation. These oscillations fall in the class of so-called "relaxation oscillations" [16], that are well known in class B lasers [17], typically solid-state ones with a small active volume and slow-inversion decay [18], where they lead to the phenomenon of spiking [19]. The effect is an important class of self-oscillations [20]. In this form, it is analogous to the Tantalus cup oscillations, used by the Romans to measure time, or in natural phenomena such as rhythmic springs. The underlying principle is the following: passed a threshold, a siphon triggers...

Polariton spin whirls

“…Due to the OSHE, the long range coherence 24 and fast spin dynamics 25 , polaritons have been proposed as a potential candidate for the realization of a new generation of spinoptronic devices 26 . In this regard, the contribution of a spin dependent exciton reservoir has not been considered thoroughly, although in non-resonant experiments and in the proximity of the excitation spot, exciton interactions dominate over other types of interactions 27 and can directly affect the spin dynamics of polaritons 28 . In this Letter, we report on the experimental observation of spin whirls in the radial expansion of a polariton condensate formed under non-resonant optical excitation in a GaAs quantum well (QW) microcavity.…”

Dephasing effects on coherent exciton-polaritons and the breakdown of the strong coupling regime

“…Meanwhile, the energy fluctuations affect neither the termψ † x (x,t)ψ x (x,t) nor its expectation value N (x,t) = ψ † x (x,t)ψ x (x,t) due to a phase cancellation [36]. Finally, we comment that the incoherent exciton population N should be interpreted as the inactive excitonic reservoir already discussed in the context of nonresonantly excited polariton condensates [38,39]. Actually, both the incoherent exciton population, featured in Fig.…”

Nonlinear spectroscopy of exciton-polaritons in a GaAs-based microcavity

“…Concerning applications operating in the THz frequency range, oscillations of the polariton population due to the dynamic Stark effect [25,26] and relaxation oscillations [27] have been observed, but unambiguous evidence for THz-lasing operation is still lacking. For an evaluation of the feasibility of the proposal of Ref.…”