Superluminal X-waves in a polariton quantum fluid

Abstract

In this work, we experimentally demonstrate for the first time the spontaneous generation of two-dimensional exciton-polariton X-waves. X-waves belong to the family of localized packets that can sustain their shape without spreading, even in the linear regime. This allows the wavepacket to maintain its shape and size for very low densities and very long times compared to soliton waves, which always necessitate a nonlinearity to compensate the diffusion. Here, we exploit the polariton nonlinearity and uniquely structured dispersion, comprising both positive- and negative-mass curvatures, to trigger an asymmetric four-wave mixing in momentum space. This ultimately enables the self-formation of a spatial X-wave front. Using ultrafast imaging experiments, we observe the early reshaping of the initial Gaussian packet into the X-pulse and its propagation, even for vanishingly small densities. This allows us to outline the crucial effects and parameters that drive the phenomena and to tune the degree of superluminal propagation, which we found to be in close agreement with numerical simulations.

Spatiotemporal continuum generation in polariton waveguides

“…These two patterns of radiation are well explained by the mechanisms of STMI 4,38,46 , CR 39 , and the formation of spatiotemporal X-waves 11,37,47 with different mechanisms dominating in the intermediate (Fig. 2e) and high (Fig.…”

“…This unusual dispersion has been exploited to demonstrate optical parametric oscillation 34 , thermal relaxation from solitonic to condensate-like states 35 , and backward-propagating Cherenkov radiation (CR) in one-dimensional microwires 36 . In both microcavities and waveguides, dispersion and diffraction can have opposite signs so that the system supports spatiotemporal X-waves 37 and exhibits an unusual curved X-shaped modulation instability gain spectrum in the frequency/angle space 4 , which we demonstrate later. The giant dispersion of polariton systems allows the characteristic dispersion and diffraction length scales to be equal for convenient picosecond pulses and ~5-μm beam widths, which are ideal for on-chip spatiotemporal optics 15 .…”

Polaritonics: from microcavities to sub-wavelength confinement

“…The anti-crossing is small compared to the line width but still visible [88]. Additional details on the parameter structures can be found in Refs [86][87][88].…”

“…To allow comparison between different resonator technologies, the confinement for planar optical cavities is considered only in the direction perpendicular to the [86]. (E) Dispersion of organic BSW polaritons.…”

Polariton panorama

“…Exciton polariton X-waves: wavepackets of exciton polaritons that sustain their shape without spreading, even in the linear regime. In a study by Gianfrate et al [164]. Selfgeneration of an X-wave out of a Gaussian excitation spot is obtained via a weakly nonlinear asymmetric process with respect to two directions of the nonparabolic polariton dispersion.…”