Spatial Resonator Solitons Based on Different Semiconductor Nonlinearities

Abstract

Optical bistability and bright and dark resonator solitons exist in semiconductor resonator at room temperature [C.O. Weis and Ye. Larionova, Rep. Prog. Phys. 70 (2007) 255-335], with the usual semiconductor nonlinearity. At low temperature in the strong coupling regime between quantum well excitons and the optical field in a semiconductor microcavity the nonlinearities are strong and based on “polariton nonlinearity”. The strong coupling results in a quasi-particle “exciton-polariton” which gives access to positive and negative reactive and dissipative optical nonlinearities, as opposed to the usual room temperature semiconductor nonlinearities possessing essentially only one sign. The type of nonlinearity and the properties of solitons can be varied widely by the detuning between the photon- and exciton resonances. We show for the first time experimentally the existence of patterns, bright and dark solitons based on polariton nonlinearity. 1. INTRODUCTION Bright and dark spatial solitons were shown to exist in such microresonators with and without population inversion [1-3]. At room temperature the nonlinearity which is at the origin of the soliton existence is the usual mixed absorptive/refractive semiconductor nonlinearity which can have transverse effects (e.g. self-focusing) and longitudinal effects such as nonlinear resonance, both ingredients for soliton stability. At low temperature (or at large exciton binding energies) the exciton, which merges with the absorption band at room temperature, is well separated spectrally from the absorption band so that the nonlinearity becomes that of a 2-level system. If the exciton resonance is strongly coupled with the field inside a resonator the exciton- and the photon resonances mix. At coincidence of the two resonance frequencies, the photon- and the exciton resonances hybridize in an avoided crossing and the two hybrid resonances are ascribed to a quasi-particle, the excitonpolariton. This quasi-particle has unusual optical properties. E.g. is the photon nonlinearity determined by exciton collisions. That the exciton-polariton system has interesting nonlinearities (e.g. induced absorption as opposed to the usual saturable absorption) has been shown previously [4]. It was realized that semiconductor microresonators in the strong field-exciton coupling regime might show optical bistability and first experimental proofs for bistability were given recently [5]. As optical bistability of a resonator is a precondition for the existence of spatial resonator solitons, the question arises whether semiconductor microresonators with polariton nonlinearity can support spatial resonator solitons, which, as a consequence of the large nonlinearity of the polaritons, could be expected to exist at rather small light intensities, as it is of importance for all applications. 2. EXPERIMENTAL ARRANGEMENT AND RESULTS The cooled resonator sample (4 K) is illuminated by light at the polariton resonance wavelengths, and the light reflected from the resonator is observed. The light source is a Ar