We experimentally demonstrate resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100. This new parametric phenomenon appears as a sharp angular resonance of the incoming pump beam, at which the moving excitonic polaritons undergo very large changes in momentum. Ultrafast stimulated scattering is clearly identified from the exponential dependence on pump intensity. This device utilizes boson amplification induced by stimulated energy relaxation.

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and $\sim 3.9\times {10}^4{\mathrm{cm}}^{-1\mathrm{}}$, is observed under conditions of continuous wave excitation of a semiconductor microcavity. The “condensation” of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.

A polariton condensate transistor switch is realized through optical excitation of a microcavity ridge with two beams. The ballistically ejected polaritons from a condensate formed at the source are gated using the 20 times weaker second beam to switch on and off the flux of polaritons. In the absence of the gate beam the small built-in detuning creates a potential landscape in which ejected polaritons are channelled toward the end of the ridge where they condense. The low-loss photonlike propagation combined with strong nonlinearities associated with their excitonic component makes polariton-based transistors particularly attractive for the implementation of all-optical integrated circuits.

Semiconductor microcavities can support quasiparticles which are half-light and half-matter with interactions possessed by neither component alone. We show that their distorted dispersion relation forms the basis of a quasiparticle “trap” and elicits extreme enhancements of their nonlinear optical properties. When driven by a continuous wave laser at a critical angle, the quasiparticles are sucked into the trap, condensing into a macroscopic quantum state which efficiently emits light. This device is thus an optical parametric oscillator based on quasiparticle engineering. In contrast to a laser, macroscopic coherence is established in the electronic excitations as well as the light field. This paves the way to new techniques analogous to those established in atomic and superconducting condensates, such as ultrasensitive solid-state interferometers.