  Two-photon absorption (TPA) allows accessing dark states of matter otherwise
inaccessible to light, which serve as important building blocks for quantum
information processing. In a semiconductor microcavity, TPA driven condensation
of strongly-coupled light-matter exciton-polaritons can enable new solid-state
quantum simulations of dark state-condensate interactions, and was predicted to
stimulate THz emission. Here, we report the first observation of two-photon
pumped polariton condensation, demonstrated by angle-resolved photoluminescence
in a GaAs-based microcavity. TPA is evidenced in the quadratic emission
dependence on pump power below and above the condensation threshold, and
second-harmonic generation is ruled out by both this threshold behaviour and by
the emission peak energy showing no dependence on pump photon energy. Our
results pave the way towards novel polariton-based sources and solid-state
coherent control of collective quantum states with individual two-level
systems.
