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

We present a systematic investigation of two-photon excitation processes in a GaAs-based microcavity in the strong-coupling regime. We observe second harmonic generation resonant to the upper and lower polariton level, which exhibits a strong dependence on the photonic fraction of the corresponding polariton. In addition, we have performed two-photon excitation spectroscopy to identify 2p exciten states which are crucial for the operation as a terahertz lasing device, which was suggested recently [A. V. Kavokin et al., Phys. Rev. Lett. 108, 197401 (2012) (http://dx.doi.org/10.1103/PhysRevLett.108.197401)1. However, no distinct signatures of a 2p exciton state could be identified, which indicates a low two-photon pumping efficiency.