Nonlinear optical processes at quadrupole polariton resonance in Cu ₂O as probed by a Z-scan technique

Employing a modified Z-scan technique at 2 K, we monitor not only the fundamental (ω) but also the frequency-doubled (2ω) and tripled (3ω) Z-scan responses in Cu2 O when the input laser frequency ω is tuned to the two-photon quadrupole polariton resonance. The Z-scan response at ω allows us to accurately estimate the absolute number of polaritons generated via two-photon absorption. A striking dip is observed near the 2ω Z-scan focus which basically arises from Auger-type recombination of polaritons. Under high excitation levels, the 3ω Z-scan shows strong third harmonic generation. Based on the nonlinear optical parameters determined, we estimate the experimental polariton density achievable and propose a direction for polariton-based Bose-Einstein condensation in Cu2 O.