Ultrasensitive and high-dynamic-range two-photon absorption in a GaAs photomultiplier tube

We demonstrate improved efficiency and dynamic range for two-photon absorption at $1.5 \mathit{\mu }\mathrm{m}$ in a photoelectron-counting GaAs photomultiplier tube. cw laser measurements show pure two-photon absorption occurring in the device from average powers of $1.3 \mathit{\mu }\mathrm{W}$ to nearly 1 mW. We use the detector to implement an autocorrelator with peak-power × average-power sensitivity of $1.7\times {10}^{-4} {\left(\mathrm{mW}\right)}^2$ and discuss practical ways to reduce this figure of merit to as low as $1\times {10}^{-5} {\left(\mathrm{mW}\right)}^2$.