Millimeter-Wave Determination of Photoinjected Free-Carrier Concentrations in Highly Excited GaAs

Measurements of millimeter-wave reflection are reported for optically injected free carriers in GaAs at 300 °K for high levels of uv laser excitation. This reflection responds directly to the total number of free carriers, giving ${10}^{14}$ electrons/${\mathrm{cm}}^2$ at an incident flux of 2.7×${10}^{23}$ photons/${\mathrm{cm}}^2$ sec. These results yield an injected free electron-hole lifetime $\tau \sim 0.5$ nsec for both pure and compensated material.