Temperature and Intensity Dependence of Intersubband Relaxation Rates from Photovoltage and Absorption

We report intersubband-scattering times $\left(T_1\right)$ in a semiconductor heterostructure with intersubband spacing below the LO phonon energy. $T_1$ is determined by simultaneous measurements of the intersubband absorption and the photovoltage induced by far-infrared radiation (FIR) near the intersubband transition frequency. At the lowest temperature $(T=10\mathrm{}\mathrm{K})$ and FIR intensity $(I=10\mathrm{}\mathrm{mW}/{\mathrm{cm}}^2)$, $T_1=1.2\mathrm{}\pm 0.4\mathrm{ns}$, several times longer than predicted theoretically. $T_1$ decreases strongly with increasing temperature and FIR intensity, to 20 ps at $T=50\mathrm{}\mathrm{K}$ in the linear regime, and to 15 ps at $T=10\mathrm{}\mathrm{K}$ and $I=2\mathrm{}\mathrm{kW}/{\mathrm{cm}}^2$.