Collective effects in intersubband transitions

We present experiments on the intersubband resonance (ISR) in InAs/AlSb quantum wells with the aim of understanding the linewidth. We find that fluctuations in the well width dominate the scattering right up to temperatures well beyond room temperature. ISR with two occupied subbands is used to gain insight into these phenomena. We find clear evidence for Landau damping and we argue generally that Landau damping represents the crucial scattering mechanism for all ISRs. The argument is strengthened by considering the intrasubband plasmon, where we also find evidence for Landau damping, in this case in a magnetic field.