Intersubband lifetime in quantum wells with transition energies above and below the optical phonon energy

A new measurement of the intersubband lifetime is presented. In a first set of experiments, electrons are pumped from the ground state to the first excited state of a n-doped quantum well using a c.w. CO₂ laser. A Fourier transform infrared spectrometer beam probes the absorption cross-section between the first and second excited state, measuring the population of optically excited electrons. From this cross-section, we find a lifetime equal to τ_s = 0.65 ± 0.15 ps for a 85 A ̊ GaAs quantum well and τ_s = 0.8 ± 0.2 ps for a 100 A ̊ Ga_0.47In_0.53As quantum well, in good agreement with theoretical predictions. In a second set of experiments we investigate an asymmetric modulation-doped GaAs/AlGaAs coupled quantum well in which the spacing between the ground and first excited state E₁₂ = 19.6 meV is below the optical phonon energy. Electrons are optically pumped on the short-lived second excited state (E₁₃ = 135 meV) and some of them, cascading down, are trapped on the first excited state. A measurement of the differential absorption between the first and second excited state proves the existence of a bottleneck effect, and we estimate an electron lifetime of about τ_s = 300 ps. The extremely narrow (FWHM = 2.6 meV) Lorentzian line of the (1-3) transition corresponds to a time τ = 0.5 ps, very close to the lifetime (0.65 ps) measured in the first experiment. This is a clear indication that we are able to observe the ultimate broadening mechanism of intersubband transitions: lifetime broadening.