Free-electron laser nonlinear spectroscopy of doubly resonant (5.5-3.0um and 4.1-2.1 um)InGaAs/AlGaAs asymmetric quantum wells

摘要

Conduction band intersubband absorption and second harmonic generation (SHG) are demonstrated in doubly resonant asymmetric step high indium content quantum wells (QW) grown on a GaAs substrate. Intersubband absorption peaks at 5.51 $mu@m and 3.05 $mu@m corresponding to the 1 to 2 and 1 to 3 transitions are measured. The susceptibility of SHG from the QW, $Chi$-QW$/$+(2)$/, is measured using a free electron laser by interference between the SHG fields generated from the QW and the GaAs substrate. A large asymmetry in the SHG power with rotation angle of the sample arising from $Chi$-QW$/$+(2)$/ is observed. The magnitude and phase of $Chi$-QW$/$+(2)$/ is measured in the 4.6 - 6.3 $mu@m pump wavelength range. $Chi$-QW$/$+(2)$/ is of maximum amplitude at 6.0 $mu@m with a value of 145 $POM 20 nm/V. A change in the sign of the phase of $Chi$-QW$/$+(2)$/ within the SHG resonance is demonstrated for the first time. Agreement of both the linear and nonlinear properties to a simple model assuming Lorentzian linewidths is discussed. SHG of 2.0 $mu@m light is also demonstrated in coupled In$-0.60$/Ga$-0.40$/As/AlAs quantum wells.