Grating-coupler-induced intersubband transitions in semiconductor multiple quantum wells

Intersubband transitions induced in a simple Faraday transmission geometry by metallic grating couplers have been studied in several GaAs/Al_0.3Ga_0.7As MQW samples lightly doped with donors and with well widths between 210 Å and 320 Å. A sensitive pumping and probing technique was employed in which chopped visible pump light and spatially modulated infrared radiation are transmitted simultaneously through the MQW. The excess free electron density in the well created by < 100 microW/cm² of red light is estimated to be ≅ 10⁹cm^-2 per well. The measured E₁ - E₀ energies are in good agreement with a simple ID model calculatio found that the grating coupling efficiency drops from 15% to 5% when the ratio of the transition wavelength to the grating period is increased from 1.5 to 3.6.