ABSORPTION AND RESONANT DISPERSION ASSOCIATED WITH NORMAL INCIDENCE INTERSUBBAND TRANSITIONS IN SI SIGE QUANTUM-WELLS/

Modulation of midinfrared beam by photoinduced intersubband absorption in undoped Si/SiGe quantum wells is investigated at normal incidence. Optical pumping of interband transitions is used to photocreate carri ers in the wells. The modulated transmission of the sample is measured by Fourier transform spectroscopy. Resonant dispersion associated wit h intersubband transitions as well as interference effects in the quan tum well stack are clearly evidenced. It is shown that the phase modul ation due to refractive index variations may compensate for amplitude modulation due to absorption. Measurements are well fitted by a phenom enological model. It is also shown that the modulation depth and the w avelength modulation profile both depend on the interband excitation w avelength. This result is attributed to the different nature of photoi nduced transitions according to the excitation wavelength. (C) 1995 Am erican Institute of Physics.