Observation of direct and phonon-assisted indirect transitions in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ multiquantum wells under hydrostatic pressure

Using photomodulated transmission spectroscopy we investigated the interband transitions in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ multiquantum wells as a function of hydrostatic pressure up to 50 kilobar. A number of spectral structures, associated with the Γ, $L,$ and $X$ critical points in GaAs wells and ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ barriers were observed, and calculations were performed to help understanding their origins. The pressure coefficients of the direct transitions in the quantum wells fall between those of GaAs and ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ bulk. The observation of phonon-assisted indirect transitions is made easy by photomodulated transmission technique, and by the band folding effect in the multiple-quantum-well systems. The indirect transitions derived from the $L$ conduction subbands and Γ valence subbands have been observed in wide pressure ranges.