Observation of photoinduced intersubband transitions in one-dimensional semiconductor quantum wires

We report on midinfrared intersubband transitions between the quantized levels of undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wires that are grown on nonplanar substrates. Using photoinduced infrared absorption spectroscopy, we were able to resolve two absorption lines in the 120–160 meV range. The first absorption line follows the polarization selection rules for intersubband transitions and exhibits photoinduced excitation profile that resembles the photoluminescence excitation spectrum of the quantum wires. The second absorption resonance, at a higher energy, is not polarized and resembles the photoluminescence spectrum of the side quantum wells. Based on these results and energy level calculations, we assign the first absorption resonance to intersubband transition between the quantized subbands of the quantum wires. The origin of the second resonance is less clear. We propose a model that attributes this resonance to an interface localized exciton mode that arises from interface roughness of the side quantum wells and the wires.