Fano Signatures in the Intersubband Terahertz Response of Optically Excited Semiconductor Quantum Wells

Absorption and transmission spectra of broadband terahertz pulses are measured to probe the intersubband response of an optically excited quantum-well heterostructure. While the terahertz absorption shows the single peak of the resonant intersubband transition, the transmission spectra display strong Fano signatures due to the phase sensitive superposition of ponderomotive and terahertz currents as predicted by our microscopic theory.

Figure 1

(a) Optical interband pump intersubband THz probe experiment. (b) The transition from the heavy-hole (hh) state to the first conduction state (c1) in the ${\mathrm{Al}}_{0.34}{\mathrm{Ga}}_{0.66}\mathrm{As}/\mathrm{GaAs}$ multi-quantum-well (MQW) sample is resonantly excited, either by a ps or a fs laser. THz pulses probe the c1-to-c2 transition 25 ps after photoexcitation. (c) The field transients of the transmitted THz pulses are detected by phase-matched electro-optic sampling with a sampling beam. The reference without photoexcitation is shown.

Figure 2

Experiment-theory comparison of the THz intersubband response. (a),(b) Measured time-resolved differential transmission signals $\Delta E(t)$ for ps and fs excitation, respectively. (c) Measured spectral amplitude of differential transmission $|\Delta E(\omega )|$ (shaded area) and calculated current density $|J_{\mathrm{tot}}(\omega )|$ (red solid line) after optical ps excitation. (d) shows the decomposition of the computed total current density (red solid line) into the ponderomotive contribution $|J_{\mathrm{A}}|$ (black dotted line) and the intersubband current $|J_{\mathrm{THz}}|$ (blue dashed line). (e),(f) Measured (shaded area) and computed (red solid line) differential transmission and THz absorption, respectively, after fs excitation. Here, $J_{\mathrm{A}}$ has been enhanced by 1.85 in order to account for substrate excitations.