Metasurfaces coupled to terahertz intersubband transitions

In this contribution, the authors present a thorough numerical study and experimental observation of time-resolved coupling of free-space single-cycle THz pulses to intersubband transitions in semiconductor QWs. To this end, we have developed a three-dimensional finite-difference time-domain code based on the standard Yee grid to simulate the interaction of single-cycle THz pulses with metasurfaces on a dielectric substrate coupled to a QW. The model incorporates the QW intersubband transitions either as simple Lorentz oscillator or via the Maxwell Bloch equations. The influence of free carriers in the QW on the coupling efficiency has been investigated using the Drude model and methods for circumventing this problem are proposed. Furthermore, the authors have compared different structures, designed for the same resonance frequency, in terms of Q-factor and energy conversion efficiency leading to an improved resonator design. To validate our simulation results, the authors have performed experiments on different metasurfaces coupled to modulation-doped GaAs/AlGaAs parabolic QWs using a conventional THz time-domain spectroscopy setup, both at room-temperature and at 5K. The observations are in good agreement to theory.