Room Temperature Tuneable THz Generation Based On Excitonic optical nonlinearities in GaAs/AlGaAs Multi-Quantum Well Structures

摘要

We report the generation of tuneable THz radiation (0.2-6 THz) and demonstrate a spectroscopic system operating at 750GHz. THz radiation is generated through second order nonlinear effects in the excitation of excitons in GaAs/AlAs multi-quantum wells (MQWs), using readily available continuous wave (CW) laser diodes at room temperature. An MBE grown 30xMQW GaAs(11.9nm)/AlAs(7.1nm) sample was designed to have excitonic resonances at a wavelength accessible by commercially available lasers (～850nm), and have E1HH1-E1LH1 splitting of 9.1meV. Two collimated, normally incident lasers were used to excite the excitonic resonances. Figure 1(a) shows THz power obtained in collinear and crossed polarisations of the lasers with one laser resonant with the HH exciton, and the other laser tuned across the excitonic bands with each laser operating at 130mW output power. A clear signal is observed in the case of collinear excitation which scales with the density of states of the excitons. Power dependence measurements confirm this is a second order non-linear effect. Using a simple interferometer, and fitting the measured power with the expected transmission of a Fabry-Perot etalon, frequency measurements confirm the ability to tune the THz radiation from 0.75-3THz. See Figs 1(b-d). Tuning over the full range of Fig 1(a) indicates that THz generation os possible from ～0.2 to 6THz. For excitation at the peaks of HH and LH, a conversion efficiency of 1.2×10~(-5) was obtained. This was achieved without the use of plasmonic effects, nor any kind of an antenna, nor an applied E-field to the structure. Spectroscopy using this new tuneable/sweepable light source will be presented at the conference. Future prospects for this technology will be discussed.