Broadband THz absorption spectrometer based on excitonic nonlinear optical effects

A broadly tunable THz source is realized via difference frequency generation, in which an enhancement to χ⁽³⁾ that is obtained via resonant excitation of III-V semiconductor quantum well excitons is utilized. The symmetry of the quantum wells (QWs) is broken by utilizing the built-in electric-field across a p-i-n junction to produce effective χ⁽²⁾ processes, which are derived from the high χ⁽³⁾. This χ⁽²⁾ media exhibits an onset of nonlinear processes at ~4 W cm^-2, thereby enabling area (and, hence, power) scaling of the THz emitter. Phase matching is realized laterally through normal incidence excitation. Using two collimated 130 mW continuous wave (CW) semiconductor lasers with ~1-mm beam diameters, we realize monochromatic THz emission that is tunable from 0.75 to 3 THz and demonstrate the possibility that this may span 0.2-6 THz with linewidths of ~20 GHz and efficiencies of ~1 × 10^-5, thereby realizing ~800 nW of THz power. Then, transmission spectroscopy of atmospheric features is demonstrated, thereby opening the way for compact, low-cost, swept-wavelength THz spectroscopy.