Transversely superimposed gratings for narrow far-field terahertz surface emission of nonlinear quantum cascade lasers

We present terahertz (THz) surface emission by difference frequency generation in nonlinear quantum cascade lasers operating at room temperature. The device comprises two separate, transversely superimposed gratings, one for selective feedback for the midinfrared (MIR) modes and one for normal surface emission of the generated THz radiation. This allows for narrow far-field THz emission with an enhanced extraction over the entire device length with improved electrical and thermal properties compared to previous devices relying on Cherenkov phase matching. The MIR grating is realized as a higher order loss-coupled distributed feedback grating for single-mode emission at two distinct wavelengths. Its position determines the position of the standing wave pattern of the MIR modes and of the nonlinear polarization wave. This allows for a precise placement of the second-order top grating, which leads to perpendicular surface emission. The device emits in a single-lobed far-field with a full width half maximum of 3.5 deg in single-mode operation at room temperature.