Terahertz quantum-cascade lasers with lateral distributed-feedback resonators
Terahertz quantum-cascade lasers (THz QCLs) are promising far-infrared sources for various applications such as local oscillators in heterodyne detectors. This requires a combination of sufficient output power, continuous-wave (cw) operation in small-sized cryocoolers, and single-mode emission at the specified target frequency. Obtaining such a combined goal requires a suitable QCL heterostructure as well as an appropriate resonator.
We developed distributed-feedback (DFB) resonators based on single-plasmon waveguides and first-order lateral DFB (lDFB) gratings [Fig. 1(a)]. These lasers exhibit single-mode emission with output powers of a few mW at operating temperatures, which are accessible by compact Stirling coolers. Based on a rigorous solution of Maxwell’s equations for the DFB unit cell, we developed a general method to calculate the coupling coefficients of lDFB gratings. This allows for an efficient simulation of the resonator properties within the framework of the one-dimensional coupled-mode equations.
One goal for THz astronomy is the heterodyne spectroscopy of the neutral oxygen (OI) fine-structure transition at 4.745 THz, which requires a local oscillator just a few GHz beside this transition frequency. Based on an optimized heterostructure and an adjusted lDFB grating, we achieved single-mode, cw operation around the target frequency [Fig. 1(b)].
Fig. 1: (a) Geometry and initial finite-element grid of the lDFB unit cell as used for simulations (periodic in the z-direction). 1: top metallization, 2: highly doped bottom contact layer, 3: bottom metallization, 4: active region, 5: semiinsulating GaAs substrate. (b) Emission spectra for a 0.1×1.2 mm2 lDFB laser (grating period: 8.46 µm). The solid vertical line indicates the OI frequency, while the dashed lines indicate the specified side-band frequencies for the local oscillator.
M. Wienold, A. Tahraoui, L. Schrottke, R. Sharma, X. Lü, K. Biermann, R. Hey, and H. T. Grahn,
Lateral distributed-feedback gratings for single-mode, high-power terahertz quantum-cascade lasers,