High performance terahertz metasurface quantum-cascade VECSEL with an intra-cryostat cavity

Abstract

A terahertz quantum-cascade (QC) vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated with over 5 mW power in continuous-wave and single-mode operation above 77 K, in combination with a near-Gaussian beam pattern with a full-width half-max divergence as narrow as ∼5° × 5°, with no evidence of thermal lensing. This is realized by creating an intra-cryostat VECSEL cavity to reduce the cavity loss and designing an active focusing metasurface reflector with low power dissipation for efficient heat removal. Also, the intra-cryostat configuration allows the evaluation of QC-VECSEL operation vs. temperature, showing a maximum pulsed mode operating temperature of 129 K. While the threshold current density in the QC-VECSEL is higher compared to that in a conventional edge-emitting metal-metal waveguide QC-laser, the beam quality, slope efficiency, maximum power, and thermal resistance are all significantly improved.

Robust Density Matrix Simulation of Terahertz Quantum Cascade Lasers

“…Two parasitic states (3,5) were also captured in the simulation. This design (and similar variants) is notable [31], as it hold records for highest peak pulsed power [1], [25] and cw power at 77 K [4].…”

“…It seems that the optical field is driving the device towards a thermal equilibrium, all by loosening the bottleneck in transport that was intentionally designed into the optical transition. Furthermore, the populations in the excited states (4,7) are significantly larger with the optical field turned on, likely due to reabsorption of photons by electrons in the lower states. There is also a noticeable decrease in the occupation of all subbands at an in-plane energy of approximately the LOphonon energy E LO (36 meV in GaAs), although this is less significant in the presence of the optical field.…”

“…We now turn to a demonstration of the model's capability to simulate widely varying designs, using fully self-consistent solutions of bandstructure, transport, and optical field strength, providing good predictions of the actual performance. This is shown by a side-by-side comparison of three different designs in Figure 2: (A) a hybrid bound-to-continuum/resonant-phonon design that has been demonstrated to have very high power [4], similar to [25], (B) a resonant-phonon design for high temperature which holds the current T max record [5], and (C) a scattering-assisted design for low frequency operation [28].…”

“…Record output power has reached 2.4 W in pulsed mode (1% duty cycle) [1] and 230 mW in cw [2]. These were, however, at a heatsink temperature of 10 K; at 77 K, the records are 1.8 W for pulsed [1] but only 5 mW in cw [3], [4]. In addition, the temperature record for any lasing has remained at 199.5 K for over half a decade [5].…”

“…THz quantum-cascade (QC) lasers are a leading practical source of radiation over approximately [1][2][3][4][5] THz which rely upon intraband radiative transitions between electronic subbands within III-V heterostructure quantum wells. Record output power has reached 2.4 W in pulsed mode (1% duty cycle) [1] and 230 mW in cw [2].…”