High-performance Frequency Combs with Planarized THz Quantum Cascade Lasers

Abstract

Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of coherent THz radiation which can also operate as frequency combs [1], making them appealing for spectroscopy and sensing. We recently developed a new planarized THz QCL platform, where the active region waveguides are surrounded by a low-loss polymer (BCB). This enables fabricating an extended top metallization and placing bonding wires over the passive area on the sides (see Fig. 1a). We have shown that the planarized waveguide geometry improves the thermal, RF and dispersion properties, all crucial for frequency comb operation [2]. Here, we present how several optimized planarized waveguide components further boost frequency comb performance (see microscope images in Fig. 1b). On the back facet side, we designed and fabricated a broadband double-chirped reflector structure [1, 3] which compensates for chromatic dispersion and reduces mirror losses. This results in broadband free-running comb operation spanning over 1 THz, as shown in Fig. 1c. In the center of the waveguide, a tapered profile results in a strong field-enhancement, which leads to several effects such as improved high-temperature comb operation up to 115 K (see Fig. 1d), strong measured RF beatnotes (up to nearly -30 dBm) and the possibility to switch between various harmonic comb states on the same device by varying the bias and temperature. Finally, on the front facet side, we can couple the propagating waveguide mode to a patch array antenna (based on the design from [4], but re-optimized for ultra-broadband operation). This increases the slope efficiency by a factor of 4–5 with output powers above 10 mW, and the surface-emitted output beam has a divergence below $(20\ \mathrm{x}\ 20)$ degrees. All the components have been optimized for an octave-spanning frequency range (2–4 THz) and are compatible to be combined in a single high-performance comb device. Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of coherent THz radiation which can also operate as frequency combs [1], making them appealing for spectroscopy and sensing. We recently developed a new planarized THz QCL platform, where the active region waveguides are surrounded by a low-loss polymer (BCB). This enables fabricating an extended top metallization and placing bonding wires over the passive area on the sides (see Fig. 1a). We have shown that the planarized waveguide geometry improves the thermal, RF and dispersion properties, all crucial for frequency comb operation [2]. Here, we present how several optimized planarized waveguide components further boost frequency comb performance (see microscope images in Fig. 1b). On the back facet side, we designed and fabricated a broadband double-chirped reflector structure [1], [3] which compensates for chromatic dispersion and reduces mirror losses. This results in broadband free-running comb operation spanning over 1 THz, as shown in Fig. 1c. In the center of the waveguide, a tapered profile results in a strong field-enhancement, which leads to several effects such as improved high-temperature comb operation up to 115 K (see Fig. 1d), strong measured RF beatnotes (up to nearly -30 dBm) and the possibility to switch between various harmonic comb states on the same device by varying the bias and temperature. Finally, on the front facet side, we can couple the propagating waveguide mode to a patch array antenna (based on the design from [4], but re-optimized for ultra-broadband operation). This increases the slope efficiency by a factor of 4–5 with output powers above 10 mW, and the surface-emitted output beam has a divergence below $(20\ \mathrm{x}\ 20)$ degrees. All the components have been optimized for an octave-spanning frequency range (2–4 THz) and are compatible to be combined in a single high-performance comb device. Fig. 1 (a) Planarized thz qcls are surrounded by a low-loss polymer (bcb) and feature improved thermal, rf and dispersion properties. (b) Optimized waveguide structures for frequency combs: dispersion-compensating reflector, tapered waveguide, and patch array antenna outcoupler. (c) Measured free-running frequency comb spectrum of a dispersion-compensated waveguide spanning over 1 thz with a single strong beatnote (inset). (d) High-temperature comb operation of a tapered waveguide up to 115 k.