Optical frequency combs based on quantum cascade lasers (QCLs) are promising broadband light sources in the mid-infrared and terahertz spectral regions. Their bandwidths are limited by two main parameters: dispersion, which originates from variation in the group velocity, and diffusion, which originates from variation in the gain. While dispersion has been extensively engineered, diffusion shaping has been elusive. We show that the addition of carefully engineered diffusive loss can enhance the bandwidth of QCL combs, demonstrating theoretically and experimentally that adding resonant loss to the cavity of a terahertz QCL can counteract the diffusive effect of the gain medium and allow broader bandwidth combs to form. Our results give a new degree of freedom for the creation of active chip-scale combs and can be applied to a wide array of cavity geometries and comb systems.
Quantum Cascade Lasers (QCLs) have immense potential for generating chip-scale frequency combs in the mid-infrared and terahertz spectral regions. In this work, we demonstrate the formation of frequency combs within ring terahertz QCLs using optical injection from a Distributed Feedback (DFB) laser. By carefully selecting a DFB design frequency that aligns with the ring cavity modes (around 3.3 THz) and employing a bus waveguide for light injection, we show that combs can be selectively formed and controlled within the ring cavity. Numerical modeling suggests that the observed comb formation is frequency-modulated in nature, with the optical injection acting as a trigger. Furthermore, we demonstrate the ring cavity's ability to function as a filter, a feature that could hold significant value for terahertz photonic integrated circuits. Our findings highlight the promise of waveguide couplers as a robust approach for injecting and extracting radiation from ring terahertz comb and offer exciting possibilities for generation of new comb states in the terahertz domain, including frequency-modulated waves, solitons, and more.
Topological crystalline insulators—topological insulators whose properties are guaranteed by crystalline symmetry—
can potentially provide a promising platform for terahertz optoelectronic devices, as their properties can be tuned
on demand when layered in heterostructures. We perform the first optical-pump terahertz-probe spectroscopy
of topological crystalline insulators, using them to study the dynamics of Pb1−xSnxSe as a function of temperature. At low temperatures, excitation of Dirac fermions leads to an increase in terahertz transmission; from this negative photoconductivity, the intrasubband relaxation rate of 6 ps is extracted. At high temperatures where only massive fermions exist, the free-carrier losses induced by the pump reduce the terahertz transmission for the duration of the 27 ps interband lifetime. Both effects are present at temperatures near the topological-to-trivial transition. Our experimental observations provide critical details for potential applications of Pb1−xSnxSe and provide a direct measurement of the topological character of Pb1−xSnxSe heterostructures.
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