Integrated longwave infrared (LWIR) photonics hold promise for enhancing on-chip molecular sensing due to the strong light-matter interaction in the LWIR spectrum, which is orders of magnitude more intense than in the near-infrared. However, conventional photonic materials suffer from high optical losses in this range. Specifically, silicon and III-V materials exhibit absorption losses due to multiphonon processes, which limit their applicability for LWIR systems. To address this issue, our work introduces a hybrid germanium-on-zinc selenide (GOZ) platform. This platform leverages the lower multiphonon absorption onset frequencies of germanium and the suitable cladding properties of zinc selenide to reduce optical losses. By employing a direct wafer bonding technique, our study achieves a waveguide system that is transparent from 2 μm to 14 μm, with measured optical losses as low as 1 cm−1 at 7.8 μm, indicating a significant improvement over traditional materials. Our findings demonstrate that the GOZ platform effectively reduces the intrinsic optical losses typical of epitaxiallygrown materials in LWIR devices, thereby paving the way for advancements in quantum and nonlinear photonic applications.
Terahertz Quantum Cascade Lasers (QCLs) are crucial for advancing research due to their high-power output, compact size, and efficiency. These lasers, designed through precise intersubband structure engineering, often show experimental outcomes that diverge from theoretical prediction. This discrepancy highlights the need for experimental gain characterization of QCLs. Using terahertz time-domain spectroscopy and a uniquely structured QCL with two beam paths of identical curvature but different lengths, self-referenced gain measurements were conducted across various temperatures and biasing voltages. A dispersion correction technique was used to distinguish signals from the two paths. The gain profile was extracted by analyzing the spectra of pulses through these different paths, providing an accurate gain profile by negating the zero-bias loss profile’s impact. At 23 K, an absorption peak was identified at 2.37 THz under low bias, shifting to lower frequencies with increased bias, aligning with density matrix simulations. Beyond the lasing threshold, the peak gain at 3.2 THz was consistently around 0 cm−1 for all bias levels. These findings underscore the self-referenced method’s significance in extracting absolute gain and dispersion information, enhancing device performance understanding.
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.
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