Mid-infrared (MIR) is a promising spectral regime for gas and biochemical sensing since each molecule exhibits a unique vibrational absorption spectrum at this band. While integrated photonics on silicon-on-insulator (SOI) has become a successful platform, it cannot cover a broad MIR range, mainly limited by oxide absorption. Recently, a germanium-on-insulator (GOI) has emerged as a promising integrated photonics platform with broadband transparency covering 2-14 μm wavelengths. Germanium (Ge) and yttrium oxide (Y2O3), which exhibit low loss at the MIR regime, are used as a core and box insulator, respectively. However, the prevailing crosstalk issue of integrated photonics becomes more problematic at MIR due to extended evanescent fields with a longer wavelength. To address this issue, we propose using subwavelength gratings (SWGs), which can be effectively represented by homogenized anisotropic metamaterials. We arranged the SWGs in the cladding and formed an extreme skin-depth (eskid) waveguide, whose skin-depth is suppressed for transverse-electric (TE) mode. We then optimized SWG parameters to achieve an exceptional coupling that can completely suppress the crosstalk, i.e., zero crosstalk. Anisotropic dielectric perturbation via SWG metamaterials allowed different field components to compensate for each other, making the overall coupling coefficient zero. We optimized our SWG-based eskid waveguide scheme near 4.2 μm wavelength, where we can directly apply it for CO2 sensing with its strong absorption. We expect our eskid scheme on the GOI platform to improve the overall performances of MIR photonic devices, especially for MIR molecular sensing applications.
For the next-generation 8 to 14μm long-wavelength infrared (LWIR) sensing, type-II superlattices (T2SLs) detectors have enormous potential to appeal to various applications including space, medical imaging, and defense. In typical absorber design, the sufficiently thick active layer (AL) is required to achieve high quantum efficiency (QE), whereas it can cause a high dark current and increase the cost. Moreover, a simple increase in AL thickness does not provide an increase in QE due to the limited carrier lifetime in T2SLs. A possible solution to the weak absorption in the AL of the T2SL-based detectors involves incorporating the AL into an optically resonant cavity. In this work, optically enhanced QE for the broadband T2SL nBn detectors will be presented through the guided-mode-resonance (GMR) structures on the top surface of the T2SL. We fabricated T2SL nBn detectors with periodic gratings on the top surface. The devices showed much-enhanced QE due to multiple resonances, as well as Fabry-Perot resonance in the thin AL with a lower dark current characteristic than the reference T2SL detector. Furthermore, we also found the broadening of the cutoff wavelength, which is typically limited by the material property, by scaling the dimension of the diffraction grating for a strong resonance beyond the cut-off region. In conclusion, the GMR-based LWIR T2SL detectors can show a significant performance enhancement in QE and extend the detection range beyond the cut-off wavelength while maintaining a low dark current.
We experimentally demonstrate the free-carrier absorption (FCA)-assisted photodetection using a waveguide-integrated bolometer on the silicon-on-insulator (SOI) platform at the near-infrared range (1520-1620 nm). A heavily-doped silicon (n + Si) plays a role as an efficient light absorption medium, which exploits the mechanism of FCA in Si. For the thermal-to-electrical conversion, a bolometric material of TiOx/Ti/TiOx tri-layer film is integrated onto the n + Si. It offers a sensitivity of -26.75 %/mW with a highly flat spectral response. In addition, a clear on/off bolometric response with the 1 kHz-modulated optical signal was obtained with the rise and fall times of 24.2 μs and 29.2 μs, respectively, which is enough for diverse sensing applications.
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