Zero-Bias Long-Wave Infrared Waveguide Photodetector via Graphene/Silicon/Halide Heterogeneous Integration

The integration of waveguides and photodetectors is an indispensable step toward the realization of on-chip spectroscopic sensing systems. However, the development of waveguide-integrated photodetectors in the attractive long-wave infrared (LWIR) range faces challenges from both waveguide platforms due to the bottom cladding material absorption and photodetection technologies due to the low LWIR photon energy. Here, we demonstrate an LWIR waveguide-integrated photodetector through heterogeneous integration of graphene photodetector and Si waveguide on CaF
2
substrate. A high-yield transfer printing method is developed for flexibly integrating waveguide and substrate materials to solve the bottom cladding material absorption issue. The fabricated Si-on-CaF
2
waveguides show low losses in the broad LWIR wavelength range of
$6.3-7.1\ \mu\mathrm{m}$
. The graphene photodetector achieves a broadband responsivity of ∼8 mA/W in these low-photon-energy LWIR wavelengths under zero-bias operation with the help of waveguide integration and plasmonic enhancement. Our strategy opens up the possibility for chip-scale, low-cost, and low-power-consumption LWIR spectroscopic sensing systems.