By coupling a strong, cavity-enhanced second-harmonic signal from a gallium phosphide (GaP) planar photonic-crystal (PPC) nanocavity to a commercial silicon photodiode, researchers at Northwestern Polytechnical University (Xían, China), Columbia University (New York, NY), Massachusetts Institute of Technology (MIT; Cambridge, MA), and Humboldt-Universität zu Berlin (Berlin, Germany) have demonstrated a near-infrared (NIR) upconversion photodetector with a responsivity of 0.81 A/W for 1 W of coupled power.
To fabricate the detector, air holes (and a central solid region of three missing air holes) are lithographically patterned in photoresist and transferred to the GaP substrate using chemical etching processes. The resultant PPC structure is then positioned over a silicon detector separated by an air gap or low-refractive-index material layer. Coupled NIR light is then upconverted by strongly confined resonant modes in the PPC cavity to submicron wavelengths that illuminate the silicon detector below for photocurrent generation. The PPC-assisted upconversion detector has higher responsivity and a smaller form factor than bulk-optic lithium niobate-based upconversion detectors and is more responsive than comparably sized indium gallium arsenide detectors. In addition, the detector can function as an efficient and compact autocorrelator for ultrafast optical pulse characterization with sub-picosecond resolution. Reference: X. Gan et al., Opt. Express, 23, 10, 12998–13004 (2015).