Photonic crystal enabled manipulation of optical and electric field in germanium avalanche photodetectors

We demonstrate the use of a photonic crystal (PhC) structure to improve the performance of a germanium avalanche photodetector (APD) by simultaneously manipulating the distribution of the optical and electric fields. The PhC is fabricated at the top center of the vertical germanium APD. For a 14 μm diameter device, the 1550 nm responsivity increases from 0.2 to 0.6 A W−1 at unity gain, owing to the resonance-enhanced absorption. Moreover, the structure separates the absorption and multiplication regions of the device, resulting in an increase of the avalanche gain and the gain-bandwidth product. Under −10 dBm input optical power, a 3 dB bandwidth of 34 GHz before avalanche and a clear 40 Gbps eye diagram under avalanche demonstrates good high-speed performance of the device.