Ge-on-Si modulators operating at mid-infrared wavelengths up to 8 μm

We report mid-infrared Ge-on-Si waveguide-based PIN diode modulators operating at wavelengths of 3.8 and 8 μm. Fabricated 1-mm-long electro-absorption devices exhibit a modulation depth of >35  dB with a 7 V forward bias at 3.8 μm, and a similar 1-mm-long Mach–Zehnder modulator has a Vπ·L of 0.47  V·cm. Driven by a 2.5Vpp RF signal, 60 MHz on-off keying modulation was demonstrated. Electro-absorption modulation at 8 μm was demonstrated preliminarily, with the device performance limited by large contact separation and high contact resistance.