  Mid-infrared silicon photonics has recently emerged as a new technology for a
wide range of applications such as optical communication, lidar, and
bio-sensing. One key component enabling this technology is the mid-infrared
optical modulator used for encoding optical signals. Here, we present a GeSn
electro-absorption modulator that can operate in the mid-infrared range.
Importantly, this device is monolithically integrated on a silicon substrate,
which provides compatibility with standard complementary
metal-oxide-semiconductor technology for scalable manufacturing. By alloying Ge
with Sn to engineer the bandgap, we observed a clear Franz-Keldysh effect and
achieved optimal modulation in the mid-infrared range of 2067-2208 nm with a
maximum absorption ratio of 1.8. The results on the Si-based mid-infrared
optical modulator open a new avenue for next-generation mid-infrared silicon
photonics.
