Silicon is an indirect-bandgap semiconductor and thus ordinarily does not allow for lasing or amplification of light without using hybrid materials. However, Stanislav Stepanov and Shlomo Ruschin of Tel Aviv University (Tel Aviv, Israel) are making silicon-on-insulator (SOI) waveguides into optically pumped gain media by diffusing gold into them. If further developed, this technology could enable practical on-chip waveguide-based silicon lasers and amplifiers for silicon photonic integrated circuits, as well as in separate components.
Using phosphorus-doped SOI wafers, the researchers thermally diffused gold into the wafer's top layer at room atmosphere and pressure, creating a range of prototypes using different diffusion temperatures from 550° to 700°C and different diffusion times from 30 minutes to 7 hours. After diffusion, the researchers patterned ridge large single-mode waveguides with 5 μm thickness, 10 μm width, and 2 cm length with a 0.5-μm-thick silicon dioxide buffer layer. The waveguides were transversely pumped from above with laser light at 532 nm, with the intent to create optical amplifiers for the near-infrared (NIR) range. The pump light was modulated by a chopper at frequencies ranging from 1 to 1000 Hz for experimentation. Two signal lasers were used: a tunable laser with a 1.527 to 1.576 μm wavelength range, and a single-wavelength 1.32 μm laser. An oscillogram of the test results using the longer-wavelength signal laser shows a gain of 6 to 7. At a 0.55 W pump power, the gain coefficient reached 30 dB/cm for the 1.55 μm signal, but only about 6 dB/cm for the 1.32 μm signal. The researchers next want to better understand the mechanisms for amplification and use that info to develop practical devices. Reference: S. Stepanov and S. Ruschin, arXiv:1611.03475v1 [physics.optics] (Nov. 6, 2016).
