Key to the achievement is a parametric amplifier architecture using a silicon nitride waveguide with far higher silicon content than usual – Si7N3.
“We have developed an optical amplifier which is able to amplify light by 17,000 times at the telecommunications wavelength,” said Assistant Professor Dawn Tan at Singapore University of Technology and Design, which worked with A*STAR and MIT.
The material has just the right band gap (2.0eV, corresponding to 3.1 refractive index) to give high non-linearity for mixing gain without promoting loss though the two-photon absorption (starts <1.7eV) which has made other attempts inefficient, said the researchers in Pushing the limits of CMOS optical parametric amplifiers with USRN:Si7N3 above the two-photon absorption edge, a Nature Communications paper. Three photon absorption was also not observed.
“The inefficiencies in highly non-linear photonic devices are overcome here, by engineering for maximum non-linearity, while still maintaining a sufficiently large bandgap to eliminate two-photon absorption. We believe this is one of the highest gains demonstrated at the telecommunications wavelength to date on a CMOS chip.” said Tan.
Cascaded four mixing using optical pumping was used to exploit the non-linearity – higher order mixing of the amplified and converted photons in a way that allows the amplifier to operate as a tunable broadband light source – therefore this device is not a demonstration of electrical pumping.
The non-linear waveguides are 550nm wide and 7mm long, hewed from a 300nm thick layer of Si7N3 deposited by inductively-coupled plasma CVD at 250°C – and therefore compatible with back-end CMOS processing.
Starting gasses were silane and nitrogen – ammonia was avoided during CVD as residual N-H bonds would absorb at 1,550nm.