Deuterated SiN drives supercontinuum
Researchers have developed a wafer-scale deuterated silicon nitride waveguide that generates broadband supercontinuum light using a low-temperature, CMOS-compatible fabrication process.
Researchers at the Singapore University of Technology and Design (SUTD) and A*STAR Institute of Microelectronics (A*STAR IME) have developed a deuterated silicon nitride (SiN) waveguide capable of generating broadband supercontinuum light directly on chip.
The wafer-scale platform replaces hydrogen in conventional SiN films with deuterium, reducing optical absorption and removing the need for high-temperature annealing typically used to eliminate silicon-hydrogen bonds.
Conventional SiN fabrication can leave Si-H bonds that absorb light around telecommunications wavelengths. Removing these bonds can require annealing at temperatures of up to 1200°C, which can exceed the thermal limits of integrated electronic circuitry and introduce stress into thick films.
The researchers instead deposited an 800-nm-thick deuterated SiN film at temperatures below 400°C in a single deposition step across an 8-inch wafer. The process shifts the principal absorption peak towards approximately 2.1 µm, enabling lower-loss operation at telecommunications wavelengths.
The resulting waveguides demonstrated propagation losses of 0.54 dB/cm. A 5.21-cm-long waveguide was sufficient to support the nonlinear interactions required for broadband spectral generation.
When 500-fs infrared pulses at 1555 nm were coupled into the waveguide, the generated spectrum extended from 587 to 1883 nm at the -30 dB level. This corresponds to a bandwidth of approximately 1.7 octaves, extending the output from the infrared into the visible red region.
The team also investigated the coherence of the generated supercontinuum, which is important for applications including precision metrology and optical frequency combs. At moderate pulse energies, the measured spectral coherence exceeded 0.81, indicating stable pulse-to-pulse operation.
At higher pulse energies, modulation instability caused the spectrum to become broader but less coherent. Simulations indicated that optimising the waveguide length could help capture a broad spectrum before noise develops during propagation.
The researchers plan to refine the waveguide geometry and length to improve spectral uniformity and bandwidth at higher power levels. Future work will also focus on integrating the waveguides with modulators, detectors and other photonic components on a single chip.
The combination of wafer-scale fabrication, low optical loss and CMOS-compatible processing could support integrated supercontinuum sources for optical communications, medical imaging, precision measurement and frequency-comb applications.
