SUTD, A*STAR IME Researchers Achieve Wafer-Scale Broadband Light Generation via Heavier Isotope

A research team in Singapore has developed a silicon nitride waveguide that can transform a narrow infrared laser pulse into an exceptionally broad spectrum of light on a chip. The device produces a supercontinuum extending from visible red wavelengths deep into the infrared, while being fabricated across a full eight-inch wafer using a process compatible with modern semiconductor manufacturing. The advance could help bring broadband light sources—now commonly produced with bulky optical fibre systems—into compact platforms for medical imaging, precision measurement, optical communications and frequency-comb technologies.

The work was led by Associate Professor Dawn Tan of the Singapore University of Technology and Design and Dr Luo Xianshu, Head of the Silicon Photonics Department at the ASTAR Institute of Microelectronics. Their team combined nonlinear photonics design with wafer-scale fabrication to address one of the central challenges in integrated optics: producing a thick, low-loss silicon nitride film without exposing the material or surrounding electronics to extreme processing conditions. The results, published in Optics Express*, demonstrate an octave-spanning supercontinuum generated in a deuterated silicon nitride waveguide.

Supercontinuum light is created when an intense, ultrashort laser pulse undergoes nonlinear interactions as it travels through a carefully engineered optical medium. Instead of remaining concentrated around a single wavelength, the pulse broadens dramatically, generating many new frequencies. The resulting spectrum can cover a large fraction of the electromagnetic region, behaving almost like a lamp with the brightness and timing precision of a laser. Such sources are valuable in optical coherence tomography, spectroscopy, frequency metrology, environmental sensing and optical clocks, where access to many precisely related wavelengths can reveal information that a single-colour laser cannot provide.

Most existing supercontinuum systems rely on specially designed optical fibres. Although fibre can produce broad spectra efficiently, the equipment generally requires substantial space, careful alignment and relatively high optical power. Integrating the same capability onto a semiconductor chip would make broadband sources easier to combine with lasers, modulators, detectors and electronic control circuits. Silicon nitride is widely regarded as one of the most promising materials for this purpose because it offers low optical loss, a broad transparency window and strong nonlinear optical behaviour. However, manufacturing high-quality, thick silicon nitride layers has traditionally required temperatures and processing steps that are difficult to reconcile with standard chip fabrication.

A major obstacle is hydrogen. Conventional silicon nitride films are often deposited using silane-based chemistry, which leaves silicon–hydrogen bonds in the material. These bonds absorb light strongly near important telecommunications wavelengths, limiting the distance over which light can travel through the waveguide. Manufacturers can reduce the hydrogen-related absorption through prolonged annealing at temperatures reaching approximately 1,200 degrees Celsius. That treatment can also generate mechanical stress in thick films, causing them to crack, while the temperature is too high for many completed electronic or photonic circuits.

The Singapore researchers addressed the problem by replacing hydrogen with deuterium, its heavier isotope. Deuterium forms chemical bonds with silicon that vibrate at lower frequencies than silicon–hydrogen bonds. This shifts the associated absorption feature away from the telecommunications region and toward approximately 2.1 micrometres. By moving the absorption rather than removing the bonds through extreme heating, the team was able to avoid the conventional high-temperature annealing step. The chemical substitution allowed the researchers to fabricate a low-loss waveguide using a comparatively low-temperature process, opening a route toward greater compatibility with established semiconductor production.

The team deposited an 800-nanometre-thick deuterated silicon nitride film in a single step at temperatures below 400 degrees Celsius across an eight-inch wafer. The resulting waveguides exhibited propagation losses of only 0.54 decibels per centimetre, meaning that relatively little optical power was lost as light moved through the structure. A fabricated device measuring 5.21 centimetres provided enough length for intense nonlinear interactions to develop. The waveguide itself was arranged in a spiral layout, allowing a long optical path to fit within a compact chip area; one microscope image showed a 1.51-centimetre-long spiral waveguide with a bending radius of 100 micrometres.

To test the device, the researchers launched infrared pulses lasting only 500 femtoseconds, or 500 quadrillionths of a second, into the waveguide. The input was centred near 1,555 nanometres, a wavelength widely used in fibre-optic telecommunications. As the pulse propagated through the silicon nitride, nonlinear effects broadened its spectrum from the original narrow infrared line to wavelengths ranging from 587 to 1,883 nanometres. The output therefore covered about 1.7 octaves, reaching into the visible red portion of the spectrum. The expansion occurs because the intense pulse changes the optical properties of the material and interacts with itself, producing new frequencies through effects including self-phase modulation, dispersive wave generation and other nonlinear processes.

The experiment also examined whether the broadened light remained coherent, a critical requirement for precision applications. A spectrum can be wide yet unsuitable for measurement if its phase and intensity fluctuate unpredictably from pulse to pulse. At moderate pulse energies, the researchers measured an overall spectral coherence greater than 0.81, indicating that much of the generated light remained stable and reproducible. At higher energies, the spectrum became broader but less coherent as modulation instability amplified fluctuations within the pulse. Simulations suggested that the first centimetre of propagation preserved high coherence before noise became more influential, pointing to a practical design strategy: future devices may be shortened or geometrically optimised to capture a broad, clean spectrum before instability takes hold.

The researchers now aim to improve the uniformity and bandwidth of the output at higher power and to integrate the waveguide with other photonic components. A chip combining the supercontinuum source with modulators, detectors and control electronics could shrink systems that currently occupy laboratory benches into compact modules suitable for deployment. Because the deuterated silicon nitride process operates below 400 degrees Celsius and can be carried out on a full wafer, it may also support more economical production than techniques requiring specialised, high-temperature fabrication. The work does not immediately replace fibre-based supercontinuum sources, but it demonstrates a significant step toward manufacturable, CMOS-compatible broadband photonics—bringing laser-like white light closer to everyday chips used in imaging, communications and precision sensing.

Web References: https://doi.org/10.1364/OE.588127

References: Optics Express, “Octave-spanning supercontinuum generation in a wafer-scale, low loss deuterated silicon nitride waveguide,” DOI: 10.1364/OE.588127

Keywords

Photonics, silicon nitride, deuterated silicon nitride, supercontinuum generation, nonlinear optics, silicon photonics, nanophotonics, optoelectronics, wafer-scale fabrication, integrated photonics, frequency combs, optical communications