Researchers Trap Light and Sound on a Chip, Adding to Photonic Toolbox

October 18, 2022 by Darshil Patel

Researchers have developed a new technique to trap light and sound in waveguides to manipulate light in large-scale circuits.

Quantum photonics is considered a next-generation technology that could surpass the limits of current semiconductor technologies—particularly by proving more efficient than classical processors for particular tasks. However, a major challenge is making a scalable, integrated platform to deploy and commercialize.

One of the drawbacks of quantum photonics is that small interfaces and quantum effects could distort data signals and make them unusable. Because they use light instead of electrons, quantum applications also require new technologies for filtering, amplifying, and conditioning optical signals.


Multilayer silicon nitride waveguides

Depiction of how University of Twente researchers trapped light and soundwaves in multilayer silicon nitride waveguides. Image used courtesy of the University of Twente


Researchers at the University of Twente recently developed a new technique to confine sound and light waves in a multilayer silicon nitride waveguide. Their solution can alter optical signals with sound in large-scale integrated circuits. The device is also compatible with current production methods. 


Stimulated Brillouin Scattering

Processing, amplifying, and filtering optical signals are at the heart of quantum photonics. One method to do this effectively is a coherent optomechanical interaction technique called stimulated Brillouin scattering. Brillouin scattering occurs when optical waves interact with acoustic lattice vibrations or phonons.

When a photon interacts with a substance, energy transfers to acoustic phonons. The incident photon scatters to slightly lower energy and propagates in the opposite direction. This scattering of photons by acoustic phonons is called Brillouin scattering.


Silicon nitride photonic chip

Silicon nitride photonic chip. Image used courtesy of the University of Twente


Maximum scattering occurs when the reflected light overlaps in phase with sound waves. Therefore, the scattering is highly frequency selective, and the reflected wave reduces in frequency by about 1–15 GHz due to the Doppler shift. This effect plays an important role in optical amplifiers and filters.

This scattering is achieved by using two finely-tuned lasers to generate a soundwave and trap it in a waveguide. Light passing through the waveguide interacts with the soundwave and undergoes scattering and Doppler shift, effectively filtering the signal.

For such applications, silicon nitride waveguides are preferred due to their nonlinear refractive index and CMOS compatibility.


Silicon Nitride Waveguides

Silicon nitride waveguides are a non-linear high refractive index and low-loss semiconductor, which is advantageous for various non-linear optical applications. The waveguides consist of a bottom reflector stacked on a silicon nitride platform and clad with an oxide buffer layer underneath. These layers are fabricated with an LCVD (laser chemical vapor deposition) device and can be patterned to a specific shape, making it CMOS-compatible.

The high refractive index of the dielectric material allows high optical confinement and control over dispersion with the waveguide structure. Despite better mode confinement, silicon nitride is less expensive than its counterparts. Therefore, this material has emerged as a workhorse for nonlinear optics in recent years.

Previous studies have suggested that implementing stimulated Brillouin scattering in silicon nitride can unlock promising new technologies like on-chip amplifiers and lasers. However, the demonstration is usually plagued by acoustic leakage from the silicon nitride core and the oxide cladding. This effect prevents strong Brillouin interactions and makes these waveguides ineffective for trapping sound waves.


Trapping Sound and Light with Multilayer Silicon Nitride Waveguides

Researchers at the University of Twente used low-loss multilayer silicon nitride nano-photonic circuits consisting of 50 cm-long spiral waveguides to confine optical and sound waves. This setup prevented acoustic leakage from occurring in a single silicon nitride core. The researchers precisely controlled the separation between silicon nitride layers, altering the acoustic waveguides and inhibiting scattering in these waveguides.


Guided scattering in multilayer silicon nitride waveguide

Guided scattering in multilayer silicon nitride waveguide. Image used courtesy of the University of Twente


The researchers showed that multilayer silicon nitride waveguides enable circuit-level stimulated Brillouin scattering. They also prepared a working proof concept in radio frequency (RF) signal processing through a notch filter with a high rejection of 66 dB using only 0.4 dB of stimulated gain.


Microwave notch filter in a silicon nitride waveguide

Microwave notch filter in a silicon nitride waveguide. Image used courtesy of the University of Twente


Professor David Marpaung, who leads the nonlinear nanophotonics research group, claimed that the new chips may be integrated with tunable lasers, programmable photonic circuits, and frequency combs, giving them an important role to play in evolving fields like quantum computing and telecommunications.