Two materials, one photonic chip: Unlocking a new way to generate light frequencies
Modern photonic chips can pack sophisticated optical functions onto devices smaller than a fingernail. They are increasingly used to generate, manipulate and measure light for applications ranging from communications to sensing. But most of these chips rely on a single material to do the heavy lifting, limiting the range of optical effects they can produce.
Researchers have developed a novel photonic chip capable of generating a broad spectrum of light frequencies through the synergistic action of two distinct materials. This breakthrough, published in Advanced Photonics, challenges the prevailing notion that a single material is sufficient for all optical functions in integrated photonic devices.
The key innovation lies in the integration of a silicon nitride core with a surrounding silica layer. While the silicon nitride excels at generating optical frequency combs—a series of precisely spaced light frequencies—silica is renowned for its strong Raman gain capability. Traditionally, silica has been used as a passive component to confine light within the photonic circuit, but this new approach harnesses its active properties.
By designing the device such that approximately 31% of the circulating optical field extends into the silica cladding, the researchers enabled a unique interaction between the two materials. This interaction gave rise to Raman lasing—a phenomenon where light interacts with molecular vibrations in silica to produce light at a different frequency. The researchers confirmed this effect by observing a 11 terahertz frequency shift in the emitted light, directly attributable to Raman scattering.
The experiments revealed a progression of optical behavior within the device. Initially, the silicon nitride core produced Raman laser sidebands, but as the input power increased, a nonlinear process called four-wave mixing became dominant. This led to the generation of additional frequencies around the original laser wavelength and the Raman-generated light, culminating in the formation of optical frequency combs.
Further optimization of the device dimensions allowed for enhanced interaction between different wavelengths, resulting in frequency combs spanning over 400 nanometers. The generated combs exhibited a power conversion efficiency of more than 32%, indicating a significant portion of incoming light was transformed into new frequencies. This efficiency suggests that the Raman effect was originating in the silica cladding, as predicted by theoretical calculations.
This pioneering work demonstrates that previously disregarded components, such as the silica cladding, can actively contribute to the optical functionality of photonic chips. By combining materials with complementary nonlinear properties, researchers can unlock new capabilities in integrated photonic circuits, such as broadband supercontinuum sources and self-referenced frequency combs.
The study opens up possibilities for designing future photonic devices that integrate multiple materials, each tailored for specific optical functions, thereby expanding the potential applications of integrated photonics.
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