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Silicon Carbide Light Source Exhibits Expansive Wavelength Range

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A research team from Carnegie Mellon University has demonstrated a chip-scale broadband light source in silicon carbide. The tiny optical devices are less than 0.1 mm in size and based on optical comb technology.

Such broadband light sources are more typically demonstrated in optical fibers and/or nonlinear crystals.

Silicon carbide, which has traditionally been used as an abrasive or for electronic devices that operate at high temperatures or high voltages, has recently emerged as a promising material due to its unique properties. The material possesses strong second- and third-order nonlinear coefficients and hosts various color centers that can be used for a wealth of quantum applications.
(a) Schematic top view (left) and cross section (right) of the 4H-silicon-carbide-on-insulator platform for frequency comb generation based on compact microring resonators. The sidewall angle (?) is estimated near 80–85 deg in our nanofabrication. Dispersion engineering is carried out by varying the ring waveguide width (RW). In addition, efficient coupling is realized using the pulley structure where the access waveguide width and coupling length are adjusted to achieve phase matching to the desired resonant mode families. (b) Scanning electron micrograph of a 36-µm-radius SiC microring. In this work, the SiC thickness is fixed at 500 nm with a pedestal layer of 50 nm. (c) Simulated integrated dispersion [Dint; see its definition in Eq. (1)] for the fundamental transverse-electric (i.e., TE00) mode with ring widths varied from 1.6 µm to 1.9 µm (solid lines). For comparison, Dint for the fundamental transverse-magnetic (i.e., TM00) mode is also provided for the ring width of 1.9 µm (magenta dashed line), exhibiting much larger values than those of the TE00 modes. (d) Simulated comb spectrum for the TE00 mode for a 36-µm-radius SiC microring with RW 1.8 µm and an input power of 100 mW, featuring a spectral bandwidth of more than one octave and dispersive-wave generation near the wavelength of 1150 nm. In the simulation, the Kerr nonlinear parameter is assumed to be ? ˜ 2.1 W-1 m-1, and the intrinsic and loaded quality factors are assumed to be 1.25 million and 0.75 million, respectively. Courtesy of Photonics Research.
(a) Schematic top view (left) and cross section (right) of the 4H-silicon-carbide-on-insulator platform for frequency comb generation based on compact micro-ring resonators. The sidewall angle (θ) is estimated near 80° to 85° in the team's nanofabrication. (b) Scanning electron micrograph of a 36-μm-radius SiC micro-ring. The SiC thickness is fixed at 500 nm with a pedestal layer of 50 nm. (c) Simulated integrated dispersion. (d) Simulated comb spectrum for the TE00 mode for a 36-μm-radius SiC micro-ring with RW 1.8 μm and an input power of 100 mW, featuring a spectral bandwidth of more than one octave and dispersive-wave generation near the wavelength of 1150 nm. Courtesy of Photonics Research
For many metrology and timekeeping-related applications, a comb bandwidth of the order of one octave more is necessary. The polytype of silicon carbide that the researchers investigated, 4H-SiC, has a strong second-order nonlinearity that can be used to double the frequency through second-harmonic generation.

“The fabrication process has been carefully optimized to define tiny ring-type structures with smooth surfaces,” said Qing Li, assistant professor of electrical and computer engineering. “Light confined in these so-called micro-rings experiences significant power enhancement due to the resonance effect, which is strong enough to induce an optical nonlinear response called the Kerr effect.”

As a result, with an input laser being a single wavelength near 1550 nm, the output light consists of multiple wavelengths spanning 1150 to 2400 nm. Because the composite frequency lines are discrete and equally spaced, this special type of light is termed as an optical comb.

“This work is the first demonstration of such wideband combs in silicon carbide, proving its competitiveness as an efficient nonlinear material against more commons choices such as silicon and silicon nitride,” Li said. “Our goal is to develop a series of device technologies, including frequency generation, modulation, and conversion, and combine them with the quantum technologies implemented in the silicon carbide platform in a seamless fashion.” 

The research was published in Photonics Research (www.doi.org/10.1364/PRJ.449267).

Photonics Spectra
Jun 2022
GLOSSARY
chip
1. A localized fracture at the end of a cleaved optical fiber or on a glass surface. 2. An integrated circuit.
kerr effect
A quadratic nonlinear electro-optic effect found in particular liquids and crystals that are capable of advancing or retarding the phase of the induced ordinary ray relative to the extraordinary ray when an electric current is applied. It varies as the square of the voltage.
metrology
The science of measurement, particularly of lengths and angles.
Research & Technologyfrequency comboptical frequency combsilicon carbidechiplaserslight sourcesopticsCarnegie Mellon Universityphotonics researchAmericasKerr effectmetrologyprecision timekeepingtimekeepingmicro-ringTechnology News

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