arXiv subjects

Lutong Cai

Publications and source records attributed to Lutong Cai.

5 recordsLinked to original sources

arXiv2024

Demonstration of 4H silicon carbide on aluminum nitride integrated photonics platform

The existing silicon-carbide-on-insulator photonic platform utilizes a thin layer of silicon dioxide under silicon carbide to provide optical confinement and mode isolation. Here, we replace the underneath silicon dioxide layer with a 1-$μ$m-thick aluminum nitride and demonstrate a 4H-silicon-carbide-on-aluminum-nitride integrated photonics platform for the first time. Efficient grating couplers, low-loss waveguides, and compact microring resonators with intrinsic quality factors up to 210,000 are fabricated. In addition, by undercutting the aluminum nitride layer, the intrinsic quality factor of the silicon carbide microring is improved by nearly one order of magnitude (1.8 million). Finally, an optical pump-probe method is developed to measure the thermal conductivity of the aluminum nitride layer, which is estimated to be over 30 times of that of silicon dioxide.

physics.optics↗

arXiv2022

Investigation of the electro-optic effect in high-$Q$ 4H-SiC microresonators

Silicon carbide (SiC) recently emerged as a promising photonic and quantum material owing to its unique material properties. Here, we carry out an exploratory investigation of the electro-optic effect in high-quality-factor 4H-SiC microresonators. Our findings confirm the existence of the Pockels effect in 4H-SiC for the first time. The extracted Pockels coefficients show certain variations among 4H-SiC wafers from different manufacturers, with the magnitudes of $r_{13}$ and $r_{33}$ estimated to be in the range of (0.3-0.7) pm/V and (0-0.03) pm/V, respectively.

physics.optics↗

arXiv2022

Measurement of the Kerr nonlinear refractive index and its variation among 4H-SiC wafers

The unique material property of silicon carbide (SiC) and the recent demonstration of low-loss SiC-on-insulator integrated photonics platform have attracted considerable research interests for chip-scale photonic and quantum applications. Here, we carry out a thorough investigation of the Kerr nonlinearity among 4H-SiC wafers from several major wafer manufacturers, and reveal for the first time that their Kerr nonlinear refractive index can be significantly different. By eliminating various measurement errors in the four-wave mixing experiment and improving the theoretical modeling for high-index-contrast waveguides, the best Kerr nonlinear refractive index of 4H-SiC wafers is estimated to be approximately four times of that of stoichiometric silicon nitride in the telecommunication band. In addition, experimental evidence is developed that the Kerr nonlinearity in 4H-SiC wafers can be stronger along the c-axis than that in the orthogonal direction, a feature that was never reported before.

physics.optics↗

arXiv2021

Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform

Silicon carbide has recently emerged as a promising photonics material due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized for a wealth of quantum applications. Here, we report the design and demonstration of octave-spanning microcombs in a 4H-silicon-carbide-on-insulator platform for the first time. Such broadband operation is enabled by optimized nanofabrication achieving >1 million intrinsic quality factors in a 36-$μ$m-radius microring resonator, and careful dispersion engineering by investigating the dispersion properties of different mode families. For example, for the fundamental transverse-electric mode whose dispersion can be tailored by simply varying the microring waveguide width, we realized a microcomb spectrum covering the wavelength range from 1100 nm to 2400 nm with an on-chip power near 120 mW. While the observed comb state is verified to be chaotic and not soliton, attaining such a large bandwidth is a crucial step towards realizing $f$-2$f$ self-referencing. In addition, we have also observed coherent soliton-crystal state for the fundamental transverse-magnetic mode, which exhibits stronger dispersion than the fundamental transverse-electric mode and hence a narrower bandwidth.

physics.optics↗

arXiv2019

Low-loss waveguides on Y-cut thin film lithium niobate: towards acousto-optic applications

We investigate the dependence of photonic waveguide propagation loss on the thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify trade-offs between optical losses and electromechanical coupling of surface acoustic wave (SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW device. Optical racetrack resonators with different lengths were fabricated by argon plasma etching to experimentally extract waveguide losses. By increasing the thickness of the oxide layer from 1 μm to 2 μm, propagation loss of 2 μm (1 μm) wide waveguide was reduced from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm), and, resonators with quality factor greater than 1 million were demonstrated. An oxide thickness of approximately 1.5 μm is sufficient to significantly reduce propagation loss due to leakage into the substrate and simultaneously attain good electromechanical coupling in acoustic devices. This work not only provides insights on the design and realization of low-loss photonic waveguides in lithium niobate, but most importantly offers experimental evidence on how the oxide thickness directly impacts losses and guides its selection for the synthesis of high-performance acousto-optic devices in Y-cut lithium niobate on insulator.

physics.app-ph↗