An Electro-Optic Modulator (EOM) based on a racetrack resonator coupled to a waveguide using butterfly Multi-Mode Interference (MMI) coupler is fabricated on Y-cut Lithium Niobate (LN) thin film. This is the first demonstration of a LN EOM in which the thin film of LN is etched in a Y-cut substrate using chlorine-based Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), a process, which is readily compatible with semiconductor fabrication facility. The Y-cut LNOI platform is interesting for the integration of electro-optic and acousto-optic components, since differently from any other LN cut it facilitates taking advantage of the maximum electro-optic and piezoelectric coefficients of LN. Coupling to the racetrack was enabled using a butterfly MMI coupler, which offered operation near the critical coupling condition, hence increasing the extinction ratio (ER) of the modulator. An unloaded quality factor of 1.3x105 was extracted for this device, which is equivalent to a propagation loss of 2.3 dB/cm. Modulation bandwidth of 4 GHz, wavelength tuning rate of 0.32 pm/V and an ER of more than 10 dB were experimentally measured for the EOM.

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 oxide layer thickness from 1 µm to 2 µm, we were able to reduce propagation loss of 2 µm (1 µm) wide waveguide from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm). Resonators with a quality factor greater than 1 million were demonstrated as well. 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 also, most importantly, offers experimental evidence of 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.

We report a vertical chirped grating coupler with high coupling efficiency (CE) fabricated on lithium niobate on insulator (LNOI). According to finite difference time domain simulations, loss in a uniform grating in LNOI comes from downward diffracted light, internal reflections, and mode mismatch between the single-mode fiber and the out-coupled light from the grating. A chirped grating coupler, whose parameters were optimized to reduce the mode mismatch loss and internal reflections, was fabricated with integrated waveguides through a single etch-step and exhibited insertion losses as low as 3.6 dB, 1.9 dB lower than a uniform grating fabricated in the same process. The demonstrated grating features high CE, ease of manufacturing, and the ability to vertically couple light—a fundamental characteristic required to facilitate the integration of on-chip laser sources and detectors.

An acousto-Optic Modulator (AOM) formed by an Arsenic Trisulfide ( $\text{As}_2 \text{S}_3$ ) Mach-Zehnder interferometer (MZI), operating in a push-pull configuration and placed inside a surface acoustic wave (SAW) cavity on a Y-cut Lithium Niobate (LN) wafer is demonstrated. This is the first demonstration of such acousto-optic (AO) modulator in this $\text{As}_2 \text{S}_3$ -LN hybrid platform. In this approach, the high index contrast of $\text{As}_2 \text{S}_3$ waveguides is exploited to attain a high optical confinement. Additionally, the placement of the photonic MZI inside the SAW cavity enhances the AO interaction due to the high quality factor (Q) of the SAW resonator. An analytical expression that describes such enhancement as a function of the elasto-optic (EO) coefficient of $\text{As}_2 \text{S}_3$ is derived in conjunction with COMSOL finite element methods to describe strain in the SAW cavity. By fitting this expression to experimental data, the EO coefficient ( $p_{11}$ and $p_{12}$ , where $p_{11} \approx p_{12}$ ) for $\text{As}_2 \text{S}_3$ is extracted to be 0.29 at the wavelength of 1550 nm. This is the first time that the EO coefficient of thin film $\text{As}_2 \text{S}_3$ is experimentally derived.

Lithium niobate thin film becomes a promising material platform for integrated nonlinear optics. Quasi-phase matching is an important way to realize optical frequency conversion. Due to the strong dispersion of the high-refractive-index-contrast lithium niobate thin film waveguides, a small poling period (several micrometers) is needed, which becomes a challenge for the poling process. In this paper, a periodic reversed domain with a period of 3.8 µm was realized by poling with an external electric field in the x-cut lithium niobate thin film. The reversed domains were characterized by piezoresponse force microscopy and the confocal Raman spectroscopy. The second harmonic generation was realized in the ridge waveguides, and the normalized conversion efficiency was 684%W⁻¹cm⁻². The waveguide loss, the irregularity of the reversed domain, and the inhomogeneities effective index along the waveguide were analyzed to be the main sources of the conversion efficiency decrease. The simulations showed that the phase matching was very sensitive to waveguide geometry. This work provided useful information for studying second harmonic generation in the lithium niobate on insulator.