Xiao-Hui Tian

Thin‐film lithium niobate (TFLN) has emerged as a powerful platform for integrated nonlinear optics owing to its large χ ⁽²⁾ nonlinearity, tight confinement and flexible tunability. To fully excavate such superior nonlinear optical properties, domain engineering is commonly adopted to fulfill the phase matching condition of χ ⁽²⁾ processes. During the past decade, various domain engineered TFLN nonlinear optical devices have been demonstrated, showing extremely high length‐normalized nonlinear optical conversion efficiencies. However, application‐driven scenarios demand absolute energy conversion in nonlinear frequency conversion rather than length‐normalized efficiencies, but the progress has been limited by imperfect fabrication processes. In this work, we realize effective on‐chip nonlinear energy conversion by developing low‐loss and high‐quality domain engineered TFLN waveguides with long interaction length. Ion beam trimming (IBT) technique and an etching‐prior‐poling workflow are adopted for such fabrication. Optical characterization yields an overall second‐harmonic generation (SHG) efficiency of 2,590 %/W. A high pump depletion of 85.7 % is demonstrated under continuous‐wave operation, which directly reflects strong nonlinear energy conversion. These results may lead to breakthroughs in applications like classical optical frequency conversion, quantum frequency conversion, and quantum light generation.

Lithium-niobate-on-insulator (LNOI) chips have shown outstanding performance in various photonic devices including modulators, lasers, nonlinear converters, and quantum sources. LNOI-based edge couplers are quite important for further promotion of the above devices in practical applications, especially for large-scale multiport photonic uses, where efficient and mode-selective coupling between chips and fibers is of necessity. Previously, several LNOI edge couplers have been demonstrated, but they mainly focus on achieving high coupling efficiency of the fundamental mode, and sub-wavelength etched lithium-niobate (LN) structures are normally needed, which increases fabrication complexity. Here we propose a new type of edge coupler with direct mode-selective excitation ability, using only SiON cladding grating structures without additional etching of LN. By introducing a cladding waveguide with periodic structures on the uniform LNOI waveguide, high-efficiency excitation of multiple modes can be realized directly with easier fabrication. For a specific simulation here, TE₀₀, TM₀₀, and TE₁₀ core modes can be excited, respectively, at optimized periods and grating lengths with a tunable central wavelength, at the launch of the TM cladding mode. The periods of the needed SiON gratings are all over 2 µm, which is feasible with i-line UV lithography. Our results provide a low-cost edge coupler for LNOI photonic circuits with the ability of flexible spatial mode selectivity, which may promote LNOI devices in large-scale multiport photonic integrated circuits in the future.

LNOI devices have emerged as prominent contributors to photonic integrated circuits (PICs), benefiting from their outstanding performance in electro-optics, acousto-optics, nonlinear optics, etc. Due to the physical properties and current etching technologies of LiNbO₃, slanted sidewalls are typically formed in LNOI waveguides, causing polarization-related mode hybridization and crosstalk. Despite the low losses achieved with fabrication advancements in LNOI, such mode hybridization and crosstalk still significantly limit the device performance by introducing polarization-related losses. In this paper, we propose a vertically etched LNOI construction. By improving the geometrical symmetry in the waveguides, vertical sidewalls could adequately mitigate mode hybridization in common waveguide cross sections. Taking tapers and bends as representatives of PIC components, we then conducted theoretical modeling and simulations, which showed that vertical etching effectively exempts devices from polarization-related mode crosstalk. This not only improves the polarization purity and input mode transmittance but also enables lower polarization-related losses within more compact structures. As a demonstration of fabrication feasibility, we innovatively proposed a two-step fabrication technique, and successfully fabricated waveguides with vertical sidewalls. Such vertical etching technology facilitates the development of next-generation high-speed modulators, nonlinear optical devices, and other advanced photonic devices with lower losses and a smaller footprint, driving further innovations in both academic research and industrial applications.

Periodically poled lithium niobate on insulator (PPLNOI) offers an admirably promising platform for the advancement of nonlinear photonic integrated circuits (PICs). In this context, domain inversion engineering emerges as a key process to achieve efficient nonlinear conversion. However, periodic poling processing of thin-film lithium niobate has only been realized on the chip level, which significantly limits its applications in large-scale nonlinear photonic systems that necessitate the integration of multiple nonlinear components on a single chip with uniform performances. Here, we demonstrate a wafer-scale periodic poling technique on a 4-inch LNOI wafer with high fidelity. The reversal lengths span from 0.5 to 10.17 mm, encompassing an area of ~1 cm² with periods ranging from 4.38 to 5.51 μm. Efficient poling was achieved with a single manipulation, benefiting from the targeted grouped electrode pads and adaptable comb line widths in our experiment. As a result, domain inversion is ultimately implemented across the entire wafer with a 100% success rate and 98% high-quality rate on average, showcasing high throughput and stability, which is fundamentally scalable and highly cost-effective in contrast to traditional size-restricted chiplet-level poling. Our study holds significant promise to dramatically promote ultra-high performance to a broad spectrum of applications, including optical communications, photonic neural networks, and quantum photonics.

The optical frequency comb based on microresonators (microcombs) is an integrated coherent light source and has the potential to promise a high-precision frequency standard; self-reference and a long-term stable microcomb are the keys to this realization. Here, we demonstrated a 0.7-octave spectrum Kerr comb via dispersion engineering in a thin-film lithium niobate microresonator, and the single-soliton state can be accessed passively with long-term stability over 3 h. With such a robust broadband coherent comb source using thin-film lithium niobate, a fully stabilized microcomb can be expected for massive practical applications.

Optical frequency comb based on microresonator (microcomb) is an integrated coherent light source and has the potential to promise a high-precision frequency standard, and self-reference and long-term stable microcomb is the key to this realization. Here, we demonstrated a 0.7-octave spectrum Kerr comb via dispersion engineering in a thin film lithium niobate microresonator, and the single soliton state can be accessed passively with long-term stability over 3 hours. With such a robust broadband coherent comb source using thin film lithium niobate, fully stabilized microcomb can be expected for massive practical applications.