Feng Gao

By employing the tip-field-induced domain direct-writing technique based on atomic force microscopy, periodically poled lithium niobate (PPLN) with a periodicity of 40 nm was successfully fabricated in a 30 nm-thick, z-cut congruent lithium niobate thin film on silicon substrate. Simultaneously, ferroelectric dot-domain arrays with a lateral dimension of a single dot domain of ∼5 nm and a minimum dot-to-dot domain spacing of ∼26 nm were also achieved, demonstrating an information-storage density of 3.7 Tbit/in2. In addition, in MgO-doped lithium niobate thin films, PPLN with a period of 50 nm and dot domains with a lateral dimension less than 40 nm were obtained. These achievements effectively facilitate the device miniaturization and performance improvement for domain-based applications such as ferroelectric domain memories in lithium niobate.

This paper presents a narrow‐linewidth, self‐wavelength sweeping laser that utilizes the thermal effect within a silica microbubble resonator. By utilizing the Fano lineshape spectrum (FLSS) of the resonator as an intracavity bandpass filter and a thermal ‐switch, the laser attained a linewidth of 8.03 kHz and an average sweeping frequency of 833 Hz using the FLSS filter with a positive slope ratio, while it obtained a linewidth of 13.87 kHz and an average sweeping frequency of 588 Hz using the filter with a negative slope ratio. The average sweeping frequency can be adjusted by tuning the coupling gap between the resonator and the tapered fiber, with a wavelength sweeping range up to 1.1 GHz. As an application example, the proposed laser was utilized to obtain a whispering gallery mode (WGM) spectrum of a microsphere. This work provides a new paradigm for leveraging the thermal effect in WGM resonators, resulting in a laser that uniquely combines narrow linewidth with self‐sweeping capability, which is extremely suitable for high‐resolution, high‐speed optical sensing applications and paving the way for dynamic photonic systems.

Fiber-to-chip coupling with ultralow loss and broadband operation wavelength range is essential in the practical applications of thin-film lithium niobate (TFLN) integrated photonic devices. However, the existing edge couplers often require electron beam lithography overlaying and multiple etching processes, which are expensive and complex. In this Letter, we demonstrate an edge coupler that includes only a vertically tapered TFLN waveguide and silicon dioxide cladding. The coupling efficiency between a lensed fiber and an on-chip LN waveguide is down to 1.43 dB/facet, while the 3-dB bandwidth exceeds the range from 1510 to 1630 nm. These edge couplers also show reliable fiber misalignment tolerance. Furthermore, the fabrication complexity is greatly reduced since only a single etching step for the TFLN waveguide is needed. The minimum waveguide width of 1.3 μm guarantees compatibility with i-line photolithography, providing a potential for massive production of TFLN devices.

Second-harmonic (SH) generation is an important way to generate short-wavelength light sources. Thin-film lithium niobate (TFLN) resonators and waveguides featuring tight light confinement are flexible to realize mode- or quasi-phase-matching (MPM or QPM) and thus efficient SH generation. Here, we report an efficient SH with an absolute conversion efficiency of 45% in a reverse-polarized double-layer TFLN waveguide. Such a high conversion efficiency without periodic poling was realized with the pump of 67 mW in the 1550 nm band, benefiting from the employment of the largest nonlinear coefficient d₃₃ and the significant nonlinear interaction of the fundamental transverse electric (TE) mode and the first-order TE mode in the vertical direction. This work paves the way toward on-chip integrated nonlinear optics in terms of harmonic generation and quantum light sources across the telecom and visible bands.

An add-drop filter (ADF) with configurable center wavelength and bandwidth was demonstrated in a tapered fiber (TF) coupled microsphere. The center wavelength was optimized utilizing the thermal effect by heating the microsphere with a control light injected from the handle of the microsphere and the bandwidth was tuned by changing of the coupling gap based on the ideal quasi-critical coupling (QCC). In the experiment, the center wavelength could be optimized from 1551.04 nm to 1551.13 nm (over 10 GHz) covering the range of X-band of microwave, with the control light power tuned between 7.41 mW to 8.51 mW. As a typical bandwidth tuning result, the band-pass peak could be tuned from 127.5 - 2529.7 MHz in the Drop port while the band-rejected notch could be tuned from 54.4 - 2681.6 MHz in the Through port, the coupling efficiency changing at the center wavelength of 1551.04 nm was less than 5%. The calculations and discussions on the range of the stable coupling efficiency when the center wavelength tuned were also presented. The demonstrated filter gains advantages of stable coupling efficiency and tunable narrow bandwidth, which will be helpful for its application in microwave photonics, narrow linewidth lasers, nonlinear optics and so on.

Resonator-enhanced electro-optical (EO) combs could generate a series of comb lines with high coherence and stability. Recently, EO comb based on thin-film lithium niobate (TFLN) has begun to show great potential thanks to the high second-order nonlinearity coefficient of lithium niobate crystal. Here we demonstrate that EO comb envelope engineering based on mode crossing induced a quality factor reduction in the TFLN racetrack microcavity both in the numerical simulation and experiment. Our method paves the way for the generation of EO combs with an arbitrary envelope.

In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have made substantial progress in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler facilitating the connection between PICs and light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride cladding. Experimental results show a low coupling loss between the TFLN PIC and a 3-μm mode field diameter (MFD) lensed fiber, measuring at 1.52 dB/facet, with theoretical potential for improvement to 0.43 dB/facet. Additionally, the coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 μm is reduced to 0.92 dB/facet. This design exhibits robust fabrication and alignment tolerances. Notably, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be readily achieved using commercially available i-line stepper lithography. This work benefits the development of TFLN integrated devices, such as on-chip electro-optic modulators, frequency combs, and lasers.

Erbium-ion-doped lithium niobate (LN) microcavity lasers working in the communication band have attracted extensive attention recently. However, their conversion efficiencies and laser thresholds still have significant room to improve. Here, we prepared microdisk cavities based on erbium–ytterbium-co-doped LN thin film by using ultraviolet lithography, argon ion etching, and a chemical–mechanical polishing process. Benefiting from the erbium–ytterbium co-doping-induced gain coefficient improvement, laser emission with an ultralow threshold (∼1 µW) and high conversion efficiency (1.8 × 10⁻³%) was observed in the fabricated microdisks under a 980-nm-band optical pump. This study provides an effective reference for improving the performance of LN thin-film lasers.

Microresonators coupled with integrated waveguides operate stably but usually lack tunability for an optimal coupling state. In this Letter, we demonstrate a racetrack resonator with an electrically modulated coupling on an X-cut lithium niobate (LN) platform by introducing a Mach–Zehnder interferometer (MZI) with two balanced directional couplers (DCs) to realize light exchange. This device provides a wide-range coupling regulation, from under-coupling and critical coupling to deep over-coupling. Importantly, it has a fixed resonance frequency when the DC splitting ratio is 3 dB. The measured optical responses of the resonator exhibit a high extinction ratio, exceeding 23 dB, and an effective half-wave voltage length V_π·L of 0.77 V·cm, suitable for CMOS compatibility. Microresonators with tunable coupling and a stable resonance frequency are expected to find application in nonlinear optical devices on LN-integrated optical platforms.

Lithium niobate (LN) thin film has recently emerged as an important platform for nonlinear optical investigations for its large χ⁽²⁾ nonlinear coefficients and ability of light localization. In this Letter, we report the first, to the best of our knowledge, fabrication of LN-on-insulator ridge waveguides with generalized quasiperiodic poled superlattices using the electric field polarization technique and microfabrication techniques. Benefiting from the abundant reciprocal vectors, we observed efficient second-harmonic and cascaded third-harmonic signals in the same device, with normalized conversion efficiency of 1735% W⁻¹ cm⁻² and 0.41% W⁻² cm⁻⁴, respectively. This work opens a new direction for nonlinear integrated photonics based on LN thin film.