Fig. 1. LN microresonator with high Q factor fabricated by the femtosecond laser writing combined with focused ion beam milling^[28-29]. (a) Procedures of fabrication of an LN microresonator; (b) SEM image of a cylindrical post formed after femtosecond laser ablation; (c) SEM image of a cylindrical post formed after the FIB milling; (d) SEM image (top view) of the 55-μm diameter microresonator after the chemical etching and high temperature annealing, in which the inset is side view of the microresonator; (e) measured transmission spectrum (dotted line) and its Lorentzian fitting curve (solid line) around the resonant wavelength of 1553.83 nm

Fig. 2. Integrated photonic structures fabricated by the femtosecond laser writing combined with focused ion beam milling. (a)-(c) Monolithic integration of an LN microdisk with a free-standing waveguide^[40]; (d) coupled LN microdisk photonic molecules^[41]; (e) vertical integration of double LN microdisks^[42]

Fig. 3. Fabrication flow schematic of LN photonic chip by the electron beam exposure combined with ion milling^[43]

Fig. 4. SEM image and transmission spectra of the fabricated LN microdisk resonators with 28-μm diameter^[43]. (a) SEM image of the LN microdisk and its magnified view, revealing that the microdisk edge has smooth sidewalls; (b) representative transmission spectrum collected from the microdisk, revealing several sets of resonant modes, in which the inset shows the optical micrograph of tapered fiber coupling on top of the microdisk resonator; (c)-(e) high-resolution transmission spectra for 2nd, 1st and 3rd order resonant modes, respectively

Fig. 5. Fabricated waveguide-coupled microring and micro-racetrack resonators, and propagation loss measurement^[44]. (a) SEM images of the microring and micro-racetrack resonators (top) and design of the micro-racetrack resonators (bottom); (b) extracted propagation loss for micro-racetrack resonators with different lengths at excitation wavelength of 1590 nm

Fig. 6. Fabrication flow schematic of LN photonic chip by the femtosecond laser photolithography assisted chemo-mechanical etching (PLACE). (a) Depositing a thin layer of Cr on the top of the lithium niobate on insulator (LNOI) wafer; (b) patterning the Cr layer by femtosecond laser ablation; (c) conducting chemo-mechanical polishing (CMP) on the sample to transfer the pattern from the Cr mask to the LNOI; (d) chemically removing the remaining Cr mask and performing a secondary CMP; (e) schematic illustration of the CMP principle and the instrument

Fig. 7. Fabricated ultra-high Q LN microresonator with a diameter of ~1030 μm^[48]. (a)(b) SEM image and optical micrograph of the microresonator; (c) zoom-in SEM image of microresonator periphery [the dashed box in Fig. 7(a)] from the top view; (d) zoom-in SEM image of microresonator periphery [the dashed box in Fig. 7(a)] from the side view; (e) Q factor measurement using the transmission spectrum from fiber taper coupling, indicating a loaded Q-factor of 7.5×10⁷ of the microresonator; (f) ring-down measurement, indicating the lifetime of the resonant photon is 64.3 ns, corresponding to the loaded (intrinsic) Q factor of 7.8×10⁷ (1.23×10⁸)

Fig. 8. Influence of different wedge angles on LN microdisk^[49]. (a)-(f) SEM images of the fabricated LN microdisks from the side view with different wedge angles of 9°, 14°, 22°, 30°, 40°, and 51°, respectively; (g) Q factor of the fabricated LN microdisks with different wedge angles

Fig. 9. LN microring cavity and optical waveguide based on PLACE technique^[50]. (a) Top-view SEM image of a LN microring resonator; (b) zoomed view of the ridge of the microring resonator in Fig. 9 (a); (c) AFM image of the ridge, revealing that the surface roughness of the microring is 0.452 nm; (d) picture of a chip consisting of an 11-cm-long waveguide captured by digital camera; (e)(f) zoomed images of the waveguides on the chip captured with an optical microscope

Fig. 10. Single-mode ridge waveguide on lithium niobate^[38]. (a) SEM image of LN waveguide covered with Ta₂O₅; (b)(c) measured and calculated TE mode profiles; (d)(e) measured and calculated TM mode profiles

Fig. 11. LNOI single-mode beam splitters^[51]. (a) Optical microscope image of the bare LNOI beam splitters fabricated with the chemo-mechanical polish lithography; (b) optical microscope image of the LNOI beam splitters covered with a layer of Ta₂O₅; (c) visibility as a function of the coupling length, showing a sinusoidal dependence of the splitting ratio on the coupling length; (d) near-field profiles captured at the output ports of the beam splitters with the different coupling lengths

Fig. 12. Reconfigurable multifunctional photonic integrated chip on LNOI^[52]. (a) Schematic of the multifunctional photonic integrated chip; (b) digital camera picture of the chip placed near by a 1 RMB coin; (c) zoom-in micrograph of the area [the dashed box in Fig. 12(b)]; (d) schematic of waveguide wiring of a 1×6 light switch; (e) measurement result of the light switch; (f) schematic of waveguide wiring of a balanced 3×3 interference beam splitter; (g) measurement result of the balanced interference beam splitter

Fig. 13. OTDL of a waveguide on LN with meter-scale lengths^[53]. (a) Schematic of the reconfigurable OTDL; (b) digital camera photo of the fabricated OTDL device; (c) measured losses of OTDL, showing a linear dependence on the length of OTDL; (d) waveform recorded on the oscilloscope after the light pulse traveling through the OTDLs with different lengths, showing two peaks separated by a time delay of 2.2 ns, in which negative time means that the pulses arrive first at the photodetector

Fig. 14. Lasing threshold characteristics of the Er³⁺-doped LN microdisk^[54]. (a) Spectrum of the Er³⁺-doped laser with the pump power at 0.92 mW, in which the inset displays the strong upconversion fluorescence of the microdisk; (b) enlarged spectrum around 1560 nm, revealing the multimode lasing spectrum; (c) dependence of the intensity of all emission lines on the absorbed pump power at 976 nm, where the experimental data are shown as circles and the curve is a linear fitting

Fig. 15. Fabricated Er³⁺-doped LN waveguide amplifier chip and characterization measurement^[55]. (a) Schematic of the Er³⁺-doped LN waveguide amplifier chip; (b) optical micrograph of the straight waveguide; (c) SEM image of the cross section of the fabricated Er³⁺-doped LN waveguide (tilt angle: 52°); (d) propagation loss curves, in which the inset is micrography of a 400-μm-diameter Er³⁺-doped LN microring; (e) measured spectra at the signal wavelength of 1530 nm for the different pump powers; (f) net gain of the Er³⁺-doped LN waveguide as a function of launched pump power at the signal wavelength of 1530 nm

Fig. 16. Monolithic high-speed electro-optic modulator^[31]. (a) Micrograph of electro-modulator chip consisted of three MZI modulators, in which the inset is cross-sectional schematic of the modulator; (b) data-transmission experiment at a rate of 70 Gbit/s; (c) CMOS driven circuit; (d)(e) measured constellation diagrams obtained with a coherent receiver and the reconstructed eye diagrams at peak-to-peak voltages of 200 mV and 60 mV

Fig. 17. High extinction cascaded MZI photonic chip^[56]. (a) Micrograph; (b) SEM of single-mode ridge waveguide; (c) extinction ratio as a function of wavelength

Fig. 18. High-speed hybrid silicon and LN electro-optic modulators^[36]. (a) Schematic of the structure of the modulator; (b) schematic of the cross-section of the hybrid waveguide; (c) SEM image of the cross-section of the LN waveguide; (d) SEM image of the metal electrodes and the optical waveguide; (e) schematic of the VAC; (f) SEM images of the cross-sections of the VAC at different positions (A, B, C) and calculated mode distributions; (g) normalized transmission of the devices with 3-mm length as a function of the applied voltage, in which the inset is measured extinction ratio on a logarithmic scale; (h) normalized transmission of the devices with 5-mm length as a function of the applied voltage; (i) optical eye diagram for the signal at data rates of 100 Gbit/s

Fig. 19. Microwave photonic chip on LNOI ^[57]. (a) Microscope image of the photonic chip; (b) microscope image of a suspended optical racetrack cavity with a thin-film acoustic resonator; (c) false-color SEM image of the thin-film acoustic resonator composed of an interdigital transducer (IDT) and an optical waveguide

Fig. 20. Broadband quasi-phase-matched harmonic generation in an X-cut lithium niobate microdisk^[39]. (a) Effective nonlinear coefficient of the microcavity varying with the azimuth; (b) conversion efficiency of SHG as a function of the in-coupled power, in which the inset is top-view optical micrograph of the SHG from the microresonator; (c) power of cascaded THG as a function of the cubic power of the in-coupled light, in which the inset is top-view image of the SHG and cascaded THG

Fig. 21. Optomechanical system on LN microresonator^[58]. (a) Top-view SEM of the 66-μm LN microresonator integrated with Cr electrodes; (b) experimental setup for characterizing the optomechanical system on microresonator, in which the inset (lower left) is simulated distribution of the electric field in LN microresonator; (c) radio frequency (RF) spectrum of the cavity transmission, in which the top-right inset is the simulation result of the mechanical mode in the LN disk; (d) mechanical frequency decreases linearly by 75, 200, 350, 500, 630, and 780 kHz at the DC voltages of 100, 200, 300, 400, 500, and 600 V, respectively

Fig. 22. Soliton frequency comb generated from a microring^[59]. (a) SEM image of the microring; (b) cross sectional schematic of mode; (c) GVD; (d) soliton comb

Fig. 23. Electro-optical tunable optical frequency comb on an LN microdisk^[60]. (a) Optical micrograph of the disk and integrated Cr electrodes under transmission illumination; (b) optical spectra of comb generation; (c) resonant wavelength shifts linearly with an electrical tuning efficiency of ~38 pm/100 V; (d) comb line shift as a function of the applied voltage

Fig. 24. Polygon coherent modes in a weakly perturbed whispering gallery microresonator^[61]. (a) Optical micrograph of an 84-μm-diameter LN microdisk; (b) close-up view optical micrograph of the edge of the microdisk; (c) triangle mode; (d) square mode; (e) pentagon mode; (f) star mode; (g)(h) intensity distribution of the tapered fiber-microdisk resonance modes when the gap between the tapered fiber and microdisk is 900 nm and 0 nm, respectively

Fig. 25. Nonlinear optical characteristic in LN microdisk photonic molecules^[41]. (a) Nonlinear spectra generated near the second harmonic (SH) wavelength at the pump powers of 10.4, 14.0, 17.1, and 21.9 mW; (b) spectra at the pump power of 23.2 mW, in which the left inset is four wave mixing (FWM) spectrum and the right inset is spectrum near the pump wavelength; (c) schematic illustration of the phase matching mechanism of the FWM process achieved by utilizing the splitting mode of the photonic molecules; (d) mode splitting due to strong coupling

Fig. 26. Double-layer LNOI microdisk^[62]. (a) Schematic of the double-layer LNOI microdisk; (b) top-view optical microscopy image of the microdisk, in which the two inner irregularly shaped circles indicate the inward etched silica buffer layers; (c) SEM image of the microdisk, with enlarged false-colored images clearly showing the well-separated upper and lower layers

Fig. 27. Energy distribution of interior and exterior modes in the double-layer LNOI microdisk^[62]. (a)(c) Transverse intensity profiles of interior WGMs and exterior slot WGMs, in which the arrows indicate the polarization of the electric field; (b)(d) intensity distribution of the respective mode along the X direction, in which the gray regions correspond to the LNOI microdisk layers

Table 1. Comparison between typical photonic integrated platforms^[23]