Fig. 1. Fabrication flow schematic of photonic structures on thin film lithium niobate by the electron beam lithography combined with ion etching^[53]

Fig. 2. Fabrication flow schematic of LN photonic structures by the femtosecond laser photolithography assisted chemo-mechanical etching (PLACE)^[56]. (a) Depositing a thin layer of Cr on the top of the lithium niobate on insulator wafer; (b) patterning the Cr layer by femtosecond laser ablation; (c) conducting chemo-mechanical polishing on the sample; (d) chemically removing the remaining Cr mask and performing a secondary chemo-mechanical polishing; (e) schematic illustration of the chemo-mechanical polishing principle and the instrument

Fig. 3. Ultra-high-speed high-resolution laser lithography^[64]. (a) Principle of the ultra-high-speed polygon laser scanner; (b) experimental setup of femtosecond laser lithography system based on the polygon scanner; (c) digital camera photograph of the color palettes at different grating periods; (d) color printing on 4-inch wafer; (e) an array of 1960 Mach-Zehnder interferers patterned on a 4-inch wafer

Fig. 4. Electro-optically tunable optical delay line in thin film lithium niobate^[64]. (a) Schematic diagram of tunable optical delay line; (b) micrograph of a tunable optical delay line integrated with microelectrodes; (c) measured transmission losses in the 10, 20, and 30 cm long waveguides; (d) measured time delay by experiment and its fitting curve, the slope of the curve is the electro-optic tuning efficiency of the delay

Fig. 5. Dual-polarization thin film lithium niobate in-phase quadrature modulators^[86]. (a) Three-dimensional schematic of the modulator; (b) measurement of half-wave voltage V_π; (c) measurement of electro-optical response

Fig. 6. High efficiency broadband electro-optic frequency comb generator^[89]. (a) Device layout; (b) measured electro-optical comb spectra at different radio frequency driving frequencies; (c) electro-optical frequency comb generation at different optical pump wavelengths

Fig. 7. Multiplexed energy-time-entangled photon generation from thin film lithium niobate on insulator chip^[92]. (a) Cross section of waveguide and mode field simulation; (b) simulation of group velocity dispersion for thin film lithium niobate waveguide and bulk lithium niobate crystal; (c) picture of domain structure captured by confocal laser scanning microscopy

Fig. 8. Giant second harmonic generation (SHG) from thin film lithium niobate metasurfaces^[113]. (a) Schematic illustration of the SHG from the thin film lithium niobate metasurface; (b) SEM image of the thin film lithium niobate metasurface; (c) linearly relationship between SHG conversion efficiency and pump beam power (the two metasurfaces MS1 and MS2 have the same period of 600 nm but different air hole diameters of 225 nm and 250 nm, respectively)

Fig. 9. Er³⁺-doped thin film lithium niobate single-mode laser based on Sagnac loop reflectors^[125]. (a) Optical microscopy image of an Er³⁺-doped thin film lithium niobate FP resonator (the bottom inset shows the green upconversion fluorescence of the Er³⁺-doped thin film lithium niobate FP resonator pumped by 980 nm laser); (b) spectrum around 1544 nm wavelength (the lasing peak is fitted with a Lorentzian line shape, the inset shows the infrared optical microscopy image of the output port of the Er³⁺-doped thin film lithium niobate FP resonator); (c) on-chip laser power of Er³⁺-doped thin film lithium niobate FP resonator laser changes with increasing pump power

Fig. 10. On-chip coherent beam combination of waveguide amplifiers on Er³⁺-doped thin film lithium niobate^[130]. (a) Schematic of the on-chip coherent beam combination of Er³⁺-doped waveguide amplifiers, where the inset shows the energy level diagram of Er³⁺; (b) electro-optic modulation of the output signal power by a triangular wave applied to a 3 mm microelectrode; (c) measured extinction ratio of 1530 nm signal light after applying the pump laser

Fig. 11. 4×4 programmable photonic circuits based on thin film lithium niobate^[133]. (a) Arbitrary SU(4) transformation matrix; (b) internal structure of MZI-unit; (c) histogram of the measured fidelity of 200 random matrices; (d) comparison between theoretical and experimental results of a random matrix

Fig. 12. On-chip arrayed waveguide grating (AWG) fabricated on thin film lithium niobate^[139]. (a) Micrograph of the fabricated 8-channel thin film lithium niobate AWG; (b) thin film lithium niobate AWG chip and the output images captured by the infrared camera at different input wavelengths; (c) measured spectra on eight channels; (d) spectrum measured at one of eight channels

Fig. 13. Four-channel waveguide amplifiers fabricated on the monolithically integrated active/passive thin film lithium niobate^[81]. (a) Illustration of the device design; (b) digital camera captured picture of the four-channel waveguide amplifiers; (c) photo of the four-channel waveguide amplifier array with 980 nm wavelength laser pumping; (d) mode profile (insets) and intensity distribution of the 1550 nm wavelength signal in the four-channel Er³⁺-doped waveguide; gain characterization of the four-channel Er³⁺-doped lithium niobate waveguide array for the signal wavelengths at 1550 nm (e) and 1530 nm (f)