Fig. 1. (a)–(d) Symmetry of the ridge waveguide with diverse materials^[50]. (a) Structure of monolithic nanophotonic waveguide. (b) High spatial asymmetry of two phase-matched modes around the z-axis component of the electric field. (c) Structure of a semi-nonlinear waveguide composed of two core materials. (d) Relative spatial symmetry of two phase-matching modes around the z-axis component of the electric field. (e)–(g) Scheme of the dual layers with reversed poling direction^[45]. (e) Design of the geometrical parameters. (f) Double-layer TFLN waveguide with reverse polarization. (g) The relationship between the effective refractive index and etch depth.

Fig. 2. (a)–(c) Experiment of the normalized conversion efficiency of 4600%/(W·cm²)^[21]. (a) Optimization of the poling process in situ using the iterative poling, depoling, and repoling sequence. (b) The relationship between the poling number and normalized conversion efficiency. (c) Measured peak conversion efficiency. (d)–(f) Experiment of the normalized conversion efficiency of the shallowly etched waveguide^[57]. (d) Cross section of the shallowly etched waveguide at a depth of 50 nm in the X-cut waveguide. (e) Poling period. (f) Conversion efficiency of the structure. (g)–(i) Experiment of the normalized conversion efficiency of 33,000%/(W·cm²)^[40]. (g) Generation of the blue light in the PPLN thin film waveguide via SHG with ultra-high efficiency. (h) Poling period. (i) Normalized spectra of the blue second-harmonic generation. (j),(k) Ultrabroadband second-harmonic generation in the nano waveguide^[54]. (j) Simulated poling period and normalized efficiency, respectively, as a function of the waveguide geometry. (k) Measured second-harmonic generation transfer function (black) for a 6-mm-long nanophotonic waveguide, showing a good agreement with the theory (blue).

Fig. 3. (a)–(c) PPLN-based non-degenerate phase-sensitive amplifier^[97]. (a) Experimental setup for the PSA. (b) Bandwidth and NF of the cascaded SHG/OPA amplifier. (c) The relationship between the gain and the SH-pump power. PSA, phase sensitive amplification; PIA, phase insensitive amplification. (d)–(f) The degenerate OPA based on the nanowaveguide^[98]. (d) Waveguides engineering for the GVD and the GVM in maximizing the OPA performance. (e) Measured saturated gain versus the input signal pulse. (f) Extracted gain versus the pump pulse energy.

Fig. 4. (a),(b) Microdisk of the high Q-factor^[113]. (a) The zoom-in SEM image of the sidewall from the side view. Upper inset: the electric field distribution; lower inset: the optical micrograph of the resonator, showing a relatively small pillar whose rough boundary is far from the periphery of the resonator, where the scale bar is 200 µm. (b) Power dependence of the OPO, showing a threshold pump power of ∼19.6 µW and a gain rate of 20%. (c)–(e) Periodically poled lithium niobate microring resonator (PPLNMR) with a low threshold in temperature tunability^[58]. (c) Illustration of the parametric oscillation using PPLNMR. (d) The experimental (red circles) and theoretical (solid line) on-chip infrared power versus the on-chip pump power for the parametric oscillation process. An example of the snapshot of the off-chip pump (blue) and parametric oscillation (red) spectra after the device as shown in the inset. (e) The OPO wavelength tuning by varying both the temperature and pump wavelength. Middle panel: simulated (solid lines) and measured (circles) OPO wavelengths versus the pump wavelength at various temperatures. As an example, the right panel plots the recorded spectra with different pump wavelengths at 125°C. The left panel indicates the occurrence of degenerate parametric oscillation by varying the temperature. (f),(g) The OPO with the racetrack^[107]. (f) Design of the OPO with the racetrack. (g) Wavelength tuning range.

Fig. 5. (a)–(c) OPO resonant device^[117]. (a) Resonant second-order nonlinear optical device. (b) Scanning the near-infrared laser shows that second-harmonic generation occurs at wavelengths corresponding to the modes of the resonator. (c) Scanning the blue pump laser across the wavelength shows that many resonances surpass the parametric oscillation threshold.