Fig. 1. Comparison of optical mode areas, index contrast, and crystal orientations in (a) conventional LN waveguides and (b) thin-film LN waveguides.

Fig. 2. Crystal orientations and corresponding preferred polarization to achieve maximum efficiency for nonlinear processes in TFLN using [(a), (b)] quasi phase matching via periodic poling and [(c), (d)] modal phase matching.

Fig. 3. Various phase-matching methods used on the TFLN platform. (a) Birefringent phase-matching.⁵⁹ (b) Modal phase matching.⁶⁰ (c) Grating-assisted quasi phase matching or mode shape modulation.⁶¹ (d) PPLN on a straight waveguide.¹³ (e) Natural quasi phase matching, which is conceptually similar to cyclic phase-matching.⁶² (f) Phase-matching-free metasurface.⁶³

Fig. 4. PPLN devices in different structures. (a) Straight waveguide.⁸² (b) Racetrack resonator that is poled on one of the straight arms.⁸³ (c) Radially poled microring resonator.⁸⁴ (d) Radially poled microdisk, poled using the PFM technique in which they also demonstrated a poling period as small as 200 nm.⁸⁵

Fig. 5. Some of the schemes used for implementing cascaded χ(2) processes in TFLN and the corresponding harmonic generations. (a) Two PPLN sections with different poling periods to enable SHG/SFG cascading for a THG device and SHG/SHG for an FHG device.¹³⁷ (b) Dual-period PPLN microdisk with demonstrated THG and FHG.⁹⁵ (c) THG and FHG on a single PPLN device via pulse pumping.⁸² (d) SHG and THG on a microdisk through cascaded SHG/SFG by taking advantage of the natural BPM.⁶⁹

Fig. 6. Using second-order nonlinear processes for generating OFCs on lithium niobate. (a) Quadratic frequency comb generation on conventional LN¹⁴³ using cascaded χ(2) processes, which has yet to be demonstrated on the TFLN platform. (b) Electro-optic frequency comb generation in TFLN using coupled microring and racetrack resonators and the resulting spectrum exhibiting a high conversion efficiency.¹⁴⁸

Fig. 7. SCG on the TFLN platform. (a) SCG spanning over two octaves in a dispersion engineered straight waveguide without poling.¹⁵⁹ (b) SCG on a periodically poled straight waveguide with a span of more than two octaves at a pulse energy of ∼11pJ.¹³⁹ (c) Using cascaded SCG and SHG for on-chip f−2f self-referencing, which is of great importance for realization of on-chip OFCs.¹⁵⁸

Fig. 8. OFC generation in TFLN using the Kerr effect. (a) Conceptual schematic of a fully integrated soliton comb system in TFLN with all the required functionalities.¹⁶¹ (b) Soliton Kerr comb generation spanning near one octave.¹⁶² (c) Cascaded Kerr and EO combs with green lines corresponding to Kerr comb lines spanning over 200 nm.¹⁶³ (d) Zoomed in spectra of the cascaded Kerr and EO combs demonstrating that EO combs with much lower repetition rates fill the gap between Kerr combs.¹⁶³

Table 1. Optical properties of some of the materials used for second-order nonlinear applications.

Table 2. Performance of various devices based on straight waveguide structure for SHG using different phase-matching methods. Power values for the pump and SH wavelength and the absolute conversion efficiency (in %) correspond to the maximum normalized conversion efficiency and are not necessarily the maximum reported numbers in these papers.

Table 3. Comparison of resonant-based structures for SHG using different phase-matching methods.