Fig. 1. Čerenkov second harmonic (SH) microscopic images of 3D NPCs constructed with femtosecond-laser-written inverted ferroelectric domains in BCT crystals^[88]

Fig. 2. Nonlinear cubic crystal system fabricated by femtosecond-laser-induced domain-inversion technique for realizing 3D QPM ^[100]. Conventional cells of cubic NPCs with (a) SC, (b) BCC, (c) FCC, and (d) DC lattices, respectively; (e)-(h) Čerenkov SH microscopic images of nonlinear cubic structures fabricated by femtosecond-laser-induced domain-inversion technique in ferroelectric SBN crystals; (i) primary reciprocal lattice vectors of cubic NPCs and corresponding nonlinear Ewald sphere for realizing SHG based on 3D QPM

Fig. 3. Piezoresponse force microscopic images of inverted nanodomains in LiNbO₃ crystals fabricated by femtosecond laser direct writing technique^[102]. (a) Circle fabricated by clockwise moving femtosecond laser, and white dashed-line shows the path of femtosecond-laser writing; (b) radial lines written by femtosecond laser from center to outside; (c) femtosecond-laser-direct-written straight lines with widths of 400, 350, 300, 250, 200, 150, and 100 nm; (d) femtosecond-laser-fabricated domain with tip of 30  nm

Fig. 4. 3D information encryption through polarization-dependent nonlinear hologram^[109]. (a) 3D information is encrypted into vertical polarization channel; (b) wavelength-dependence of Δk at horizontal (black) and vertical (red) polarizations; (c)-(f) experimental results at SH waves (with vertically-polarized fundamental light input, two letters 3 and D at distances of 657 μm and 866 μm can be reconstructed, respectively; with horizontally-polarized fundamental light input, only SH speckles present)

Fig. 5. Dynamic 3D information reconstruction based on wavelength-temperature joint modulation^[109]. (a) Dynamical projections of 3D objects at SH waves by adjusting crystal temperature and input wavelength, and the input fundamental light is vertical polarization and output SH light is horizontal polarization; (b)-(d) experimental results [at initial state (900 nm, 45 ℃), two arrows present at different propagation depths of SH waves; at middle state (975 nm, 45 ℃), SH patterns become a square and a triangle at different distances; at end state (975 nm, 350 ℃), two arrows shown in Fig. 5(b) present again]

Fig. 6. Frequency up-conversion edge enhancement^[109]. (a) Experimental diagram, a 1342 nm light (possessing object information) and a 1064 nm light are frequency-summed in designed nonlinear hologram, and the output light at 593 nm presents the edge-enhanced image; (b) presentation of convolution kernel; (c) amplitude and phase of χ⁽²⁾-super-pixel hologram; (d)(g)(j) near-infrared (NIR) images of three objects at 1342 nm; (e)(h)(k) experimental results of frequency up-conversion edge enhancement at 593 nm; (f)(i)(l) intensity contrasts on dotted lines marked in (e), (h), and (k), respectively

Fig. 7. Characterization of 3D NPCs in LiNbO₃ crystals fabricated by femtosecond-laser-induced domain-erasing technique^[37]. (a) Čerenkov SH microscopic images of 3D NPCs; (b) SH image in x-y plane through a confocal SH microscopic system; (c) SH intensity distribution along black line marked in Fig. 7(b)

Fig. 8. Switchable reconstruction of different SH structured light beams by femtosecond-laser-induced 3D NPCs^[114]. (a) QPM schemes for reconstruction of SH structured light beams at different wavelengths; (b) diagrams of three-sequential 3D NPCs with different QPM structures; (c) SH structured light beams output from different functional parts of three-sequential 3D NPCs at the pumping wavelengths of 857 nm, 827 nm, and 784 nm, respectively

Fig. 9. Calculated SH build-up inside nonlinear materials with PM structure, QPM structure, and femtosecond-laser-modified periodic domain structure (not domain inversion)^[117]

Fig. 10. Čerenkov SH microscopic images of 3D conic nonlinear photonic crystals (3D conic NPCs) ^[119]. (a) Side view of 3D NPCs;(b) front view of 3D NPCs; (c)-(f) 3D conic NPCs at different truncation factors

Fig. 11. Scheme of APP matching for nonlinear frequency conversion^[120]. (a) Diagram of birefringent phase-matching in negative uniaxial crystals; (b) schematic of QPM in ferroelectric crystals; (c) schematic of APP theory in arbitrary nonlinear optical crystals; (d) amplitude of SH field under phase-mismatching and various phase-matching conditions

Fig. 12. Second-to-fifth harmonic generations based on femtosecond-laser-direct-written APP quartz crystals^[123]. (a) Microscopic image of femtosecond-laser-direct-written 2D APP quartz crystals; (b) distribution of period with various widths in 2D APP quartz crystals; (c) experimental setup for generating second-to-fifth harmonics; (d) photograph of second-to-fourth harmonics