The fabrication strategy for nonlinear photonic crystals has drawn substantial research interest because of their highly efficient nonlinear optical interactions. Femtosecond laser engineering has distinct advantages over conventional methods for the fabrication of nonlinear structures. These advantages include its high precision, resolution, and flexibility. This paper summarizes the research progress of femtosecond laser processing technology for constructing nonlinear photonic crystals and provides a brief introduction to the quasi-phase matching theory involved. The processing mechanism of femtosecond-laser-induced ferroelectric domain inversion and laser erasure of second order nonlinear polarization coefficients (χ2) are discussed, and the experimental results and applications of nonlinear photonic crystals in different dimensions realized by these two approaches are demonstrated. Finally, the challenges of the femtosecond laser technique in the processing of nonlinear photonic crystals are analyzed, and the prospects for future development are presented.

This paper summarizes the research progress of femtosecond laser processing technology for constructing nonlinear photonic crystals and also provides a brief introduction to the quasi-phase matching theory involved. The processing mechanism of femtosecond-laser-induced ferroelectric domain inversion and laser erasure of χ2 are discussed, and the experimental results and applications of nonlinear photonic crystals in different dimensions realized by these two approaches are demonstrated. Finally, the challenges faced by the femtosecond laser technique for processing nonlinear photonic crystals are analyzed, and the prospects for future development are discussed.

Tightly focused femtosecond laser pulses can induce a thermoelectric field in the ferroelectric crystal that inverts the direction of spontaneous polarization. On the basis of this mechanism, an arbitrary arrangement of 2D inverted domains can be constructed to enhance the second-harmonic emission from the crystal, and quasi-phase-matching structures can be integrated in the LiNbO₃ waveguide to achieve efficient frequency conversion (Fig. 4). This technique can also be used to fabricate 3D nonlinear photonic crystals in multi-domain/single-domain Ba_0.77Ca_0.23TiO₃(BCT)/ Ca_0.28Ba_0.72Nb₂O₆(CBN) crystals, which demonstrate second harmonic diffraction with 3D quasi-phase matching (Fig. 5). Another technique that relies on the laser-induced amorphization of the crystal to partially erase χ2 shows versatility for processing non-ferroelectric crystals. Multiple quasi-phase-matching structures can be inscribed into the waveguide core to realize a parallel multiwavelength output (Fig. 6). A 3D nonlinear photonic crystal has also been obtained using this technique in LiNbO₃ to provide abundant 3D reciprocal vectors for second-harmonic generation in various directions (Fig. 7).When processing inside the crystal, the aberration resulting from the mismatch of the refractive index causes an axial shift of the focal spot, which seriously limits the axial resolution as well as the fabrication quality of the structures. Reasonable diffractive optical components for aberration compensation must be implemented during fabrication. One method is to introduce a spatial light modulator into the femtosecond laser processing system, thereby eliminating the effect of aberration by loading a specific phase hologram.To date, few attempts have been made to combine nonlinear photonic crystals with other optical devices to extend their functionalities. Various functional optical devices, such as electro-optic modulators, resonators, waveguides, and nonlinear frequency converters, can be integrated within a single ferroelectric crystal by combining the flexibility of the femtosecond laser and other processing techniques. The integrated photonic chips exhibit more powerful functions in modern optical signal processing and quantum computing.Currently, χ2 can only be reduced by 20% using the femtosecond laser erasure technique, which restricts the modulation efficiency of the structures. Therefore, a deep understanding of the femtosecond laser interaction mechanism with the lattice is required to determine the optimal fabrication parameters for large-amplitude χ2 erasure, thereby improving the frequency conversion efficiency of the as-prepared structures.In addition to the aforementioned development trends, certain topics, such as the development of a fabrication strategy with high efficiency to lay the foundation for mass production, must be investigated further. With improved femtosecond laser processing technology, nonlinear photonic crystals show promising prospects.

The spatial distribution ofχ2can modulate wavefronts in a new wavelength range; thus, it can be applied in optical communication, optical storage, and quantum information processing. Nonlinear patterns constructed flexibly by a femtosecond laser are capable of the nonlinear generation of vortices and Hermite–Gaussian beams. The as-prepared 3D nonlinear photonic crystal can realize the simultaneous conversion of the fundamental beam into multiple structured beams (Fig. 9) or efficient beam shaping based on full-dimensional phase matching and nonlinear volume holography (Fig. 10). In the past few years, researchers have also introduced detour phase encoding by femtosecond laser fabrication into nonlinear holography to realize the reconstruction of arbitrary target images (Fig. 11). Moreover, the strategy of erasing nonlinear coefficients using femtosecond lasers can be applied to quartz crystals to obtain efficient frequency doubling in the challenging deep-ultraviolet region (Fig. 13).

While substantial progress has been made in the femtosecond laser processing of nonlinear photonic crystals, some challenges remain.

When processing inside the crystal, the aberration resulting from the mismatch of the refractive index causes an axial shift of the focal spot, which seriously limits the axial resolution as well as the fabrication quality of the structures. Reasonable diffractive optical components for aberration compensation must be implemented during fabrication. One method is to introduce a spatial light modulator into the femtosecond laser processing system, thereby eliminating the effect of aberration by loading a specific phase hologram.

To date, few attempts have been made to combine nonlinear photonic crystals with other optical devices to extend their functionalities. Various functional optical devices, such as electro-optic modulators, resonators, waveguides, and nonlinear frequency converters, can be integrated within a single ferroelectric crystal by combining the flexibility of the femtosecond laser and other processing techniques. The integrated photonic chip will exhibit more powerful functions in modern optical signal processing and quantum computing.

Currently, χ2 can only be reduced by 20% using the femtosecond laser erasure technique, which restricts the modulation efficiency of the structures. Therefore, a deep understanding of the femtosecond laser interaction mechanism with the lattice is required to determine the optimal fabrication parameters for large-amplitude χ2 erasure, thereby improving the frequency conversion efficiency of the as-prepared structures.

In addition to the aforementioned development trends, certain topics, such as the development of a fabrication strategy with high efficiency to lay the foundation for mass production, must be investigated further. With improved femtosecond laser processing technology, nonlinear photonic crystals show promising prospects.