飞秒激光制备非线性光子晶体研究进展

Research Progress on Femtosecond Laser Fabrication of Nonlinear Photonic Crystals

非线性光子晶体能够实现高效的非线性光学过程,其制备手段吸引了该领域研究者的高度关注。飞秒激光加工技术具有极高的精度、分辨率和灵活性,相比传统的非线性结构制备工艺具有独特的优势。总结归纳了利用飞秒激光加工技术构建非线性光子晶体的研究进展,并对涉及的准相位匹配原理进行了简要介绍。讨论了飞秒激光反转铁电畴和擦除非线性系数的加工机理,论述了这两种方式在多种维度非线性光子晶体加工方面的实验成果和应用。最后分析了目前飞秒激光加工非线性光子晶体所遇到的挑战,并展望了未来的发展前景。

Significance 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(χ^((2)))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.Progress 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 ofχ^((2))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 Ba0.77Ca0.23TiO3(BCT)/Ca0.28Ba0.72Nb2O6(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 eraseχ^((2))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,χ^((2))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-amplitudeχ^((2))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 ofχ^((2))can 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).Conclusions and Prospects 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,χ^((2))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-amplitudeχ^((2))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.