| With the rapid development of high-capacity and high-speed information systems,there is an urgent need for faster,more integrated,and more functional micro information processing systems.Lithium niobate crystals have excellent electro-optic,acousto-optic,piezoelectric,and nonlinear optical properties due to their wide transmission window,high refractive index,high non-linear coefficient,and electro-optic coefficient.Lithium niobate optoelectronic devices have important applications in nonlinear optical fields such as electro-optic modulation,optical switching,photon integration,and frequency conversion.Therefore,lithium niobate is known as the"silicon of photonics".Lithium niobate is often used in microdisk resonators for electro-optical tuning,optical waveguides and grating structures.In addition,lithium niobate is also one of the commonly used materials for photonic crystals.Up to now,researchers have conducted extensive research on the micro-nano-processing technology of lithium niobate.Commonly,there are electron beam lithography,focused ion beam etching,photolithography-assisted dry/wet etching,and chemical mechanical polishing.Conventional methods are complex and difficult to achieve the preparation of complex three-dimensional structures of lithium niobate.Due to its ultra-short pulse width and ultra-high instantaneous power,femtosecond lasers can achieve maskless direct writing processing of materials with high precision.It is worth mentioning that high precision direct writing processing inside lithium niobate materials can be achieved due to the nonlinear absorption of the material that can be achieved with femtosecond lasers.Therefore,femtosecond laser micro-nano-processing is an effective way to achieve high-precision fabrication of lithium niobate materials.However,the high surface roughness of lithium niobate structures after high-energy laser irradiation is difficult to meet the optical requirements.Moreover,the existing lithium niobate processing methods make it difficult to fabricate internally buried devices based on wet etching as well as complex three-dimensional structures on the surface,and these challenges affect the fabrication and application of lithium niobate optoelectronic devices.Therefore,it is important to develop processes for high precision as well as high quality fabrication of lithium niobate based on femtosecond lasers.To solve the problems of complex processes and difficulty in achieving complex structure fabrication in the field of micro-nano-processing of lithium niobate optoelectronic devices.This paper proposes strategies such as femtosecond laser direct writing assisted liquid back etching technology,wet etching assisted femtosecond laser modification technology,and wet etching assisted femtosecond laser polarization domain inversion technology,successfully achieving high-precision,high surface quality,and high fidelity complex three-dimensional structures fabrication of lithium niobate materials.It provides a variety of solutions for the micro-nano fabrication of lithium niobate optoelectronic devices.In addition,second harmonic enhancement devices,internal microchannels and nanogratings,pyramid-like anti-reflective structures,and cone-tip mid-infrared broad-spectrum anti-reflective structures have been successfully fabricated on lithium niobate with the techniques mentioned above.And demonstrated its applications in the fields of nonlinear optics and mid-infrared anti-reflection.The main research results are as follows:1.Femtosecond laser direct writing assisted liquid back etching technique for the fabrication of second harmonic enhancement devices.Aiming at the problem of difficulty in achieving the fabrication of complex three-dimensional structures on the surface due to debris accumulation during the laser processing of lithium niobate.Femtosecond laser direct writing assisted liquid back etching technique is proposed.It solves the problem that due to the influence of surface debris during lithium niobate processing,it is difficult to achieve microfabrication of complex three-dimensional structures.Three-dimensional fabrication of lithium niobate with high surface quality(Ra=0.422 nm)was achieved.A variety of planar patterned structures were successfully fabricated on the surface of lithium niobate crystals.In addition,the fabrication of cruciferous array structures with depths of up to 20 microns and sidewall perpendicularity of up to 90 degrees has been achieved.This technology solves the problem of non-perpendicular sidewalls in conventional lithium niobate lithography.Further,the fabrication of lithium niobate helical arrays demonstrated the capability of true three-dimensional processing.Finally,cone-tipped second-harmonic enhancement devices were fabricated on lithium niobate crystals and successfully achieved nearly 12times enhancement of second harmonic signals.This technique provides a new method for precise fabrication of lithium niobate crystals in the field of surface nonlinear devices and photonic integrated devices.2.Wet etching assisted femtosecond laser modification technique for the fabrication of pyramid-like anti-reflective structures.Aiming at the difficulty in achieving high-quality and high-precision fabrication of the internal structure of lithium niobate by the traditional photolithography process and the femtosecond laser direct writing assisted liquid back etching technology.Wet etching assisted femtosecond laser modification technique is proposed.By femtosecond laser modification of lithium niobate with subsequent wet etching to remove the modified area,microchannel structures with internal lengths up to 1.4 mm,length-to-width ratios of 280:1,and cross-vertical ratios of 1:267 have been fabricated in lithium niobate.Meanwhile,high-precision fabrication of nanoscale grating structures with periods of 800 nm and 400nm was achieved internally,demonstrating the potential application of this technique for the high-quality fabrication of lithium niobate internally buried devices.Subsequently,high quality fabrication of planar and true 3D structures on lithium niobate surfaces was achieved by laser modification from the interior to the upper surface.Finally,pyramid-like array structures were fabricated by this technique for lithium niobate mid-infrared anti-reflective applications,and the transmittance was successfully increased from 79%to 86%at the wavelength of 4μm.This technique provides an effective method for the high-quality fabrication of internal microchannels,buried devices and surface structured devices of lithium niobate crystals,and develops a high-quality process that is universal for lithium niobate interiors and surfaces.3.Wet etching assisted femtosecond laser polarization domain inversion technology.Aiming at the problem of the difficulty in fabrication of nanoscale complex structures on the surface of lithium niobate by femtosecond laser direct writing.Wet etching assisted femtosecond laser polarization domain inversion technique is proposed.Based on domain engineering,lithium niobate was subjected to polarized domain inversion by femtosecond laser,and subsequently,with the help of the different etching rates of the laser domain inversion region and the unpolarized region,the laser-polarized domain inversion region was retained in the subsequent wet etching,while the unmodified region was removed,resulting in a high-quality fabrication of the lithium niobate structure(with a roughness of 0.34 nm).This technique enables the fabrication of lithium niobate surface-patterned structures and the high-precision fabrication of nanocone arrays with a minimum period of up to 200 nm.As a demonstration,the fabricated high-precision cone-tip arrays were used for second harmonic enhancement applications.Finally,combined this technique with wet etching assisted femtosecond laser modification proposed a three-dimensional composite lithography of lithium niobate.Areas preserved by femtosecond laser polarization and removed by laser modification allow high fidelity fabrication of lithium niobate surface structures in one step of wet etching.For the first time,the strong potential of domain engineering in the field of lithium niobate micro-nano-fabrication has been demonstrated,providing a novel approach to the high-precision micro-nano-fabrication process of lithium niobate based on domain engineering.4.Design and fabrication of lithium niobate mid-infrared subwavelength broad-spectrum anti-reflective structures.We have analysed the effect of parameters such as morphology,height,period and filling factor of the subwavelength structures on the anti-reflective properties of the devices.And the surface structures of lithium niobate were fabricated by wet etching assisted femtosecond laser polarization domain inversion technique.Subsequently,large-area periodic cone-tip array structures with a period of 1.3μm and a height of 1.7μm were designed and fabricated on the surface of lithium niobate and exhibited excellent broad-spectrum anti-reflective performance in the 2.6-5.2μm band in anti-reflective performance test.In particular,the average transmittance of lithium niobate at 3-5μm was increased from 78%to 85%,and a high transmittance of 88%was achieved at the wavelength of 5μm.Finally,the results of the device stability test show that the fabricated lithium niobate subwavelength broad-spectrum anti-reflective structures are able to maintain good mid-infrared broad-spectrum anti-reflective effect under the high temperature of 800°C and the high-energy laser irradiation with a single-pulse laser energy density of 3.5 J/cm~2.This research promotes the development of lithium niobate mid-infrared lasers and infrared sub-wavelength broad-spectrum anti-reflective applications in hard materials and also provides new ideas for the efficient and high-precision fabrication of micro-nano structures on the surface of ferroelectric crystals.In summary,this paper addresses the problems in the high-precision three-dimensional fabrication of lithium niobate optoelectronic devices.Femtosecond laser direct writing assisted liquid back etching,wet etching assisted femtosecond laser modification,and wet etching assisted femtosecond laser polarization domain inversion techniques were proposed,which successfully realized the fabrication of complex three-dimensional structures of lithium niobate materials with high precision,high quality as well as high fidelity.We have also demonstrated its application in the fields of second harmonic enhancement and mid-infrared anti-reflection.These results provide new methods for the precision processing of lithium niobate crystal optoelectronic devices,which are expected to promote the development of lithium niobate in the field of nonlinear optics and photonic integration. |
