飞秒激光制备非线性光子晶体研究进展 下载: 655次
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 (
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
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 erase
The spatial distribution of
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,
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.
1 引言
自20世纪60年代以来,激光技术的日臻成熟推动了众多新兴光学领域的发展,非线性光学就是其中之一。利用非线性晶体中的二次谐波生成、和频生成、差频生成、光学参量振荡等非线性频率转换过程,能够利用原有激光波长获得新的波长,从而拓宽了激光器的输出波长范围。然而,由于晶体的色散效应,能流会在基频光波与生成的新频率光波之间交替变化,限制了非线性过程的频率转换效率。为了解决这个问题,研究者们提出了准相位匹配的概念[1]:通过周期性地调制晶体的非线性系数,在非线性过程中引入倒格矢,弥补参与频率转换光波之间的相位失配,从而能够使生成的新频率光波获得持续的能量增长。基于准相位匹配技术能实现高效的频率转换,极大地拓宽了非线性光学的研究范畴和应用领域。
准相位匹配概念被提出后,其理论经历了从一维、二维到三维的发展过程,基于该理论的非线性光子晶体也在实验上得到了长足的发展。最初的准相位匹配结构只在一维方向上进行非线性系数的周期调制,仅能满足共线的非线性过程,输出光方向单一。后来,研究者提出了二维非线性光子晶体的想法[2],即非线性系数在整个平面内被周期调制,因此能够提供更为丰富多样的倒格矢以满足复杂的非线性过程,使多波长频率转换和多方向输出成为可能。近年来,三维非线性光子晶体的出现标志着准相位匹配理论的发展达到了新的高度,在空间三个正交方向上都能够提供倒格矢,具有更高的灵活性[3]。
铁电晶体的自发极化特性为非线性过程提供了可行性,铁电晶体成为制备非线性光子晶体的理想材料。早期的制备手段包括晶片堆积、晶体生长、化学扩散等,往往存在工艺复杂、效率低的问题,且只能制备简单的一维结构。目前主流的加工手段是外加电场极化,通过图案化电极施加电场来选择性地反转铁电晶体极化方向[4],可以简单高效地获得一维、二维非线性光子晶体,但仍只能在单个平面内进行非线性调制,缺少在晶体内部任意深度处的调制能力。因此,三维非线性光子晶体的制备在很长一段时间内一直是个难题。飞秒激光的独特优势使其在微纳米制造领域发挥着越来越重要的作用[5]。其通过紧聚焦超短脉冲激光得到超高能量密度的焦点,在极微小尺度范围内与材料发生非线性作用,加工分辨率能突破衍射极限,加工区域热影响小,穿透性好,极适合在透明材料内部加工三维复杂精细结构。飞秒激光的引入为非线性光子晶体制备开辟了崭新的发展空间。利用近红外波段的飞秒激光配合高数值孔径物镜和三维移动工作台,可在铁电晶体表面或内部任意位置反转或擦除非线性系数,从而突破了三维非线性光子晶体的制备技术瓶颈。近年来国内外科研团队开展了诸多工作。
本文主要介绍了飞秒激光制备非线性光子晶体的研究进展。首先介绍了非线性光子晶体的基本理论,阐述了从一维到三维准相位匹配的原理和特性;接着将飞秒激光与非线性晶体的作用机理分为反转铁电畴和擦除非线性系数,分别论述了其原理和在加工方面的研究进展,进一步介绍了所制备结构在非线性光束整形、深紫外相干光源方面的应用。最后讨论了飞秒激光加工非线性光子晶体所面临的挑战,展望了这项技术的发展前景。
2 非线性光子晶体中的相位匹配理论
在晶体材料的非线性倍频光学过程中,频率为
式中:
图 1. 完全相位匹配、准相位匹配和非相位匹配情形下,非线性晶体中二次谐波强度随传播距离的变化
Fig. 1. Second-harmonic intensity versus propagation distance in nonlinear crystal under conditions of perfect phase matching, quasi-phase matching,and non-phase matching
2.1 准相位匹配基本理论
1962年,哈佛大学Armstrong研究组和密歇根大学Franken研究组先后提出了准相位匹配的概念[1,6]。在晶体中每隔奇数倍的相干长度,基波和二次谐波的相对相位改变
通过分析典型光学非线性过程中二次谐波的演变,可以清晰地揭示出准相位匹配的基本原理[7-8]。假设基频光为连续或长脉冲激光且在整个转换过程中无损耗,不考虑两波在传播过程中的消耗,那么在近似平行入射的基频光的作用下,晶体中主导二次谐波增长的电场强度变化方程为
式中:
式中:
式中:
式中:
进一步将周期函数
式中:
式中:
只有
将
对于理想的常规双折射相位匹配,
基于上述分析可知,在准相位匹配条件下,通过调控极化反转畴结构的周期能够改变
2.2 非线性光子晶体的理论发展
准相位匹配理论的发展指导了后续的结构设计和制备。1998年,法国的Berger[2]将准相位匹配结构和传统意义上的光子晶体类比,提出了非线性光子晶体的概念。常规光子晶体具有随空间周期性变化的线性极化系数(
图 2. 非线性光子晶体和相位匹配的示意图[8]。(a)一维非线性光子晶体和相位匹配条件情形;(b)二维非线性光子晶体和相位匹配条件;(c)三维非线性光子晶体和相位匹配条件
Fig. 2. Schematics of nonlinear photonic crystal and involved phase-matching[8]. (a) 1D nonlinear photonic crystal and its phase-matching condition; (b) 2D nonlinear photonic crystal and its phase-matching condition; (c) 3D nonlinear photonic crystal and its phase-matching condition
为弥补一维非线性光子晶体的不足,Berger[2]提出了二维非线性光子晶体的概念,
式中:
2009年,上海交通大学陈险峰研究组将二维非线性光子晶体进一步推广[3],提出了三维非线性光子晶体的概念,随后更多研究者对其理论进行了探讨[17-19]。在三维非线性光子晶体中,
式中:
式中:
式中:
式中:
3 飞秒激光制备非线性光子晶体的方法和机制
随着微纳加工技术的快速发展,研究者们已经能够在铁电晶体中构建非线性光子晶体,实现波长变换、光学混频、光学参量振荡等光学过程。根据上文所述理论,为了补偿基频光和倍频光的相位差,需要周期性地调制材料的
得益于极短的脉冲持续时间和超高的峰值功率,飞秒激光在微纳制造领域中占有重要地位,被广泛应用于微机械[26]、微光学[27]、微流控[28]、生物医学[29]等方面的器件加工。相比传统的连续激光或纳秒激光利用热效应进行材料的去除,飞秒激光脉冲持续时间只有10-15 s数量级,远小于材料中受激电子的能量释放时间,使材料免受热扩散的不利影响,热影响区域可以忽略不计,大大提高了加工精度[5]。由于飞秒激光焦点具有超高的能量密度,在被加工材料的相互作用过程中产生了各种非线性光学效应,使加工分辨率突破了衍射极限,且加工材料不受限制,易于在非线性晶体中实现微米甚至亚微米级别的周期性畴结构。此外,飞秒激光作用于材料时,只有焦点处的能量大小才能满足多光子非线性吸收要求,激光无损耗地通过透明材料,聚焦于材料内部。这种在材料内部任意三维位置处实现精细加工的能力超越了外加电场极化等传统方法,使三维非线性光子晶体的制备成为可能。因此,研究者们采用近红外波段飞秒激光,结合三维移动工作台,实现激光焦点在非线性晶体中的扫描加工,目前已经提出了两种飞秒激光与非线性晶体的作用机制,一种是基于非线性吸收效应产生的热释电场反转铁电畴极化方向的“反转”法,另一种是利用激光照射晶体,使晶体非晶化以降低甚至完全去除
3.1 飞秒激光反转铁电畴的机理
早在1994年,爱尔兰科克大学Fahy研究组从声子动力学的角度论证了高强度光脉冲反转铁电畴的可行性[30]。将LiNbO3中的Li离子看作简谐耦合的阻尼振子,如果激光脉冲传递给振子足够的能量使其能越过能量势垒,系统就能从一个稳定状态切换到另一个稳定状态,从而造成自发极化方向的反转。陈险峰研究组在此模型基础上,把激光脉冲和晶体的作用考虑进去,分析了晶体的电场时空分布对离子加速过程的影响[31]。他们发现,随着光脉冲电场振幅的增加,铁电畴的反转与反转后的回转交替出现,只有能量位于某些“反转窗”范围内的脉冲才能引起铁电畴反转,而脉冲能量越高,反转窗的能量宽度也越大。实验也证实了只有超过特定阈值能量的飞秒激光照射y向切LiNbO3晶体才能在样品表面形成圆形的反转畴区域,且区域面积随着能量的增大而增大。为了促使锂离子振荡,激光偏振方向必须和晶体自发极化方向一致,因此实验中只有偏振方向与晶体z轴平行的光才能形成反转畴区域。该研究组随后进一步研究了不同激光能量对反转区域半径大小的影响,发现铁电畴的反转只有在特定的能量密度范围内才能实现,与能量的大小无关[32]。
上述观点认为,铁电畴的反转原因是飞秒激光脉冲的强电场引发了Li离子稳定状态的切换。与上述观点不同,有研究组观察到反转过程持续时间较长,但不是如Fahy模型那样在皮秒级别的时间内完成,表明铁电畴反转是随时间累积的激光脉冲加热造成的[33]。飞秒激光焦点部位材料的多光子吸收产生极高的温度,温度梯度产生热电场或热释电场,如果电场方向和材料自发极化方向相反而强度又超过矫顽场的强度,该区域自发极化方向就能发生反转。此前有研究者基于类似机理,用高强度紫外激光辐射铁电晶体局部,从而生成反转电畴[34-36]。根据上述机理,只有焦点从垂直于z轴的-z面沿z轴向垂直于z轴的+z面移动时,焦点周围热梯度的方向才与晶体自发极化方向相反,在热释电场的作用下铁电畴反转区域能持续生长,而光束聚焦在+z面上时,热释电场方向和自发极化方向相同,无法生成有效的反转区域。因此,理论上飞秒激光加工铁电畴反转结构的位置和方向会受到限制,但加工过程只是反转了晶体的自发极化方向,保留了晶格结构的完整性,因此与飞秒激光擦除法相比,制备的准相位匹配结构中光的传输损耗较小。
2018年,德国明斯特大学Denz研究组提出了一种新型的飞秒激光反转极化机理[37]。如
图 3. 飞秒激光和热处理相结合在LiNbO3晶体中制备反转畴结构[37]。(a)飞秒激光在样品中加工丝状结构;(b)经过热处理后在丝状结构下方形成反转畴;(c)反转畴晶格结构下表面的Čerenkov型二次谐波显微镜图;(d)反转畴晶格结构的Čerenkov型二次谐波显微镜三维图
Fig. 3. Formation of inverted domains in LiNbO3 crystals by combining femtosecond laser processing with thermal treatment[37]. (a) Femtosecond-laser induced filaments in sample; (b) inverted domains below filaments after thermal treatment; (c) Čerenkov second-harmonic generation micrograph of lower surface of inverted domain lattice; (d) 3D Čerenkov second-harmonic generation micrograph of inverted domain lattice
3.2 飞秒激光反转铁电畴制备非线性光子晶体
2015年,澳大利亚国立大学Yan研究组首次实验演示了飞秒激光反转铁电畴[38]。实验使用波长为800 nm、脉宽为180 fs、重复频率为76 MHz、脉冲能量为4 nJ的飞秒激光,激光焦点从LiNbO3晶体-z面向+z面进行扫描加工。随后对加工区域进行Čerenkov型二次谐波显微表征,观察到了锥形二次谐波信号,证明了反转铁电畴的存在。用周期性反转畴可以构建出方形晶格和其他排布方式的阵列结构,当相邻反转畴间距小于1.5 μm时,反转畴之间相互融合,因此能实现的最小结构周期为1.5 μm。晶格结构的二次谐波显微成像显示,反转铁电畴区域延伸到晶体表面以下60 μm,但随着深度继续增加,反转畴区域逐渐缩小,难以保持初始形状,如
图 4. 飞秒激光在LiNbO3晶体中反转铁电畴加工一维和二维非线性光子晶体。(a)表层15 μm深度和(b)晶体内部大深度下方形晶格反转畴结构的Čerenkov型二次谐波显微镜三维图[38];(c)波导中反转畴结构的光学显微镜图[47];(d)反转畴结构的Čerenkov型二次谐波显微镜三维图[47]
Fig. 4. Fabrications of 1D and 2D nonlinear photonic crystals in LiNbO3 crystals by femtosecond laser domain inversion. 3D Čerenkov second-harmonic generation micrographs of square lattice inversion domain structure (a) at depth of 15 μm in surface layer and (b) at large depth inside crystal[38]; (c) optical micrograph of inverted domain structure in waveguide[47]; (d) 3D Čerenkov second-harmonic generation micrograph of inverted domain structure[47]
飞秒激光反转
2018年,Yan研究组发挥飞秒激光在平面之外第三个维度上调制
图 5. 飞秒激光在CBN晶体中反转电畴加工三维非线性光子晶体[49]。(a)3D反转畴结构的Čerenkov型二次谐波显微镜图;(b)多畴晶体中制备的六边形反转畴结构生成的二次谐波图案;(c)单畴晶体中制备的反转畴结构生成的二次谐波图案
Fig. 5. Fabrication of 3D nonlinear photonic crystals in CBN crystal by femtosecond laser domain inversion[49]. (a) 3D inverted domain structure visualized by Čerenkov second-harmonic generation micrograph; (b) second-harmonic pattern obtained for hexagonal inverted domain structures fabricated in multidomain crystal; (c) second-harmonic pattern obtained for inverted domain structures fabricated in monodomain crystal
3.3 飞秒激光擦除 的机理
早期一些研究者利用飞秒激光对LiNbO3的结构进行修饰,发现激光修饰区域折射率改变的同时还伴随着材料非线性的降低[50]。也有研究者发现,在LiNbO3中,飞秒激光直写Ⅰ型波导区域非线性的降低制约了输出二次谐波的转换效率[51-52]。进一步研究发现,这种非线性的降低与晶体结构的改变有关。Deshpande研究组详细研究了飞秒激光对LiNbO3的物理化学改性,对激光加工单点的形貌进行分析,发现中央烧蚀区域外围存在一圈非晶态区域[53]。他们认为可能是瞬时的双光子吸收加热使晶体熔化,但是光束边缘区域的能量不足以使晶体再结晶,冷却后仍保持非晶态。Nolte研究组将超快激光对铁电晶体结构的修饰作用分为两种:一种情况是焦点区域积累的能量较低,折射率升高,晶格结构会受到较弱的损伤,造成的非线性降低能通过高温处理恢复;另一种情况是较高的能量造成晶格结构被强烈破坏,应力诱导折射率降低的同时材料的非线性特性被完全去除,无法通过高温处理恢复[54]。综合以上研究,飞秒激光擦除
理想的擦除工艺使晶体周期性地引入
式中:
3.4 飞秒激光擦除 制备非线性光子晶体
2013年,Nolte研究组第一次演示了利用激光擦除
图 6. 飞秒激光在LiNbO3晶体中制备一维非线性光子晶体。(a)飞秒激光线扫描擦除非线性系数示意图[56];(b)制备的准相位匹配结构的横截面显微镜图[56];(c)飞秒激光直写波导和在波导中擦除非线性系数[57];(d)波导中集成单周期和四周期准相位匹配结构[58];(e)并行集成四段准相位匹配结构波导的温度调控曲线[58]
Fig. 6. 1D nonlinear photonic crystals fabricated inside LiNbO3 by femtosecond laser. (a) Schematic of erasing nonlinear coefficients by femtosecond laser line scanning[56]; (b) microscopy image of cross section of fabricated quasi-phase matching structure[56]; (c) femtosecond laser direct writing waveguide and erasing nonlinear coefficients in waveguide[57]; (d) quasi-phase matching structures with one period and four periods embedded in waveguides[58]; (e) temperature tuning curves of waveguide embedded with four parallel quasi-phase matching structures[58]
2015年,Denz研究组利用飞秒激光在LiNbO3晶体中同时完成了波导的直写和准相位匹配结构的集成[57],如
2018年,张勇研究组用飞秒激光擦除
图 7. 飞秒激光通过在LiNbO3晶体中擦除非线性系数加工三维非线性光子晶体[55]。三维非线性光子晶体的(a)前两层结构和(b)局部三层结构的Čerenkov型二次谐波显微镜图;(c)三维非线性光子晶体上层结构的光学显微镜图;(d)三维准相位匹配输出二次谐波示意图
Fig. 7. 3D nonlinear photonic crystal fabricated by erasing nonlinear coefficients in LiNbO3 crystals with femtosecond laser[55]. (a) First two-layer structure and (b) partial three-layer structure of 3D nonlinear photonic crystal visualized by Čerenkov second-harmonic generation micrographs; (c) optical micrograph of top layer structure of 3D nonlinear photonic crystal; (d) schematic of second harmonic emission enabled by 3D quasi-phase matching
4 飞秒激光制备非线性光子晶体的应用
与传统电场极化方法相比,飞秒激光加工方法在简化实验设备和工艺步骤的同时,不受限于特定的加工方位和晶体厚度,可在晶体内部任意位置加工任意分布的
4.1 非线性结构光生成
通过飞秒激光对晶体
图 8. 飞秒激光加工二维非线性光子晶体用于非线性结构光生成。(a)飞秒激光选择性擦除铁电畴加工得到的HG10、HG11、HG12全息图案的二次谐波图[59];(b)HG10、HG11、HG12结构输出的第一衍射级光束形貌[59];(c)将叉形光栅和锥透镜功能集成在非线性光子晶体中用于生成完美涡旋光二次谐波[60];制备的带不同拓扑荷结构的(d)非线性显微图像和(e)生成的二次谐波强度分布[60]
Fig. 8. Femtosecond laser fabrication of two-dimensional nonlinear photonic crystals for nonlinear structured light generation. (a) Second-harmonic images of HG10, HG11, HG12 holographic patterns fabricated by femtosecond laser selective erasing ferroelectric domain[59]; (b) beam profiles at first diffraction order in output from HG10, HG11, HG12 structures[59]; (c) integrating function of fork grating and axicon into nonlinear photonic crystal to generate second harmonic beam of perfect vortex light[60]; (d) nonlinear micrographs and (e) intensity distribution of emitted second harmonic beam of fabricated structures with different topological charges[60]
平面
图 9. 飞秒激光反转铁电畴加工的三维非线性光子晶体用于非线性光束整形[61]。(a)由不同方向叉形光栅组成的三层结构;(b)三层叉形光栅结构的Čerenkov型二次谐波显微镜三维图;(c)三层叉形光栅结构输出的远场二次谐波图样;(d)由叉形光栅、线性光栅和环形光栅组成的三层结构;(e)三层不同光栅结构的Čerenkov型二次谐波显微镜三维图;(f)三层不同光栅结构输出的远场二次谐波图样
Fig. 9. Nonlinear beam shaping with 3D photonic crystals fabricated by femtosecond laser domain inversion[61]. (a) Three-layer structure comprised of fork gratings with different orientations; (b) 3D three-layer fork structure visualized by Čerenkov second-harmonic generation micrograph; (c) far-field second-harmonic pattern emitted from three-layer fork structure; (d) three-layer structure comprised of fork, linear and circular gratings; (e) 3D three-layer structure comprised of different gratings visualized by Čerenkov second-harmonic generation micrograph; (f) far-field second-harmonic pattern emitted from three-layer structure comprised of different gratings
在上述工作制备的二维和三维非线性光子晶体中,对应每个调制光束的
图 10. 飞秒激光加工三维非线性光子晶体用于高效光束整形。(a)三维非线性光子晶体x-z和x-y面的二次谐波显微镜图[62];(b)不同输入波长下产生的二次谐波衍射图样和对应的准相位匹配形式[62];(c)设计得到的非线性体全息结构[63];(d)在CBN晶体中实际加工的非线性体全息结构;(e)非线性体全息重建的二次谐波涡旋光束[63]
Fig. 10. High-efficient beam shaping with 3D nonlinear photonic crystals fabricated by femtosecond laser. (a) 3D nonlinear photonic crystal visualized by second-harmonic micrographs in x-z and x-y planes[62]; (b) second-harmonic diffraction patterns at different input wavelengths and their corresponding quasi-phase matching configurations[62]; (c) designed nonlinear volume holographic pattern[63]; (d) fabricated nonlinear volume hologram in CBN crystal; (e) second harmonic vortex beam reconstructed from nonlinear volume hologram[63]
2020年,Yan课题组继续拓展三维非线性光子晶体的设计思路,首次制备了非线性体全息结构并用于非线性涡旋光束生成[63]。之前报道的三维非线性光子晶体由沿纵向排列的基本平面全息
4.2 非线性全息成像
除了在新波段输出具有特定振幅和相位分布的结构光之外,非线性光束整形还可以重建更为复杂的图像信息。传统的非线性计算全息采用二值化编码,计算得到的是复杂而无序的铁电畴图案,而飞秒激光极化或擦除
图 11. 飞秒激光加工迂回相位编码全息图用于非线性全息成像[64]。(a)所制备的迂回相位编码全息图整体和4种基本单元的二次谐波显微镜图;(b)实验测得的远场二次谐波字母图像;(c)通过增大全息图像素数量仿真得到的更高质量的远场二次谐波字母图像
Fig. 11. Detour phase coded holograms processed by femtosecond laser for nonlinear holographic imaging[64]. (a) Fabricated whole detour phase coded hologram and four basic units visualized by second-harmonic micrographs; (b) experimentally measured second harmonic holographic image of letter in far field; (c) simulated second harmonic holographic image of letter in far field with improved quality obtained by increasing numbers of hologram pixels
通过在非线性全息中引入多通道复用技术,可以在单个非线性光子晶体中编码多个图案信息,并在特定条件下分别再现出来。2021年,张勇研究组提出了准相位匹配分区复用的概念[66]。他们将多个目标图案分别编码在3D倒空间的不同Ewald球面上,每个球面都是一组满足特定准相位匹配条件的倒格矢集合,通过改变输入的基频光波长来实现相应的准相位匹配条件,从而在远场成像区域选择性地重建不同球面上的图案,如
图 12. 飞秒激光加工三维非线性光子晶体用于准相位匹配分区复用全息[66]。(a)准相位匹配分区复用非线性全息原理示意图;(b)利用飞秒激光擦除非线性系数在LiNbO3晶体内部实现3D迂回相位编码;(c)三通道准相位匹配分区复用全息的远场二次谐波成像结果
Fig. 12. 3D nonlinear photonic crystals processed by femtosecond laser for quasi-phase-matching-division multiplexing holography[66]. (a) Schematic of quasi-phase-matching-division multiplexing nonlinear holography; (b) detour phase encoding in LiNbO3 realized by femtosecond laser erasing nonlinear coefficients; (c) far-field second harmonic imaging results of three-channel quasi-phase-matching-division multiplexing holography
4.3 深紫外激光光源
非线性晶体的频率转换特性常被用于拓宽激光器的输出波长。尽管目前常用的各种非线性晶体能通过变频获得宽广的波长范围,但一些特殊波段,如深紫外波段仍具有挑战性。实用化的深紫外相干光源需要非线性晶体有足够的透明波段下边缘和较大的双折射特性,以满足基频光和倍频光的相位匹配条件[67-68]。目前只有KBe2BO3F2(KBBF)一种晶体能通过级联频率转换获得实用化的深紫外相干输出[69-70],但这种晶体生长困难且原材料具有毒性。凭借飞秒激光擦除
图 13. 飞秒激光在石英晶体中加工准相位匹配结构用于深紫外相干输出。(a)深紫外二次谐波产生实验装置[71];(b)加工非线性结构的石英晶体和原生石英晶体产生的二次谐波信号[71];(c)石英中多层准相位匹配结构的角度调控[72];(d)改变相位匹配角度调控二次谐波输出波长和晶体的有效二阶非线性系数[72]
Fig. 13. Femtosecond laser processing quasi-phase-matching structure in quartz crystal for deep-ultraviolet coherent output. (a) Experimental setup of deep-ultraviolet second harmonic generation[71]; (b) second harmonic signals in quartz crystal with nonlinear structure and as-grown quartz crystal[71]; (c) angle regulation of multilayer quasi-phase-matching structure in quartz[72]; (d) tuning second harmonic generation wavelength and effective second-order nonlinear coefficient of crystal by varying phase-matching angle[72]
2022年,该课题组在石英准相位匹配结构中引入角度调控以扩宽输出波长范围[72],将飞秒激光加工的一维周期结构在深度方向上由一层扩展为多层以满足大范围角度调控的需求。基频光以不同的空间角度入射晶体,在经过相邻晶态/非晶态区域后,光程发生变化,从而满足不同波长二次谐波的准相位匹配条件。加工周期为4.2 μm的五层结构时,将
5 结束语与展望
作为一种新兴的微纳米制造手段,飞秒激光加工技术具有极高的精度和分辨率,具有在多种材料内部任意位置加工精细三维结构的能力,因此成为修饰非线性晶体材料的理想工具。介绍了飞秒激光构建非线性光子晶体的研究进展,基于飞秒激光反转铁电畴和擦除
在晶体内部进行加工时,飞秒激光需要经过两层折射率不同的介质,折射率失配引起的像差会导致聚焦光斑在光轴方向上的位置偏移和焦斑畸变,严重限制了加工的轴向分辨率和位置精确性,影响了
在已报道的工作中,鲜有将非线性光子晶体与其他光学器件结合以扩展其功能的尝试。利用飞秒激光加工多种结构的灵活性以及与其他加工技术的良好兼容性,有望实现单一铁电晶体基底上电光调制器、谐振器、波导、非线性频率转换器等多种功能性光学器件的高效集成。此外,基于飞秒激光将单纯的准相位匹配结构扩展为具有更复杂结构、更强大功能的集成式光子芯片,该芯片有望在现代光通信、光信号处理、量子计算等领域中获得广泛的应用。
目前,飞秒激光擦除工艺只能使
除上述方向外,还有一些方向值得进一步探索,如利用飞秒激光制备形式多样化的径向对称型非线性光子晶体及准周期和非周期非线性光子晶体以提供更丰富的倒格矢,支持更复杂的非线性光学过程;研究结合空间光整形技术的高效率飞秒激光加工技术[75-76],解决单点直写耗时长的问题,为产业化奠定基础;将飞秒激光加工工艺推向更丰富的非线性材料中,拓宽此技术的适用范围。总之,在飞秒激光加工技术的支持下,非线性光子晶体的发展前景不可估量,将在更多领域的科学研究和应用开发方面发挥越来越重要的作用。
[1] Armstrong J A, Bloembergen N, Ducuing J, et al. Interactions between light waves in a nonlinear dielectric[J]. Physical Review, 1962, 127(6): 1918-1939.
[2] Berger V. Nonlinear photonic crystals[J]. Physical Review Letters, 1998, 81(19): 4136-4139.
[3] Chen J J, Chen X F. Phase matching in three-dimensional nonlinear photonic crystals[J]. Physical Review A, 2009, 80(1): 013801.
[4] Yamada M, Nada N, Saitoh M, et al. First-order quasi-phase matched LiNbO3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generation[J]. Applied Physics Letters, 1993, 62(5): 435-436.
[5] Sugioka K, Cheng Y. Ultrafast lasers: reliable tools for advanced materials processing[J]. Light: Science & Applications, 2014, 3(4): e149.
[6] Franken P A, Ward J F. Optical harmonics and nonlinear phenomena[J]. Reviews of Modern Physics, 1963, 35(1): 23-39.
[7] Fejer M M, Magel G A, Jundt D H, et al. Quasi-phase-matched second harmonic generation: tuning and tolerances[J]. IEEE Journal of Quantum Electronics, 1992, 28(11): 2631-2654.
[8] 马博琴, 王霆. 非线性光子晶体的研究[M]. 北京: 北京理工大学出版社, 2013: 17-19.
MaB Q, WangT. Researches on nonlinear photonic crystals[M]. Beijing: Beijing Institute of Technology Press, 2013: 17-19.
[9] Zhu S N, Zhu Y Y, Ming N B, et al. Quasi-phase-matched third-harmonic generation in a quasi-periodic optical superlattice[J]. Science, 1997, 278(5339): 843-846.
[10] Zhu S N, Zhu Y Y, Qin Y Q, et al. Experimental realization of second harmonic generation in a Fibonacci optical superlattice of LiTaO3[J]. Physical Review Letters, 1997, 78(14): 2752-2755.
[11] Wang X H, Gu B Y. Nonlinear frequency conversion in 2D χ(2) photonic crystals and novel nonlinear double-circle construction[J]. The European Physical Journal B - Condensed Matter and Complex Systems, 2001, 24(3): 323-326.
[12] Saltiel S M, Kivshar Y S. Phase matching in nonlinear χ(2) photonic crystals[J]. Optics Letters, 2000, 25(16): 1204-1206.
[13] Saltiel S M, Kivshar Y S. All-optical deflection and splitting by second-order cascading[J]. Optics Letters, 2002, 27(11): 921-923.
[14] Saltiel S m, Krolikowski W, Neshev D N, et al. Generation of Bessel beams by parametric frequency doubling in annular nonlinear periodic structures[J]. Optics Express, 2007, 15(7): 4132-4138.
[15] Saltiel S M, Neshev D N, Fischer R, et al. Generation of second-harmonic conical waves via nonlinear Bragg diffraction[J]. Physical Review Letters, 2008, 100(10): 103902.
[16] Arie A, PeriodicVoloch N. quasi-periodic, and random quadratic nonlinear photonic crystals[J]. Laser & Photonics Reviews, 2010, 4(3): 355-373.
[17] Zhang J, Zhao X H, Zheng Y L, et al. Universal modeling of second-order nonlinear frequency conversion in three-dimensional nonlinear photonic crystals[J]. Optics Express, 2018, 26(12): 15675-15682.
[18] Pogosian T, Lai N D. Theoretical investigation of three-dimensional quasi-phase-matching photonic structures[J]. Physical Review A, 2016, 94(6): 063821.
[19] 杜金恒, 宋伟, 张怀金. 三维准相位匹配研究进展[J]. 中国激光, 2021, 48(12): 1208001.
[20] Chen J J, Chen X F. Generation of conical and spherical second harmonics in three-dimensional nonlinear photonic crystals with radial symmetry[J]. Journal of the Optical Society of America B, 2011, 28(2): 241-246.
[21] Feng D, Ming N B, Hong J F, et al. Enhancement of second-harmonic generation in LiNbO3 crystals with periodic laminar ferroelectric domains[J]. Applied Physics Letters, 1980, 37(7): 607-609.
[22] Wang W S, Zou Q, Geng Z H, et al. Study of LiTaO3 crystals grown with a modulated structure I. Second harmonic generation in LiTaO3 crystals with periodic laminar ferroelectric domains[J]. Journal of Crystal Growth, 1986, 79(1/2/3): 706-709.
[23] Lim E J, Fejer M M, Byer R L, et al. Blue light generation by frequency doubling in periodically poled lithium niobate channel waveguide[J]. Electronics Letters, 1989, 25(11): 731-732.
[24] Lim E J, Fejer M M, Byer R L. Second-harmonic generation of green light in periodically poled planar lithium niobate waveguide[J]. Electronics Letters, 1989, 25(3): 174-175.
[25] Webjorn J, Laurell F, Arvidsson G. Blue light generated by frequency doubling of laser diode light in a lithium niobate channel waveguide[J]. IEEE Photonics Technology Letters, 1989, 1(10): 316-318.
[26] Maruo S, Ikuta K, Korogi H. Submicron manipulation tools driven by light in a liquid[J]. Applied Physics Letters, 2002, 82(1): 133-135.
[27] Wu D, Wu S Z, Niu L G, et al. High numerical aperture microlens arrays of close packing[J]. Applied Physics Letters, 2010, 97(3): 031109.
[28] Amato L, Gu Y, Bellini N, et al. Integrated three-dimensional filter separates nanoscale from microscale elements in a microfluidic chip[J]. Lab on a Chip, 2012, 12(6): 1135-1142.
[29] Narayan R, Goering P. Laser micro- and nanofabrication of biomaterials[J]. MRS Bulletin, 2011, 36(12): 973-982.
[30] Fahy S, Merlin R. Reversal of ferroelectric domains by ultrashort optical pulses[J]. Physical Review Letters, 1994, 73(8): 1122-1125.
[32] Lao H Y, Zhu H S, Chen X F. Threshold fluence for domain reversal directly induced by femtosecond laser in lithium niobate[J]. Applied Physics A, 2010, 101(2): 313-317.
[33] Chen X, Karpinski P, Shvedov V, et al. Two-dimensional domain structures in lithium niobate via domain inversion with ultrafast light[J]. Photonics Letters of Poland, 2016, 8(2): 33-35.
[34] Muir A C, Sones C L, Mailis S, et al. Direct-writing of inverted domains in lithium niobate using a continuous wave ultra violet laser[J]. Optics Express, 2008, 16(4): 2336-2350.
[35] Steigerwald H, Ying Y J, Eason R W, et al. Direct writing of ferroelectric domains on the x- and y-faces of lithium niobate using a continuous wave ultraviolet laser[J]. Applied Physics Letters, 2011, 98(6): 062902.
[36] Sones C L, Valdivia C E, Scott J G, et al. Ultraviolet laser-induced sub-micron periodic domain formation in congruent undoped lithium niobate crystals[J]. Applied Physics B, 2005, 80(3): 341-344.
[37] Imbrock J, Hanafi H, Ayoub M, et al. Local domain inversion in MgO-doped lithium niobate by pyroelectric field-assisted femtosecond laser lithography[J]. Applied Physics Letters, 2018, 113(25): 252901.
[38] Chen X, Karpinski P, Shvedov V, et al. Ferroelectric domain engineering by focused infrared femtosecond pulses[J]. Applied Physics Letters, 2015, 107(14): 141102.
[39] Liu S, Switkowski K, Chen X, et al. Broadband enhancement of Čerenkov second harmonic generation in a sunflower spiral nonlinear photonic crystal[J]. Optics Express, 2018, 26(7): 8628-8633.
[40] Zhang S G, Yao J H, Shi Q, et al. Fabrication and characterization of periodically poled lithium niobate waveguide using femtosecond laser pulses[J]. Applied Physics Letters, 2008, 92(23): 231106.
[41] Huang Z C, Tu C H, Zhang S G, et al. Femtosecond second-harmonic generation in periodically poled lithium niobate waveguides written by femtosecond laser pulses[J]. Optics Letters, 2010, 35(6): 877-879.
[42] Campbell S, Thomson R R, Hand D P, et al. Frequency-doubling in femtosecond laser inscribed periodically-poled potassium titanyl phosphate waveguides[J]. Optics Express, 2007, 15(25): 17146-17150.
[43] Zhang S G, Yao J H, Liu W W, et al. Second harmonic generation of periodically poled potassium titanyl phosphate waveguide using femtosecond laser pulses[J]. Optics Express, 2008, 16(18): 14180-14185.
[44] Wang L, Zhang X T, Li L Q, et al. Second harmonic generation of femtosecond laser written depressed cladding waveguides in periodically poled MgO: LiTaO3 crystal[J]. Optics Express, 2019, 27(3): 2101-2111.
[45] Triplett M, Khaydarov J, Xu X Z, et al. Multi-watt, broadband second-harmonic-generation in MgO: PPSLT waveguides fabricated with femtosecond laser micromachining[J]. Optics Express, 2019, 27(15): 21102-21115.
[46] 丁烨, 李强, 李靖怡, 等. 超快激光在无源光波导器件制造中的应用综述[J]. 中国激光, 2021, 48(8): 0802020.
[47] Chen X, Karpinski P, Shvedov V, et al. Quasi-phase matching via femtosecond laser-induced domain inversion in lithium niobate waveguides[J]. Optics Letters, 2016, 41(11): 2410-2413.
[48] Xu T X, Switkowski K, Chen X, et al. Three-dimensional nonlinear photonic crystal in ferroelectric Barium calcium titanate[J]. Nature Photonics, 2018, 12(10): 591-595.
[49] Mazur L M, Liu S, Chen X, et al. Localized ferroelectric domains via laser poling in monodomain calcium Barium niobate crystal[J]. Laser & Photonics Reviews, 2021, 15(9): 2100088.
[50] Burghoff J, Hartung H, Nolte S, et al. Structural properties of femtosecond laser-induced modifications in LiNbO3[J]. Applied Physics A, 2007, 86(2): 165-170.
[51] Lee Y L, Yu N E, Jung C, et al. Second-harmonic generation in periodically poled lithium niobate waveguides fabricated by femtosecond laser pulses[J]. Applied Physics Letters, 2006, 89(17): 171103.
[52] Osellame R, Lobino M, Chiodo N, et al. Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient[J]. Applied Physics Letters, 2007, 90(24): 241107.
[53] Deshpande D C, Malshe A P, Stach E A, et al. Investigation of femtosecond laser assisted nano and microscale modifications in lithium niobate[J]. Journal of Applied Physics, 2005, 97(7): 074316.
[54] Burghoff J, Nolte S, Tünnermann A. Origins of waveguiding in femtosecond laser-structured LiNbO3[J]. Applied Physics A, 2007, 89(1): 127-132.
[55] Wei D Z, Wang C W, Wang H J, et al. Experimental demonstration of a three-dimensional lithium niobate nonlinear photonic crystal[J]. Nature Photonics, 2018, 12(10): 596-600.
[56] Thomas J, Hilbert V, Geiss R, et al. Quasi phase matching in femtosecond pulse volume structured x-cut lithium niobate[J]. Laser & Photonics Reviews, 2013, 7(3): L17-L20.
[57] Kroesen S, Tekce K, Imbrock J, et al. Monolithic fabrication of quasi phase-matched waveguides by femtosecond laser structuring the χ(2) nonlinearity[J]. Applied Physics Letters, 2015, 107(10): 101109.
[58] Imbrock J, Wesemann L, Kroesen S, et al. Waveguide-integrated three-dimensional quasi-phase-matching structures[J]. Optica, 2020, 7(1): 28-34.
[59] Zhu B, Liu H G, Chen Y P, et al. High conversion efficiency second-harmonic beam shaping via amplitude-type nonlinear photonic crystals[J]. Optics Letters, 2019, 45(1): 220-223.
[60] Liu D W, Liu S, Mazur L M, et al. Smart optically induced nonlinear photonic crystals for frequency conversion and control[J]. Applied Physics Letters, 2020, 116(5): 051104.
[61] Liu S, Switkowski K, Xu C L, et al. Nonlinear wavefront shaping with optically induced three-dimensional nonlinear photonic crystals[J]. Nature Communications, 2019, 10: 3208.
[62] Wei D Z, Wang C W, Xu X Y, et al. Efficient nonlinear beam shaping in three-dimensional lithium niobate nonlinear photonic crystals[J]. Nature Communications, 2019, 10(1): 4193.
[63] Liu S, Mazur L M, Królikowski W, et al. Nonlinear volume holography in 3D nonlinear photonic crystals[J]. Laser & Photonics Review, 2020, 14(11): 2000224.
[64] Wang B X, Hong X M, Wang K, et al. Nonlinear detour phase holography[J]. Nanoscale, 2021, 13(4): 2693-2702.
[65] Zhu B, Liu H G, Liu Y A, et al. Second-harmonic computer-generated holographic imaging through monolithic lithium niobate crystal by femtosecond laser micromachining[J]. Optics Letters, 2020, 45(15): 4132-4135.
[66] Chen P C, Wang C W, Wei D Z, et al. Quasi-phase-matching-division multiplexing holography in a three-dimensional nonlinear photonic crystal[J]. Light: Science & Applications, 2021, 10(1): 146.
[67] Halasyamani P S, Rondinelli J M. The must-have and nice-to-have experimental and computational requirements for functional frequency doubling deep-UV crystals[J]. Nature Communications, 2018, 9: 2972.
[68] 赵智刚, 玄洪文, 王景冲, 等. 真空紫外193 nm波段固体激光器研究进展综述[J]. 光学学报, 2022, 42(11): 1134010.
[69] Togashi T, Kanai T, Sekikawa T, et al. Generation of vacuum-ultraviolet light by an optically contacted, prism-coupled KBe2BO3F2 crystal[J]. Optics Letters, 2003, 28(4): 254-256.
[70] Dai S B, Chen M, Zhang S J, et al. 2.14 mW deep-ultraviolet laser at 165 nm by eighth-harmonic generation of a 1319 nm Nd∶YAG laser in KBBF[J]. Laser Physics Letters, 2016, 13(3): 035401.
[71] Shao M C, Liang F, Yu H H, et al. Pushing periodic-disorder-induced phase matching into the deep-ultraviolet spectral region: theory and demonstration[J]. Light: Science & Applications, 2020, 9: 45.
[72] Shao M C, Liang F, Yu H H, et al. Angular engineering strategy of an additional periodic phase for widely tunable phase-matched deep-ultraviolet second harmonic generation[J]. Light: Science & Applications, 2022, 11: 31.
[73] Jesacher A, Booth M J. Parallel direct laser writing in three dimensions with spatially dependent aberration correction[J]. Optics Express, 2010, 18(20): 21090-21099.
[74] Cumming B P, Jesacher A, Booth M J, et al. Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate[J]. Optics Express, 2011, 19(10): 9419-9425.
[75] 孙伟高, 季凌飞, 郑锦灿, 等. 飞秒贝塞尔光束直写铌酸锂高深径比光子晶体结构[J]. 中国激光, 2022, 49(10): 1002503.
[76] 丁铠文, 王聪, 罗志, 等. 超快激光光束整形原理与方法及其在功能性微结构制造中的应用[J]. 中国激光, 2021, 48(2): 0202005.
Article Outline
黎隆富, 张乐然, 徐力群, 李欣, 廖常锐, 王义平, 吴东. 飞秒激光制备非线性光子晶体研究进展[J]. 中国激光, 2023, 50(8): 0802401. Longfu Li, Leran Zhang, Liqun Xu, Xin Li, Changrui Liao, Yiping Wang, Dong Wu. Research Progress on Femtosecond Laser Fabrication of Nonlinear Photonic Crystals[J]. Chinese Journal of Lasers, 2023, 50(8): 0802401.