飞秒激光与透明硬质材料的相互作用:从相变机理到永久光存储 下载: 627次特邀综述
Since the beginning of the 21st century, with the rise of the internet, artificial intelligence, cloud computing, and cloud storage, the amount of data generated has grown exponentially, thus leading to a sharp increase in the demand for data storage. According to the International Data Corporation (IDC), by 2025, the world is expected to produce a total of 175 ZB (1 ZB=1012 GB) of data, of which approximately 10%‒15% is expected to eventually evolve into cold data, which does not require frequent access but needs to be stored for a long time. Such data include the National Archives Center data, human civilization events, and scientific achievements.
Cold data can be stored in several ways, including in mechanical hard drives, solid-state drives, and tape storage. Among these, magnetic storage has problems, such as vulnerability to damage and high environmental requirements. In comparison, optical storage technology is durable, lighter, more energy-efficient, and has lower environmental requirements. Optical disk storage uses lasers to modify and engrave 0 and 1 on the surface of photosensitive materials to achieve storage. However, further reducing the point size of optical storage is challenging owing to the diffraction limit; consequently, increasing the capacity of optical disks is challenging.
A possible solution for increasing the storage capacity and reducing the refresh rate involves shifting from 2D storage to 3D storage, and from organic storage to inorganic optical storage. Femtosecond laser optical storage, which uses ultrafast lasers with extremely high peak powers to store data inside hard materials such as glass, sapphire, and diamond, has become a potential solution. Since 2003, several research institutions, including Kyoto University in Japan; the University of Southampton in the UK; and Tsinghua University, Huazhong University of Science and Technology, and Jilin University in China have introduced five-dimensional optical storage technology. Hitachi and Microsoft Research in Cambridge have also introduced unique ultrafast laser optical storage solutions.
However, femtosecond laser-based optical storage cannot meet industrial demands in terms of writing speed and storage capacity because of the complexity of the ultrafast action mechanism. Different powers, pulse numbers, and pulse widths produce different types of phase changes, thus affecting the optical properties of storage units and resulting in complex refractive index changes in the spatial distributions and birefringence properties. This correspondingly requires dozens or even hundreds of pulses, with pulse energies of tens to hundreds of nanojoules. Additionally, it prohibits both further reduction in the point spacing and interlayer spacing for high-density storage, and efficient processing through beam splitting with limited ultrafast laser energy, thus increasing the difficulty of industrialization. A study on the formation mechanism can offer ideas for accelerating the introduction of nanograting. Therefore, the formation mechanisms of the laser-induced modifications must be summarized.
Herein, we reviewed nearly 20 years of ultrafast mechanisms of interaction between lasers and transparent hard materials. First, we analyzed the physical process of the interaction between the femtosecond laser and transparent hard materials on a timescale. To specify the interaction between the femtosecond laser and transparent materials, the modification process and influencing factors induced by the femtosecond laser were investigated in detail. Based on the different types of structures induced by the femtosecond laser, the structures were divided into positive refractive index changes (Type Ⅰ), negative refractive index changes (Type Ⅱ), and scattering-based modification types (Types Ⅲ and Ⅳ). The formation mechanism and applications of each modified structure were discussed, and the application of nanogratings to 5D optical storage, including the principles, progress, and prospects, was explained in detail.
However, some problems remain in optical data storage based on nanogratings, such as low storage capacity and slow data-writing speed, must be overcome. To compensate for the deficiencies of this technology, new types of structures, namely Types X and S, were proposed herein. Type X nanopores are randomly distributed and generated by laser pulse width modulation, and are transient states between Types I and Ⅱ. These structures have very high transmittance, which is key for improving the storage capacity. Type S is an anisotropic nanolamella-like structure produced by fast pulse energy modulation, which can minimize unnecessary thermal effects caused by femtosecond pulses. These structures can improve the data writing speed.
Finally, we presented the challenges of 5D eternal optical storage as well as feasible solutions. Combined with the theory of far-field-induced near-field breakdown, the development of new materials is a promising research direction.
Eternal optical storage is a feasible solution for the demands of the big data era. However, the capacity and the writing speed both limit the practical use of this technology. Many researchers are attempting to overcome these problems and have made progress. We believe that external optical storage will be practical in the future.
1 引言
从石头上的雕刻到纸张上的书写,再到目前的数字化信息存储,信息存储方式的演变昭示着人类文明的发展历程。进入21世纪以来,伴随着互联网、人工智能、云计算和云存储的兴起,数据的产生量呈现指数增长[1-3],对数据存储的需求急剧增加。据国际数据公司(IDC)测算,到2025年,全球将产生175 ZB(1 ZB=
数据的存储有多种方式,机械硬盘(HDD)和固态硬盘(SSD)由于存储容量不断增加并且价格逐渐走低,逐渐占据了主流数据中心市场。然而,Google公司的数据显示[5],在大量数据应用的环境下,磁盘阵列的硬盘损坏率极高,6块硬盘同时工作2年的存活率为54%,3年存活率仅为30%,12块硬盘同时工作5年的存活率则低至1%。此外,无论是HDD还是SSD,都容易受到强磁影响,需要额外进行防磁处理,而且它们运行时会产生大量的废热,需要精密空调提供恒温恒湿环境。通常,HDD的使用寿命平均只有5~8年,而SSD的寿命则更是低至2~3年。因此,为避免数据丢失,在HDD和SSD的生命周期内要求每两到三年进行一次数据迁移[3]。苛刻的运行环境以及频繁的数据迁移带来了高能耗、高排放以及较多工业垃圾产生等问题[6]。开放数据中心委员会(ODCC)的测算数据显示,在数据中心加快建设部署的大背景下,2020年中国数据中心能耗总量为939×108 kW·h,碳排放量为64万吨。预计到2030年,中国数据中心能耗总量将达到3800×108 kW·h左右,碳排放增长率将超过300%[7]。高能耗导致HDD和SSD的总体持有成本(T.C.O)较高,因此,对于需要长时间存放的冷数据,磁带存储(TPE)是一种更好的方法。盒式磁带不易受到物理损坏,存储介质使用多年不轻易退磁,数据错误率也比HDD低4~5个数量级[8]。磁带盒可以简单地存放在机器人库的插槽中,不消耗电量[9]。然而,磁带对运行环境的要求较高,需要精密空调提供恒温恒湿环境,需要防磁、防尘,而且需要定期绕带。盒式磁带的使用寿命一般为10~30年[10-11]。
相比而言,光存储介质更耐用、更轻便、更省电,对环境要求更低,在20世纪80年代被发明后迅速流行,尤其是在国内引进蓝光等生产线后成为国内冷数据存储的主要方式之一。光存储是利用激光在感光材料表面通过改性刻划记录0、1来实现存储的。目前,采用405 nm激光光源和数值孔径为0.85的物镜,在蓝光光盘(BD)单面单层的盘面上可刻录25~27 GB的数据。然而,随着大数据时代的到来,尤其是考虑到降耗能、减耗材等方面的需求,光存储点尺寸由于衍射极限而难以继续缩小,导致光盘容量难以提升[12-15]。特别地,由于光盘上的有机感光材料老化,存储数据必须在5~10年内进行刷新,导致物耗以指数级数增加,数据迁移准确率大大降低。
从二维存储到三维存储(增加容量)、从有机存储到无机光存储(增加寿命),这是减小刷新率、降低物耗和能耗的重要发展趋势。利用具有极高峰值功率的超快激光在玻璃、蓝宝石、金刚石等硬质材料内部实现数据存储的飞秒激光光存储成为潜在的解决方案[16-18]。早在1997年,哈佛大学的Glezer等[8]就采用飞秒激光在熔融石英玻璃中通过微爆炸的方式实现了数据存储。自2003年以来,日本京都大学、英国南安普顿大学以及我国的清华大学、华中科技大学、吉林大学等科研院校相继推出了多维光存储技术[19-24],日立公司、微软英国剑桥研发中心也推出了独特的超快激光光存储方案。然而,基于飞秒激光的光存储在写入速度、存储容量方面还无法达到产业化需求,最根本的原因在于超快作用机制的复杂性:不同的功率、脉冲数、脉宽会产生不同类型的相变,影响着存储单元的光学性质,导致复杂的折射率改变量和双折射特性。具体表现为所需的脉冲数从几十到几百,单脉冲能量从几十到几百纳焦,无法通过减小点间距和层间距来实现高密度存储,在有限的超快激光能量内无法通过分束实现高效加工,导致产业化难度激增。
为此,本文梳理了近20年来超快激光诱导透明硬质材料的相变机制,从电子激发开始,分析了光与物质的相互作用过程,总结了飞秒激光诱导相变的类型和作用过程,分别就Type Ⅰ、Type Ⅱ、Type Ⅲ、Type X、Type Ⅳ等多种类型结构进行了讨论,并展示了其在光存储应用方面的初步应用;接着从写入速度和存储容量的角度出发,介绍了最新的脉宽调制效应和热调制高重复频率直写技术;最后阐述了五维永久光存储目前面临的挑战以及可行的解决办法。
2 飞秒激光与透明硬质材料的相互作用
飞秒激光的脉宽极短,导致其功率密度极高,局域电场强度高达109 V/m,这一数值可与氢原子核外电场强度相比拟[25]。飞秒激光与物质的相互作用具有非线性、非平衡等特征,自应用于材料加工以来就受到了广泛关注[26-27]。
在研究飞秒激光与透明介质的相互作用时,必须要综合考虑光子-电子、电子-电子、电子-晶格、晶格-晶格等多个作用体系。按照发生时间的先后,可简略地将上述过程总结为
图 1. 飞秒激光与物质相互作用。(a)飞秒激光激发固体内电子和晶格过程的时间尺度[28];(b)根据Keldysh理论计算出的间隙宽度为9 eV的熔融石英的电离率与激光强度的依赖关系(实线),以及其与多光子近似(虚线)、隧道电离近似(点划线)的对比[30]
Fig. 1. Interaction between femtosecond laser and material. (a) Timescale of a femtosecond laser exciting electron and lattice processes in solid[28]; (b) ionization rate for fused silica with a gap of 9 eV from Keldysh’s theory (solid line) as function of laser intensity, and their comparison between multiphoton ionization (dotted line) and tunnel ionization (dash-dotted line)[30]
1)光吸收和载流子激发。透明材料的禁带宽度通常要远大于单光子能量,因此,飞秒激光与透明介质相互作用一般会经历双光子甚至是多光子吸收过程,在几十到几百飞秒的超短脉冲辐照时间内,光能被转化为载流子(通常为电子)的动能和势能。然而,电子的弛豫时间通常只有几飞秒,激发的电子会进一步增强光子吸收,导致更多的电子“活化”。通常可通过半经典的能带理论将此过程分为3种机制[29]:①初期激发电子浓度较低,可只考虑价带顶对导带底的填充过程,即带间跃迁过程,此时电子的电离表现为多光子电离;②随着导带载流子浓度的增加,必须要考虑导带内电子的单光子吸收过程,即通过逆轫致辐射吸收过程实现雪崩电离等过程;③其他机制,如缺陷中的施主或受主的光吸收机制等。
特别地,光电离过程会受到激光峰值功率密度的严重影响,即:在带内光跃迁过程中,若单脉冲能量足够高,费米能级在带间填充过程中会导致能带倾斜,进而导致隧道电离发生。
多光子电离或隧道电离将价带的电子激发到导带,产生低能量的自由电子。低能量的自由电子通过逆轫致辐射线性地吸收能量,获得足够大的动能后与价带的电子发生碰撞,从而在导带内产生两个低能量电子。这一过程不断重复,使得自由电子的浓度呈指数形式增长。该过程被称为雪崩电离[31]。通常认为,当激光脉宽在100 fs以下时,雪崩电离的效应不显著[32-33]。
以上过程都可借鉴半导体材料中的电子激发过程进行理解。可以利用电子浓度的激发公式结合描述介电常数变化的Drude模型或者Lorentz-Drude模型表示上述过程,从而构建整个光吸收、电子激发到介电常数(复折射率)变化的过程。值得注意的是,上述3个过程在整个脉宽内也会彼此影响。材料性质(介电常数)、波长、脉宽等都会影响最终积累的电子浓度,这也为不同脉宽下的结构产生构建了一定的理论基础和调控基础。
2)能量传导和结构成形。能量传导过程主要发生在光辐射消失后,积聚的高浓度自由电子通过共振传播、碰撞等最终将能量传递给晶格。这一过程根据能量的传递形式一般分为非热(能)传导和热(能)传导两种,其中:前者以载流子-载流子散射为主,在库仑力作用下发生库仑爆炸或等离子体效应,实现表面非热去除[28];后者以晶格-晶格碰撞为主,导致晶格的部分体积变形或者间距变短,通过熔化、汽化、升华实现材料去除[34]。
具体而言,对于非热传导来说,由于飞秒激光与透明硬质材料的相互作用时间仅为几百飞秒,可在固体材料中激发出大量的电子,同时将大量价电子从成键态激发到反成键态,改变了原子间作用力,使电子和晶格远离平衡态,从而导致固体迅速转变成由原子核和外层电子以共同体形式组成的物质第四态——等离子体态,并在静电力作用下脱离表面形成材料去除。由于晶格在上述过程中没有经历熔化、汽化或者升华等的固液气相变化,热扩散较少,故而一般被称为冷加工[35]。近年来,随着对上述过程认识的加深,人们发现飞秒强电场还会诱导电子产生共有化振荡,形成等离子激元[36],实现更精细的相变变化[37]。
对于热传导来说,很容易从热的微观本质是粒子的运动强度这一角度来理解。电子-晶格相互作用一般可通过电子对晶格温度传递的双温模型或者考虑光激发的改进双温模型来描述[35],即:通过碰撞作用(热扩散),晶格离开平衡位置振动,并最终以熔化、汽化、升华的方式将能量转化为部分材料的动能,使材料脱离本体,形成烧蚀坑或者微爆炸。对于长脉宽的皮秒激光、纳秒激光甚至脉宽更长的激光来说,此过程占绝对主导;但对于短脉宽的飞秒激光来说,也不应忽视热作用的影响,尤其是在高重复频率下,由于晶格弛豫时间较长,热积累效应非常显著,成为各种改性和相变的主要因素。
对于其他作用类型,如冲击波、热扩散及重新固化/重结晶等,在特定的加工中也需要综合考虑。如在水中及溶液中,必须要考虑激光诱导的等离子冲击波的作用[37],因为它也会导致材料形成缺陷[38-39]。特别需要指出的是,由于超快激光的作用一般都是多脉冲的,这些缺陷联合前序脉冲形成的微纳结构共同构成了所谓的“种子”结构,在后续脉冲的持续作用下,这些“种子”结构会经历不同于前序脉冲与物质作用的过程[40],产生不同于单脉冲作用的新效果,从而形成超快激光改性/烧蚀的复杂图谱[34],构成了激光与物质相互作用的孵化/演变过程[41]。
3 飞秒激光诱导结构的形成机制和应用
为了更具体化飞秒激光与透明材料的作用过程,笔者详细调研了飞秒激光诱导材料改性的过程及影响因素。可将飞秒激光诱导的结构类型分为以下4种:1)正折射率变化类型(Type Ⅰ改性);2)负折射率变化类型,通常为纳米光栅(Type Ⅱ改性);3)以散射为主的改性类型,通常为微孔(Type Ⅲ改性)和微裂纹(Type Ⅳ改性);4)其他的过渡类型,如随机分布的纳米孔(Type Ⅹ改性)等。
当脉冲能量较小、脉宽较短时,根据脉冲的时间高斯性,即
3.1 Type Ⅰ型结构的形成机制和应用
当材料中沉积的光能达到化学键断裂或晶格结构破坏的能量阈值时,材料会发生一系列永久性的改变。1996年,Davis等[51]将810 nm飞秒激光聚焦于玻璃时发现了折射率增大的现象,并获得了Type Ⅰ型结构。当飞秒激光脉冲的光强高于多光子电离阈值但低于自聚焦阈值时,介质就会发生光滑的材料改性,材料折射率呈均匀的正变化并伴随着暗化现象[49],如
图 2. Type Ⅰ型结构的形成机制及应用。(a)通过激光改性所带来的折射率变化来直写波导;(b)Type Ⅰ型材料折射率呈均匀的正变化并伴随着暗化现象[49];(c)元素分布的形成是由温度梯度驱动的扩散引起的热迁移导致的[52];(d)重复频率为25 MHz、脉宽为30 fs、能量为5 nJ的激光经数值孔径为1.4的物镜聚焦后形成的结构[56];(e)Type Ⅰ型改性形成的X定向耦合器[60]
Fig. 2. Forming mechanisms and applications of Type I structure. (a) Laser beam is used to modify the refractive index of the material to create the waveguide; (b) refractive index of Type Ⅰ material shows a uniform positive change accompanied by darkening[49]; (c) element distribution caused by the heat transfer induced by temperature-gradient-driven diffusion[52]; (d) the structure is formed after the laser with repetition of 25 MHz, pulse width of 30 fs, energy of 5 nJ is focused by an objective lens with a numerical aperture of 1.4[56]; (e) X directional coupler formed by Type Ⅰ modification[60]
Type Ⅰ型改性的原因目前还并不明确,尚未形成统一的解释,主流观点是光致色心。2011年,Shimizu等[52]利用光学显微镜观察发现激光照射产生的热膨胀引起的应力会诱发黏弹性变形,并改变辐照区域内的元素分布,如
飞秒激光的脉宽、能量、波长、重复频率、偏振状态、写入深度等各项加工参数决定了透明介质的改性效果[57-58]。Taylor等[58]根据脉冲能量与脉冲持续时间之间的关系定义了材料改性的三个阶段。Stankevič等[57]重点测量了Type Ⅰ型改性对脉冲能量、聚焦深度和偏振状态的依赖性。激光在材料内部聚焦的深度以及扫描速率也会改变改性的类型。
飞秒激光诱导正折射率改变的Type Ⅰ型改性已被广泛用来制备光波导等光电器件,如
3.2 Type Ⅲ/Ⅳ型结构的形成机制和应用
利用更高功率的飞秒激光在透明材料内部可以诱导出微纳米结构[63],如
图 3. 超快激光诱导的Type Ⅲ/Ⅳ型结构。(a)蓝宝石中纳米孔横向截面的扫描电镜图像[63];(b)将二进制数据存储在熔融二氧化硅内[73];(c)将图像存储在熔融二氧化硅内[73];(d)超短激光脉冲触发空间受限微爆炸生成高密度相[67];(e)高折射率LiNbO3晶体中三维光子晶体的光学制备[71]
Fig. 3. Ultrafast laser-induced Type Ⅲ/Ⅳ structures. (a) SEM images of the cross-section of nanopores in sapphires[63]; (b) storing binary data in molten silicon dioxide[73]; (c) storing images in molten silicon dioxide[73]; (d) ultrashort laser pulses triggering space-limited micro-explosion to form high density phases[67]; (e) optical preparation of three-dimensional photonic crystals in high refractive index LiNbO3 crystals[71]
利用飞秒激光微爆炸产生Type Ⅲ/Ⅳ型结构为人工新高压相的制备提供了新策略。2013年,Rapp[65]将单晶硅暴露于激光微爆炸引起的强冲击波中,实现了激光诱导的不透明材料的微爆炸,这表明超快激光脉冲可以为高压材料相的形成创造极端的压力和温度条件。2014年,Buividas等[66]发现飞秒激光在橄榄石中的紧密聚焦单脉冲产生的微爆炸可使橄榄石铁的NEXAFS光谱发生细微变化,即形成了具有不同化学键的新铁相,为制备新型晶体和非晶纳米材料以及超密度和超硬材料的形成提供了一条全新途径。2011年,Vailionis等[67]在蓝宝石内部通过飞秒激光诱导的微爆炸合成了一种全新的致密BCC-Al相,该相在快速淬火后以压缩状态存在,如
微空隙结构还可以被应用于光子晶体的制造。2001年,Sun等[26]在Ge掺杂的二氧化硅中利用飞秒激光焦点处的多光子吸收效应,产生了折射率剧烈变化的空腔,并将之构建成了可与原子晶格比拟的光子晶格。2002年,Juodkazis等[70]基于超短激光脉冲对材料的非线性光学激发,讨论了基于全息记录和透明介电材料光致损伤这两种不同类型的三维激光微加工,并实现了0.2~1 µm尺度微结构的制备。2006年,Zhou等[71]使用飞秒激光诱导微爆炸方法在高折射率的LiNbO3晶体中成功地制备了三维光子晶体[如
将飞秒激光在熔融石英内部诱导的微孔洞记录为“1”,将没有作用的区域记录为“0”[如
除了存储数字数据外,熔融石英存储技术还可以利用点来绘制字母、艺术品和照片,如
3.3 Type Ⅱ型结构的形成机制和应用
与Type Ⅲ型结构的发现较为相似,人们发现飞秒脉冲改性区域具有双折射特性[75]。2003年,京都大学的Shimotsuma等[19]发现这种改性区是由硅氧周期性变化的纳米光栅组成的,如
图 4. 纳米光栅形成机制。(a)纳米光栅SEM图[19];(b)入射光-极化子干涉模型[19];(c)纳米等离子体各向异性生长模型[76];(d)表面等离子共振模型[78];(e)周期纳米结构电磁形成机制示意图[81]
Fig. 4. Formation mechanism of nano-grating. (a) SEM image of nano-grating[19]; (b) incident light-polaron interference model[19]; (c) anisotropic growth model of nano-plasma[76]; (d) surface plasmon resonance model[78]; (e) schematic diagram of electromagnetic formation mechanism of periodic nanostructures[81]
2003年,京都大学的 Shimotsuma等[19]提出了入射光-极化子干涉模型,如
2015年,Liao等[78]提出了表面等离子共振模型,如
Type Ⅱ型结构具有双折射和可擦除等特点,在光存储和微光学元件等方面被广泛应用。利用光学原理的平行平板模型[如
其中,
式中:ne为e光折射率;no为o光折射率;n1为纳米光栅相对高浓度氧缺陷的薄层的折射率;n2为纳米光栅厚层的折射率;f1为折射率为n1区域的填充系数;f2为折射率为n2区域的填充系数;
图 5. 基于纳米光栅的五维光存储。(a)基于纳米光栅的五维光存储示意图[85];(b)纳米光栅双折射模型;(c)时间胶囊样品照片[21]
Fig. 5. Five-dimensional (5D) optical storage based on nanograting. (a) Schematic diagram of 5D optical storage based on nano-grating[85]; (b) birefringence model of nanograting; (c) pictures of time capsule samples[21]
这种双折射区别于应力产生的双折射和晶体产生的双折射,故而又被称为“形式双折射”,它包含慢轴角度和光程延迟两个参数。定义光程延迟为
式中:
实验发现:光程延迟可以通过控制激光功率或者脉冲数来改变;而双折射的慢轴角度,即纳米光栅方向,可以通过激光的偏振方向进行控制。利用超快激光诱导材料产生的双折射效应,可以制备有别于传统动力学相位的几何相位棱镜、透镜和偏振转换器件。2021年,Xu等[83]在蓝宝石内部实现了双折射效应以及偏振转换器件和几何相位透镜。
2007年,Taylor等[84]发现了纳米光栅的可擦除重写特性:当飞秒激光的偏振方向改变时,旧的纳米光栅被擦除,同时被新的纳米光栅取代,且其方向完全由重写光束的偏振决定。纳米光栅可以重写1000次,而且其质量几乎没有退化。该特性使得纳米光栅在光存储领域具有非常广阔的发展前景。2014年,Zhang等[20]利用双折射相位延迟量和慢轴方位角构成了两个维度的参数调控,再结合空间X、Y、Z方向的三维调控,实现了5个维度的信息复用光存储技术[85],如
到目前为止,大部分Type Ⅱ型结构都是由纳米光栅结构(头部,沿向激光传播方向)和一部分Type Ⅰ型结构组成(尾部)的。特别地,纳米光栅负折射率的改变,尤其是激发等离子体的强反射作用,会导致纳米光栅纵向不连续,从而导致双折射误差增大、慢轴方位角精度变差[86]。无独有偶,大部分现有的Type Ⅲ/Ⅳ型结构也会掺杂Type Ⅱ和Type Ⅰ型结构。结构的不纯导致改性体元的光学性质变得复杂并出现波动,器件的光学透过性能变差。这也是当前研究要解决的一个课题[87]。
3.4 Type X型结构的形成机制及应用
虽然纳米光栅在光存储方面具有非常广阔的前景,但是加工过程中已形成结构对光的散射作用导致纵向纳米光栅结构不连续性,透过率约为70%左右,不利于百层甚至千层数据的写入。为此,必须要探索更低损耗的双折射结构,其中人们发现了一种新型的有Type Ⅰ型向Type Ⅱ型结构过渡的结构——Type X型结构。
2020年,Sakakura等[88]报道了一种随机分布的纳米孔,它的散射损耗极低,在可见光范围内的透射率能达到99%,在0~330 nm紫外光谱范围内的透射率高于90%。该结构是通过调节生成纳米光栅的脉宽得到的。
图 6. 脉宽调制效应[88]。(a)不同持续时间下写入的双折射结构的延迟图像(左)和透射图像(右);(b)不同脉宽下写入的改性区域的折射率变化;(c)不同脉宽对应的改性区域的延迟量和透射率;(d)Type X和Type Ⅱ型结构抛光刻蚀后的SEM图
Fig. 6. Pulse width modulation effect[88]. (a) Retardance (left) and transmission (right) images of birefringent structures written at different durations; (b) refractive index changes of modified regions written with different pulse widths; (c) retardance and transmittance of modified regions corresponding to different pulse widths; (d) SEM images of Type X and Type Ⅱ structures after polishing and etching
3.5 Type S型结构的形成机制及应用
如何提高写入速度和存储密度是石英光盘五维光存储面临的主要挑战,提高激光重复频率和减少单点脉冲数是关键。然而,脉冲间的热积累效应也会随着脉冲重复频率的提高而增加,甚至会导致材料局域熔化,影响纳米光栅的形成及双折射效果。
2021年,Lei等[86]通过快速脉冲能量调制的方法实现了10 MHz重复频率的双折射数据写入。具体做法[90]为:采用快速脉冲能量调制装置使前两个脉冲的能量稍大于微爆炸阈值,产生各向同性的纳米孔洞(尺寸约为130 nm),然后通过低能量脉冲的近场增强效应[如
图 7. 纳米光栅热调制[90]。(a)调制能量前后写入的双折射结构的对比;(b)焦点中心的温度演变模拟;(c)不同直径的纳米体周围的光强分布模拟;(d)2个种子脉冲和8个后续脉冲诱导的结构的双折射照片;(e)抛光和KOH刻蚀后的纳米层状结构的SEM图像
Fig. 7. Thermal modulation of nanograting[90]. (a) Comparison of birefringence structures before and after modulating energy; (b) simulation of temperature evolution in the focus center; (c) simulation of laser intensity distribution around nanocrystals with different diameters; (d) birefringence photo of the structures induced by two seed pulses and eight subsequent pulses; (e) SEM images of nano-layered structures after polishing and KOH etching
3.6 挑战与机遇
提高永久光存储写入速度和写入容量始终是飞秒激光五维光存储的重要方向,解决的手段有缩短曝光时间、减小单点脉冲数和缩小体素间的点间距和层间距。虽然近20年来人们对超快激光与物质的相互作用有了进一步的认识,而且对各种类型的结构进行了较深入的探索,但对飞秒激光击穿透明材料内部的机制认识得还不够清晰。现有的能用于光存储的Type Ⅱ、Type X结构在产生过程中实际上还伴随着Type Ⅰ型区域的产生,能量稍大或脉冲数过多时,Type Ⅲ/Ⅳ型结构还会出现在纳米光栅区域中,这都会影响Type Ⅱ结构相位延迟量的大小,进而对数据的存储精度产生很大影响。Type Ⅱ型结构是如何产生的,如何抑制Type Ⅱ型结构产生过程中的Type Ⅰ/Ⅲ/Ⅳ等型结构,是当前亟须解决的两个难题。
2020年,Li等[41]针对纳米光栅是如何产生的提出了一种远场控制近场击穿的理论(O-FIB)。他们在实验中发现,光学近场的增强效应是形成纳米光栅的关键。单个脉冲只能形成尺寸为20~50 nm的烧蚀坑,但在下一个脉冲的照射下,受限于麦克斯韦方程组的电场连续性边界条件,光场会在垂直于偏振的方向上产生定向拉伸,使原来的圆形结构变成狭缝纳米结构;随着缝隙的拉伸,会产生类似于Slot-waveguide的光场调控,即在纳米狭缝两边形成次级增强结构,从而诱导产生次级纳米烧蚀孔。这两个烧蚀孔会在接下来的脉冲照射下继续上述过程,直到形成稳定的纳米光栅结构。他们进行了一个概念性验证,如
图 8. 纳米光栅永久光存储所面临的挑战的解决办法。(a)电场强度分布模拟图[41];(b)利用远场控制近场击穿(OFIB)技术以自由形式书写的纳米槽的慢轴定向图(光源波长为546 nm,双折射延迟量为9 nm)[41];(c)皮秒激光时空调控光路图[92];(d)延迟量和脉冲数的关系[91]
Fig. 8. Solutions for the challenges which eternal optical storage is facing. (a) Simulations of the electric field intensity distribution[41]; (b) slow-axis orientation map of the free-form written nanogrooves by OFIB technology (the retardance at a wavelength of 546 nm is 9 nm)[41]; (c) schematic of picosecond laser space-time control optical path[92]; (d) relationship between retardance and pulse number[91]
除了光调制技术之外,还需要考虑材料本身的特性和未来的工业化需求。2018年,Fedotov等[91]将高硅酸盐纳米多孔玻璃作为材料,仅利用3个脉冲间隔为98 ns的飞秒激光脉冲诱导了具有均匀双折射效应的结构。同时,利用4个脉冲诱导出了双折射延迟量可达到35 nm的结构,如
4 结束语
随着大数据时代的到来,飞秒激光永久光存储作为一种高容量、长寿命冷数据的存储方案,具有很大的发展潜力。光存储技术的基本存储单元是飞秒激光诱导透明硬质材料产生的微结构,因此,若要从根本上提升永久光存储媒介的性能,必须厘清各种结构的形成机制。为此,笔者梳理了近20年来各种飞秒激光诱导微结构形成机制的研究,阐述了纳米光栅双折射特性和永久光存储的原理。同时,针对提高写入速度和存储容量的脉宽调制方法和热调制方法进行了解释,总结了激光在不同种类玻璃中产生的微结构的分类。远场控制近场增强理论指导模型对于突破目前永久光存储的瓶颈具有一定的参考意义。通过时空调制皮秒激光,有望实现六维永久光存储。相信未来会有更多模型帮助人们解释飞秒激光诱导微结构的形成机制,优化纳米光栅的形成条件,提高永久光存储的写入速度和容量,从而加快超快激光永久光存储应用的商业化脚步。
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Article Outline
刘姿廷, 袁一鸣, 李子越, 龚伟, 张栩, 赵新景, 王熠, 李臻赜, 王磊. 飞秒激光与透明硬质材料的相互作用:从相变机理到永久光存储[J]. 中国激光, 2023, 50(18): 1813005. Ziting Liu, Yiming Yuan, Ziyue Li, Wei Gong, Xu Zhang, Xinjing Zhao, Yi Wang, Zhenze Li, Lei Wang. Interaction Between Ultrafast Laser and Transparent Hard Materials: from Phase Change Mechanism to Eternal Optical Data Storage[J]. Chinese Journal of Lasers, 2023, 50(18): 1813005.