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摘要:
随着数字化时代的发展,人类正进入一个大数据纪元。然而海量产生的数据已很难被全部记录,据估算,现有的数据载体已不足以存储人类产生数据的一半,如果该现状得不到改善,那么大量数据将会被强制舍弃。但改善这样的现状面临着难题,阿贝衍射极限的存在制约了光存储单元的大小,这使得传统光存储的容量被激光波长和物镜的数值孔径所限制。这个难题可以通过引入多维复用技术替代性地解决,本文针对多维存储中材料三维空间、偏振、波长等复用技术,综述了维度扩展在光存储中的研究成果以及未来的发展趋势。其中,着重介绍了基于纳米光栅的五维度光存储的发展历史、当前现状及亟需解决的问题。
Abstract:Human beings are entering a Big Data era, which has significantly boosted the current digital economy and society. But in reality, all current data storage technologies and mediums can only store less than half of what we generate, which means most of the data will be forcibly lost if without breakthrough in high-capacity storage technologies. However, Ernst Abbe set a fundamental barrier that limits the smallest feature size of a recording voxel to approximately half of the wavelength. Alternatively, this limitation could be overcome by implementing multiplex technology. In this review, techniques employed various multiplex dimensions such as 3D space, polarization and wavelength are briefly introduced. Especially, we highlight the development history, current state of the art and urgent challenges of five-dimensional optical data storage based on laser-induced nanogratings.
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Overview: Human beings are entering a big data era, which has significantly boosted the current digital economy and society. According to an estimation by the International Data Corporation (IDC), the information generated and consumed is nearly doubled every two year. Human being have already generated data onto an amount of 35 ZB (1 ZB=1000 EB=1000, 000 PB=1000, 000, 000 TB=1000, 000, 000, 000 GB) globally in 2017 and in the year of 2020 the total amount will reach 44 ZB. However, all current data storage technologies and mediums can only store less than half of this amount, which means most of the data will be forcibly lost if without breakthrough in high-capacity storage technologies. The infrastructure of the current information technology and the sustainability of the current information economy has been constantly challenged by the thirst for more storage capacities as well as low energy consumption. These challenges set a fundamental obstacle to the longevity and sustainability of the current information technology. Known for its green features, optical data storage is regarded as an excellent candidate for long-term data archiving. However, Ernst Abbe set a fundamental barrier that limits the smallest feature size of a recording cell to approximately half of the wavelength, leading to a capacity of hundreds of Gigabytes per disc. This capacity limitation could be overcome by implementing multiplex technology. This technology enables the potential for storing more than one bit of data in a single memory cell. It can be applied to materials which exhibit sensitivity to not only the intensity but also other parameters of light like polarization, wavelength, and fluorescence. Limited by the material response, only five multiplex dimensions have been achieved in gold nanorods embedded polymer and fused silica glass. The nanogratings, generated by femtosecond laser writing in fused silica, behave as a uniaxial optical crystal with negative birefringence. The two parameters of birefringence, the slow axis orientation and retardation can be independently controlled by the polarization and intensity of the incident laser beam. Thanks to the effect of multi-photon excitation, 3D space of the medium volume can be simultaneously utilized by focusing femtosecond laser in fused silica. Such memory, encoding data in 5 dimensions, is capable of recording 360 TB data per disc for billions of years. It is believed that 5D optical data storage based on nanogratings in fused silica opens a new era of eternal data storage.
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图 3 (a) 银纳米颗粒的原子力显微镜图,其中左侧的银纳米颗粒为55 nm高,右侧为46 nm高;(b)银纳米颗粒高度和其对应LSPR波长最大值的实验结果,图中斜线为其实验结果的线性拟合[42]; (c) A、B、C三种不同摆放的银纳米颗粒的扫描电子显微镜图;(d)对应图 3(c)中三种不同情况下的散射光谱图[43]
Figure 3. (a) Atomic force microscopy profiles of silver nanoparticles; (b) Plot of LSPR peak maximum wavelength vs. nanoparticle height, the slope is the linear fitting of the experimental data[42]; (c) SEM images of specially designed silver nanoparticles; (d) The corresponding scattering spectra of silver nanoparticles in Fig. 3(c)[43]
图 4 (a) 利用三种不同极化方向的偏振光在偶氮染料共聚物中完成的数据写入演示[19]; (b)利用双光子吸收,在偶氮染料中完成的数据写入、擦除及重写,其中字母I和J被写入后擦除,并在同一位置写入了字母F和E[45]; (c)含有金纳米棒的薄膜材料在不同波长和偏振光照射下,显示出不同的图案[46]
Figure 4. (a) Data writing in azo dye copolymer by irradiating three different linearly polarized light[19]; (b) Data writing, erasing, and rewriting in azo dye by two-photon absorption process[45]; (c) Microscope images of the gold-nanorod nanocomposite films irradiated by light with different wavelengths and polarizations[46]
图 5 (a) 不同偏振态的飞秒激光聚焦到掺锗熔融石英内部时产生的各向异性散射[51]; (b)纳米光栅结构的扫描电子显微镜图[52];(c)纳米光栅的场发射枪扫描电子显微镜(FEG-SEM)图,可以看到形成的纳米多孔层状结构[53]; (d)纳米光栅表现出双折射特性的示意图,纳米光栅可以简化为周期排布的折射率不同(n1, n2),厚度不同(t1, t2)的层状结构[54]
Figure 5. (a) Anisotropic scatterings produced by focusing femtosecond laser of different polarizations inside the germanium doped fused silica[51]; (b) Scanning electron microscopy image of nanogratings structure[52]; (c) FEG-SEM image of nanogratings[53]; (d) Schematic diagram showing birefringence characteristics of nanogratings[54]
图 7 (a) 左侧和右侧分别为在熔融石英中,不同结构在明场显微镜和双折射显微镜观察下的图片,红色箭头代表偏振态[62]; (b)在纳米多孔玻璃中,不同能量和脉宽情况下产生的结构在双折射显微镜观察下的图片;(c)光程延迟值随激光能量和脉宽改变的关系图。其中激光脉冲数为6个,重频为200 kHz[63]
Figure 7. (a) Bright field and birefringence microscopic images of structures produced in fused silica by different burst energies and polarization directions, the red arrows indicate the polarization directions[62]; (b) Slow axis orientation images of the laser-induced birefringent spots in nano-porous glass; (c) The plot of retardance vs pulse energy. The number of pulses is 6 and pulse repetition is 200 kHz[63]
表 1 传统光存储技术的参数
Table 1. Parameters of conventional optical data storage technologies
CD DVD 蓝光 波长/nm 780 650 405 数值孔径 0.45 0.6 0.85 轨距/μm 1.6 0.74 0.32 单层容量/GB 0.7 4.7 23.5 最多层数 1 2 6 最大容量/GB 0.7 8.5 500 -
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