Ultrahigh density

As has been known for some time, the limit on the optical resolution of a high-numerical-aperture (high-NA) objective, which was discovered by the German physicist and entrepreneur Ernst Abbe, sets a fundamental barrier that limits the smallest size of a recorded bit to approximately half the wavelength of the light used for recording in the lateral direction and approximately one wavelength in the axial direction. Hence, the theoretical maximum storage density for an aberration-free objective with a high NA of 1.4 is only on the order of TB per disc.8,9 Recent advances in nanophotonics can facilitate either the encoding of information in physical dimensions, such as those defined by the frequency and polarization parameters of the writing beam, or the achievement of three-dimensional (3D) super-resolution recording, breaking the conventional storage-capacity limit.

Multidimensional storage

Nanophotonics allows for sharp color and polarization selectivity in light–matter interactions at the nanometer scale. For example, light can couple to surface plasmons, the collective oscillations of free electrons in metallic nanoparticles, which yield deterministic spectral responses associated with their sizes and shapes.22 These appealing properties make nanoparticles suitable for the implementation of spectrally encoded memory. Consequently, based on the principle of light-induced shape transitions, three-color spectral encoding by using gold nanorods of various sizes has been demonstrated.23 Similarly, polarization anisotropy can be created in nanocomposite materials by means of selective excitation by different polarization states of a writing beam.24,25 It has been recently demonstrated that when such polarization selectivity is combined with the sharp spectral selectivity of nanophotonics, gold nanorods can enable information recording in five dimensions, encoded across three wavelengths and two polarization states with an equivalent capacity of 1.6 TB in a single disc,23 as illustrated in Figure 3. Another possible dimension that can be exploited, which is also the most straightforward to control, is the intensity of the optical beam; control of the beam intensity forms the basis of the principle of gray-scale-encoded ODS. Instead of recording binary information, each voxel stores a range of gray-scale values by varying the discrete intensity levels of the writing beam. With a precise control of its focal intensities through vectorial Debye focusing of the beam using a high-NA objective, up to eight levels can be recorded and distinguished in photobleaching polymers26 and glass materials.27,28

Figure 3 Scheme of multidimensional optical data storage. In the case of 5D data storage, the information bits are multiplexed in the polarization and spectrum domains of the writing beam and in multiple layers inside the medium. One recorded layer, indicated by the red dashed line and accessed using a randomly polarized broadband source as illustrated in the middle, cannot separate the polarization- and spectrum-coded information. The multiplexed information can be individually addressed by using the appropriate polarization state (indicated by the arrow) and wavelength, as illustrated in the right-hand column.4 The inset table lists four physical dimensions of the writing beam that could be employed for multiplexed storage. 5D, five-dimensional. Full size image

The dimension of the beam that has received the least attention is the angular momentum (AM), which can be carried by photons in the forms of spin AM and orbital AM. As a demonstration of the principle of AM information multiplexing, the AM of light has been used to create orthogonal and spatially distinct streams of data-transmitting channels multiplexed in a single fiber.29 It is therefore expected that AM can be used to multiplex information and thereby boost the storage capacity. On the other hand, circularly polarized beams with different handedness (spin states) can be employed to introduce different magnetization orientations in all-optical magnetic recording.30 In this case, if the recording spots could be reduced to their smallest, diffraction-limited size,31 the optical magnetic response could act as another dimension to allow for further increase in the storage capacity. Limited by the material response, the multiplexing of information in six individual dimensions has never been achieved. Recent advances in nanotechnology, which have enabled a variety of new nanocomposites, such as nitrogen vacancy centers in diamonds19,32,33 with optically detected magnetic resonance,34,35 might provide a solution for exploiting the maximal number of information channels by simultaneously employing all physical dimensions of the writing beam.

3D super-resolution recording

A variety of methods have been proposed and demonstrated to break the diffraction barrier in the near-field region and achieve super-resolved areal memories.36,37 However, these approaches do not exhibit the ability to record information in the volume of a medium. Recently, inspired by a diffraction-unlimited far-field imaging approach,38 scientists have developed super-resolution photoinduction-inhibition nanolithography (SPIN), which can break the diffraction barrier and achieve 3D super-resolved writing.

In contrast to conventional optical memory, SPIN is achieved by employing dual beams during recording; the behavior of each beam is still governed by its diffraction. In general, the two beams operate at different frequencies enduing different functions, as illustrated in the insets of Figure 4. One of the two beams, with a Gaussian shape, falls within the transition bands of the materials and thus is responsible for photoinduction. The other beam, with a spatially modulated intensity distribution that is usually a doughnut shape (with zero light intensity in the center), is responsible for inhibiting photoinduction everywhere in the focal region except at its center. Consequently, the effective focal spot can be made much smaller than the diffraction barrier by the spatial superposition of the two beams and varying the intensity ratio between the two beams.

Figure 4 Projection of the maximal capacity that a single disc can hold as a function of the feature size of the recorded bits. In the projection, the lateral separation and the axial separation are set to 2.5 times and 8 times the feature size, respectively. The insets illustrate the principle underlying SPIN's ability to break the diffraction barrier and achieve super-resolution recording/lithography. Top inset: comparison of direct laser recording and super-resolution recording by using SPIN methods. Bottom inset: comparison of conventional laser lithography and super-resolution lithography. SPIN, super-resolution photoinduction-inhibition nanolithography. Full size image

By employing the principle of SPIN, researchers have successfully demonstrated super-resolved line fabrication as well as dot recording in a variety of photoinduction systems, as schematically illustrated in the right-hand panel of insets in Figure 4. Table 1 summarizes the state of the art of SPIN-based super-resolution recording methods for the production of ultrafine features. In general, photoinduction can refer to any photoinduced chemical or physical processes that initiate a change in material properties, such as photochromism,39 photopolymerization20 and photoreduction,48 and that can also be terminated by an inhibition beam operating at a different wavelength. As such, the photochromism process has been used to inhibit photoabsorption, and the smallest line-feature size of 30 nm (approximately one-tenth of the wavelength λ) has been successfully demonstrated.39 Recently, a photopolymerizable material with improved photosensitivity and mechanical stability has been developed and successfully applied in using the SPIN method for the fabrication of suspended lines with the smallest feature size down to 9 nm, or λ/42.20 These exceptional features open a new avenue toward diffraction-unlimited laser fabrication, as well as for the development of ultrahigh-density optical memory.

Table 1 The development of SPIN methods Full size table

The development of 3D super-resolved writing methods will enable further decrease in the effective size of recorded bits to below 50 nm and will eventually boost disc capacity to approach or even surpass the PB scale in the near future, as projected in Figure 5. In the meantime, sophisticated techniques for light manipulation on the focal plane are necessary to maintain an effective focal spot smaller than the diffraction limit with enriched physical dimensions. The combination of super-resolution techniques and multiplexing in the physical dimensions can further expand disc storage capacity beyond hundreds of PBs. Clearly, nanophotonics-enabled storage techniques are far superior to the current HDD and flash memory techniques, which are encountering density bottlenecks (Figure 5).

Figure 5 Comparison of the development of storage capacities using the HDD (squares), flash (triangles) and ODS (circles) techniques. The capacity of flash memory is calculated based on a storage area of 1 cm2. The HDD technique is currently subject to a 30 nm technical limitation (∼1.5 TB) and is expected to approach its theoretical limitation of 10 nm (∼12 TB). However, 5D storage has already broken the technical limitation of the HDD technique, achieving a capacity of 1.6 TB in 2009.23 The super-resolution focal-volume technique has pushed the capacity further to 3 TB.9 Moreover, the application of the SPIN method in 3D optical storage has enabled a feature size below 100 nm with an equivalent capacity of approximately 10 TB.49 The latest result of a recorded bit size of 32 nm has surpassed the theoretical limitation of the HDD technique and demonstrated a new world record with an equivalent capacity of approximately 100 TB.50 3D, three-dimensional; 5D, five-dimensional; HDD, hard disk drive; ODS, optical data storage; PB, petabyte; SPIN, super-resolution photoinduction-inhibition nanolithography. Full size image

Ultrahigh throughputs

Another challenge confronting big data storage is low writing and reading throughputs. Based on the current throughput (∼20 MB s−1) of Blu-ray discs, it might take more than one and a half years to access 1 PB of information.3 Even taking the current HDD technique into consideration (∼100 MB s−1), it would still require more than 100 days to access such a massive amount of data. Therefore, it is equally as important to boost writing/reading throughputs as it is to increase storage capacity.

Various optical parallelism methods for generating multifocal arrays in single laser shots have been proposed and experimentally demonstrated, including microlens arrays,51 diffractive optical elements,52 Debye-based Fourier transformation53 and dynamic computer-generated holograms.54 Among these methods, Debye-based Fourier transformation enables the formation of diffraction-limited multifocal spots using a high-NA objective wherein each focal spot can be dynamically programmable, which is a necessity for ultrahigh-density optical recording. The implementation of this method by using a spatial light modulator for parallel recording is illustrated in Figure 6a.53 The single beam carrying the modulated phase pattern calculated by the Debye-based Fourier transform can be diffracted into the designed multifocal array in the focal plane of the objective. With a few hundred programmable focal spots for parallel writing, the throughput of optical storage could be significantly increased to ∼10 GB s−1; therefore, the accessing time for 1 PB could be dramatically reduced to only a few days. The Debye-based Fourier transform method can generate not only in-plane multifocal arrays but also high-quality 3D multifocal arrays for volumetric parallel writing,55 and it can even be integrated with other physical parameters to increase the throughput beyond tens of GB s−1, as illustrated in Figure 6b. Thus, polarization states of individual focal spots in 3D multifocal arrays have been controlled to achieve parallelism in the dimension of the polarization states. As an example, the generation of cylindrically polarized multi-focal arrays has been demonstrated by applying the vectorial Debye-based Fourier transform method.56 Furthermore, the combination of polarization-controllable multifocal arrays with SPIN methods can allow for super-resolution parallel recording and readout, which will therefore be a core platform for the development of PB optical discs (Figure 6c).

Figure 6 (a) Scheme for parallel optical recording using an SLM. (b) Schematic illustrations of in-plane parallel (2D) recording, volumetric 3D recording and 3D-combined-with-polarization parallel recording. The arrows indicate the polarization orientations. (c) The overall throughput as a function of the in-plane parallelism and physical dimensions. BS, beam splitter; 2D, two-dimensional; 3D, three-dimensional; FS, femtosecond; ICCD, intensified charge-coupled device; SLM, spatial light modulator; SS, scanning stage. Full size image

Ultrahigh security

The physical dimensions of a writing beam can be used not only as information channels to increase the storage capacity but also as versatile means to encrypt information for data security, which is one of the most important aspects of any memory system.

As an electromagnetic wave, light can selectively interact with optical materials with physical anisotropic properties via its polarization, or the oscillation of its electric fields. The response of optical materials strongly depends on the orientation of the electric dipoles with respect to the polarization state of the light. As such, information can be encrypted using a specific polarization state of a writing beam, which cannot be retrieved without pre-knowledge of the polarization key. By rotating the polarization orientation, flexibility of the encryption key can be achieved, and multiple states of information can be encrypted in the same spatial region. Thus, information can be encrypted in the two polarization states of writing beams at 0°- and 45°-oriented polarizations through polarization-induced birefringence.24 Recently, it has been demonstrated that by employing the depolarization effect through the tight focusing of a radially polarized beam, one can generate a longitudinal polarization state within the focal region.57,58 Through the superposition of weighted radially and azimuthally polarized beams, a 3D arbitrary polarization orientation can be achieved in the focal region of a high-NA objective. In combination with the sharp polarization sensitivity of gold nanorods, 3D polarization encryption within a solid angle in the focal plane has been demonstrated for ultrahigh security,59 as illustrated in Figure 7a.

Figure 7 (a) Scheme for 3D polarization encryption using 3D oriented polarization states.60 (b) Illustration of hologram-encoded multimode recording for enhanced data security. Information bits can be randomly accessed through the 2P fluorescence readout and retrieved in the diffraction mode of the recorded holograms.61 (c) Projection of levels of information security by combining multimode recording with other physical dimensions. 3D, three-dimensional; FL, fluorescence; FS, femtosecond; 2P, two-photon. Full size image

Additionally, the enrichment of the physical dimensions for light–matter interactions offered by nanophotonic approaches enables information to be recorded in multimode material responses, which opens a new avenue toward a high level of information security. For example, simultaneous changes in the refractive index and fluorescence intensity of graphene-based materials have been demonstrated to hold the potential for hologram-encoded multimode recording,61 as shown in Figure 7b. On the one hand, the information can be randomly accessed through two-photon fluorescence readout. On the other hand, the information can be simultaneously encrypted in holograms, ensuring its integrity and durability. It is possible to integrate hologram-encrypted multimode recording with other physical dimensions, such as polarization and AM, to achieve an even higher level of information security, as illustrated in Figure 7c. However, the experimental achievement of such a high level of security strongly depends on material properties of the recording medium, specifically, its deterministic responses to light with various characteristics. Nanotechnology that offers the ability to engineer and control material properties on the nanoscale may provide a solid platform for future ultrahigh-security optical storage.

Ultralong lifetime

As we have noted, magnetization-based HDD techniques have a limited lifetime of 2–5 years, and therefore, frequent data migration is needed to avoid potential data loss. However, ODS has a longer lifetime than HDDs, which dramatically reduces the required frequency of data migration. In addition, ODS only consumes energy when the data are written or read out and does not consume any energy when the optical disc is in idle state. For comparison, Figure 8 illustrates the power consumption and operational cost of information storage incurred by a single HDD (4480 kWh) and a single disc (35 kWh) over 50 years. Clearly, optical technology greatly reduces the waste generated by frequent data migration, reduces the energy consumption in idle status and reduces the cost for the replacement of new units associated with short lifetimes. Moreover, increasing the lifetime of ODS to greater than 50 years can dramatically increase the savings in overall expenditures, including electricity and costs for storage devices, by more than 96% compared with HDD techniques.

Figure 8 Reduction in the operational cost for a single ODS unit compared with that of a single HDD unit as a function of years elapsed. The inset shows a comparison in terms of the electricity consumption for reading/writing and in idle status for a single unit over 50 years. In this comparison, a single HDD unit typically consumes 20 W for reading/writing, and it is used in this manner for 100 h per year. In the idle status, it consumes 10 W per hour. A single ODS unit consumes 7 W for reading/writing and is used in this manner for 100 h per year. The cost is calculated based on an assumption of $0.12 per kWh, and the unit prices of the HDD and the ODS are $80 and $40, respectively. HDD, hard disk drive; ODS, optical data storage. Full size image

Consequently, the development of ODS with ultralong lifetimes has been a subject of intensive research. Permanent laser-induced physical changes, such as voids in polymers62 and glass materials,63 provide an approach to long-lifetime storage without information degradation. Laser inscriptions in glass materials27,28 can withstand temperatures of up to 1000 °C and maintain data stability and readability for up to thousands of years, in principle. The demonstrated physical changes including voids and inscription are limited by a low capacity of approximately 50 GB per disc because the physical size of each bit cannot be smaller than the diffraction barrier. The top-down nanocomposite approach offers an alternative method that may allow the development of SPIN methods to break the diffraction limit for ultrahigh capacity ODS with thousands of years of lifetime.