In the cause of cramming more and more data onto optical storage devices, materials scientists have sought to add extra dimensions to recording media, literally. Now a group from Melbourne's Swinburne University of Technology has developed a five-dimensional optical recording technique with the potential to increase storage capacities by several orders of magnitude. The extra dimensions are the wavelength and polarization of light, which integrated with the familiar three spatial dimensions creates true five-dimensional recording within one volume. The result is a theoretical 1.6 terabytes capacity for a DVD-sized disk. The new system makes use of surface plasmon resonance (SPR)-mediated photothermal reshaping of a substrate of gold nanorods immersed in a polymer layer. Crosstalk-free readout is via two-photon luminescence. Immediate applications can be found in security patterning and multiplexed optical storage. By exploiting not only the three spatial dimensions but also other ways to record information, it is theoretically possible to store much more onto an optical device such as a DVD than has hitherto been possible. Here, a five-dimensional optical recording technique using polarization of light and its wavelength as the two additional dimensions, is demonstrated. The method consists of using a substrate of gold nanorods immersed in polymer. Multiplexed optical recording provides an unparalleled approach to increasing the information density beyond 1012 bits per cm3 (1 Tbit cm-3) by storing multiple, individually addressable patterns within the same recording volume. Although wavelength1,2,3, polarization4,5,6,7,8 and spatial dimensions9,10,11,12,13 have all been exploited for multiplexing, these approaches have never been integrated into a single technique that could ultimately increase the information capacity by orders of magnitude. The major hurdle is the lack of a suitable recording medium that is extremely selective in the domains of wavelength and polarization and in the three spatial domains, so as to provide orthogonality in all five dimensions. Here we show true five-dimensional optical recording by exploiting the unique properties of the longitudinal surface plasmon resonance (SPR) of gold nanorods. The longitudinal SPR exhibits an excellent wavelength and polarization sensitivity, whereas the distinct energy threshold required for the photothermal recording mechanism provides the axial selectivity. The recordings were detected using longitudinal SPR-mediated two-photon luminescence, which we demonstrate to possess an enhanced wavelength and angular selectivity compared to conventional linear detection mechanisms. Combined with the high cross-section of two-photon luminescence, this enabled non-destructive, crosstalk-free readout. This technique can be immediately applied to optical patterning, encryption and data storage, where higher data densities are pursued.

/pdf/five-dimensional-optical-recording-mediated-by-surface-3ffqnut6no.pdf

Five-dimensional optical recording mediated by surface plasmons in gold nanorods

We present an overview of our research effort on volume holographic digital data storage. Innovations, developments, and new insights gained in the design and operation of working storage platforms, novel optical components and techniques, data coding and signal processing algorithms, systems tradeoffs, materials testing and tradeoffs, and photon-gated storage materials are summarized.

Holographic data storage

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.

Photomodulation of liquid crystal orientations for photonic applications

Holograms, the optical devices to reconstruct predesigned images, show many applications in our daily life. However, applications of hologram are still limited by the constituent materials and therefore their working range is trapped at a particular electromagnetic region. In recent years, the metasurfaces, an array of subwavelength antenna with varying sizes, show the abilities to manipulate the phase of incident electromagnetic wave from visible to microwave frequencies. Here, we present a reflective-type and high-efficiency meta- hologram fabricated by metasurface for visible wavelength. Using gold cross nanoantennas as building blocks to construct our meta-hologram devices with thickness ∼ λ/4, the reconstructed images of meta-hologram show polarization- controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. The flexibility demonstrated here for our meta-hologram paves the road to a wide range of applications related to holographic images at arbitrary electromagnetic wave region.

https://nocweba.ntu.edu.sg/laq_mems/journal%20papers/2014/High-Efficiency%20Broadband%20Meta-Hologram%20with%20Polarization-Controlled%20Dual%20Images.pdf

High-Efficiency Broadband Meta-Hologram with Polarization-Controlled Dual Images

Photorefractive materials are being widely investigated for applications in holographic data storage1. Inhomogeneous illumination of these materials with an optical interference pattern redistributes charge, builds up internal electric fields and so changes the refractive index. Subsequent homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern. A range of inorganic and organic photorefractive materials are known2, in which thousands of holograms of high fidelity can be efficiently stored, reconstructed and erased. But there remains a problem with volatility: the read-out process usually erases the stored information and amplifies the scattered light. Several techniques for ‘fixing’ holograms have been developed3,4,5,6, but they have practical disadvantages and only laboratory demonstrators have been built7,8,9,10. Here we describe a resolution to the problem of volatility that should lead to the realization of a more practical system. We use crystals of lithium niobate — available both in large size and with excellent homogeneity — that have been doped with two different deep electron traps (iron and manganese). Illumination of the crystals with incoherent ultraviolet light during the recording process permits the storage of data (a red-light interference pattern) that can be subsequently read, in the absence of ultraviolet light, without erasure. Our crystals show up to 32 per cent diffraction efficiency, rapid optical erasure of the stored data is possible using ultraviolet light, and light scattering is effectively prevented.

/pdf/non-volatile-holographic-storage-in-doubly-doped-lithium-3y2ugfesqd.pdf

Non-volatile holographic storage in doubly doped lithium niobate crystals

A multiple page fully digital holographic data storage system is demonstrated. This system is used to store and retrieve digital image and compressed video data with a photorefractive crystal. Architecture issues related to spatio-rotational multiplexing and novel error-correcting encoding techniques used to achieve low bit-error rates are discussed.

https://science.sciencemag.org/content/sci/265/5173/749.full.pdf

Volume Holographic Storage and Retrieval of Digital Data

An encryption method and apparatus for holographic data storage are disclosed. In a system using orthogonal phase-code multiplexing, data is encrypted by modulating the reference beam using an encryption key K represented by a unitary operator. In practice, the encryption key K corresponds to a diffuser or other phase-modulating element placed in the reference beam path, or to shuffling the correspondence between the codes of an orthogonal phase function and the corresponding pixels of a phase spatial light modulator. Because of the lack of Bragg selectivity in the vertical direction, the phase functions used for phase-code multiplexing are preferably one dimensional. Such phase functions can be one-dimensional Walsh functions. The encryption method preserves the orthogonality of reference beams, and thus does not lead to a degradation in crosstalk performance.

Encrypted holographic data storage based on orthogonal phase code multiplexing

We propose a technique for data detection in a two-dimensional page-access optical memory. The technique combines sequence detection by the use of the Viterbi algorithm with decision feedback to improve the bit-error-rate performance in a system corrupted by intersymbol interference. It has an advantage in that it can be operated on a row-by-row basis as data are output from the optical detector. Use of the proposed scheme might ease the design tolerances of the optical components or permit the use of large data pages.

Signal detection for page-access optical memories with intersymbol interference

We compare the characteristics of cross talk for angular multiplexing and several phase-encoded multiplexing strategies for volume holography. We discuss the implications of these characteristics for holographic data storage and compare noise arising from cross talk with noise arising from scattering that is due to inhomogeneities within a holographic medium.

Cross-talk considerations for angular and phase-encoded multiplexing in volume holography

Various channel codes, including binary, gray-scale, threshold, and differential techniques, are compared for digital holographic data storage. The tradeoffs among bit error rate, storage capacity, and system complexity are discussed.

J. F. Heanue

Papers

Volume Holographic Storage and Retrieval of Digital Data

Encrypted holographic data storage based on orthogonal phase code multiplexing

Signal detection for page-access optical memories with intersymbol interference

Cross-talk considerations for angular and phase-encoded multiplexing in volume holography

Channel codes for digital holographic data storage