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Digital storage in five dimensions 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 10 12 bits per cm 3 (1 Tbit cm -3 ) by storing multiple, individually addressable patterns within the same recording volume. Although wavelength 1 , 2 , 3 , polarization 4 , 5 , 6 , 7 , 8 and spatial dimensions 9 , 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.

We describe a method for storing sequences of optical data pulses by converting them into long-lived acoustic excitations in an optical fiber through the process of stimulated Brillouin scattering. These stored pulses can be retrieved later, after a time interval limited by the lifetime of the acoustic excitation. In the experiment reported here, smooth 2-nanosecond-long pulses are stored for up to 12 nanoseconds with good readout efficiency: 29% at 4-nanosecond storage time and 2% at 12 nanoseconds. This method thus can potentially store data packets that are many bits long. It can be implemented at any wavelength where the fiber is transparent and can be incorporated into existing telecommunication networks because it operates using only commercially available components at room temperature.

Electromagnetically induced transparency 1 , 2 , 3 is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a ‘coupling’ laser is used to create the interference necessary to allow the transmission of resonant pulses from a ‘probe’ laser. This technique has been used 4 , 5 , 6 to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud 4 . Here we use electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively stopped; coherent information initially contained in the laser fields is ‘frozen’ in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the ‘read’ and ‘write’ operations. We anticipate applications of this phenomenon for quantum information processing.

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging 1 , optical data storage 2 , 3 and lithographic microfabrication 4 , 5 , 6 . These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ 3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10 −50 cm 4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures 7 .

Several types of quantum memory protocols have been presented over the last ten years, including photon echoes 1 , 2 , 3 , 4 , off-resonant Raman scattering 5 , 6 , ultraslow light-based quantum mapping processes 7 , 8 , 9 , 10 and resonant Raman optical echoes 11 . These quantum optical memory protocols are limited by a storage time on a scale as short as milliseconds, determined by the spin phase decay time of the storage medium. For applications of long-distance quantum communications, a quantum repeater composed of quantum entanglement swapping and quantum memory must be used 12 , 13 . Achieving longer storage times in quantum memory therefore brings a definite advantage to applications of quantum repeaters for long-distance quantum communications. Here, we propose a quantum optical data storage protocol to extend the storage time by several orders of magnitude beyond the conventional limitation of the order of milliseconds. The present ultralong quantum optical storage technique is achieved by introducing an optical locking method to the resonant Raman optical echo protocol 11 . Quantum optical memory protocols are currently limited to storage times in the millisecond range. A quantum optical data storage protocol that extends the storage time by several orders of magnitude is proposed. The method introduces an optical locking technique to the resonant Raman optical echo approach.

Photonic signals were efficiently stored in a semiconductor-based memory cell. The incident photons were converted to electron-hole pairs that were locally stored in a quantum well that was laterally modulated by a field-effect tunable electrostatic superlattice. At large superlattice potential amplitudes, these pairs were stored for a time that was at least five orders of magnitude longer than their natural lifetime. At an arbitrarily chosen time, they were released in a short and intense flash of incoherent light, which was triggered by flattening the superlattice amplitude.