In spite of a small and indirect bandgap, silicon features properties that are potentially very attractive for optical applications. The unsurpassed level of impurity control leads to suppression of nonradiative recombination paths and provides for a very long minority carrier lifetime. Up to the present, observation of an optical memory effect has not been reported for crystalline silicon (c-Si). In contrast, it has been observed in GaAs and GaN. In this study, we have used c-Si doped with erbium (Er) ions, since the trivalent Er³⁺ ion emits at a wavelength of 1.54 μm that is suitable for telecommunication applications. In a two-color experiment, utilizing a Nd:YAG pulse excitation as a writing beam and a free-electron laser (FEL) as a reading beam, we revealed that Si exhibits optical memory and afterglow effects at low temperatures T<50 K. Both effects are, in fact, manifestations of the properties of Si itself, conveniently revealed by the optical dopant. Detailed investigations of the FEL-induced emission at 1.54 μm show that this excitation mechanism is governed by the release of a single type of charge carrier. This implies that hole and electron capture events necessary for the Er excitation could, in principle, be separated in time. Therefore it appears possible to combine optical and electrical write/read procedures for the reported memory effect.

Spectral hole burning material parameters that impact optical memory architecture and performance are investigated. Optical power budget analysis for the data storage process reveals that although narrow homogeneous linewidth can confer high data density, it does not optimally support high data rates. Trading narrow linewidth for higher oscillator strength would be desirable, if attainable. Experimental measurement of oscillator strengths, quantum yield into alternative hyperfine ground state levels, and persistence of the hyperfine level populations in Eu³⁺:Y₂SiO₅ and Pr³⁺:Y₂SiO₅ are presented and discussed. Quantum yield measurements of less than 25% indicate that spin projections are strongly preserved during excitation and relaxation processes. The hole depth consequently attainable from single π-pulse illumination requires trade-offs in memory system design.

The approach to increase optical recording density has become very popular research subject in these years. One direct and effective method is to increase the recording layer stack number. That is to say, to add one more recording layer can get one more recording capacity. In this paper, we will propose a new method for manufacturing read only type multi-layered disc. The process is described in the following. This first recorded data layer (called L0) still follows the traditional DVD disc manufacturing process. We obtain the polycarbonate substrate by replicating from Ni stamper. Then polycarbonate substrate is sputtered thin silicon film for semi-reflection layer. As for second layer (L1) and even more layer (Ln-1) producing, one special kind of duplication (called SKD) method is proposed. The duplication (or replication) source of second or nth recorded data is not only limited from Ni stamper. Even polycarbonate or PMMA substrate has recording data are also acceptable sources. At next step, the duplication source is deposited by thin gold film. Then we apply spin coating to bond the first layer (L0) substrate and second layer (L1) duplication source by choosing suitable UV curing glue. After being emitted by UV lamp for several seconds, we can easily separate the duplication source of second layer (L1) from (L0) substrate. Then we find the thin second data layer (L1) is replicated and stacks upon the first layer. On the same way, we sputter thin AgTi layer on the thin second data layer for another semi- reflective layer. By following the above manufacture step, we can produce more layers. In our experimental, we prepare triple layered read-only type disc. The total capacity is almost 12GB for one side of disc, and 24GB for two side of disc. The read-out intensity of laser from each data layer is expected to be similar. Thus we have designed particular reflectance and transmittance for each data layer by controlling the thickness of thin silicon film. We can verify our design by checking the focusing error signal in S-curve search of optical pickup head. The signal quality for each layer can be found from the signal eye pattern and jitter. For compatibility with present drive system, the requirement of the readout signal from each layer should be same as DVD or CD specification

As computer processing and memory elements continue to shrink in size, quantum effects will no longer be negligible. This is especially important for memories, since these are expected to reach quantum size limits first. For example, if current trends continue, magnetic memory domains will become so small that the energy required to flip the domain orientation will become comparable to the Boltzmann energy, kT, at room temperature. To address these issues, the field of quantum computing has undergone substantial recent growth. However, quantum computers will require quantum memories and while many potential quantum computer designs are being explored, relatively few quantum memories are being developed. With this in mind, the prospects for storing quantum information in rare earth doped crystals are explored. In particular, the promising ultra-slow and stopped light storage schemes are described in detail, and projections are made based on experimental results.

Spatial-spectral holography using spectral hole burning materials is a powerful technique for performing real-time, wide-bandwidth information storage and signal processing. For operation in the important 1.5 μm communication band, the material Er³⁺:Y₂SiO₅ enables applications such as laser frequency stabilization, all-optical correlators, analog signal processing, and data storage. Site-selective absorption and emission spectroscopy identified spectral hole burning transitions and excited state T₁ lifetimes in the 1.5 μm spectral region. The effects of crystal temperature, Er³⁺-dopant concentration, magnetic field strength, and crystal orientation on spectral diffusion were explored using stimulated photon echo spectroscopy, which is the "prototype" interaction mechanism for device applications. The performance of Er³⁺:Y₂SiO₅ and related Er³⁺ materials has been dramatically enhanced by reducing the effect of spectral diffusion on the coherence lifetime T₂ through fundamental material design coupled with the application of an external magnetic field oriented along specific directions. A preferred magnetic field orientation that maximized T₂ by minimizing the effects of spectral diffusion was determined using the results of angle-dependent Zeeman spectroscopy. The observed linewidth broadening due to spectral diffusion was successfully modeled by considering the effect of one-phonon (direct) processes on Er³⁺ - Er³⁺ interactions. The reported studies improved our understanding of Er³⁺ materials, explored the range of conditions and material parameters required to optimize performance for specific applications, and enabled measurement of the narrowest optical resonance ever observed in a solid-with a homogeneous linewidth of 73 Hz. With the optimized materials and operating conditions, photon echoes were observed up to temperatures of 5 K, enabling 0.5 GHz bandwidth optical signal processing at 4.2 K and providing the possibility for operation with a closed-cycle cryocooler.

We demonstrate the effects of the applied field orientation during the writing period in electrical fixing. The dependency of the diffraction efficiency on the direction of the applied electric field while writing a hologram is studied. Our experiments show that during writing, when the electric field is applied opposite to the c-axis the grating can be successfully revealed with both positive and negative dc voltages. However, when the grating is written with a field parallel to the c-axis, the grating can only be revealed with a field applied in the opposite direction. This situation generates the largest diffraction efficiency of nearly 78%. Theoretical analyses of these observed phenomena are performed.

Holographic data storage (HDS), which makes use of the full volume of the recording medium, possesses high potential by promising fast transfer rates of hundreds of Megabytes/sec and storage densities greater than 200 Gbytes per 120mm disk. The restrictions that are placed on the holographic media, however, are stringent. Described here is a high performance photopolymer based medium that has the properties necessary to enable this technology. Through the use of several different holographic techniques, the material characteristics that are necessary for holographic storage products may be determined. The two different systems that are discussed here include Plane Wave and Digital Holographic Data Storage. These measured characteristics include high dynamic range (M/#), sensitivity, and small recording-induced Bragg detuning. In addition, results of archival and shelf-life environmental testing of the media will be discussed.

We explain and compare two different methods (two-step and two-center recording) for gated holographic recording in lithium niobate crystals. We first compare the holographic recording performance of the two schemes based on the experimental results published in the literature. Then, we use a general model to compare the essential physics of the two methods theoretically, and we show that two-center recording has better performance in low light intensities. Global optimization of two-center recording as well as a unique feature of gated holography (i.e., localized recording) for new applications will also be discussed.

In order to read out correct data from a disk-type holographic storage system, it is very important to implement a servo control in pickup modules, like as in conventional CD-ROM and DVD-ROM. We propose a novel servo-control method by use of a glass-plate on optical axis, which is able to compensate readout errors mainly coming from mechanical wobbling of the holographic disk. By rotating the glass-plate within ± 10 degrees, we can reduce the readout position-errors of ± 200 μm to ± 15 μm.

In phase change recording, higher linear densities can be achieved with materials in which crystallization is dominated by growth. This is due to the fact that marks can be written with sharper edges, which give rise to lower jitter. Therefore AgInSbTe alloy based thin films appear to be one of the latest promising materials for optical data storage that has drawn worldwide attention. In the present paper (AgSbTe) _x (In1-ySby) _1-x quaternary alloy based films for x = 0.2, 0.3, 0.4 and y = 0.7, were deposited using thermal evaporation technique under a high vacuum of 10^-6 torr. The potentiality of the above mentioned films for a phase change optical memory was confirmed using Differential Thermal Analysis (DTA). The results show that this material has good glass forming ability. Further the micro-structural details of the films were studied using SEM (Scanning Electron Microscopic) technique. We also investigated the effect of 1hour thermal annealing on grain size of the films. Thermal annealing of the prepared films was done at different temperatures ranging between 200°c-400°c through radiant heating in vacuum at a pressure of ~10^-5 torr. The micro-structural analyses of the as-deposited and annealed films are presented here. This also explains the effect of change in composition as well as change in annealing temperature on the crystalline phases formed on the film.

Photochemical hole-burning phenomenon is discussed with regard to its application in high-density optical memory systems. A theory is introduced to express the recording density limit of this memory as a function of readout time and material parameters. Experiments in dye-doped polymer systems show that the main factor limiting the attainable recording density is hole filling brought about by the irradiation of another wavelength. This hole filling occurs because the doped chromophore in an amorphous matrix has various energy levels. Extent of hole filling is related to the molecular structure of the chromophore. When a chromophore is rigid and does not have a low energy vibration mode, the extent of filling becomes smaller.

The design, performance analysis and experimental demonstration for an analog, broadband, high performance electro-optical signal processor are presented. The Spatial Spectral (S²) Coherent Holographic Integrating Processor, or S²-CHIP, has been developed recently as a broadband core-component for range and mid-to-high pulse repetition frequency radar-signal processing systems, as well as for lidar and radio astronomy applications. In a range radar system, if the transmit and receive RF waveforms are modulated onto a stable optical carrier, the S² material will perform the analog correlation of the transmit and receive signals to yield the target’s range, and also coherent integrate multiple return results to increase the signal-to-noise-ratio and provide for target velocity determination. Preliminary experimental results are shown of S²-CHIP range processing using a 1.0 Gb/s data rate with 512-bit BPSK pulses. Good range resolution is observed for delays up to 1.0 microsecond. The ability of the processor’s to handle dynamic coding on the transmit RF waveforms is demonstrated.

Current performance in two-photon WORM volumetric write once read many data storage systems is presented and future directions discussed. Influence of numerical aperture in a 3-D multi-layer optical data storage system is analyzed.

We review the crucial properties of a phase-encoded volume holographic storage system in terms of data quality and security, which are the key issues of any bulk memory system. Two major problems which need to be tackled in holographic storage systems in terms of data quality are the hologram erasure during readout and the data encoding schemes for error-free reconstruction. We present a novel storage material, (bismuth tellurite crystals - Bi₂TeO₅), which has the potential to overcome the volatility problem and avoiding the need of any further fixing. Regarding data encoding schemes, we present a general approach of gray scale modulation coding in order to improve the data capacity in comparison to normal modulation coding, while the bit error rate maintains low. Data security in a phase encoded system can be realized by exploiting its special multiplexing characteristics. We present different encryption techniques and investigate their decryption probability.