Fluorescence readout of near-field photochromic memory

The fluorescence- and transmittance-detection readout methods of a near-field photochromic optical memory by a scanning-probe technique were studied theoretically. Shot noise, as the principal noise, was taken into account in an analysis of the signal-to-noise ratio (SNR). Under most conditions, fluorescence-detection readout yielded a better SNR than did transmittance-detection readout. For a transmittance change of 0.9-1.0 as a result of recording, a readout light power of approximately 100 nW, and a system bandwidth of 1 MHz, only the fluorescence-detection readout method, under the condition that the fluorescent layer of the medium have a fluorescence quantum yield greater than 0.4, can produce a sufficiently large SNR.     

Photochromic diarylethene for two-photon 3D optical storage

We had accomplished recording and readout of bit patterns by two-photon 3D optical storage technology using a new photochromic diarylethene, 1,2-bis(2-methyl-5-methylene-n-butylamido-thien-3-yl) perfluorocyclopentene (BMMBTP), as memory medium. The photochromic reaction, both in solution and in poly(methyl methacrylate) (PMMA) amorphous film, and fluorescence property of BMMBTP were investigated.     

Comparison of readout characteristics between magnetooptic transferheads and magnetic heads

The readout characteristics of a magnetooptic transfer (MOT) head were compared with those of a magnetic head. Magnetic recording/readout was done on a CrO₂ flexible disk by using a head with a track width of 5 μm. A Bi-substituted garnet film with a domain width of 1.2 μm and an He-Ne laser spot focused down to 3 μm were used as the MOT head. Readout waveforms from both heads were surprisingly similar. The maximum carrier-to-noise ratio obtained was 50 dB (bandwidth: 30 kHz) for both heads. Experimental data for off-track and crosstalk characteristics demonstrated that the MOT head was suitable for use as a high-track-density readout head. The potential advantages of multitrack readout using MOT heads are described     

Photochromic diarylethene for polarization holographic optical recording

Photochromic diarylethene, 1,2-bis[2-methyl-5-(3-fluorophenyl)-3-thienyl] perfluorocylcopentene (1a), was synthesized. The compound showed good photochromic reactions both in solution and in PMMA matrix by photo-irradiation. Using the diarylethene 1b/PMMA film as recording medium and a He-Ne laser for recording and readout, four types of polarization holographic optical recording were accomplished for the first time. The results show that the orthogonal circular polarization recording is the best method for holographic optical recording when the target photochromic diarylethene is used as recording material.     

Pulse-read on erasable thermal phase-change superresolution disks

New erasable thermal phase-change superresolution (EPSR) disks composed of mask and recording layers can increase recording density by the detection of the below-diffraction-limited marks within the readout spot. The formation of the aperture and the readout signal on the EPSR disk were analyzed. The feasibility of optically designed EPSR disks was evaluated by thermal simulation. A carrier-to-noise ratio of 32 dB at a mark size of 0.4 mum, 8 dB higher than that of a conventional disk, was obtained by application of a pulse-read method to the EPSR disks at a wavelength of 780 nm and a numerical aperture of 0.55.     

Three-dimensional optical random access memory materials for use as radiation dosimeters

This article describes the development of the first three-dimensional optical random access memory (3D-ORAM) material and readout system for monitoring energetic neutrons. Two different photochromic dyes, 5'-chloro-6-nitro-1',3',3'-trimethylspiro-[2H-1-benzopyran-2,2'-in doline] (spirobenzopyran) and anthracene, have been investigated for use in these 3-D ORAM dosimeter materials. These dyes were immobilized in a poly(methyl methacrylate) support, and the resulting dosimeter materials were irradiated with neutrons from a Cf-252 source. Fluorescence measurements from the dosimeter show a dramatic decrease in the overall fluorescence intensity of the 3D-ORAM dosimeter exposed to the Cf-252, relative to a nonirradiated dosimeter. In addition, a two-photon excitation readout system has been developed for determining characteristics of the radiation that are necessary for estimating dose.     

Wide angular aperture holograms in photorefractive crystals by the use of orthogonally polarized write and read beams

We demonstrate a method of simultaneous holographic recording and readout in photorefractive crystals that provides high write-read beam isolation and wide angular bandwidth. The method uses orthogonally polarized read and write beams and parallel tangent diffraction geometry near the equal curvature condition to provide spatially separable, orthogonally polarized diffracted output beams with high isolation and wide Bragg-matched angular bandwidth. The available angular bandwidth of this read-write technique is analyzed, simulated, and experimentally investigated. The measured angular bandwidth internal to the crystal is approximately 18° × 6° for our 45°-cut BaTiO(3) crystal, yet the entire hologram still demonstrates high Bragg selectivity. In contrast, traditional nonparallel-tangent geometries yield angular apertures of the order of 1° × 4°.     

Readout characteristics of a near-field optical probe as a data-storage readout device: submicrometer scan height and resolution

A photodiode-embedded near-field scanning microscope cantilever (photocantilever) was used to scan in a noncontact, constant-height mode at a range common in hard disk drives to examine its readout capabilities when mounted on a flying-slider head. The intensity ratios of spatial frequencies that compose the obtained near-field image were analyzed by use of the fast Fourier transform. A simplified model was developed as a guiding principle for estimating the readout characteristics of the near-field optical probe in the above-proximity scan-height range.     

Spot and mark-size characterization in magneto-optic recording

The attainable performance of a magnetooptic (MO) recording system is determined by the laser intensity profile and the size and shape of the written magnetic domain. The authors present a computer model that provides MO readout waveforms based on an overlap integral of the scanning laser spot and the written domains. This model and regression analysis are applied to experimental magnetooptic readout waveforms, and the laser profile and mark size are determined. A nonlinear least-squares fit of readout data yields a unique determination of the laser-spot profile and written-domain size. The effects of groove width and thermal blooming on domain shape can be included in the modeling. The results are compared to those of other spot and domain measurements, and close agreement is obtained     

Shift-multiplexed microhologram fabrication with photoisomeric chromophores

Microholographic memory is an attractive storage system for its capability to hold high-density data and for its access time. Using a photochromic chromophore (diarylethene)-doped recording medium can give rise to microholographic memory's durability and contrast. In addition, it is possible to increase the microholographic memory's density by shift-multiplexed recording, since a hologram pit is constructed in a small area. The microhologram was fabricated in the diarylethene-based sample with two counterpropagating focused beams. Also, surface images and cross-sectional images scanned by a confocal microscope indicated that shift-multiplexed recording was achieved in high contrast.     

Near-field optical recording with nanocomposite cover-layer for numerical aperture of 1.85

Near-field optical recording media with a nanocomposite cover-layer were prepared and tested using a gap servo, near-field recording system with an effective numerical aperture of 1.85. The refractive index of the nanocomposite cover-layer, which has a smooth surface, is 1.86, yielding a successful gap servo operation. However, the recording density cannot be maximized due to the refractive index of the cover-layer being lower than the required value for the effective numerical aperture. A recording density of 95?GB per disc can be achieved with a nanocomposite cover-layer having a refractive index of 1.86. The readout signal from 110?GB is not clear due to the reduced eventual numerical aperture in conjunction with the limited refractive index of the cover-layer.     

Holographic memory with localized recording

We experimentally demonstrate and characterize a memory module that features selective page erasure and readout persistence using the localized recording method in doubly doped LiNbO(3). Pages of information can be selectively erased without partially erasing the whole memory. Data pages can be written over erased pages multiple times. Information is read millions of times before refreshing is required. We quantify the optical quality of the holograms by measuring their signal-to-noise ratio for a memory size up to 100 holograms. A compact phase-conjugate readout architecture is also presented and experimentally demonstrated.     

Experimental analysis in recording transmission and reflection holograms at the same time and location

Holographic recording media with a reflection layer are useful because they make it possible to maintain backward compatibility with CDs and DVDs, and a conventional servo system is easily attachable. The incident beam is fed back to the recording layer by the reflection layer, so there are four beam pairs to record the transmission and reflection holograms. We analyze the basic property of the transmission and reflection holograms and evaluate the problem when the transmission and reflection holograms are recorded at the same time. It is shown that the shrinkage in the photopolymer medium has a different effect on each hologram, so the readout image from the two holograms is misaligned. Those diffraction beams make the interference pattern, and the signal-to-noise ratio (SNR) of the output image decreased. Taking into account the difference in wavelength selectivity between the transmission and the reflection holograms, we propose a way to select one hologram to get the diffraction beam and eliminate the interference pattern using the tuning readout wavelength. By using this method, we can eliminate the diffraction beam from the reflection hologram and keep a high SNR.     

Digital beamforming in ultrasound

The effects on array gain and sidelobe level of a practical digital beamforming (DBF) processor under the wideband conditions typical of ultrasound is discussed. It is concluded that a relatively simple design that replaces each analog delay line with a tapped, digital shift register (DSR) and a digital phase shift operation adjusted for midband will provide the desired performance, provided that the sampling rate of the signal at the input to the DSR is 4 to 10 times the bandwidth. More realistically, when nonidealized passbands are taken into account and the typical condition whereby the transducer frequency is about twice the bandwidth is considered, the rule of thumb for the sampling rate is that it must be 4 to 10 times the transducer frequency.     

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.     

Evanescent coupling in magneto-optical and phase-change disk systems based on the solid immersion lens

Results of numerical computations pertaining to evanescent wave coupling for near-field magneto-optical and phase-change disks based on the concept of the solid immersion lens are presented. We investigated the relation between the coupling efficiency and the width of the air gap in terms of the throughput of the recording process and the resolution of the readout signal. The simulations show a drastic decrease with a widening air gap of the coupling efficiency by means of evanescent waves into the recording medium. In magneto-optical readout, loss of the signal may be attributed to the reduction of magneto-optical interaction, the rise of reflectance, and the variation of the relative phase between the two components of polarization. In the phase-change readout the reduced reflectivity contrast between crystalline and amorphous marks is the cause of signal reduction.     

Theoretical study of near-field optical storage with a solid immersion lens

Both the reflection inside a hemisphere solid immersion lens (SIL) and the reflection inside the gap between the SIL and the optical recording medium are considered. The near-field SIL imaging theory for high numerical aperture is developed by using the vector diffraction and thin-film optics. Numerical results show that the spot size, Strehl ratio, and sidelobe intensity have an oscillatory behavior with the change of thickness of the air gap, which results from the interference effect of the transmitted field. We find that for smaller spot size, the Strehl ratio is smaller but the sidelobe intensity is larger. A certain thickness of air gap is useful for optical storage, which is less than 63 nm for the system in the simulated examples.     

Storage of 1000 holograms with use of a dual-wavelength method

We demonstrate the storage of 1000 holograms in a memory architecture that makes use of different wavelengths for recording and readout to reduce the grating decay while retrieving data. Bragg-mismatch problems from the use of two wavelengths are minimized through recording in the image plane and using thin crystals. Peristrophic multiplexing can be combined with angle multiplexing to counter the poorer angular selectivity of thin crystals. Dark conductivity reduces the effectiveness of the dual-wavelength method for nonvolatile readout, and constraints on the usable pixel sizes limit this method to moderate storage densities.     

Recording and readout using clock marks premastered by groovewobbling

A new recording and readout technique for land and groove recording on a magnetic super-resolution (MSR) disk is described. The technique uses specifically premastered clock marks. To generate a stable clock signal, the clock marks are fabricated with short bursts in grooves by wobbling at a different frequency from address information. The clock marks and the address information can be separated from a track error signal. When the extracted clock signal is applied to precise recording and readout of the magneto-optical data marks, the bit error rate becomes lower than that of the conventional clock recording/readout system. Additionally, the crosstalk of the wobbled address information to the magneto-optical signal can be canceled out electronically. This new format is not only suitable for high-density recording but also convenient in the disk manufacturing process     

Local near-field enhancement of random Sb-SiN films

Local near-field enhancement of random Sb-SiN films has been studied. Specimens consisting of a random Sb-SiN film and an optical recording layer were prepared and exposed to a focused laser beam. Laser-induced ablation occurred on the recording layer adjacent to random Sb-SiN film much faster and at much lower power than on a single recording layer. These results indicate that an optical field can be enhanced by random Sb-SiN films. The enhanced field was subsequently investigated by scanning near-field optical microscopy, and the pictures revealed that the enhanced field was localized.