Table of contents

Phase-change alloys constitute the basis for a widening collection of storage technologies both optical and electrical. These uses of phase-change alloys are characterized by switching material properties either by laser irradiation or by an electric current on the nanosecond time scale. Considering the conflicting requirements for high-speed switching, yet long term data storage integrity, a deeper understanding of the switching processes in these materials is essential for insightful application development. We have used synchrotron-based time-resolved X-ray absorption fine structure spectroscopy (XAFS), a technique equally suitable for amorphous and crystalline phases to elaborate details in structural changes in the phase-change process on a sub-nanosecond time scale using optical pump/X-ray probe techniques. In this work, we present initial results of sub-nanosecond laser excitation of the laser-reamorphized state of Ge₂Sb₂Te₅. The technique is general and can be applied to a wide variety of nanoscale structures.

In this paper, we propose an "L-shaped" rotary voice coil motor (VCM) actuator for an ultra slim optical disk drive (ODD) with a CF II card size using the integrated design method that integrates coupled-field analysis and design methods.

Super hemispherical solid immersion lens system becomes a matter of interest due to its high numerical aperture (NA) gain. However, because of the instability of the aplanatic condition, even small amount of alignment error can easily lower the optical performance. To overcome the instability while maintaining high NA gain, we suggest an optimum solid immersion lens (opti-SIL) system which combines the advantages of both super hemispherical SIL (hyper-SIL) and hemispherical SIL (hemi-SIL). Exemplary designs and simulation results of the tolerance analysis show that opti-SIL system has much higher tolerances to various performance-lowering factors than hyper-SIL, even with relatively small NA resignation.

To improve the recording rate of optical disk equipment, we have been developing an optical device that can be used with a multi-beam light source by employing waveguides for visible light to align the beams at a narrow pitch. In this study, an optical device composed of eleven bent waveguides was fabricated using amorphous polymer, which is highly transparent throughout the visible region. The waveguides transmitted approximately 70% of a blue-violet beam with a wavelength of 405 nm, obtaining a maximum intensity of approximately 20 mW (continuous wave). By controlling the curvature of each waveguide, the optical device narrowed the pitch of blue-violet beams from 125 to 12 µm, acting as an optical system to reduce the pitch of beams by a factor of approximately 10. Using the optical device, a blue-violet four-beam light source with a narrow pitch of 12 µm was demonstrated.

Concept, design and analysis of several solid immersion lens (SIL) mounting systems utilizing high-index refractive micro SILs are described. This paper proposes several designs for wafer-scale SIL assemblies capable of a numerical aperture of 2.2.

Mechanical resonance of optical pick-up actuators can cause errors in reading information from high-speed optical discs. Ferrofluid on the surface of magnets is retained in a magnetic field and its viscosity provides the desired mechanical damping to a moving mass. A ferrofluidic damper that is controlled by saturation magnetization for an optical pick-up actuator not only improves system performance such as settling time and access time of a drive on warped or eccentric discs but also remarkably delays temperature increase due to the induction of overcurrent on the coils.

As the traditional method for increasing the readout and write data rate in optical storage by increasing the disc rotational speed is at its upper bound, due to factors such as disc vibration, servo bandwidth, signal noise ratio, etc. multi-beam systems that enable higher data rates are emerging as an important option in the speed race. By using our new wiggling method for multi-beam optical storage, the extra link-overhead caused by traditional jumping is avoided. Using this method, a certain data rate can be achieved with one laser spot less than when using the traditional jump method. This means a considerable reduction in cost and power dissipation.

We developed a brand-new stabilizer with a cylindrically concaved active surface for a flexible optical disk system. The unique design enabled extremely stable driving of the flexible disk at rotational speeds over 10,000 rpm. We actually demonstrated the driving at rotational speeds of up to 15,000 rpm, the spindle motor limit of our optical disk tester. This highest rotational speed promises a maximum data transfer rate of more than 600 Mbps for the recording density of a Blu-ray Disc. This stable state was achieved using a simple control that just adjusts the relative axial position of the stabilizer against the flexible disk. Once the adjustment was made, high stability was maintained over a wide rotational speed, ranging from 4,000 to 15,000 rpm. In this stable state, the axial runout on the pickup scanning line was suppressed to less than 10 µm at all rotational speeds. By achieving this high performance with simplified stabilizer control, we have come close to putting our system into practical use.

Using rotary transformers, we developed a contactless power supply mechanism for a layer-selection-type recordable multi layer optical disk. This method provides stable resistance during disk rotation and requires minimum space for power supply mechanism. Our prototype transmits sufficient electrical power to change the optical reflectivity of the rotating disk. We studied the miniaturization of an optical drive and concluded that a slim-type drive is feasible with this contactless power supply mechanism.

In this paper, we employed full vector diffraction treatment to study the dependence of the servo signals to the doughnut-shaped focus radiation of radial polarization, which provides a smaller spot size than linear polarization, and the resistance to the primary aberrations in a numerical aperture (NA)=1.0 optical data storage system. Compared with linear polarization, at strong focusing, a radially polarized beam leads to 14% asymmetry reduction in the linear range of the focus-error signal (FES) curve and to at least 56% variation reduction in crosstalk in a weakly aberrated system. These characteristics make radial polarization a potential candidate for next-generation high-density optical systems.

In this paper we look at the issues involved in the drive design for multi-layer Blu-ray Disc. We will discuss the layer thickness correction and signal to noise ratio. Furthermore we will experimentally investigate the layer cross-talk. The cross-talk from data on other layers, and the consequences of the layer cross-talk for the radial push–pull signal of the main spot and the satellite spots are addressed. It is found that a simple drive design is still possible, provided that for radial tracking only the main spot is used.

We have realized a high-speed and precise tracking servo using a feed-forward control with the zero phase error tracking method (ZPET-FF control) on an optical disk at the disk rotation speed of 10800 rpm. To realize a high-data-transfer-rate recording of more than 200 Mbps over the entire disk area, the optical head must be controlled with an accuracy of better than 9 nm at the maximum rotation speed of about 10800 rpm for track following on a high-density optical disk. We performed high-speed tracking, and the experimental results show that the ZPET-FF control is more precise than feedback control. The residual tracking error is suppressed to 6.3 nm at a disk rotation speed of 10800 rpm on optical disk drives having an object lens with a numerical aperture (NA) of 0.85 and media with a track pitch of 0.32 µm, such as the Blu-ray disc and broadcast-use optical disk.

A new efficient four-ary (1,4) run-length limited (RLL) code is presented, which is intended for high density multi-level optical recording channels. By using proposed hybrid DC control scheme, the new code can achieve acceptable DC suppressing characteristics while maintaining high code rate. The bit error rate (BER) performances of this new code on blue laser recording systems are evaluated by using partial-response maximum-likelihood (PRML) detection. The feasibility of using four-ary RLL modulation and PRML detection is studied in this paper, and these techniques show great potential to be applicable in high-density multi-level optical recording channels.

In this paper, we propose an encryption method, in which a multi reference pattern is used in coaxial holographic data storage. The signal data patterns are multiplexed at one spot with multi reference patterns, which are the key code for secure readout. This method is easy to implement in coaxial holographic storage drives, and provides higher security against unauthorized readout and copying of the hologram disk. We experimentally demonstrate the encryption and decryption process with the proper and improper keys. Next, we analyze the degree of security provided by this method, and demonstrate a method of increasing the security further by recording more than 10 holograms at one spot. Finally, we record multiplexed holograms to reach 50 and 100 Gbit/in.² with this encryption method, and analyze the degradation of the error rate.

Multilevel recording with run-length limited (RLL) modulation is a novel way to significantly increase the recording density without changing the optical or mechanical units of current optical disc systems. In this paper, a new efficient 4-ary RLL(2,13) code with special constraint is designed for four-level RLL modulation on read-only digital versatile disc (DVD-ROM). This code can result in alternate multilevel pits and lands on the disc, and reduce the difficulty of multilevel disc replication. Further more, simulation results show the symbol error rate (SER) performance can be improved about 3 dB when the proposed code is used for optical recording channel with partial-response maximum-likelihood (PRML) detection.

Holographic data storage capacity is proportional to the thickness of a photopolymer recording medium. To maximize the use of the performance of a photopolymer recording medium, a multiplexing technique that can record holograms in the thickness direction of such a medium is demanded. To this end, we propose a new multiplexing method that combines shift, peristrophic, and focus-shift multiplexings, and successfully records holograms in the thickness direction of the medium by controlling the incidence angle of the reference beams. We confirm that the holograms recorded are well spatially separated in the thickness direction of the medium, and that up to 50 holograms can be recorded.

Common-aperture holography is presented as an alternative shift multiplexing concept. We measured the shift selectivity of holograms recorded with this novel concept. The experimental results are discussed and compared to theoretical predictions calculated with the beam propagation method.

We propose an error correctible balanced code for holographic data storage. As the transitions of a message word are considered and parity bits that check for errors in the section are appended on encoding, we can decode message words with the Viterbi algorithm and correct errors. The effectiveness of the proposed method is validated by numerical simulations and experiments. An improvement in bit error rate (BER) is achieved.

A holographic data storage system has the advantages of a high data rate, rapid access and a multiplexing method. The two-dimensional page-oriented nature of holographic data storage also utilizes the information capacity of an optical wavefront to allow data to be recorded and retrieved in parallel, a page at a time, rather than serially as in conventional storage. This affords the potential for extremely high data rates subject only to the limitations imposed by the input/output (I/O) at devices (SLM, detector arrays) and electronic channels. A further distinguishing feature is the speed by which data can be accessed. In this paper, we propose the integration of overall error reduction algorithm for a holographic data storage system. Therefore, it is possible for a disk tilt error, position error and IPI noise error to be corrected by our error reduction algorithms. Furthermore, we obtain a good performance for writing and retrieving in the holographic data storage system.

A modified low-density parity-check decoding scheme for holographic data storage has been developed and evaluated. It compensates the negative effect from neighboring pixels during the iterative decoding process, which improves the overall error-correction performance when pixels are misaligned. A simulation shows that the proposed scheme outperforms a conventional log-likelihood-ratio (LLR) belief-propagation (BP) algorithm.

In this paper, we propose a new parallel hologram copying system in which multiplexed holograms are coherently copied from a master medium to a copy medium. The diffraction efficiency in spatial spread-spectrum multiplexing, which is a type of phase multiplexing based on phase modulation onto signals, can be significantly enhanced by coherent parallel copying. We theoretically and experimentally showed that the diffraction efficiency of M multiplexed holograms can be increased up to 1/M of that of a single hologram, which is an improvement by a factor of M compared with the 1/M² formula in conventional multiplexing systems.

A novel selective erasure method for multiplexed holograms in a photorefractive crystal using a phase conjugator is presented. In this method, the π-phase shift effect on the diffracted beam due to the photorefractive effect is used. There is a π-phase shift between the interference pattern formed by the reading and diffraction beams, and that formed in the recording process. By returning the reading and reconstructed beams to a recording medium with a phase conjugator, the hologram can be selectively erased in parallel with the readout. Therefore, a dynamic selective erasure can be realized without requiring the accurate alignment of the selective erasure beams. The calculations using coupled-wave equations and experiment results demonstrate the implementability of our method. Furthermore, we analytically determine the requisite conditions of incident beams in the selective erasure process for the reduction of the decay of other multiplexed holograms.

To reduce the loss of the dynamic range M/# of medium, we have proposed and demonstrated a high-pass filtering method for coaxial holographic data storage. Total exposure power decreased to 1/2 by removing the DC components of the Fourier-transformed signal and reference beams with a high-pass filter. Our proposed method can reduce the wasted dynamic range of medium. We evaluated the bit error rate of a high-pass-filtered signal beam to be 5.0×10^-4 and that of an unfiltered signal beam to be 1.3×10^-3. Thus, we confirmed that the quality of the signal beams without the DC component is comparable to that with the DC component. Next, we successfully recorded and reconstructed the filtered signal beam with the coaxial holographic configuration. The bit error rate of the retrieved signal beam was 1.4×10^-3, showing the absence of degradation without the DC component of the retrieved signal beam. We also evaluated the shift selectivity improved from 5 to 2 µm by high-pass filtering.

In reflection-type holographic memory, speckle-shift multiple recording along the axial direction of a storage medium is investigated experimentally and numerically. We record three data pages along the axial direction of an iron-doped lithium niobate medium with a thickness of 0.5 mm and an interval of the first null distance of shift selectivity, and each hologram was read out at the recording position. We also numerically evaluate the multiple recording along the axial direction. The numerical analysis supports the fact that the speckle size determines the three-dimensional recording interval. The numerical results are in qualitative agreement with the experimental results. We numerically examine the bit error rate as a function of the recording interval. The results show that multiplexing along the axial direction can increase the storage capacity.

In a two-beam angle multiplexing holographic memory, there is a problem that the tilt margin is small. Therefore, we propose a radial tilt compensation method (radial tilt is a tilt in the vertical direction to a surface that is made by reference and signal beams). When the radial tilt occurred during the reading, we brought the relative angles of interference fringes and the reference beam near the state at the time of recording. Concretely speaking, we shifted the disc to the circumferential direction slightly to obtain the optimum signal-to-noise ratio (SNR) and then recovered the hologram. In our experiments, we recorded 1 hologram (no multiplexing) and 51 multiplexing holograms, and recovered them using this compensation method. As a result, we confirmed that our method improved the tilt margin markedly in both cases. This compensation method is thus effective for interchangeability without the need for an additional mechanism and making for smaller drives.

We have proposed and demonstrated a new holographic medium that can act as a write once read many (WORM) polarization-recordable medium. The new medium, composed of azobenzene-containing polymer and photocurable liquid crystal, exhibited birefringence when irradiated by polarized light. Additionally, the diffraction efficiency and the birefringence of this recorded complex increased to several times those of the simple rewritable azopolymer. This occurred because the orientation of photocurable liquid crystals was as good as that of the azopolymer because of the interaction with the mesogens. By measuring the birefringence, we confirmed that unlike the azopolymer, the orientation of the liquid crystal polymer was maintained even after circularly polarized beam irradiation. In this medium, only the photocurable liquid crystals were cured and became polymers with fixed orientation. Therefore, we propose that the new medium, which is polarization-recordable, WORM and highly sensitive, is very promising and practical for digital holographic storage.

There are two known competitive arrangements for reflection-type coaxial holographic storage systems: the optical layout developed by optware, which is a so-called collinear or "split-aperture system", and the "common-aperture system" developed by the European consortium named athos. We modeled the reference diffraction noise for both arrangements by the beam propagation method (BPM), and compared the results from the M number (M#) consumption point of view. We suggest the use of reference beam apodization for the split aperture system to reduce the diffraction noise caused by the reference beam at the object area.

We have developed a new coaxial interference method for the next-generation storage system. A signal beam and a reference beam are coaxially arranged and focused into a holographic medium using a bifocal lens. We can easily downsize the system because the optics is very simple. The reference beam is focused on the mirror surface of the medium, and the signal beam is defocused on this surface. A semicircular reference pattern is used to select normal playback or phase conjugate playback in a reproducing process. We have recorded a single hologram and a multiplexed hologram in a photopolymer medium with this method. These holograms are reconstructed successfully. We have found that our interference method has a 4 µm shift selectivity and excellent characteristics for the next-generation high-capacity optical memory.

We have proposed a novel structure of holographic data storage media, employing a phase-change (PC) reflector for optical disc applications. The PC layer, which is initially amorphous and has a low reflectivity in a write process, works as an absorber and inhibits reflected stray beam exposure. After the write process, the PC layer is crystallized and its reflectivity is switched to a high state, realizing a sufficient signal beam intensity in a read process. Experimental results show a low noise and a high signal-to-noise ratio, and a potential to have a large multiplexing number in the PC reflector.

A selective erasure/update method of multiplexed photorefractive and photopolymer holograms by use of the double Mach–Zehnder (DMZ) interferometric arrangement is proposed. The DMZ arrangement is used as the optical phase-control device for holographic recording/erasure/update operations, which can produce a pair of π-phase-shifted interference patterns with a simple optical setup. We intend to erase the holograms recorded in not only the inherently rewritable photorefractive crystal but also the photopolymer material generally used for the write-once (i.e., unerasable) holographic recording. It is experimentally confirmed that our method can erase the write-once type photopolymer holograms as well as the photorefractive holograms, although there will be a certain degree of restriction on the rewrite cycles limited by the monomer density, the degree of multiplexing and so on.

The characteristics of holographic recording in the presence of phase noise caused by air disturbances and vibrations were studied. Diffraction efficiencies were calculated by a finite-difference time-domain method. Simulation results showed that the amplitudes of fringes decrease with increasing phase noise level, and thus, diffraction efficiencies decrease. A novel method of phase compensation for holographic data storage was proposed. Interference fringes were impinged on a charge-coupled device (CCD) camera symmetrically positioned relative to a recording medium, and the fringes in the medium were estimated using the fringes on the camera. Phase information was obtained by fast Fourier transform, and a phase modulator was set in the reference path and driven to reduce the phase error, so that the phase on the camera was stabilized. By this method, the standard deviation of the phase error was reduced to less than 1/10 that obtained by a conventional method. The diffraction efficiencies of angle-multiplexing in a photopolymer were measured and the relative multiplexing number with feedback control was 50% larger than that without it. As the holographic data storage device records the phase, the proposed method is an important and feasible technology.

In this paper, we reveal a new device for use as an optical data storage system. The system is made up of Cassegrain-like structures. The volume (35×20×20 mm³) of the system is the size of a pickup head used in conventional optical storage. It has compatibility with several conventional optical disk technologies. Namely, the system provides a platform for integration with compact disc (CD), digital versatile disc (DVD), Blu-ray disc (BD) and holographic systems. The full width half maximums (FWHMs) of the spot size for a CD have been measured in this system and are 0.98 and 0.95 µm in the tangential and radial directions, respectively.

A new data-reproducing scheme is presented and implemented by field programmable gate array (FPGA) for higher density data reproduction on a commercial Blu-ray Disc. By introducing a signal waveform phase detector, the phase locked loop (PLL) circuit shows good performance against the inter-symbol interference (ISI) effect. From the experiment results, the new scheme can be used to reproduce data with a linear density of 40 GB using a commercial single-layer Blu-ray Disc.

We studied the feasibility of two-photon absorption recording and reflective reproduction for three-dimensional (3D) optical memories by comparison between bulk and super-multilayered media. The basic design of such a super-multilayered structure for achieving a high reflectance from a pit and for reducing layer thickness was described. To evaluate basic optical characteristics, two types of sample with a 100% diarylethene recording layer were fabricated. It was experimentally confirmed that reflective reproduction can be possible in the super-multilayered structure, whereas reflective signals were not obtained in the bulk structure as indicated by the results of theoretical analysis. Therefore, 3D recording and reproducing in a super-multilayered medium would be promising for a future high-capacity memory system.

A decision feedback partial response maximum likelihood (PRML) for super-resolution media was developed. Decision feedback is used to compensate for nonlinear distortion in the readout signals of super-resolution media, making it possible to compensate for long-bit nonlinear distortion in small circuits. An field programmable gate array (FPGA) was fabricated with a decision feedback PRML, and a real-time bit error rate (bER) measuring system was developed. As a result, a bER of 4×10^-5 was achieved with an actual readout signal at the double density of a Blu-ray disc converted to the optical properties of the experimental setup using a red-laser system. Also, a bER of 1.5×10^-5 was achieved at double the density of an a high definition digital versatile disc read-only memory (HD DVD-ROM), and the radial and tangential tilt margins were measured in a blue-laser system.

We present a simple self-assembly process for fabricating a nanohole array via a nanodot array on a glass substrate by dripping ethanol onto the nanodot array. It is found that well-aligned arrays of nanoholes as well as nanodots are formed on the whole surface of the glass. A dot is transformed into a hole, and the alignment of the nanodots strongly reflects that of the nanoholes. We find that the change in the depth of holes agrees well with the change in the surface energy with the ethanol concentration in the aqueous solution. We believe that the interfacial energy between the nanodots and the dripped ethanol causes the transformation from nanodots into nanoholes. The nanohole arrays are directly applicable to molds for nanopatterned media used in high-density near-field optical data storage. The bit data can be stored and read out using probes with small apertures.

We proposed a concept of a roll-type optical advanced memory (RoCAM). RoCAM is a multilayered optical memory, in which recording layers are wound onto a shaft. Multilayered media are fabricated easily by winding a two-layered film, which is composed of a photosensitive layer and a pressure-sensitive adhesive (PSA) layer. We showed certain advantages of RoCAM and also demonstrated a prototype of RoCAM and the recording and reading results for two recording layers.

Near-field recording using an actuated solid immersion lens is an attractive solution for a next-generation optical storage product. By applying advanced servo techniques, like feed forward control and active disc tilt detection and compensation, robust and reliable servo operation can be ensured, even at high disc rotation speeds. Furthermore, using media with a protective cover-layer has a number of important advantages. As demonstrated in this paper, crucial advantages are superior data protection and robustness, which are essential for realizing a reliable product. Using a lens with a numerical aperture (NA) of 1.45 and discs with a 3 µm polymer cover-layer, very good recording results are obtained at tangential densities up to 36.5% more than the nominal NA-scaled density. These results strongly indicate that capacities over 100 GB per layer are feasible in this system.

We advocate the use of a polymeric cover layer for protecting the data layer and the tip of the solid immersion lens in near-field optical recording system. With a cover on top of the data layer, the numerical aperture (NA) of the objective lens is limited to the refractive index of the cover material. This means that the maximum attainable NA of cover-incident near-field systems and therefore the maximum achievable storage density is lower compared to that for first surface systems. This lower storage capacity per layer can be more than compensated for by using multiple data layers which is not possible in first-surface systems with bare discs. In this paper we present first experimental results for near-field recording with a solid immersion lens that focuses through a cover layer and a spacer layer onto a data layer, as in a dual-layer near-field disc.

We propose a novel super-resolution near-field structure (super-RENS) read-only-memory (ROM) disc with a narrow track pitch using a group tracking technique. The group tracking technique enables the tracking control of an optical disc because it consists of track pitch sizes that are less than the diffraction limit. The group tracking technique to increases capacity in the radial direction of the super-RENS ROM disc. By using this technique, carrier-to-noise ratio (CNR) was obtained at 33 dB and the minimum crosstalk level was -23 dB at the 100 nm pit length. A readout track was selected by controlling the offset level of the tracking error signal in a high definition digital versatile disc (HD-DVD) system. It was confirmed that the capacity of the super-RENS ROM disc with the group tracking structure was improved in the radial direction.

Multilayer recording in volumetric two-photon absorbing fluorescent Al₂O₃:C,Mg media is reported. Dynamic random mark-length recording and readout in a single crystalline disk was performed at rotation speed up to 3600 rpm and linear velocity of 14 m/s. Clear eye diagrams were obtained at data clock from 0.32 to 6.5 MHz. Monotones with carrier-to-noise ratio (CNR) from 22 to 45 dB were demonstrated at different data rates.

A new rhodamine derivative with red light absorption was designed and synthesized. The absorption properties and thermal stability of the rhodamine derivative were discussed. We also studied the characteristics of a super-resolution recordable disk with a modified rhodamine recording layer and a poly(methyl methacrylate) (PMMA) contrast enhanced layer. A sample disk, which consists of a spin-coated rhodamine derivative recording layer and a PMMA contrast enhanced layer, showed excellent recording and readout characteristics with a carrier-to-noise ratio (CNR) of about 43 dB for a mark length of 200 nm (laser wavelength = 635 nm, numerical aperture = 0.60). These results indicate that the rhodamine derivative dye is a promising candidate for optical recording by a super-resolution near-field structure (super-RENS) technique.

We have measured the temperature dependence of the thermal conductivities of Ge₂Sb₂Te₅ (GST) and ZnS–SiO₂ using a nano second thermoreflectance measurement system from room temperature to 500–600 °C. The specific heat capacities of these materials also have been determined from -130 to 500 °C for GST and from room temperature to 600 °C for ZnS–SiO₂. The Debye temperature was obtained from specific heat capacity measurement. Using the obtained temperature dependence of the thermal conductivities, a temperature simulation inside a simple structured optical disk with and without considering its temperature dependence was carried out, and the difference in maximum temperature was approximate 80 °C.

A dielectric material (SiO₂) was added to the PtO_x recording layer of a recently developed super-resolution near-field structure (super-RENS) disc to enhance the readout durability. The PtO_x–SiO₂ (Pt_8.4Si_21.1O_70.5) composite layer was prepared by a chip-on-target sputtering method. The recorded mark became significantly solid-filled when the prepared layer was used to replace the PtO_x layer of the original disc. The carrier-to-noise ratio obtained for the 100 nm marks (laser wavelength: 405 nm, numerical aperture: 0.65) was 44 dB. It was possible to readout 7.5×10⁴ times until the initial carrier-to-noise ratio (CNR) decreases by 3 dB; this is a more than one-order-of-magnitude enhancement compared with at in the simple PtO_x case. Readout laser power was not reduced. This enhancement is achieved as a result of improving the recording layer of the super-RENS disc.

The damascene process using chemical mechanical polishing was introduced to embed a super-resolution material in the pits of a read-only memory (ROM) substrate to fabricate discs using a next-generation large-capacity optical-disc technique called three-dimensional pit selection. GeSbTe was used as the super-resolution material. Experimental results using scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that GeSbTe remained only in the pits. The readout signal from a disc tester demonstrated that GeSbTe in the damascene disc changed its phases between amorphous and crystal. The polishing rate could be controlled by adjusting the conditions for the concentration of colloidal silica and the pH of the slurry. The test equipment we fabricated for mass producing the discs demonstrated that the degree of polishing could automatically be determined by detecting the distortion in the polishing arm caused by friction between the polishing head and the sample surface.

We have developed a new inorganic write-once optical disk for an optical recording system with a numerical aperture of 0.85 and a 405 nm blue-violet laser diode. The recording layer is composed of multi-layered structure of alternating Mg-based alloy layer and Si layer. Our experimental results showed that the recording sensitivity of the optical disk is strongly dependent on the stack structure of the recording layer, which can be optimized through adjusting the number of the component layers. Dynamic experiment results showed that the write-once optical disk has a low-to-high polarity and jitter value below 7.5% can be achieved at both 1× and 2× speeds. Effects of the multi-layered recording layer structure and the low-to-high polarity on recording performance are discussed.

A new labeling technology, i.e., "Labelflash^TM", is reported. This technology uses the digital versatile disc (DVD) drive data recording head to burn high-quality images directly into a specialized dye layer on the label side of DVD discs. The basic structure of the disc is similar to a conventional double-sided recordable digital versatile disc (DVD-R). Consequently, a Labelflash disc shows various features as follows: non consumable, no special optical path or drive control system, the same working distance as that for data recording, manufactured using conventional equipment for double-sided DVD-R, fast drawing speed, high durability, and professional appearance. Moreover, four color variation types and mat-type discs were developped. The graphic quality performance index for Labelflash was newly proposed. The values were closely matched by the subjective evaluations of contrast.

A novel method of optical data storage was proposed using the rotation of polarization. The optical data was recorded as a chiral structure formed in a film made of an azobenzene copolymer using elliptically polarized light irradiation. The elliptically polarized light of a reading light was digitized into two states, "0" and "1". The initial state is defined as "0", while state "1" is observed by an optical rotation of the reading light, which is achieved by the photoinduced chiral structure formation. The recording characteristics were investigated by varying the intensity of the recording light and the recording time. Since the chiral structure was erased by circularly polarized light, state "1" could be reversed to state "0". The possibility of using our proposed method for achieving next-generation rewritable, multilevel, and parallel optical data storage was discussed.

We report the error rate improvement of super-resolution near-field structure (super-RENS) write-once read-many (WORM) and read-only-memory (ROM) discs in a blue laser optical system [laser wavelength (λ), 405 nm; numerical aperture (NA), 0.85]. We prepared samples of higher carrier level WORM discs and wider pit width ROM discs. Using controlled equalization (EQ) characteristics and an adaptive write strategy and an advanced adaptive partial response maximum likelihood (PRML) technique, we obtained a bit error rate (bER) of 10^-4 level. This result shows the high feasibility of super-RENS technology for practical use.

We have developed a new inorganic write-once disc using low-cost and environmentally friendly recording materials of Si oxynitride and Al oxynitride with high transmittance. Data-to-clock jitters of 5.6% for Layer 1, 5.5% for Layer 2, and 6.3% for Layer 3 of the individual single-layer discs were obtained using a limit equalizer. This newly developed recording material is immensely promising for a multilayer disc system.

This study is intended to establish a standard technique for measuring high-density optical disk life expectancy. The life expectancy of archival performance for high-speed recordable optical disks [digital versatile disc-recordable (DVD-R) disk] was examined using two acceleration test models, namely, the Arrhenius and Eyring models, and an analysis of statistical techniques based on the ISO method. Results demonstrate that the statistical distribution of failure time using both acceleration test models follows a lognormal distribution. Thus, we can estimate a standard life expectancy of 95% survival probability with a 95% confidence level for each acceleration test model.

We report the resin material dependence of the shape of pits obtained by thermal direct mastering (TDM), which is a heat-mode mastering method utilizing the thermal decomposition of resins. By applying suitable resins, both circular and crescent pits can be obtained by TDM. This difference in shape was considered to originate from the temperature difference between the glass transition temperature and the thermal decomposition temperature of the resins. The resin with a relatively small temperature difference was suitable for the fabrication of the circular pits and random patterns, and the resin with a large temperature difference was suitable for the fabrication of the crescent pits and monotone patterns with high linear density. By using a deep ultraviolet (DUV) laser mastering system with a wavelength of 257 nm and a numerical aperture of 0.90, a monotone pattern of 40-nm-length pits was fabricated by applying a novolak-type resin with a relatively large temperature difference.

Doping Ag into the fast-growth Sb₇₀Te₃₀ recording film can increase the crystallization temperature and activation energy for crystallization so that the archival stability can be improved, however, the initialization will become more difficult. The activation energy for melting of the Ag-doped Sb₇₀Te₃₀ recording films will decrease with Ag concentration so that the recording sensitivity can also be improved. New compounds such as Ag₅Te₃ and AgSbTe₂ will form due to the addition of Ag. When the concentration of Ag was increased, the crystallization mechanisms of the Ag-doped Sb₇₀Te₃₀ recording films will gradually transfer from polyhedral growth to linear growth. Under pulsed laser irradiation, the melt-quenched amorphous zone for the annealed Ag-doped Sb₇₀Te₃₀ recording films will enlarge as the Ag content was increased, representing that the recording sensitivity will increase. However, once the concentration of Ag was increased to 10.8 at. %, the as-deposited Ag-doped Sb₇₀Te₃₀ recording film will become difficult to be initialized.

In this study, we demonstrate the gold nanorods can be successfully incorporated into common optical recording media such as phthalo-cyanine, azo-dye, and phase change media, and operate as a nano-heat sensitizers. Since the recording mechanism for optical recording media is optothermal energy conversion, high absorption cross section of gold nanorods tuned at recording laser wavelengths is shown to be extremely useful in nano-heat generation, which facilitate the efficient recording on those media.

We have developed a completely novel inorganic write-once disc using a hydrogen-containing dielectric recording material. We demonstrate the recording characteristics of a newly developed write-once disc on the basis of the Blu-ray disc format (dual-layered, 4× speed).

A tungsten-oxide single-layer film can be colored densely by using dilute sulfuric acid as a liquid electrolyte instead of the multilayer electrochromic cell. The transmissivity contrast before and after coloration reaches 70% at a wavelength of 405 nm. The bleaching relaxation time of a colored tungsten-oxide single-layer film is in the order of 1000 hours and depends on the sputtering power.

We constructed an HD DVD mastering process simulator on the basis of the cell removal model. In the exposure process, we simulated the exposure profile in the photoresist film. In the development process, we defined the density and development rate of the unit cell. We carried out iterative calculation for each unit cell dissolution. The development rate was approximated as the function of the exposure intensity profile. From the results, we were able to simulate the three-dimension (3D) pit profiles of HD DVD-ROM (read only memory). We clarified that our development rate equation is similar to Hirai et al. and Trefonas and Daniels' type equation in semiconductor lithography.

The influence of interface layers on the chemical and electronic states of a phase-change recording material, GeBiTe (GBT) alloy, used in high-speed rewritable HD DVD media was investigated for the first time by hard-X-ray photoelectron spectroscopy (HX-PES). The binding state of elements for the amorphous state of the phase-change recording film with interface layers is closer to that of the crystalline state than the amorphous film without interface layers. The density of states (DOS) for the valence band of the amorphous state without an interface layer was smaller than that of the crystalline state. The band-edge energy of the amorphous state without an interface layer was lower than that of the crystalline state by about 0.5 eV. On the other hand, the DOS and the band-edge energy of the amorphous state of GBT with interface layers were almost the same as those of the crystalline state. This result may lead to almost the same carrier density for electrical conduction for the crystalline state as the amorphous state, which is totally unexpected, thus very interesting, because the atomic arrangements should differ from each other. We speculate that these effects are a factor allowing high-speed crystallization.

The dual-layer recordable digital versatile disc (DVD-R DL) fabricated by an inverse stack method has not been released in the market because of unstable land pre pit (LPP) addressing in an L1 layer due to its peculiar disc structure. We have developed a DVD-R DL disc that has the novel shape of the protruded LPP in the L1 layer fabricated by an inverse stack method. The height of the protruded LPP is larger than the land height of the L1 substrate to restrain extensions in the radial direction of recording marks. We have confirmed that the protruded LPP disc satisfies the specifications of DVD-R DL and enables stable LPP addressing.

We simulated a temperature change in thin phase-change optical discs that have various substrate thicknesses in order to evaluate the thermal tolerance of a polycarbonate substrate during DC sheet beam initialization. We simulated the temperature change by applying a finite difference method to the partial differential equation for heat conduction. In discs with substrates on both sides, the difference between the sums of the given heat while the temperature of each substrate is more than or equal to glass transition temperature for a disc with a 100-µm-thick substrate on both sides and a disc with a 5-µm-thick substrate on one or both sides, was within 5%. The sum of the given heat to the substrate for a disc with a substrate on one side, was about 1.5 times larger than that for a disc with substrates on both sides. These results showed that the initialization margin needed to obtain a good-quality initialization state with no thermal damage to the substrate is smaller for a disc with a substrate on one side than for that with substrates on both sides.

We describe the method of controlling the refractive index of pressure sensitive adhesives (PSAs) from 1.461 to 1.531 by changing the ratio of component monomers. The difference in refractive index between recording and PSA layers determines the penetration depth for recording and reading light due to multiple reflections at layer interfaces. We fabricated multilayered media that contained one, five and ten recording layers and measured their transmittances. The readout signal intensities of ten-layered media, which were fabricated using PSA films of various refractive indices, were measured. We found that a smaller refractive-index difference between recording and transparent layers led to a larger transmittance of the medium and that a refractive index difference smaller than 0.01 provided a signal sufficient for distinguishing each layer. We also calculated reflection intensity distribution for the optimization of the structure of multilayered media.

Near-field technology has been expected as one of the promising techniques for increasing the recording density in an optical storage disk system since it was introduced. In a near-field optical disk drive system (NFDD), the gap between a disk and a solid immersion lens (SIL) is required to be less than one-tenth laser wavelength with a high accuracy in order to generate an evanescent wave for reading or writing a signal. This is because the gap performance has detrimental effects on the RF amplitude vibration in a reading channel and on the signal laser peak power vibration in a writing channel. In this study, we first clarify whether specific disk resonances cause the deterioration of the gap performance in the NFDD. We then demonstrate an approach of improving the disk mechanical performance in order to achieve a highly precise gap performance. Finally, we investigate a high-performance gap servo system by utilizing the disk substrate suitable for the NFDD.

A new phase-transition mastering (PTM) process was developed for Blu-ray Disc read-only memory (BD-ROM) mastering. Results obtained with both a 266 and 405 nm laser beam recorder (LBR) are discussed in this paper. The feasibility of BD-ROM mastering was successfully demonstrated on both LBRs. With the insight that 25 Gbytes BD-ROM can be mastered with a 405 nm wavelength LBR, the availability of the 266 nm wavelength LBR opened the route to explore PTM of near-field data densities. First experiments indicate that the PTM process is also suitable for mastering data densities beyond 25 Gbytes data density.

TiO₂ nanoparticles are added into UV-curable resin to increase the refractive index of the cover-layer laminated for cover-layer incident near-field recording media. A high refractive index is required for the cover-layer operating with an optical head with a high numerical aperture. The eye pattern from a cover-layer coated 20 GB read-only memory disc in which the refractive index of the cover-layer is 1.75 is achieved, but the gap servo is unstable owing to the rough surface of the cover-layer. Even though the light loss due to the nanoparticles is negligible, a rough microstructure is developed by adding the nanoparticles into an organic binder material. To achieve a smooth surface for a stable gap servo, the solubility of the nanoparticles should be enhanced by the optimization of the surface of the nanoparticles.

In this research, we confirm that a near-field recording (NFR) system, particularly a solid immersion lens-based NFR system, is so weak to the slightest external shock that we should solve this problem to realize NFR technology for a real product. For this, we first define the disturbances caused by the external shock as the displacement of a disc and an actuator. We modify a general NFR servo system to deal with these disturbances by applying a safety mode to a switch mode, which is one of the servo systems in general NFR. To protect a disc from a collision between the actuator and the disc, we add a protector to the cover of the actuator and show that the NFR system with the applied safety mode can avoid the collision between a SIL and the disc for a severe external shock, such as the amount of 240 G based on the results of simulations.

A novel biosensor utilizing an interference of light reflected at the interfaces of a multilayer structure is proposed. This biosensor detects analytes by monitoring the changes in reflection intensity due to their adsorption to the sensor surface, on which functional biomolecules are immobilized to specifically bind to the analytes. The proposed biosensing instrument is based on a commercial digital versatile disc (DVD) system, which allows the instrument to be small and inexpensive. For the preliminary examination, SiO₂ thin films with a well-defined thickness were deposited on the sensor surface. The reflection intensity varied almost linearly depending on the thickness of the SiO₂ films in a thickness range of 2–10 nm. Subsequently, it was demonstrated that biotin–streptavidin binding events were clearly detectable on a rotating disc substrate at a constant linear velocity of 4.0 m/s. We named this interference-based biosensor BioDVD, which is expected to be useful for high-throughput multi-analyte bioassays.

In this paper recent high speed recording results are presented for Blu-ray Disc CuSi recording. An optical pulse timing scheme is discussed based on power controlled transitions (PCT), using the physics and dynamics of the semiconductor lasers. The applied PCT scheme allows for a timing resolution in the 10 ps range and does not require special ultra-high speed electronics, making it a very suitable candidate for future drive implementation. In this way high speed recording experiments became feasible with recording speeds up to 12×. The recording material CuSi has proven to be a good candidate for high speed Blu-ray Disc recording, without the need for complex write strategies.