Near-field Optical Storage Research Articles

Modeling, design and analysis of a micro flying slider in near-field optical storage system

A micro flying slider is developed to satisfy the demands for large capacity, high data density, and fast transfer rate of mass storage systems. The air-bearing surface of the micro flying slider is designed based on micro fluid theory of hydrodynamics to obtain positive and negative pressures, thereby enhancing flying height stability. The lubrication model is investigated based on the generalized Reynolds equation. The finite volume method is utilized to solve the revised Reynolds equation that governs the air-bearing pressure and flying system behavior. The dynamic model for the slider–disk interface of the micro flying slider is developed, and the air-bearing system is modeled as a 3-degrees-of-freedom, lumped-parameter model to simulate flying characteristics. The air-bearing pressures with flying attitudes in the loading/unloading processes are characterized and calculated using the proposed model, with flying height and pitch angle as the key variables. The sample data obtained via accurate triggering and superposed averaging are processed to acquire modal frequencies and damping ratios based on the parameter identification method of modal analysis technique. System experiments are presented to illustrate the effectiveness of the proposed micro flying slider and its dynamic model for flying-height control. Theoretical analysis and experimental results show that a stable storage working process can be obtained at 1600 rpm and a pitch angle of 1.5°. Moreover, the micro flying slider exhibits satisfactory flying stability, with a near-field working distance of less than 75 nm. Good correlation between the experimental and simulation results is achieved.

Optimizing the optical field distribution of near-field SIL optical storage system using five-zone binary phase filters

Five-zone binary phase filters (FBPFs) are proposed for decreasing the spot size and/or increasing the focal depth of the near-field optical storage system with a hemisphere solid immersion lens (SIL). The design of filters is based on the vector diffraction theory and the MATLAB optimizing toolbox. Three FBPFs with rotationally symmetrical pupil function have been designed, where the one FBPF is for increasing the focal depth as big as possible, the second FBPF is for improving the resolution as high as possible, and the third FBPF integrate the increase of focal depth with the improvement of resolution. Numerical results show that compared with the three-zone amplitude filter, the designed five-zone binary phase-only filters have more prominent performances in improving the focal depth and the resolution of the near-field SIL optical storage system.

Theoretical study of optical recording with a solid immersion lens illuminated by focused double-ring-shaped radially-polarized beam

Based on the Richards–Wolf vector diffraction theory, the intensity distributions in the recording sample near a solid immersion lens are calculated for two different radially-polarized beams ( R- TEM 11 ∗ and R- TEM 01 ∗ modes). Numerical results show that a double-ring-shaped R- TEM 11 ∗ mode focusing has some excellent features in near-field optical storage, compared with a single-ring-shaped R- TEM 01 ∗ mode focusing. The recording density is markedly improved, the focal depth of the near-field recording system is substantially increased, and a subsurface recording is effectively obtained using the R- TEM 11 ∗ mode focusing.

Feasibility study of the application of radially polarized illumination to solid immersion lens-based near-field optics

We analyzed the behavior of the electric field in a focal plane consisting of a solid immersion lens (SIL), an air gap, and a measurement sample for radially polarized illumination in SIL-based near-field optics with an annular aperture. The analysis was based on the Debye diffraction integral and multiple beam interference. For SIL-based near-field optics whose NA is higher than unity, radially polarized light generates a smaller beam spot on the bottom surface of a SIL than circularly polarized light; however, the beam spot on the measurement sample is broadened with a more dominant transverse electric field. By introducing an annular aperture technique, it is possible to decrease the effects of the transverse electric field, and therefore the size of the beam spot on the measurement sample can be small. This analysis could have various applications in near-field optical storage, near-field microscopy, lithography at ultrahigh resolution, and other applications that use SILs for high resolution.

Improving the recording ability of a near-field optical storage system by higher-order radially polarized beams

Distributions of the optical field in a solid immersion lens recording system are calculated for higher-order radially polarized modes of the incidence. Results show that two higher-order radially polarized modes of R-TEM(11) (* )and R-TEM(21) * are useful to near-field optical recording, but further higher-order modes such as R-TEM(31) (* ), R-TEM(41) (* ), and R-TEM(51) (* ) are not useful due to the strong side-lobe intensity. Compared with R-TEM(01) (* ) beam focusing, the full width at half-maximum of the recording spot is decreased markedly and the focal depth is increased substantially by using R-TEM(11) (* )beam focusing. The effect of the beam width of the R-TEM(11) (* ) mode is also discussed.

Progress of super-resolution near-field structure in near-field optical storage technology

Super-resolution near-field structure (Super-RENS) is one of the most promising near-field optical recording schemes with significant application prospects. The development of Super-RENS from the basic type to the third-generation is introduced. The development of mask material and the application of Super-RENS in different recording systems are summarized.

Optical data storage system with a planoellipsoidal solid immersion mirror illuminated directly by a point light source

A new solid immersion mirror called the planoellipsoidal (PE) solid immersion mirror (SIM) for the near-field optical storage is proposed and developed. The PE SIM has a small aperture on the apex of the ellipsoidal surface. The intensity distribution of the transmitted field is calculated by using the vector diffraction theory. Compared with a conventional solid immersion lens (SIL), the proposed PE SIM has the following features. A PE SIM replaces three optical elements of the collimator, objective, and SIL in a conventional SIL optical storage system, so that the optical system equipped with the PE SIM is not only simple in its assembly but is also effective in making an optical head unit. The PE SIM obtains light from a point light source and focuses it directly on the recording layer, which may be useful for a compact optical data storage system. The convex ellipsoidal surface of the PE SIM can reduce the risk of the SIM touching the surface of the recording medium. In addition, the spreading of the spot size with the increase of distance is very small in the PE SIM.

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.

Rigorous transmittance analysis of a sub-wavelength Sb thin film lens

We quantitatively analysed the factors contributing to the optical transmission enhancement of a sub-wavelength Sb thin film lens, using the finite-difference time-domain (FDTD) method. The results show that the transmission enhancement of the dielectric with a Gaussian distributed refractive index loaded in a sub-wavelength circular hole is not only due to the high refractive index dielectric, but also due to the specific distributions of refractive index. It is the first study about the effects of the refractive index distribution on the transmission of a sub-wavelength aperture. This kind of lens has practical applications in the very small aperture lasers and for near-field optical storage and lithography.

Ultrahigh-density storage media prepared by artificially assisted self-assembling methods

Two types of recording media possessing nanodot structures were investigated. The media were prepared by an artificially assisted self-assembling (AASA) method, which includes simple nanopatterning using a nanoimprint and fine nanopatterning using self-assembling organic molecules. One type of recording media is circumferential magnetic patterned media prepared on a 2.5-in.-diam glass plate. A Ni master disk possessing spiral patterns with 60-250 nm width lands and a 400 nm width groove was pressed to a resist film on a CoCrPt film to transfer the spiral patterns. A diblock copolymer solution was cast into the obtained grooves, and then annealed to prepare self-assembling dot structures aligned along the grooves. According to the dot patterns, the lower magnetic films were patterned by ion milling to yield patterned media with 40 nm diameter. We have also prepared FePt dot media with high magnetic anisotropy for near-field and magnetic-field hybrid recording aiming at more than 1 Tbin.2 density. A Ni stamp disk with aligned dot structures was also prepared by the AASA method to produce patterned media at the lowest cost. The other type of media was organic patterned media for X-Y type near-field optical storage. Bulky dye molecules were evaporated in vacuum to produce self-assembling amorphous nanodots. The dots were arranged by the AASA method, i.e., according to the polymethylmethacrylate film hole arrays or chemically patterned surface.

Complex Padé approximants for bidirectional and nonparaxial beam propagation method

The nonparaxial and bidirectional beam propagation method suitable for modelling near-field and high numerical aperture (NA) optical storage systems is suggested for 2D geometry and TE polarization of incident light beam. The complex Pade approximants are introduced for correct approximation of evanescent field in the near-field optics. Pole–zero shifting and branch-cut rotation methods of building complex Pade approximants are studied and compared.

Near-field optical storage system using a solid immersion lens with a left-handed material slab

A new near-field optical storage system utilizing a left-handed material (LHM) is introduced by attaching an LHM slab to the lower surface of a conventional solid immersion lens (SIL). The performance of the present storage system is compared with a conventional SIL system through numerical simulation. The LHM slab in the present storage system can image very well the focused spot at the lower surface of the SIL to the surface of a disc. It allows a large air-gap for the mechanical convenience while keeping a large signal contrast and a high storage density. The tolerance of the air-gap is also improved.

High-density optical data storage with one-photon and two-photon near-field fluorescence microscopy

A spatially localized photochemical reaction induced by near-field femtosecond laser pulses is demonstrated on a nanometer scale and used for high-density optical data storage. Recorded domains down to 120 and 70 nm are obtained with one-photon and two-photon excitation, respectively. It is shown that the local-field confinement that is due to the quadratic dependence of two-photon excitation on light intensity has the potential to increase the near-field optical storage density.

Nanometric aperture arrays fabricated by wet and dry etching of silicon for near-field optical storage application

We fabricated nanometric aperture arrays in order to apply to an optical probe in high-density near-field optical storage for increase of data-transmission rate. The aperture arrays were fabricated by forming concave pyramidal grooves on a silicon-on-insulator wafer with electron beam lithography and wet anisotropic etching. Modification of the apex shape of the grooves by re-oxidation and subsequent reactive ion dry etching was able to increase the uniformity of the aperture size remarkably. The light transmission efficiency of the fabricated apertures was measured to be ∼10−3 when the aperture size was 100 nm, which was higher than that of a conventional optical fiber probe by several orders of magnitude.

Silicon planar-apertured probe array for high-density near-field optical storage

We propose a novel, to our knowledge, silicon planar-apertured probe array as an optical head for high-density near-field optical storage. In comparison with a conventional fiber probe employed for near-field optical storage the apertured probe array has a higher readout data-transmission rate and better mechanical durability. A probe array with an aperture size of 100 nm was fabricated by use of photolithography and wet etching of a silicon wafer. Subwavelength-readout capability was demonstrated by use of one aperture of the probe array. Furthermore, we achieved a 16 times increase in the light-transmission efficiency of the probe array by installing glass-sphere microlenses on each aperture. The increase was confirmed by measurement of the near-field optical intensity.

High-density data storage using proximal probe techniques

We describe some of the achievements and problems associated with proximal probe-based approaches to high-density data storage. While STM-based methods have demonstrated spectacular areal densities dwarfing anything achievable with today's storage technologies, reliability and data rate issues present serious obstacles. These problems have led us to focus on techniques based on AFM and near-field optics. First, we have developed a thermomechanical writing scheme using an AFM tip. We have addressed many of the practical issues involved, including data rate. With custom low-mass cantilevers, we have demonstrated readback on real data with a data rate of 1.2 Mb/s. We have also pursued nontopographic storage techniques based on charge storage in nitride-oxide semiconductor structures and near-field optical storage. These techniques should be able to achieve densities comparable to those reached with the AFM scheme, with the added advantage that they are fast and reversible. Although it is not yet clear whether any of these probe-based approaches can ever be made practical, they do represent potential pathways to the higher densities that will be needed in the decades ahead.

New applications of near-field optical storage

Some time ago, we described a new type of near-field imaging system using the Solid Immersion Lens. A confocal microscope using this principle has demonstrated sub-100 nm resolution with visible light. In this paper, we will describe how this device is being applied to increasing the density of optical storage in CD ROMs and magneto-optical storage systems.The SIL is placed between the objective and the disk. As illustrated in Fig. 1(a), if the SIL is in the form of a hemisphere, rays enter it along its radii and converge at its center. If the refractive index of the SIL is n, the wavelength in this material is reduced by 1/n and consequently increases the effective NA of the objective lens by the refractive index n and decreases the spot size by 1/n. With this configuration, we were able to obtain good quality images of a grating with 100 nm gaps and strip widths.