Near-field Optical Data Storage Research Articles

A Hyperlens-embedded solid immersion lens for beam focusing beyond the diffraction limit

A novel beam focusing device called hyperlens-embedded solid immersion lens (HESIL) is proposed in this letter. In the device, a hyperlens which supports propagating waves with very large wavenumbers is embedded into a super-hemispherical solid immersion lens and a super-resolution focusing spot can be formed at the bottom planar plane of the HESIL. The working principles and characteristics of the device are discussed in detail. Numerical simulations verify that a nano-scale spot with a diameter of 57.5 nm (1/6.5 of the vacuum wavelength <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\lambda_{0}$</tex> </formula> ) is achieved (the numerical aperture of the objective is 0.45) and the system transmission exceeds 34%, which validates the effectiveness of our proposed device for beam focusing. The device is expected to find numerous applications in high-density near-field optical data storage, nano-photolithography, and so on.

Fabrication of Pure Silica Core Multimode Ultraviolet OpticalFibre Probes by Tube Etching

As a light wave-guide component for transmitting ultraviolet (UV) laserpulses, pure silica core UV fibre probes have attracted a great deal of attentionin the near-field optical data storage and bio-medical studies. Wefabricate UV fibre probes with tips in dimension of about 2–5 μm andtaper angle 16° by the tube etching method, using 40% HF acid asetching solution and xylene as overlayer. Probes produced have curvyconfiguration with smooth surface. The yield of fine probes is ratherhigh and etching operation greatly simplified. With higher damagethreshold, pure silica core multimode UV fibre probes can be coupledinto more laser power. In addition, using UV light reduces the cutoffwavelength of the fibre probes, which is in favour of increasing thetransmission efficiency of the probe. Furthermore, the larger tipdimension helps to enhance the light throughput either. The advancesof fabrication technique of UV optical fibre probe may further supportthe studies of UV light data storage, pulsed laser biosurgery and UVphotolithography.

Fabrication of Nanohole Array via Nanodot Array Using Simple Self-Assembly Process of Diblock Copolymer

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.

High-intensity C-shaped nanoaperture vertical-cavity surface-emitting laser with controlled polarization

The authors designed and demonstrated a C-shaped nanoaperture (C aperture) vertical-cavity surface-emitting laser (VCSEL) with a maximum far-field power of 157μW coming from a 70nm C aperture. Simulation shows that the near-field full width at half maximum spot size at 30nm away from the C aperture is 94×108nm2 in X and Y directions. The authors estimate the peak near-field intensity from the C-aperture VCSEL to be as high as 19mW∕μm2. This high intensity and small spot size are promising for realizing near-field applications such as near-field imaging and ultradense near-field optical data storage.

Cover-layer effects on contamination in near-field optical data storage

Contamination at the head-disk interface (HDI) was observed for first-surface near-field recording (NFR) under accumulation over a long writing time with a high writing power. The contamination was caused by laboratory dust coupled with the heat from the focus of the writing laser at the HDI. One suggested solution to the problem is that a cover layer should be laminated for cover-layer-incident NFR. A uniform and edge-bead-free UV-curable cover layer was laminated by using an outer-ring technique. Diamondlike carbon films were optimized and coated on the cover layer, and then a lubricant film was applied to reduce the tribological problems at the HDI.

High transmission through ridge nano-apertures on Vertical-Cavity Surface-Emitting Lasers

We report high-intensity nano-aperture Vertical-Cavity Surface- Emitting Lasers (VCSELs) with sub-100nm near-field spots using ridge apertures. Power transmission efficiency through different ridge apertures, including bowtie, C, H and I-shaped apertures on VCSELs were studied. Significantly higher transmission efficiencies were obtained from the ridge apertures than those from conventional square apertures. Mechanisms for high transmission through the ridge apertures are explained through simulation and waveguide theory. A new quadruple-ridge aperture is proposed and designed via simulation. With the high-intensity and small spot size, VCSELs using these ridge nano-apertures are very promising means to realize applications such as ultrahigh-density near-field optical data storage and ultrahigh-resolution near-field imaging etc.

Design of high-performance supersphere solid immersion lenses

Two types of novel solid immersion lens are designed and investigated theoretically using the vector diffraction theory. The advantages of these so-called high-performance supersphere solid immersion lenses (HPSILs) are that they can improve the Strehl ratio of the focused spot and increase the focal depth of near-field optical systems. Both the spot size and the sidelobe intensity are not increased, however, compared with those of the standard Weierstrass solid immersion lens. These HPSILs will be useful for near-field optical data storage and photolithography.

Field enhancement by surface plasmon polariton in self-assembling nanopatterned media

We present the fabrication of nanodot arrays on metal films by self-assembly of diblock copolymer. The nanodots are uniform and aligned regularly with hexagonal close-packed structure on both gold film and glass. Gold is specifically a good candidate as the metal film excites surface plasmon polariton in the near-field optical media. Surface plasmon polariton is excited with the Kretschmann configuration at the gold-nanodot interface. We found that the intensity at the interface was enhanced 53.8 times to that of the incident light. It is expected to improve the carrier-to-noise ratio of the readout signal in the case of nanopatterned near-field optical data storage under collection mode.

Effects of Solvent and Ambient Humidity on Nanodot Structures for Near-Field Optical Data Storage Using Self-Assembled Diblock Copolymer

We present the formation of nanodot structures on a glass substrate by the self-assembly of a diblock copolymer. The structures can be used as nanopatterned media for near-field optical data storage with high data density. Improvements in the carrier-to-noise ratio (CNR) and in the uniformity of the recorded bit marks are expected with these structures. It is found that the structure of nanodots depends strongly on the ambient humidity during dripping and subsequent spin-casting and also on the solvent used. Nanodots of uniform size can be formed on a glass substrate with benzene, toluene, or toluene blended with acetone as a solvent when the ambient humidity is in the range of 60% to 70%. The size of the individual nanodots as well as the distance between two consecutive nanodots can be controlled, and the nanodots may be aligned like regular hexagonal close-packed structures by adjusting the volume ratio of acetone to toluene.

Enhanced resolution induced by random silver nanoparticles in near-field optical disks

The super-resolution near-field structure (super-RENS) is a high-density near-field optical data storage medium which can achieve superior spatial resolution beyond the diffraction limit. Our previous studies found that enhanced local optical intensity occurred at the near fields of super-RENS disks, and the nonlinear near-field optical enhancement is related to the localized surface plasmons of silver clusters dissociated from the AgOx layer in the super-RENS disks. In this paper, we studied the near-field and far-field properties of AgOx-type super-RENS with different distributions of silver nanoparticles using finite-difference time-domain (FDTD) simulations. Highly localized enhancements are found between adjacent silver nanoparticles in the near fields. The far-field signals of different silver nanoparticles distributions confirm the super-resolution capability of AgOx-type Super-RENS disks, and a simplified Fourier optics model is used to describe the relation between highly localized near-field distributions and enhanced resolution of far-field signals.

Near-Field Evanescent Wave Enhancement with Nanometer-Sized Metal Grating and Microlens Array in Parallel Optical Recording Head

To increase the data capacity and data transfer rate in optical data storage, a parallel near-field optical array head has been designed and prepared using the finite-difference time-domain (FDTD) simulation and nanofabrication process. Since the vertical cavity surface emitting laser (VCSEL) microprobe array head reported in our previous paper does not satisfy the required recording power for conventional phase change optical media, we have studied a new array head structure designed to enhance the optical throughput using the surface plasmon resonance effect between the incident light and the metal grating. The theoretical analysis and fabrication of a new integrated optical head were discussed with emphasis on the FDTD simulation for the metal grating structure. An approximately 500 times increase in the resonant power enhancement of evanescent waves was observed when a grating periodicity of 118 nm was maintained with the half-wavelength of the incident light inside the GaP substrate. Grating width and depth also affect the evanescent light enhancement between the incident light and metal grating; such result can be used as a guideline for future applications of near-field optical data storage.

Diffractive super-resolution elements applied to near-field optical data storage with solid immersion lens

The intensity distribution in near-field optical data storage with a solid immersion lens (SIL) and a binary phase-only diffractive super-resolution element (DSE) is expressed in a single definite integral by using angular spectrum theory. The super-resolution of binary two-zone phase DSEs for SIL systems is numerically studied for low and high numerical aperture (NA) systems. The results for the low-NA systems show that optimizing the zone boundary and phase of binary two-zone phase DSEs can decrease the spot size. The Strehl ratio, sidelobe intensity and axial characteristic length are also discussed. In addition, a binary two-zone phase filter can change the position of focus that shifts from the SIL–air interface to air, but the spot size increases. For the high-NA systems, the y- and z-polarized components of the transmitted field increase as the boundary and depth of phase of the DSE increase. When the phase boundary is smaller and the depth of phase depth is close to π, super-resolving effect of DSE is more obvious but the intensity of sidelobes is larger for the high-NA system. In this way, it may be possible to improve both the resolution and focal depth of the SIL with high-NA systems.

Study on the Mechanism of Super-Resolution Near-Field Structure High-Density Optical Data Storage

The super-resolution near-field structure (Super-RENS), glass/SiN/Sb/SiN, which is promising for near-field ultrahigh-density optical data storage (ODS), has been proposed and investigated in recent years. The recording mark size is a few tens of nanometers, far beyond the diffraction limit of traditional optics. The mechanism of this Super-RENS, including the working principle of the nonlinear optical layers and the size reduction of the recording marks, has not been clearly understood. In this study, the action of sandwich multilayer films in Super-RENS is mainly investigated. The behavior of the optical wave at the boundary between two dielectric media is discussed in detail, and the resonance characteristic of the surface plasmon wave (SPW) is analyzed. The mechanism of ultrahigh-density ODS using Super-RENS is explained quantitatively. It is demonstrated that the wave effect is larger than the thermal effect in Super-RENS.

Fabrication of High-Accuracy Microtranslation Table for Near-Field Optical Data Storage Actuated by Inverted Scratch-Drive Actuators

We proposed a microtranslation table to convey a recording medium to a near-field optical head. The microtranslation table was fabricated on a silicon substrate with surface micromachining technology and consisted of inverted scratch-drive actuators (SDAs) actuated by electrostatic force. An object was translated by mounting it on the inverted SDA array. We observed that a micro object was translated in the direction of the fabricated SDAs by applying AC voltage of ±600 V at 100 Hz. The dimension of the SDA plate was 70 µm long, 70 µm wide, and 1.3 µm thick and it has 1.5-µm-high bushing. We proposed a novel mechanism for releasing the SDAs from the object by retracting them. By applying the voltage of 65 V between the SDA plate and the lower electrode, the plate was retracted 160 nm down. Therefore, the X-Y translation is possible by placing the inverted SDAs in four different directions.

High Spatial Resolution and Throughput Potential of an Optical Head with a Triangular Aperture for Near-Field Optical Data Storage

A near-field optical head with a triangular aperture has been precisely analyzed on the basis of a three-dimensional finite-difference time-domain method in order to obtain both higher spatial resolution and signal output performance for a near-field optical data storage system. The numerical analysis revealed that in contrast to a conventional aperture, this triangular-shaped aperture can successfully enhance energy distribution intensity at only the side perpendicular to the incident polarization direction, generating an extremely small optical spot not limited by its entire aperture size. The read-out performance was also simulated for the aperture passing over a space-patterned metal medium, resulting in both superior spatial resolution and signal output.

A virtual optical probe based on evanescent wave interference

A virtual probe is a novel immaterial tip based on the near-field evanescent wave interference and small aperture diffraction, which can be used in near-field high-density optical data storage, nano-lithography, near-field optical imaging and spectral detection, near-field optical manipulation of nano-scale specimen, etc. In this paper, the formation mechanism of the virtual probe is analysed, the evanescent wave interference discussed theoretically and the sidelobe suppression by small aperture is simulated by the three-dimensional finite-difference time-domain method. The simulation results of the optical distribution of the near-field virtual probe reveal that the transmission efficiency of the virtual probe is 102-104 times higher than that of the nano-aperture metal-coated fibre probe widely used in near-field optical systems. The full width at half maximum of the peak, in other words, the size of virtual probe, is constant whatever the distance in a certain range so that the critical nano-separation control in the near-field system can be relaxed. We give an example of the application of the virtual probe in ultrahigh-density optical data storage.

Mechanisms for Enhancing Power Throughput from Planar Nano-Apertures for Near-Field Optical Data Storage

Near-field optical data storage systems are important for ultrahigh density data storage. One type of near-field optical data storage system uses a planar nano-aperture to define the data storage resolution. However, conventional nano-apertures have a tradeoff between resolution and power throughput. Recently, we reported a novel “C”-aperture design to overcome this problem. In this paper, we present our study on the mechanisms for power throughput enhancement, based on a detailed study of nano-slits using numerical simulations. We find that generating a propagation mode is essential for high power throughput, and by optimizing the design of the aperture thickness, resonant transmission can be achieved for even higher power throughput.

Piano-Convex Solid Immersion Mirror with a Small Aperture for Near-field Optical Data Storage

We proposed and developed a new solid immersion mirror (SIM) for near-field optical data storage. The SIM has some advantages, especially in manufacturing, because its shape is similar to a general plano-convex lens. Making an aperture of about 130 nm in diameter just on the focus area of the SIM, a bit-data recording onto a photochromic thin film was carried out. Minimum diameter of the written marks was estimated to be 180nm. We also demonstrated readout experiments where a recorded medium was rotating and a gap between the SIM and the disk was controlled with capacitance sensing. This is the first result of reading with a rotating disk using an SIM containing a small aperture.

Near-field optical data storage: avenues for improved performance

Because they produce small spot size, near-field techniques are applied to optical data storage systems to increase recording density. Two practical near-field implementations are aperture probes and solid immersion lenses (SILs). The basic signal modulation characteristics of these systems are reviewed, and some considerations for improving performance are discussed. Combinations of SILs and apertures could produce data storage systems with ultra-fine resolution and good detection characteristics.

Near-field recording with high optical throughput aperture array

To create and utilize a strong light source with sub-wavelength size has opened up a new frontier in optics, the near-field optics. One of the most attractive applications of the near-field optics is near-field optical data storage. The optical memory with high density and high data transfer rate is highly demanded to utilize an array of high throughput nano-scaled light sources for writing and reading bits on a medium. In this paper, we propose a hybrid structure of vertical cavity surface emitting laser (VCSEL) and aperture array for optical near-field memory head, namely VCSEL/NSOM. A technological solution for fabrication of the aperture array with Si micromachining technology is presented. An investigation of the optical performance of the near-field light at the apertures of the Si micromachined aperture array is presented. The primary result of writing and reading bits on a phase change (PC) medium using the fabricated structure is demonstrated.