Super-resolution Near-field Structure Research Articles

Research progress of super-resolution optical data storage

With the rapid development of Big Data and artificial intelligence, emerging information technology compels dramatically increasing demands on data information storage. At present, conventional magnetization-based information storage methods generally suffer from technique challenges raised by short lifetime and high energy consumption. Optical data storage technology, in comparison, is well known for its advantages of low energy consumption and high security. However, the disc capacity of optical data storage technology inevitably gets stuck in the physical fundamental barrier-optical diffraction limit. How to break optical diffraction barrier and improve the resolution of optical storage system, thereby increasing the data storage capacity of the optical storage system is the key to incorporating optical storage technology with information technology trend such as big data and cloud computing. In this review, we present the principle of optical storage techniques beyond diffraction-limited and recent progress in high capacity optical data storage, including far field super-resolution three dimensional optical (3D) storage techniques (such as two-photon absorption-based process and saturation stimulated emission depletion fluorescence-inspired approaches) and near field super-resolution two dimensional (2D) optical storage techniques (such as near field scanning probe methods, solid immersion lens approaches, and super-resolution near-field structure methods). Eventually, the here-and-now problems confronted by the super-resolution optical data storage and future development of optical storage technology towards ultra-high capacity optical disc based on optical super-resolution techniques are discussed.

The mechanisms and research progress of laser fabrication technologies beyond diffraction limit

Laser is recognized as one of the top technological achievements of 20th century and plays an important role in many fields, such as medicine, industry, entertainment and so on. Laser processing technology is one of the earliest and most developed applications of laser. With the rapid development of nanoscience and nanotechnology and micro/nano electronic devices, the micro/nanofabrication technologies become increasingly demanding in manufacturing industries. In order to realize low-cost, large-area and especially high-precision micro-nanofabrication, it has great scientific significance and application value to study and develop the laser fabrication technologies that can break the diffraction limit. In this article, the super resolution laser fabrication technologies are classified into two groups, far-filed laser direct writing technologies and near-field laser fabrication technologies. Firstly, the mechanisms and progress of several far-field laser direct writing technologies beyond the diffraction limit are summarized, which are attributed to the lasermatter nonlinear interaction. The super-diffraction laser ablation was achieved for the temperature-dependent reaction of materials with the Gaussian distribution laser, and the super-diffraction laser-induced oxidation in Metal-Transparent Metallic Oxide grayscale photomasks was realized by the laser-induced Cabrera-Mott oxidation process. Besides, the multi-photon polymerization techniques including degenerate/non-degenerate two-photon polymerization are introduced and the resolution beyond the diffraction limit was achieved based on the third-order nonlinear optical process. Moreover, the latest stimulated emission depletion technique used in the laser super-resolution fabrication is also introduced. Secondly, the mechanisms and recent advances of novel super diffraction near-field laser fabrication technologies based on the evanescent waves or surface plasmon polaritons are recommended. Scanning near-field lithography used a near-field scanning optical microscope coupled with a laser to create nanoscale structures with a resolution beyond 100 nm. Besides, near-field optical lithography beyond the diffraction limit could also be achieved through super resolution near-field structures, such as a bow-tie nanostructure. The interference by the surface plasmon polariton waves could lead to the fabrication of super diffraction interference fringe structures with a period smaller than 100 nm. Moreover, a femtosecond laser beam could also excite and interfere with surface plasmon polaritons to form laser-induced periodic surface structures. Furthermore, the super-resolution superlens and hyperlens imaging lithography are introduced. Evanescent waves could be amplified by using the superlens of metal film to improve the optical lithography resolution beyond the diffraction resolution. The unique anisotropic dispersion of hyperlens could provide the high wave vector component without the resonance relationship, which could also realize the super resolution imaging. Finally, prospective research and development tend of super diffraction laser fabrication technologies are presented. It is necessary to expand the range of materials which can be fabricated by laser beyond the diffraction limit, especially 2D materials.

High-density channel model and detection method for signal readout from super-resolution near-field structure discs

Although a readout method using the super-resolution near-field structure (super-RENS) effect can overcome diffraction limits, readout characteristics for greatly surpassed high-density conditions do not become clear, because a high-density channel function having a differential response property is superimposed on a normal readout function. We propose a high-density channel model to indicate the properties of the super-RENS effect directly. This model can be expressed as a differential response function using the finite impulse response (FIR) filter model. It expresses the super-RENS readout process, which is divided on the basis of recording densities such as high and normal Blu-ray Disc™ densities. We estimated the properties of super-RENS readout signals by comparison between theoretical expressions and experiments. Results show that good signal quality require readout signals having sharp peaks and smaller offsets. We also evaluated the channel model by adding an adaptive FIR filter and a Viterbi decoder by simulations. Results show that the super-RENS disc can achieve a fourfold higher recording density if the signal-to-noise ratio (S/N) is improved to 6 dB in the case of partial response (PR) (1 + D + D2).

Design principle of super resolution near-field structure using thermally responsive optical phase change materials for nanolithography applications

The super resolution near-field structure (Super-RENS) which is composed of a thin layer of a thermally responsive optical phase change material (PCM) between two dielectric layers, can be a means of resolving the limited resolution of the laser beam for direct laser lithography. In Super-RENS, incident laser irradiation induces the direct, reversible opening and closing of a nanoaperture in the PCM layer, and a nanoscale pattern is realized in the lithography system. Here, we first introduce the complete modeling procedures and optimization methodology for Super-RENS in nanolithography based on a rigorous analysis of near-field structure, thermal analysis in the finite-element method, and analysis of the corresponding feature size on the photoresist (PR) layer. Multiple combinations of the PCM layer and the two dielectric layers with varying dimensions are considered as design parameters to achieve the required resolution in the nanolithography system. The feasible line profiles are investigated at the general operating conditions of the pulsed laser beam, based on varying dimensions of the PCM layer (5–30nm) and the two dielectric layers (10–200nm). This work will provide a detailed methodology for the design and optimization of the Super-RENS for applications in the nanolithography system.

Progress of super-resolution near-field structure and its application in optical data storage

The era of big data has necessitated the use of ultra-high density optical storage devices. Super-resolution near-field structure (super-RENS), which has successfully surpassed the fundamental optical diffraction limit, is one of the promising next generation high-density optical storage technologies. This technology combines the traditional super-resolution optical disk with a near-field structure, and has the advantages of structural simplicity, strong practicability, and, more importantly, compatibility with the current optical storage pickup. The mask layer in super-RENS functions as the key to realizing super- resolution. Development of suitable materials and stack designs has greatly been improved in the last decade. This paper described several types of super-RENS, such as aperture-type, light scattering center-type, bubble-type, and other types (e.g., WOx and ZnO), particularly the newly proposed super-RENS technology and research achievements. The paper also reviews the applications of super-RENS in high-density optical data storage in recent years. After analyzing and comparing various types of super-RENS technology, the paper proposes the aperture- type based on the mechanism of nonlinear optics as the most suitable candidate for practical applications in the near future.

A special issue on Optical Storage

The use of optics for data storage could trace back to 40 years ago when the researchers explored the possibilities of optical technology to record and playback audio signals. Optical storage devices have great advantages in low cost, high reliability and good compatibility, which make them suitable for data backup, digital publications, audio & visual products, and so on. To meet with the increasing demand for information storage in the era of big data, optical storage technologies are actively involved in continuous improvement of ultra-high densities and data transfer rates. Several promising technologies have grained growing interest, such as holographic data storage (HDS), multi-layer optical storage, multi-dimensional optical storage and super-resolution near-field structure (super-RENS) optical storage, they are with great expectations because they utilize the volume of optical materials to store information and show great potentials in capacity, density and speed. These technologies, emerging with the revival of optical storage, are now providing a platform for people from both academic and industrial fields to overcome the challenges and realize the practical applications for next generation optical data storage in near future. In this “Special Issue on Optical Storage”, 5 review articles and 3 research articles are specially presented, which covers the research frontiers in relevant subjects. In the review articles, Dr. Naoyasu Miyagawa from Panasonic Corporation gave an overview of the research and development progress on Blu-ray disc (BD) for more than 20 years, and presented the key technologies in optical disc system, rewritable media, recordable media, multi-layer media. Prof. Xiaodi Tan at Beijing Institute of Technology introduced high density collinear HDS system, both media structure and recording methods of HDS are discussed. Prof. Hao Ruan at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences reviewed the major achievements of HDS in the past 10 years and discussed about the key technologies and challenges. Prof. Yiqun Wu at same institute reported the progress of super-RENS and its application in optical data storage, analyzed and compared various types of super-RENS technologies, concluded the aperture-type media as the most suitable candidate. Prof. Changsheng Xie at Wuhan National Laboratory for Optoelectronics proposed the optical storage as an emerging option for long-term digital preservation, compared optical media based solutions with magnetic media, and summarized the techniques to improve the optical media based archive system. In the research articles, Prof. Yuzuru Takashima from University of Arizona in USA reported optical design in high density and high capacity multi-layer data storage system, and demonstrated sufficiently small as-built wavefront error (less than 0.1 waves) while implementing the focusing and tracking capabilities. Prof. Din Ping Tsai at Taiwan University studied the feasible concept of high density optical data storage by employing plasmonic resonances in metallic nano-structures and demonstrated information storage through the modulation of amplitude and phase of light. Prof. Jing Pei at Tsinghua University presented a novel multi-dimensional optical storage reading strategy, the readout rate could be increased by 10 times over the traditional disc. All the excellent works from the authors are appreciated and we hope these selected articles can serve as enlightenment and guidance to arouse the readers’ interests in optical data storage.

Sparse modeling of super-resolution near-field structure read-out signals

In this study, super-RENS disc read-out signals were modeled with various nonlinear models. In the results of modeling the nonlinearity, DC offset, and domain bloom phenomena of the super-RENS channel with a second-order Volterra filter, it was identified that the number of significant quadratic coefficients was relatively small. Therefore, a sparse Volterra model and a truncated Volterra model are applied to effectively model the nonlinear channel. In addition, the Hammerstein-type asymmetric conversion model is employed to model the nonlinear channel for the purpose of comparison. The results demonstrate that the sparse model of a simplified polynomial form achieves good modeling performance for the super-RENS disc read-out signals.

Domain Bloom in Super-Resolution Near-Field Structure Read-Out Signals

Super-resolution near-field structure (super-RENS) read-out samples are affected by a nonlinear and noncausal channel, which results in intersymbol interference. Some of these nonlinearities are caused by domain bloom or asymmetry, which is an imperfection in the disc. In this study, we investigate the effect of the domain bloom on the super-RENS read-out signal. For this, we employ the asymmetric symbol conversion scheme to generate asymmetric symbols corresponding to a bit pattern and apply the linear modeling approach to model the read-out channel with a finite impulse response filter. The modeling performance is verified with the causal and noncausal filters in terms of normalized mean square error. The performance of the approach considered here is compatible with that of the Volterra filter, whereas the approach maintains a lower complexity than the Volterra filter.

Super-Resolution Optical Disc with Radial Density Increased by Narrowed Track Pitch Corresponding to Diffraction Limit

In–Sb-based super-resolution near-field structure read-only-memory discs with radial density increased by introducing a narrow track pitch corresponding to the diffraction limit of an optical system were developed. Using an optical system with a laser diode with a wavelength of 405 nm and an objective lens with a numerical aperture of 0.85, we confirmed that differential phase detection (DPD) could detect track errors from disc samples recorded random data including a minimum pit length of 75 nm in a 240 nm track period. It has higher capability of track error detection than push–pull detection at a narrowed track pitch. Moreover, bit error rates satisfying the criterion of 3.0×10-4 were experimentally obtained for 66.7-GB-capacity disc samples with a 240 nm track pitch through signal processing with the partial response maximum likelihood of the (1,2,2,1)-type, by applying DPD to tracking servo control. The feasibility of increasing the track density of the Blu-ray DiscTM physical format by 1.33 times was indicated.

Influence of the Numerical Aperture on the Superresolved InSb Focused Spot

A detailed study on the influence of the focusing lens numerical aperture on the InSb super resolved spot is presented. The super resolution effect is described using the threshold model, where the material permittivity locally changes under laser exposure, being rigorously simulated in a finite element method program. The superresolved InSb focused spot is investigated for a wide range focusing lenses systems, from high to low numerical apertures. An extension of the model to account for disc motion is discussed. Such dynamic super-resolution near-field structure (Super-RENS) threshold model is suitable for readout signal analysis when combined with a "quasi-scalar" readout program, also described. Interesting properties of the InSb Super-RENS focused spot, such as sub-diffraction limited spot size and the general conditions to achieve it, are shown. An analysis on the Super-RENS readout signal is also given, with useful applications in optical data storage. Thus, this work greatly helps in understanding the InSb scatterer type of Super-RENS effect for optical data storage and other super resolution applications.

Influence of the Numerical Aperture on the Superresolved InSb Focused Spot

A detailed study on the influence of the focusing lens numerical aperture on the InSb super resolved spot is presented. The super resolution effect is described using the threshold model, where the material permittivity locally changes under laser exposure, being rigorously simulated in a finite element method program. The superresolved InSb focused spot is investigated for a wide range focusing lenses systems, from high to low numerical apertures. An extension of the model to account for disc motion is discussed. Such dynamic super-resolution near-field structure (Super-RENS) threshold model is suitable for readout signal analysis when combined with a “quasi-scalar” readout program, also described. Interesting properties of the InSb Super-RENS focused spot, such as sub-diffraction limited spot size and the general conditions to achieve it, are shown. An analysis on the Super-RENS readout signal is also given, with useful applications in optical data storage. Thus, this work greatly helps in understanding the InSb scatterer type of Super-RENS effect for optical data storage and other super resolution applications.

Nonlinear Equalization of the Super-Resolution Near-Field Structure Read-Out Signal Using the Adaptive Amplitude Nonlinear Gradient Descent Algorithm with the Sigmoid Function

In this study, to mitigate nonlinearity in super-resolution near-field structure (super-RENS) read-out signals, we investigate nonlinear equalization based on the adaptive amplitude nonlinear gradient descent (AANGD) algorithm, which is suitable for processing nonlinear and nonstationary input signals with a large dynamic range. Note that the sigmoid function is employed as the nonlinear activation function since it is commensurate with the experimental data and entails simple implementation. The experiment results regarding bit error rate (BER), convergence speed, lookup table (LUT) size, and computational complexity show that the proposed equalizer outperforms the Volterra filter and linear finite impulse response filter with the normalized least-mean square (NLMS) algorithm.

Shrinkage of picosecond laser beam by plasma reflection in super-resolution near-field structure of SiN/Sb/SiN thin film

The transmittive and reflective Z-scan technique is used with a 10 Hz, frequency doubled, Q-switched, and mode-locked Nd:YAG laser to verify that the reflectivity of the super-resolution near-field structure of an SiN/Sb/SiN thin film increases as incident intensity decreases. This intensity-dependent reflection, called nonlinear reflection, reflects a TEM(00) mode laser beam more strongly at its periphery than at its center and so shrinks the transmitted laser beam. The observed nonlinear reflection is attributed to laser-induced change of carrier densities in Sb, to justify quantitatively the experimental results.

Direct measurement of the near-field super resolved focused spot in InSb

Under appropriate laser exposure, a thin film of InSb exhibits a sub-wavelength thermally modified area that can be used to focus light beyond the diffraction limit. This technique, called Super-Resolution Near-Field Structure, is a potential candidate for ultrahigh density optical data storage and many other high-resolution applications. We combined near field microscopy, confocal microscopy and time resolved pump-probe technique to directly measure the induced sub-diffraction limited spot in the near-field regime. The measured spot size was found to be dependent on the laser power and a decrease of 25% (100 nm) was observed. Experimental evidences that support a threshold-like simulation model to describe the effect are also provided. The experimental data are in excellent agreement with rigorous simulations obtained with a three dimensional Finite Element Method code.

Nonlinear Equalization for Super-Resolution Near-Field Structure Discs Using a Simplicial Canonical Piecewise-Linear Model

In this paper, we propose a simplicial canonical piecewise-linear (SCPWL) model-based equalizer to mitigate nonlinear distortion resulting in inter symbol interference (ISI) in a super-resolution near-field structure (super-RENS) disc system. The proposed equalizer consists of the linear part represented by a finite impulse response (FIR) filter and the nonlinear part implemented using a SCPWL function. Its validity is tested using radio frequency (RF) signal samples obtained from a super-RENS disc. The raw bit error rate (BER) performance with the proposed equalizer is measured for various key parameters of the SCPWL model. The simulation results demonstrate that the proposed equalizer achieves outstanding raw BER performance improvement by effectively compensating for the nonlinear distortion introducing the ISI.

Simple Readout Channel Model of Super-Resolution Near-Field Structure Disk System

The readout channel response of super-resolution near-field structure (super-RENS) disks have been experimentally investigated using a 63-bit pseudorandom sequence and the autocorrelation of the readout sequence. The time domain analysis reveals that the super-RENS disk has two responses: one is the response of the normal readout beam and the other is the sharp differentiated response of the super-RENS effect. The numerical analysis proves that the sharp differentiated response offers an excellent spatial resolution of the super-RENS disks.

Simple Readout Channel Model of Super-Resolution Near-Field Structure Disk System

The readout channel response of super-resolution near-field structure (super-RENS) disks have been experimentally investigated using a 63-bit pseudorandom sequence and the autocorrelation of the readout sequence. The time domain analysis reveals that the super-RENS disk has two responses: one is the response of the normal readout beam and the other is the sharp differentiated response of the super-RENS effect. The numerical analysis proves that the sharp differentiated response offers an excellent spatial resolution of the super-RENS disks.

Nonlinear Modeling for Super-RENS Disc Using a Simplicial Canonical Piecewise-Linear Adaptive Filter

A correct and accurate model of an optical storage system is very important in the development and performance evaluation of various data detection algorithms. In this paper, we present a nonlinear modeling of a super-resolution near-field structure (super-RENS) read-out signal using the simplicial canonical piecewise-linear (SCPWL) model. The validity of this model is tested using radio frequency (RF) signal samples obtained from a super-RENS disc. The experiment results on modeling indicate that the SCPWL model can be efficiently utilized for the nonlinear modeling of the super-RENS systems.

A Compressed Sensing Approach for Modeling the Super-Resolution Near-Field Structure Disc System With a Sparse Volterra Filter

In this paper, we investigate the compressed sensing (CS) algorithms for modeling a super-resolution near-field structure (super-RENS) disc system with a sparse Volterra filter. It is well known that the super-RENS disc system has severe nonlinear inter-symbol interference (ISI). A nonlinear system with memory can be well described with the Volterra series. Furthermore, CS can restore sparse or compressed signals from measurements. For these reasons, we employ the CS algorithms to estimate a sparse super-RENS read-out channel. The evaluation results show that the CS algorithms can efficiently construct a sparse Volterra model for the super-RENS read-out channel and that observable nonlinear interactions take place among restricted components in the read-out channel.

超分辨近场结构光盘的传统以及差分读出方法研究

超分辨近场结构光盘的传统以及差分读出方法研究