Near-field Optical System Research Articles

Optical skyrmions and tunable fine spin structures in deep-subwavelength scale at metal/graded index material interfaces

A skyrmion is a topological quasiparticle that has been studied widely in nuclear physics, condensed matter physics, cosmology, and optics. Previously, the optical skyrmions in the surface plasmon polaritons platform were not tunable because the dielectric properties of the material were fixed. In the study, we introduce the graded refractive index materials into the near-field optical system and systematically investigate the propagation properties, dispersion relations, and spin-orbit decomposition of the surface waves at the metal/graded refractive index materials interface. Our theoretical results exhibit that the topological spin skyrmions can be formed in the system and the dimensions of optical skyrmions can be tuned by varying the central permittivity and exponent of the graded refractive index materials. Additionally, the spin fine structure, in which the spin state varies sharply from the ‘up’ state to the ‘down’ state, can be also controlled by adjusting the materials properties of the graded refractive index materials. The minimal full width of the spin fine structure is 0.254λ, which has the potential for achieving the displacement metrology with a sensitivity of 2.54 × 10−7λ theoretically. Our findings provide an extra degree of freedom to control the formation and scale of fine spin structures in optical skyrmions and open an avenue for next-generation pico-photonics.

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.

Phase-sensitive near field Investigation of Bloch surface wave propagation in curved waveguides

Bloch surface waves (BSWs) are electromagnetic surface waves excited in the band gap of a one dimensional dielectric photonic crystal. They are confined at the interface of two media. Due to the use of dielectric material, the losses are very low, which allows the propagation of BSWs over long distances. Another advantage is the possibility of operating within a broad range of wavelengths. In this paper, we study and demonstrate the propagation of light in ultra-thin curved polymer waveguides having different radii fabricated on a BSWs-sustaining multilayer. A phase-sensitive multi-parameter near-field optical measurement system (MH-SNOM), which combines heterodyne interferometry and SNOM, is used for the experimental characterization. Propagating properties, bending loss, mode conversion and admixture are investigated. We experimentally show that when light goes through the curved part of the waveguide, energy can be converted into different modes. The superposition and interference of different modes lead to a periodically alternating bright and dark beat phenomenon along the propagation direction. Experimental optical phase and amplitude distributions in the curved waveguide show a very good agreement with simulation results.

Ultraviolet tip-enhanced nanoscale Raman imaging

We have constructed an ultraviolet (UV)-apertureless near-field scanning optical microscope-Raman spectroscopy system by using an aluminum tip for the simultaneous measurement of topography and Raman scattering of nanomaterials with high spatial resolution. The topography, Rayleigh scattering image, and tip-enhanced Raman scattering image of the carbon nanotube film showed that a spatial resolution of around 19 nm was achieved. This spatial resolution of UV-Raman mapping image exceeds that of previous approaches, which have several hundred nanometers of spatial resolution. Copyright © 2012 John Wiley & Sons, Ltd.

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.

Control Methods in Data-Storage Systems

The recording performance of data-storage devices, in which write/read elements move relative to a storage medium to reliably store and retrieve information, depends on the capability of servo mechanisms to provide the necessary positioning accuracy. The desired characteristics of servo mechanisms for data-storage systems include robustness against variations of environmental parameters, high resolution, accuracy, stability, and fast response. This paper presents a comprehensive overview of advanced servo-control methods for data storage. The applications are to well-established recording technologies, including magnetic tape and magnetic disk systems as well as CD/DVD/Blue-Ray optical data-storage systems. Moreover, newer holographic and near-field optical systems and the emerging probe-storage technology are also addressed. Emphasis is given to the potential exhibited by the technologies considered for achieving ultra-high storage capacity, as required by the exploding demand in data-storage capacity for archival systems and massive multimedia data storage.

Nano-Particle Manipulated with Near-Field Optical Tweezers

By applying the direct calculation of Maxwell stress tensor using three-dimensional finite difference time domain method, the feasibility of using a metal-coated fiber probe to create near-field optical tweezers is investigated. Numerical results indicate that these schemes are able to trap nano-particles with lower laser intensity than that required by conventional optical tweezers. The near-field optical trapping systems that are more flexible than conventional optical tweezers are built. In experiments, 120-nm polystyrene particles are trapped in a multi-circular shape with a minimum size of 400 nm. The realization of trapping particles in the range of tens of nanometers largely promotes the role of near-field optical manipulation at the nanometer scale.

High Spatial Resolution Photodetectors Based on Nanoscale Three-Dimensional Structures

A high spatial resolution photodetector is designed and realized using a newly developed two-sided nanoscale three-dimensional fabrication technique. Two types of nanophotodetectors with slab and channel structures are fabricated and characterized. By comparison, nanophotodetectors with channel structures show a 10 times lower dark current, higher detection resolution, and signal-to-noise ratio than nanophotodetectors with slab structures. The smallest nanophotodetector fabricated is as small as 100 nm wide. A responsivity of 0.19 A/W at 1310-nm wavelength and 3-V bias for a channelized nanophotodetector is registered, with a photocurrent of 135 nA over a dark current of 1.25 nA. In addition, using a modified near-field scanning optical microscopy-based photocurrent mapping system, we demonstrated a high spatial resolution photodetection of 400 nm using a nanophotodetector with channel structure fabricated.

High-Density Recording with a Near-Field Optical Disk System Using a Medium with a Top Layer of High Refractive Index

A near-field optical technology using a solid immersion lens (SIL) has been actively studied to expand the storage capacity higher than 100 Gbytes per layer in a 12-cm-sized optical disk. However, the working distance of an objective lens in a near-field optical disk system should only be 25 nm or less. Therefore, from the practical viewpoint, a topcoat layer is required to protect the recording layer when the SIL collides with a disk surface because of disturbances such as dust, shock and vibration. From a mechanical viewpoint, the topcoat should have the mechanical toughness to protect the disk surface. Moreover, from an optical viewpoint, it should have refractive index higher than the numerical aperture of an SIL to achieve a sufficient evanescent couple between the SIL and disk surface. In this study, we describe a topcoat for a near-field optical recording. First, we investigate the topcoat performance from the optical viewpoint. Second, we evaluate the topcoat performance from a mechanical viewpoint. Finally, we report the results of recording experiments for a disk with the topcoat and discuss its performance.

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.

纳米孔径垂直腔面发射激光器的研究进展

This paper reviews the progress on nano-aperture vertical-cavity surface-emitting lasers (VCSELs). The design, fabrication, and polarization control of nano-aperture VCSELs are reviewed. With the nano-aperture evolving from conventional circular and square aperture to unique C-shaped, H-shaped, I-shaped, and bowtie-shaped aperture, both the near-field intensity and near-field beam confinement from nano-aperture VCSELs are significantly improved. As a high-intensity compact light source with sub-100-nm spot size, nano-aperture VCSELs are promising to realize many new near-field optical systems and applications.

Performance Investigation of Soft-Decodable Runlength-Limited Codes With Different Minimum Runlength Constraints in High-Density Optical Recording

This paper investigates the performance of runlength-limited (RLL) codes with different d constraints in high-density optical recording, for both the low-density parity-check (LDPC)-coded Bliss scheme and the standard concatenation scheme. For the d = 5 constraint, we propose a low-complexity compression/decompression scenario that maps each 6-bit channel to a 3-bit compressed codeword by Gray-alike coding of the position of the transition in the 6-bit codeword, to compensate the density loss introduced by the redundancy of LDPC coding in the LDPC-coded Bliss scheme. Using a scalar diffraction channel model, we ran simulations to compare the performance of RLL codes with four different d constraints: d = 0,1,2, and 5, in near-field optical recording systems. The simulation results address the average bit error number in erroneous symbols after soft-input soft-output channel detection. Further, we evaluated the net density with different sets of simulation parameters, for both the LDPC-coded Bliss scheme and the standard concatenation scheme, considering jitter noise and additive white Gaussian noise, respectively.

Determination of the R-line luminescence spatial resolution of a near-field optical spectroscopy system for piezospectroscopy

The spatial resolution of a near-field optical spectroscopy system in the R-line photoluminescence mode has been evaluated to assess the resolution with which stress measurements can be made by piezospectroscopy. The effect of the probe tip–sample distance as well as the aperture size of the near-field probe on the luminescence R-line intensity was obtained. The luminescence intensity was found to increase linearly with the third power of the aperture size, whereas the lateral spatial resolution was proportional to the aperture size. These findings are consistent with a simple model for the electric dipole moment interaction between the tip and the sample. The minimum lateral spatial resolution was found to be 160nm using the smallest available aperture size of ∼100nm. Images of the stress distribution in a polycrystalline alumina are presented, showing variations at this scale.

A Write Head Integrated With a Solid-Immersion-Lens System for Heat-Assisted Magnetic Recording

We have developed for heat-assisted magnetic recording a novel write head in which a magnetic writer and a near-field optical lens system are integrated. The single-pole type (SPT) write head is embedded in the hemispherical lens of a solid immersion lens (SIL) system, which has an effective NA of 1.44 at a wavelength of 405 nm. The SPT head, which has high magnetic efficiency and minimizes light scattering, consists of a main pole and vertical-turns type (VTT) coils. With an FWHM spot diameter of 130 nm and a main pole 2 mum wide, the head is estimated to be capable of small-spot writing

Realization of femtosecond-resolved near-field optical system and its application

By combining femtosecond laser with the scanning near-field optical microscope (SNOM), we have successfully realized the femtosecond-resolved near-field optical system. Through high frequency acousto-optic modulation and difference frequency lock-in detection, the signal-to-noise ratio was greatly improved and the background signals of the pump and probe light were removed. Therefore, we can measure a femtosecond-resolved weak transmission change in the collection mode SNOM. Spatial resolution of 80nm and temporal resolution better than 200fs were achieved simultaneously. Using this system, we studied the relaxation processes of hot electrons in gold nanostrucures and the difference in relaxation dynamics was observed at different sample positions.

A novel self-aligned process between a nano-aperture and a solid immersion lens for a near-field optical system

For near-field optical systems, an aperture in nano-size and a solid immersion lens (SIL) are two popular methods to overcome the diffraction limit and reduce the optical spot size. In previous research, combining a nano-aperture and SIL has been proposed to provide high throughput and ultra-high resolution. However, alignment between the nano-aperture and SIL is very critical in the fabrication process, and no effective alignment technique has been investigated yet. In this research, a novel self-alignment technique is proposed for the combination of the nano-aperture and SIL, where the nano-aperture is fabricated with the focused ion beam (FIB) system and the SIL is formed by the thermal reflowing process. The self-alignment technique is based on the backside exposure and the surface tension self-modulation technique during the thermal reflowing process. The maximum deviation of the fabricated SIL is less than 3% in comparison with the designed values. A SIL of 15 µm diameter has been measured with 65.2% transmission efficiency. The polymer SIL combined with the circular aperture of 329 nm diameter has 1.68 times enhancement on throughput compared with the circular aperture of 329 nm alone. This result shows that the SIL can effectively enhance the throughput of the nano-aperture, and the feasibility of the proposed self-alignment method is also verified.

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.

Strategies for employing surface plasmons in near-field optical readout systems

The enhanced transmission of light through subwavelength-size holes in a metal plate is well-known to be associated with surface plasmons. We have undertaken a systematic theoretical study of several strategies for applying these plasmon effects in a near-field optical readout system using an exact Green's tensor formulation. Based on the results of our simulations with light of wavelength lambda = 500 nm, data structures separated by 120 nm could be clearly resolved, and asymmetries of about +/-10 nm in the optical readout system could be tolerated without serious degradation of the performance. Advantages and disadvantages of each strategy are discussed.

The intensity profiles of the focused spot in the near-field optical SIL plus objective lens system

The approximation of quasi-vector diffraction theory is employed to calculate the intensity profiles and the transmittance of numerical aperture (NA) 0.85 and 0.65 near-field optical solid immersion lens systems. The numerical results of the calculation are obtained. It is found that the near-field optical system with higher NA objective lens can obtain smaller optical spot size, but lower transmittance, compared with the one with lower NA objective lens.

High Density Near-Field Optical Disc System

Key technologies for near-field recording/readout systems using a solid immersion lens (SIL) are summarized in this paper. Our system employing a SIL of 1.84 numerical aperture (NA) and a laser diode (LD) of 405 nm wavelength has realized the capacity of a 112 GB in a 12 cm diameter phase-change disc along with a write-power margin of ±11.2% and a data transfer rate of 36 Mbps. Such a larger-capacity data storage system should provide a high data transfer rate. A preliminary result of 1.84 NA dual-channel near-field recording/readout using a monolithic dual-beam LD with 412 nm wavelength is presented.