Yong-Joong Yoon

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.

Improved Air-Gap Control for SIL-Based Near-Field Recording System

We propose a gap servo method to reduce the overshoot in the near-field (NF) region. In general, we use a mode-switching servo which consists of approach, hand-over, and gap-control modes. However, there is a critical problem, such as an overshoot at the turning point from the approach mode to the hand-over mode, which may cause a collision between the SIL and the disk. In this paper, we propose an improved NF gap servo system using an exponential input in the approach mode which can reduce the overshoot and settling time of the mode-switching servo

Effects of optical variables in immersion lens-based near-field optics

We analyze the effects of optical variables, such as illumination state, focal position variation, near-field air-gap height, and refractive index mismatch, in immersion lens-based near-field optics on the resultant field propagation characteristics, including spot size, focal depth, and aberrations. First, to investigate the general behaviors of various incident polarization states, focused fields near the focal planes in simple two- or three-layered media structures are calculated under considerations of refractive index mismatch, geometric focal position variations, and air-gap height in a multi-layered medium. Notably, for solid immersion near-field optics, although purely TM polarized illumination generates a stronger and 15% smaller beam spot size in the focal region than in the case of circularly polarized incident light, the intensity of the focused field decreases sharply from the interface between air and the third medium. For the same optical configurations, we show that changes in geometric focal position to the recording or detecting medium increases focal depth. Finally, through focused field analysis on a ROM (read-only memory) and a RW (rewritable) medium, compound effects of considered variables are discussed. The resultant field propagation behaviors described in this study may be applicable to the design of either highly efficient reflection or transmission near-field optics for immersion lens based information storage, microscopy and lithographic devices.

Aspherical Solid Immersion Lens of Integrated Optical Head for Near-Field Recording.

New solid immersion lenses (SILs) have been studied for a high-density optical storage system by the near-field process that can overcome the far-field diffraction limit. We have proposed the aspherical SILs, named elliptic SIL (ESIL) and Cartesian SIL (CSIL) according to geometrical optics. The SILs have a high numerical aperture (NA), for instance, the NAs of the ESIL and the CSIL are over 1, with a refractive index of 1.56 of the disc cover layer. The SILs that include the function of the objective lens are able to read/write the signals inside the disc substrate. The optical heads employing an internal recording method are expected to be utilized in an unsealed environment. Experimental results of the application of an ESIL are presented. The replicated ESIL (RESIL) has been proposed to solve critical issues such as the problems of the thickness error of the SILs or the disc substrate. These problems need to be solved for the commercialization of the near field recording (NFR) technology.

Cover-layer-protected solid immersion lens-based near-field recording with an annular aperture

Currently, data recording density in cover-layer-protected near-field-recording (NFR) and multiple-recording layered NFR optical data storage technology is limited by the difficulty in obtaining high-refractive-index cover layer materials. In addition, with the exception of improved resolution, the higher the numerical aperture (NA), the poorer the optical characteristics. However, in this study, we present novel cover-layer-protected solid immersion lens (SIL)-based NFR optics that provide superior optical performance with higher recording density, greatly enhanced focal depth, and less sensitivity to near-field air-gap-distance variation by modulating the amplitude and phase in the entrance pupil using annular pupil zones. Using an annular aperture consisting of three concentric annular zones to effect amplitude and phase modulation, the 1.45 NA cover-layer-protected SIL-based NFR optics achieved a data recording density as high as that of conventional 1.80 NA SIL-based NFR optics. These 1.45 NA optics yielded a full-width at half-maximum (FWHM) spot size of 0.315 lambda, a focal depth of 0.82 lambda, a focused beam spot sensitivity to air-gap-distance within the near-field region of 0.04 lambda, and a sidelobe intensity lower than 7%. In comparison with conventional 1.80 NA SIL-based NFR optics, the annular aperture optics achieved 3.5 times longer focal depth and much lower focused beam spot sensitivity to air-gap distance while maintaining the same high resolution. The introduction of this novel specially designed NFR optics could greatly improve data capacity in multiple-recording layered NFR.

Analysis on Effect of External Shock in Near-Field Recording System

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.

Design and Analysis of Replicated Solid Immersion Lens for Large Thickness Tolerance in Near-Field Recording

We have designed a novel solid immersion lens (SIL) optical head to increase SIL thickness tolerance for the fabrication and assembly of a SIL optical head. We have applied an aspherical replicated lens to a general SIL optical head to improve SIL thickness tolerance more than that of a hemispherical SIL and to maintain as high a numerical aperture as a super-hemispherical SIL. By using an aspherical replicated lens, we were able to reduce the sensitivity to spherical aberration with respect to variations in the SIL thickness, which are the main cause of narrow SIL thickness tolerance. This SIL optical head design shows that the SIL thickness tolerance is larger than 30 µm and the numerical aperture is 1.88, which is as high as that for a super-hemispherical SIL.

Solid Immersion Lens Optical Head for High-Numerical-Aperture Cover-Layered Incident Near-Field Recording

For increasing data recording density and reducing spherical aberration in cover-layered incident near-field recording (NFR) systems, a high-refractive-index cover layer is necessary and the assembly and evaluation technologies of a solid immersion lens (SIL) optical head for a high-numerical-aperture (NA) cover-layered incident NFR system are also required. To assemble a SIL optical head for the high-NA cover-layered incident NFR system, a modified Twyman–Green interferometer is developed. In this paper, we present the design and assembly results for a SIL optical head with a high-refractive-index cover-layered disk. We also compare evaluation results with those of a simulation to confirm the feasibility of the assembly. Through this research, we can improve the effective NA to 1.84, which is the highest NA reported for a cover-layered incident NFR system, and consequently, the data recording capacity per layer can be increased. # 2009 The Japan Society of Applied Physics

A study on the realization of high resolution solid immersion lens-based near-field imaging optics by use of an annular aperture

We report on the realization of solid immersion lens (SIL)-based near-field (NF) optics with an annular aperture, which is targeted to achieve high optical resolution. A numerical aperture (NA) = 1.84 hemisphere SIL-optics with an annular aperture achieves higher optical resolution than the conventional NA = 2.0 SIL-optics. The designed aperture is fabricated by photo-lithography and dry-etching technique. Experimental verification of the designed optics was performed through beam spot profile measurement under NF imaging conditions. A 15% smaller full-width-at-half-maximum spot diameter is obtained by the aperture. We verified that this method gives an improvement of the resolution in the optical imaging systems requiring higher resolution.

Design of Hybrid Lens for Compact Camera Module Considering Diffraction Effect

We introduce the design method of a hybrid lens composed a refractive lens and a diffractive optical element (DOE) considering the diffraction effect for a compact camera module. When imaging optical system is designed using the DOE, diffraction efficiency of the DOE and stray light due to the unintended diffraction light should be considered. Therefore, to analyze the effect of diffraction efficiency of the DOE on image characteristics, we evaluate the performances of two designed compact camera lenses, which have different construction wavelengths, using the modulation transfer function (MTF) considering diffraction efficiency. The stray light that is induced by the diffraction effect of the DOE is analyzed by calculating the focal position according to each diffraction order, and a method for the reduction of the stray light is proposed.