Wan-Chin Kim

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.

Effects of motion of an imaging system and optical image stabilizer on the modulation transfer function

This paper analyzes the effects due to the angular motion of a small-sized imaging system equipped with an optical image stabilizer (OIS) on image quality. Accurate lens moving distances for the OIS required to compensate the ray distortion induced by the angular motion are determined. To calculate the associated modulation transfer function, the integrated and the compensated point spread functions are defined. Finally, the deterioration of the image resolution due to angular motion and the restorative performance of the OIS are analyzed by isolating seven types of angular motion.

Anti-Shock Air Gap Control for SIL-Based Near-Field Recording System

The aim of this research is to improve the robustness of the air gap controller for solid immersion lens (SIL)-based near-field recording (NFR) servo systems against dynamic disturbances such as external shocks. Stable control is essential in these systems because the air gap distance between the SIL and the rotating disk is less than 100 nm. To detect gap error and read-out signals for the SIL and the disk, the air gap control performance must be increased to avoid collisions due to external shocks. We propose an anti-shock air gap control system using a disturbance observer (DOB) and a dead-zone nonlinear controller. Experimental results demonstrate that a DOB with a Q filter bandwidth of 500 Hz reduced the minimum air gap distance due to an external 1 g@10 ms shock by 70.1%. A DOB and a dead-zone nonlinear controller maintained the optical head distance for a 2 g@10 ms shock. Thus the control performance in the presence of external shocks was improved.

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.

Development of Integrated Small-Form-Factor Optical Pickup with Blu-ray Disc Specification

In this study, a small-form-factor optical pickup (SFFOP), corresponding to the Blu-ray disc (BD) specifications has been developed. The developed SFFOP is composed of an integrated optical pickup and a swing-arm-type actuator. The integrated optical pickup has been developed in the form of an array using wafer-level fabrication technology. The developed SFFOP is composed of the fundamental optics which yields a numerical aperture (NA) of 0.85 and uses a blue laser diode having a short wavelength of 407 nm and a silicon optical bench, which consists of a laser diode, a photodiode and several mounts for the laser diode, an objective lens and mirrors. The micro objective lens is bonded to the lens holder on the SFFOP by an active alignment using a modified Mach–Zehnder interferometer. Also, a static focus error signal was detected to assemble a polarized holographic optical element. Through the measurement of the focus error signal with a swing-arm-type actuator, which was developed for the SFFOP, it is estimated that the developed SFFOP satisfies the BD specifications with the balance of the focus error signal below 10%.

Design and fabrication of diffractive optical elements by use of gray-scale photolithography.

Diffractive optical elements (DOEs) are key components in the miniaturization of optical systems because of their planarity and extreme thinness. We demonstrate the fabrication of DOEs by use of gray-scale photolithography with a high-energy-beam sensitive glass photomask. We obtained DOE lenses with continuous phase profiles as small as 800 microm in diameter and 5.9 microm in the outermost grating pitch by selecting a suitable optical density for each height level and optimizing the process variables. Microlenses patterned with eight levels and replicated by UV embossing with the polymer master mold showed a diffraction efficiency of 81.5%, which was sufficiently high for the devices to be used as optical pickups. The effects of deviations in diffraction efficiency between the DOE height and profile design were analyzed.

Development of Microlens for High-Density Small-Form-Factor Optical Pickup

A microlens of numerical aperture (NA) 0.85 a small-form-factor optical pickup, following the specifications of the Blu-ray disc (BD), was designed, fabricated and evaluated. To avoid difficulties in the fabrication of a high-NA objective lens and to obtain a low chromatic aberration, a new hybrid lens unit was designed to have a refractive lens and a diffractive lens. The micro-plano-aspheric refractive lens was fabricated using glass molding technology, and the diffractive lens was fabricated in a two-dimensional array using the electron beam mastering and consecutive UV embossing process. For the evaluation of the developed lens unit, diffraction efficiency was measured with the proposed diffraction efficiency measurement method, and the wavefront error of the lens unit was evaluated using a modified Mach–Zehnder interferometer. The measured average efficiency of the diffractive lens was approximately 85% and the RMS wavefront error of the lens unit was 0.0376 λrms.

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.

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