Near-field Optical Technology Research Articles

Comparative study on the paraxial approximation errors of tightly focused fundamental Gaussian beams

When the beam waist size is on the order of or smaller than a wavelength, the traditional Gaussian solution exhibits noticeable errors in describing a tightly focused laser beam of the fundamental mode. The spatial distributions of the paraxial approximate errors of the Gaussian solution to the paraxial Helmholtz equation, as compared with Sapozhnikov’s exact solution to the scalar Helmholtz equation, are illustrated, tabulated, and discussed systematically. The maximum and average errors of the Gaussian solution are presented for a list of different specified waist sizes of laser beams. We found that the maximum error distribution is of an annular structure and contains the largest error ring on the waist plane. The laser beam waist should be >1.2 wavelengths for a 1% error, while the beam waist should be >3.5 wavelengths for a 0.1% error. This study provides a useful criterion to ascertain whether the traditional Gaussian solution or the exact solution is to be used to describe the fundamental laser beam in optical engineering. The results exhibit potential applications in micronano technology, near-field optical technology, and other fields where tightly focused laser beams are utilized.

Transmission efficiency analysis of near-field fiber probe using FDTD simulation

A fiber probe is the key component of near-field optical technology which is widely used in high resolution imaging, spectroscopy detection and nano processing. How to improve the transmission efficiency of the fiber probe is a very important problem in the application of near-field optical technology. Based on the results of 3D-FDTD computation, the dependence of the transmission efficiency on the cone angle, the aperture diameter, the wavelength and the thickness of metal cladding is revealed. The authors have also made a comparison between naked probe and the probe with metal cladding in terms of transmission efficiency and spatial resolution. In addition, the authors have discovered the fluctuation phenomena of transmission efficiency as the wavelength of incident laser increases.

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.