ABSTRACT
The data density of bit-by-bit optical recording depends on the smallest mark that a given recording mechanism can generate. Ernst Abbe found in 1873 that light with wavelength, traveling in a medium with refractive index n and converging to a spot with angle q will make a spot with Full width at half maximum (FWHM) d not smaller than d = l/(2n sin q). The factor n sin q is often used as the numerical aperture (NA) of the lens. In this chapter, optical super-resolution techniques that are potentially useful for bit-by-bit optical storage are reviewed. 3.1 Introduction
Direct laser modification been one of the cutting-edge techniques vital in the nanotechnology sector. Compared with other techniques, the optical technique stands out for its features such as fast, versatile, and particular its capability of dynamic recording data three-
dimensionally, which has the potential to achieve high data-density and capacity. The most challenging problem in applying super-resolution techniques in direct-laser-writing optical data storage is how to generate a smallest possible laser focal spot and at the same time maintains high energy conversion. The former dictates the data density and the later determines speed of recording.Super-resolution techniques that aiming to engineer a small laser spot can be generalized into several different approaches: The first approach and relatively traditional approach is so called far-field technique, which is to modulate the phase and/or amplitude of a laser beam before it entering the focusing objective lens. The modulation can be achieved by using phase masks [1, 2], adaptive optics element such as liquid crystal phase modulators, deformable mirrors, etc. The idea behind it is that through the modulation the wavefront, the light at the focal center can be maximized, so that a smaller laser spot is created. In the past decade, this approach has been extended to modulate other optical properties of a laser beam [3-7]. One typical example is the modulation of polarization for sub-diffraction limited focal spot. It has been demonstrated that the radial polarized beam can produce a smaller focal spot than traditionally used linearly polarized beam under the high numerical aperture objective lenses [3, 5-7]. One feature that needs to be aware of is that with each small laser spot is created, the redistribution of light could result in strong side lobes in either perpendicular or parallel to the laser propagation direction, which can cause the elongation or even splitting of the focal spot.
