Laser Nano-Machining Using Near-Field Optics

A near-field optical technique, using a new type of solid immersion lens (SIL), has been developed and applied to various areas, for example, high-density optical storage, near-field-scanning-optical-microscope probes, photolithography. Solid immersion microscopy offers a method for achieving resolution below the diffraction limit in air with significantly higher optical throughput by focusing light through a high refractive-index SIL held close to a sample [1]. The minimum resolution of a focusing system is inversely proportional to numerical aperture (NA), where NA = n sinθ, θ is the maximum angle of incidence, and n is the index of refraction at the focal point. Light with vacuum wavelength λ can be focused by an aberration-free lens to a spot whose full width at half maximum (FWHM) is λ/(2 NA) in the scalar diffraction limit, equivalent to Sparrow’s criterion for spatial resolution. In a medium of refractive index n, the effective wavelength is λ_eff = λ/n and corresponding effective numerical aperture is NA_eff = n²sinθ. When a SIL is used, improvements in NA_eff and spatial resolution are proportional to the refractive index of the SIL material. Fletcher et al. demonstrated imaging in the infrared with a microfabricated SIL [1, 2]. Baba et al. analyzed the aberrations and allowances for an aspheric error, a thickness error, and an air gap when using a hemispherical SIL for photoluminescence microscopy with submicron resolution beyond the diffraction limit [3]. Terris et al. developed and applied a SIL-based near-field optical technique for the writing and reading domains in a magneto-optic material [4]. Song et al. proposed the new concept of a SIL for high density optical recording using the near-field recording technology [5]. In this paper, we propose a sub-micron scale laser processing technique with spatial resolution beyond the diffraction limit in air using near-field optics. Our goal is to eventually develop a massively parallel nano-optical direct-write nano-manufacturing technique.