Subwavelength-size solid immersion lens

Abstract

We report on the fabrication and characterization of nanoscale solid immersion lenses (nano-SILs) with sizes down to a subwavelength range. Submicrometer-scale cylinders fabricated by electron-beam lithography are thermally reflowed to form a spherical shape. Subsequent soft lithography leads to nano-SILs on transparent substrates for optical characterization. The optical characterization is performed using a high-resolution interference microscope with illumination at 642 nm wavelength. The focal spots produced by the nano-SILs show both spot-size reduction and enhanced optical intensity, which are consistent with the immersion effect. © 2011 Optical Society of America OCIS codes: 220.4241, 310.6628, 180.3170. Hooke first discussed the immersion technique to improve the imaging performance of a microscope in 1678 [1]. Although the concept of homogeneous immersion preceded Abbe's pioneering work, he constructed the first oil-immersion lens in developing the imaging theory of microscopy [2]. Abbe also developed the standard measure of performance of an objective lens, the numerical aperture (NA), which is defined as [3]where θ is the angle a ray makes with the optical axis (half the angle of the focusing cone), and n is the refractive index of the medium through which the rays pass. In 1990, Mansfield et al. developed a new immersion concept, which is termed solid immersion lens (SIL) [4]. Focusing in the center of a hemispherical solid leads to normal incidence of the rays with respect to the spherical interface. Therefore, there is no refraction at the interface between the spherical solid and the surrounding medium (e.g., air), leading to an increase in the NA by a factor of n (the refractive index of the solid medium) as shown in Eq. (1). At the beginning, the fabrication of SILs was limited to macroscopic size (i.e., millimeter scale). Advances in micro-and nanofabrication technologies enabled the development of different types of SILs, including diffractive SILs [5], micrometer-size SILs [6-9], nanoscale spherical lenses [10], and wavelength-scale SILs [11].The goal of this Letter is to report on the fabrication and optical characterization of nanoscale SILs (nanoSILs) with sizes down to subwavelength range. In general, for structures smaller than the optical wavelength, design methods for larger devices, such as ray optics, are not applicable. More specifically, subwavelength-scale lenses cannot simply be considered to be refractive optical surfaces. However, recent modeling work has shown that subwavelength-scale SILs are still expected to produce a reduced size focal spot [11], the so-called immersion effect. To the best of our knowledge, in this Letter we report the first experimental demonstration of the immersion effect in subwavelength-scale nano-SILs.Electron-beam lithography (EBL) was used to form cylindrical nanostructures with two dimensions: (1) 600 nm diameter and 200 nm height; and (2) 450 nm diameter and 150 nm height, which are shown in the scanning electron microscope (SEM) images of Fi...

Numerical verification of light confinement in a nano solid immersion lens

“…Recent experimental work has shown that subwavelength-scale SILs demonstrate a reduced-size focal spot [4]. Further investigation of nano-SILs of different shapes was reported as well [5].…”

“…Initially, macroscopicsize SILs were the main subject due to the state of fabrication technologies. Recent advances in micro-and nano-fabrication technologies led to the development of different types of SILs, including diffractive SILs [2], micrometer-size SILs [3] and wavelength-scale SILs [4].…”

Extension of solid immersion lens technology to super-resolution Raman microscopy

“…SILs with diameters in the micrometer region [12] down to sub-wavelength size [13] are described in the literature. Brunner et al propose a diffraction-based solid immersion lens (dSIL) as an alternative to the conventional design based on refraction [14].…”

Advanced optical characterization of micro solid immersion lens

“…millimeter range dimensions, due to the lack of advanced fabrication technologies. The advance of micro-and nano-technology boosts the developments of different types of solid immersion lenses, for instance, diffractive SILs 6 , micro-size SILs 7,8 , nano-size SIL-like elements 9,10 , and subwavelengthsize SILs 11 . The reported micro-size SILs in Refs.…”

“…When the SILs are large, the fabrication and alignment errors produce substantial aberrations because the amount of aberrations is proportional to the size of the optical elements. Therefore, when the characteristic size of the SIL becomes smaller and falls within the wavelength size, aberrations, especially the spherical aberration, become negligible 11 . In the visible, the size of a few micrometers is a good target value to avoid aberration effects and at the same time to facilitate the fabrication processes.…”