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...

We present the fabrication and optical characterization of nano-scale solid immersion lenses (nano-SILs) with sizes down to a subwavelength range. Submicron-scale cylinders fabricated by electron-beam lithography (EBL) are thermally reflowed to form a spherical shape. Subsequent soft lithography leads to nano-SILs on transparent substrates, i.e. glass, for optical characterization with visible light. The optical characterization is performed using a high-resolution interference microscope (HRIM) with illumination at 642 nm wavelength. The measurements of the 3D amplitude and phase fields provide information on the spot size and the peak intensity. In particular, the phase measurement is a more convincing proof of the Airy disc size reduction rather than the full-width at half maximum (FWHM) spot size. The focal spots produced by the nano-SILs show both spot-size reduction and enhanced optical intensity, which are consistent with the immersion effect. In this way, we experimentally confirm the immersion effect of a subwavelength-size SIL (d = 530 nm and h = 45 nm) with a spot reduction ratio of 1.35, which is less than the expected value of 1.5, most likely due to the slightly non-ideal shape of the nano-SIL.