Imaging beyond the diffraction limit with self-assembled nanoscale lenses

The conventional optical microscope is one of the most important tools of modern science. It is found in practically every scientific laboratory. Unfortunately, no matter how well a microscope is built, a fundamental constraint remains that prohibits exploration of the details of an object below some threshold size. This constraint is the so-called diffraction limit,¹ discovered by Ernst Abbe in the late 19^th century.

Beating the diffraction limit has been one of the main focuses of research in optics for several decades. These efforts have spurred a variety of technologies that currently exist at differing stages of development. Popular examples include the solid-immersion lens¹ (SIL), the super-² and hyperlens,³ scanning-probe optical near-field microscopy,¹ and stimulated emission depletion microscopy.¹ Each of these techniques has its own advantages and shortcomings. Yet the SIL, aside from the less-effective liquid-immersion microscopy, remains the only well-developed means of projecting sub-diffraction-limit information directly into a far-field image without the need for further data processing. Commonly employed SILs are macroscale (~1mm), but the ever-increasing drive for miniaturization of optical systems argues for reducing them to the nanoscale. In our laboratory, we are developing new means of fabricating nanoscale optical elements, such as nanoscale SILs (nSILs), and investigating their functionality for imaging and a range of other optical applications.⁴

In its simplest implementation, the SIL is a hemispherically shaped dielectric (i.e., electrical insulator) that sits atop an object surface. Images are obtained by focusing a regular optical microscope through the SIL, which provides local enhancement of the refractive index at the object surface. The result is a shortening of the effective wavelength of the imaging radiation and an increase in resolution by a factor of ~1/n, where n is the SIL's refractive index. Reducing the size of a SIL to ~100nm presents the challenge of fabricating almost perfectly shaped, optically transparent plano-convex structures. We have produced such nSILs using a novel self-assembly technique employing calix[4]hydroquinone (CHQ) molecules.⁴ Self-assembly is a bottom-up nanofabrication technique that produces structures by virtue of precise intermolecular interactions. The technique has been used to make structures with nano- and even atomic-scale resolution with almost no defects. In recent experiments,⁴ we showed that these self-assembled nanoscale lenses could be employed like regular SILs to obtain high-quality images with resolution beyond the diffraction limit.

The self-assembly of CHQ molecules produces a wide variety of optically transparent, plano-convex lenses with diameter D = 0.05−3μm and thickness H < 800nm. The spherical side of the lens deviates less than 3% from that of a perfect sphere, and the surface roughness is just ~1nm (see Figure 1). To study the imaging properties of the nSILs, we fabricated lithographically patterned metal-stripe arrays on which CHQ lenses were deposited randomly by spin coating.

Figure 1. (top left) Scanning-electron-microscope (SEM) image of a self-assembled nanolens. (top right) Example result of numerical simulations of light focusing by a nanolens. (bottom) Atomic-force-microscope profile (X, Z: Horizontal, vertical) showing the near-perfect spherical face of the nanolens with the corresponding SEM image. D: Diameter. H: Thickness. R: Radius of curvature.

Figure 2 shows images of a 250 and 220nm-pitch stripe array atop which sits an individual nSIL (a, c) face up and (b) face down, obtained using a conventional optical microscope with a numerical aperture (NA) of 0.9 and 472nm wavelength illumination. These images clearly demonstrate the resolution enhancement provided by the nSIL. The Rayleigh criterion predicts the resolution of the microscope objective to be δ~0.61λ/NA for two pointlike objects or δ~0.5λ/NA for line patterns, where δ is the smallest distance between two objects such that they can be resolved individually in the final image, and λsignifies wavelength. In our experiments, δ = 262nm. Figure 2(c) makes clear that the nSIL resolves the individual stripes that are separated by 220nm, which is ~20% smaller than the Rayleigh resolution limit. In addition, we have confirmed that the stripe array in the area outside the nSIL cannot be resolved through any further adjustment of the microscope focus. Implementation of the nSIL has clearly provided an improvement to the resolution beyond the diffraction limit of ordinary optical microscopes.

Figure 2. Optical (λ = 472nm, NA = 0.9) microscope (top) and corresponding SEM (bottom) images of individual nanoscale solid-immersion lenses (nSILs) sitting on metallic stripes separated by 250nm (a, b) and 220nm (c). The stripes are resolved under the nSIL regardless of face-up or face-down orientation. Even the stripes separated by 220nm can be resolved under the nanolens (c), which is a ~20% improvement on the diffraction limit of the optical microscope.⁴ λ: Wavelength. NA: Numerical aperture.

In summary, while beating the diffraction limit has been one of the major goals of research in optics for several decades, aside from the SIL, few well-developed devices exist that are simple to implement for high-quality projection imaging beyond the diffraction limit. Our recent experiments are the first to demonstrate high-quality projection imaging using nSILs. Our future research will include developing new practical applications of nSILs and other self-assembled nanoscale optical devices.