Scattering Lens Resolves Sub-100 nm Structures with Visible Light

The smallest structures that conventional lenses are able to optically resolve are of the order of 200 nm. We introduce a new type of lens that exploits multiple scattering of light to generate a scanning nanosized optical focus. With an experimental realization of this lens in gallium phosphide we imaged gold nanoparticles at 97 nm optical resolution. Our work is the first lens that provides a resolution better than 100 nm at visible wavelengths.

Figure 1

(a) Principle of light coupling to high transversal $k$ vectors into a high-index material. Without scattering refraction would strongly limit the angular range to which light could be coupled. By exploiting strong scattering at the interface, incident light $k_{\mathrm{in}}$ is coupled to all outgoing angles $k_{\mathrm{out}}$ in the high-index material. (b) Schematic of a HIRES lens that uses light scattering to achieve a high optical resolution. This HIRES lens consists of a slab of gallium phosphide (GaP) on top of a strongly scattering porous layer. By controlling the incident wave front, a small focus is made in the object plane of the HIRES lens.

Figure 2

Overview of the setup. A ${\lambda }_0=561\mathrm{nm}$ laser beam is expanded and illuminates a phase-only spatial light modulator. The modulated reflected beam is first imaged onto a two-axis steering mirror and then onto the porous surface of the GaP HIRES lens. A variable aperture controls the extent of the illuminated area and a light stop places the setup in a dark field configuration by blocking the center of the light beam. We image the object plane onto a CCD camera using an oil immersion microscope objective.

Figure 3

Experimental imaging demonstration with a GaP HIRES lens. (a) A reference image taken with conventional oil immersion microscope ($\mathrm{NA}=1.49$). The image shows a blurred collection of gold nanoparticles. The scale bar represents 300 nm. (b) A high-resolution image acquired with our GaP HIRES lens. The image was obtained by scanning a small focus over the objects while monitoring the amount of scattered light and deconvoluted with Eq. (1) [25]. (c) A vertical cross section through the center of the left sphere in (a) and (b) shows the increase in resolution. The dashed lines are Gaussian fits to the data points.

Figure 4

Optical resolution of a GaP HIRES lens for different radii $R_{\max }$ of the illumination area. Red circles: measured resolutions of the HIRES lens. Solid blue line: expected theoretical resolution deduced from Eq. (1). Green squares: measured resolution of the oil immersion microscope. Dashed green line: mean measured resolution. Black arrow: expected resolution for an infinitely large illumination area.