Far-Field Optical Microscopy with a Nanometer-Scale Resolution Based on the In-Plane Image Magnification by Surface Plasmon Polaritons

A new far-field optical microscopy capable of reaching nanometer-scale resolution is developed using the in-plane image magnification by surface plasmon polaritons. This approach is based on the optical properties of a metal-dielectric interface that may provide extremely large values of the effective refractive index $n_{\mathrm{e}\mathrm{f}\mathrm{f}}$ up to ${10}^3$ as seen by surface polaritons, and thus the diffraction limited resolution can reach nanometer-scale values of $\lambda /2n_{\mathrm{e}\mathrm{f}\mathrm{f}}$. The experimental realization of the microscope has demonstrated the optical resolution better than 60 nm at 515 nm illumination wavelength.

Figure 1

(a) SPP-based far-field microscope: SPPs are excited by laser light and propagate inside a parabolically shaped droplet. Placing an object near the focus of a parabola produces a magnified image in the metal plane, which is viewed from the top with a conventional optical microscope. (b) SPP dispersion on the gold-glycerine interface and the positions of the Ar-ion laser lines. Also shown are the guided optical modes inside the thin layer of glycerine.

Figure 2

(a) Image demagnification by a SPP mirror. (b) Ray optics is used to show that the pinholes $A$ and $B$ in (a) artificially created inside a glycerine droplet are imaged into $a$ and $b$, which are located near the geometrically defined position of the focus [shown by the green dot in (b)]. The scale bar is $7\mu \mathrm{m}$.

Figure 3

(a), (c), (e) 2D images of a $30\times 30\mu {\mathrm{m}}^2$ rectangular nanohole array with 500 nm hole spacing, obtained with SPPs excited with ${\lambda }_0=502\mathrm{n}\mathrm{m}$ using various glycerine droplets as SPP optical elements. The scale bars are $30\mu \mathrm{m}$. (b), (d), (f) Image reconstructions via ray tracing of SPP in droplets. Individual nanoholes of the array are shown as individual dotes in model images. The cross section (g) through the row of nanoholes in (e) shows the edge resolution of at least 100 nm. (h) The spatial resolution is lost with ${\lambda }_0=458\mathrm{n}\mathrm{m}$ at which SPPs are not excited.

Figure 4

Resolution test of the microscope. The $30\times 30\mu {\mathrm{m}}^2$ array of triplet nanoholes (100 nm hole diameter, 40 nm distance between the hole edges in the triplet, 500 nm period) shown in (c) is imaged using a SPP optical element formed by a glycerine droplet (a). (b) The image of the triplet array obtained at ${\lambda }_0=515\mathrm{n}\mathrm{m}$ and (f) its ray-tracing reconstruction. The scale bar is $30\mu \mathrm{m}$ in (a) and $5\mu \mathrm{m}$ in (b). (d) Magnified image shows subwavelength triplet structure. (e) The cross section through the line of nearest nanoholes.