Microsphere nanoscope and experimental imaging performance

Figure 1 illustrates the schematic of a transmission mode white-light microsphere nanoscope. The microspheres are placed on the top of the object surface by self-assembly13. A halogen lamp with a peak wavelength of 600 nm is used as the white-light illumination source. The microsphere superlenses collect the underlying near-field object information, magnify it (forming virtual images which keep the same orientation as the objects in the far-field) and pick it up by a conventional ×80 objective lens (numerical aperture NA=0.9, Olympus MDPlan). In the experiments, gratings consisting of 360-nm-wide lines, spaced 130 nm apart, were imaged using 4.74-μm-diameter microspheres (Fig. 2a). The virtual image plane was 2.5 μm beneath the substrate surface and inside substrate. As can be seen from Figure 2a, only those lines with particles on top of them have been resolved. The lines without particles on top mix together and form a bright spot, which cannot be directly resolved by the optical microscope because of the diffraction limit (for the lowest visible wavelength λ=400 nm, the best diffraction-limited resolution is estimated to be 215 nm in air using the vector theory of Richards and Wolf14, and to be 152 nm by taking the solid immersion effect of a particle into account. For the main peak of a white-light source at λ=600 nm, the limits are 333 nm in air and 228 nm with solid immersion effect, respectively. Here, one should also note that the focal planes for lines with and without particles on top are different). The magnified image in Figure 2a corresponds to a ×4.17 magnification factor. Figure 2b shows a fishnet gold-coated anodic aluminium oxide (AAO) membrane imaged with 4.74-μm-diameter microspheres. The pores are 50 nm in diameter and spaced 50 nm apart. As it can be seen, the microsphere nanoscope resolves these tiny pores that are well beyond the diffraction limit, giving a resolution of between λ/8 (λ=400 nm) and λ/14 (λ=750 nm) in the visible spectrum range. It is important to note that the magnification in this case is around ×8, which is almost two times of that in the grating samples as shown in Figure 2a. This implies that the performance of microsphere superlens is affected by the near-field interaction of the sphere and the substrate. In our experiment, we have confirmed that the gold coating layer on the AAO surface not only enhanced the resolving power but also increased the magnification factor of the microsphere superlens. As self-assembled particles are easy to spread over a large surface area and meanwhile each particle can work as a superlens, the images produced by each particle can be stitched together to form a large image. These are the cases seen in Figure 2a,b, in which a hexagonal array of particles functions as an array of superlens covering a large area.

Figure 1: Experimental configuration of white-light microsphere nanoscope with λ/8–λ/14 imaging resolution. Schematic of the transmission mode microsphere superlens integrated with a classical optical microscope. The spheres collect the near-field object information and form virtual images that can be captured by the conventional lens. Full size image

Figure 2: Microsphere superlens imaging in transmission mode. (a) Microsphere superlens imaging of 360-nm-wide lines spaced 130 nm apart (top left image taken by scanning electron microscope (SEM)), the optical nanoscope (ON) image (top right image) shows that the lines are clearly resolved. (b) A gold-coated fishnet AAO sample imaged with a microsphere (a=2.37 μm, borders of two spheres are shown by white lines) superlens. The nanoscope clearly resolves the pores that are 50 nm in diameter and spaced 50 nm apart (bottom left SEM image). The size of the optical image between the pores within the image plane is 400 nm (bottom right ON image). It corresponds to a magnification factor of M≈8. Scale bar, 5 μm. Full size image

Our previous calculations reveal the fact that the Poynting vector of the radiation reflected by the surface transfers through the particle15, which permits the formation of an image in the reflection mode as well. Figure 3a demonstrates a Blu-ray DVD disk (200-nm-wide lines separated 100 nm apart) imaged with 4.74-μm-diameter microspheres in the reflection mode using the halogen light illumination. The subdiffraction-limited lines are clearly observed. Figure 3b shows another example of reflection mode imaging of a star structure made on SbTe DVD disk. The complex shape of the star, including the 90 nm corners of the star, was clearly resolved by the microsphere superlens. Further experiments have confirmed that complex shape structures can also be well imaged in the transmission mode and that both the transmission and reflection modes can achieve 50 nm resolution. Indeed, the microsphere nanoscope has proven its practicability and versatility in nanoimaging of various samples.

Figure 3: Microsphere nanoscope reflection mode imaging. (a) Microsphere superlens reflection mode imaging of a commercial Blu-ray DVD disk. The 100-μm-thick transparent protection layer of the disk was peeled off before using the microsphere (a=2.37 μm). The subdiffraction-limited 100 nm lines (top left SEM image) are resolved by the microsphere superlens (top right ON image). (b) Reflection mode imaging of a star structure made on GeSbTe thin film for DVD disk (bottom left SEM image). The complex shape of the star including 90 nm corner was clearly imaged (bottom right ON image). Scale bar: SEM (500 nm), ON (5 μm). Full size image

Experimental comparison with SILs imaging

It is important to mention that we have also conducted comparison experiments using two SILs (Supplementary Fig. S1), and confirmed that none of them can be used to resolve our samples with feature sizes between 50 and 130 nm (Supplementary Fig. S2). Immersion of samples 'into' solids, attained by positioning the samples in close contact with the flat bottom surface of the lens, provides a means to access a shorter working wavelength in lens materials, and thus higher resolution beyond that in air. The resolution limit of the SILs used in our experiments was ∼152 nm, which makes it impossible to resolve the 100 nm objects.

The imaging mechanism of the microsphere nanoscope

In principle, the imaging resolution and magnification of the microsphere superlenses are fundamentally related to their focus properties16. It is well known that small spheres can generate 'photonic nanojets' with super-resolution foci17, less well known is that such super-resolution foci are only achievable for a narrow window of (n, q) parameters, where n is the refractive index of the sphere and q is the size parameter defined as q=2πa/λ, according to Mie theory16. Figure 4a shows the calculated super-resolution window for different n and q parameters for spheres immersed in air. The y axis was calculated as (focus spot size−Rayleigh diffraction limit)/radius, which we name as super-resolution strength. For the n=1.46 spheres used in this study, super-resolution occurs for q<70, which corresponds to smaller than 9.0-μm-diameter spheres at a wavelength of λ=400 nm. From our experiments, it was verified that 10 and 50 μm spheres are not successful in 100-nm-resolution imaging tests, whereas 3.0 μm spheres produce clear 50-nm-resolution images as achieved by the 4.74 μm spheres. We also examined the use of 1 μm spheres for imaging. It is found that because of the small-view windows of such particles, high-resolution imaging was not successful. Therefore, the limit for 1 μm sphere is a practical conclusion rather than a theoretical one. The practical size window for n=1.46 microspheres is recommended as 2 μm<diameter<9 μm for 50-nm-resolution imaging. From Figure 4a, it can also be seen that refractive index has a strong effect on super-resolution foci; with n=1.8, the size window for super-resolution extends up to q∼250, which implies that particles as big as 30 μm could be used for nanoimaging. This would facilitate the experiments because of wider view windows offered by bigger particles. Moreover, one can see that the super-resolution strength is maximized at n=1.8. When refractive index increases further to n=2.0, the super-resolution strength reduces and super-resolution window shrinks, making it undesirable to use n>1.8 high-index materials for nanoimaging in our technique. On the contrary, high-index (n>1.8) materials are important for SILs as their imaging resolution is determined by the refractive index of lens materials because of the solid immersion mechanism.

Figure 4: Super-resolution foci and virtual magnification factor analyses. (a) Super-resolution strength, defined as (focus spot size−Rayleigh limit)/radius, as a function of size parameter q for different refractive index particles. The inset shows q up to 300 for n=1.46. (b) The intensity distributions calculated for SIL (left image, height H=a(1+n−1)), sphere (middle image) and particle on surface(right image) of a 40-nm-thick gold film for the sphere with radius a=2.37 μm and refractive index n=1.46 at the wavelength λ=600 nm. (c) Full width at half maximum of foci for SIL (blue solid), sphere (red dot) and sphere on substrate (green solid). (d) Virtual image magnification versus particle size for sphere with n=1.46 at the wavelength λ=600 nm. Full size image

Figure 4b,c compares the |E|2 intensity distribution for SIL, sphere and particle on surface calculated with the same parameters, that is, n=1.46, diameter=4.74 μm and λ=600 nm. Here, one important difference between SIL and sphere was demonstrated: a super-resolution focus outside of sphere and a diffraction-limited focus for the same-diameter SIL. Truncating of sphere into SIL causes the loss of super-resolution focus, and diffraction-limited spot of SIL makes it impossible to resolve below 100 nm objects. Super-resolution foci are the key requirement of our technique. With the presence of a substrate, the focus at particle–substrate contact region generally becomes sharper. This is evidenced by our particle on surface calculation (Fig. 4b,c). Such effects could enhance the imaging resolution according to the reciprocity principle18.

Magnification factor of the microsphere nanoscope