Fluorescent Nanowire Ring Illumination for Wide-Field Far-Field Subdiffraction Imaging

Here we demonstrate an active method which pioneers in utilizing a combination of a spatial frequency shift and a Stokes frequency shift to enable wide-field far-field subdiffraction imaging. A fluorescent nanowire ring acts as a localized source and is combined with a film waveguide to produce omnidirectional illuminating evanescent waves. Benefitting from the high wave vector of illumination, the high spatial frequencies of an object can be shifted to the passband of a conventional imaging system, contributing subwavelength spatial information to the far-field image. A structure featuring 70-nm-wide slots spaced 70 nm apart has been resolved at a wavelength of 520 nm with a 0.85 numerical aperture standard objective based on this method. The versatility of this approach has been demonstrated by imaging integrated chips, Blu-ray DVDs, biological cells, and various subwavelength 2D patterns, with a viewing area of up to $1000\mu {\mathrm{m}}^2$, which is one order of magnitude larger than the previous far-field and full-field nanoscopy methods. This new resolving technique is label-free, is conveniently integrated with conventional microscopes, and can potentially become an important tool in cellular biology, the on-chip industry, as well as other fields requiring wide-field nanoscale visualization.

Figure 1

Schematic of the NWRIM. (a) Schematic of the configuration and imaging process. A hexagon array is used as an example object to show the effect of NWRIM. Under an omnidirectional evanescent illumination from the fluorescent NWR, the subdiffraction nested features of the central hexagon show up in the far-field image. (b) The basic mechanism of the NWRIM method. ${\mathbf{k}}_{\mathbf{c}}$ and ${\mathbf{k}}_{\mathbf{c}}^{\prime }$ are the TWVs of the illuminating and scattered light, respectively, under conventional illumination. ${\mathbf{k}}_{\text{eva}}$ and ${\mathbf{k}}_{\text{eva}}^{\prime }$ are the TWVs of the illuminating and scattered light, respectively, under NWR evanescent illumination.

Figure 2

Imaging of a pair of 85-nm-wide slots spaced 85 nm apart on an ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{SiO}}_2\text{−}\mathrm{Si}$ substrate. (a) Scanning electron microscopy (SEM) image of the slot pair. (b) Far-field optical microscopy image of the slot pair under NW illumination. The images under NW illumination (lower inset) and conventional illumination (upper inset) are magnified for clarity. $\mathrm{NA}=0.85$. (c) Cross-sectional intensity distribution of images under NW illumination (black curve) and conventional illumination (red curve). (d) Simulated cross-sectional view of the near field to far-field scattering process. The field intensity is shown on a log scale. (e) Far-field electromagnetic field distribution ($2\mu \mathrm{m}$ above the object plane) of the scattered light. (f) Frequency spectrum of the slot pair. The green and orange circles are centered at (0, 0) and ($-k_{\text{eva}}/2\pi$, 0), respectively. Their radius is $1/\lambda$. (g) Fourier transformation of the scattered light. (h) Simulated far-field image with an ideal aberration-free 0.85 NA imaging system. (i) Schematic of the amplitude distribution of the image for each slot (green and orange lines) and the total image (gray line). OD and ID are abbreviations for “object distance” and “image distance,” respectively.

Figure 3

(a),(b) Simulated cross-sectional field distribution when the NW works (a) passively and (b) actively on an ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{SiO}}_2\text{−}\mathrm{Si}$ substrate, respectively. Intensity is shown on a log scale. (c) Simulated field magnitude in the ${\mathrm{Al}}_2{\mathrm{O}}_3$ film along the propagation direction. (d) Image of a stair-shaped slot-pair array under NW illumination.

Figure 4

NWRIM images of various fabricated 2D patterns with subwavelength features. (a)–(c) SEM image (a) of a triangular pattern on a ${\mathrm{SiO}}_2$ film and its images under illumination from a straight NW (b) and an NWR (c), respectively. $\mathrm{NA}=0.75$. Lines perpendicular or nearly perpendicular with the NW fail to be imaged in (b). (d),(e) SEM (d) and NWRIM (e) images of a circular pattern on an ${\mathrm{SiO}}_2$ film. $\mathrm{NA}=0.75$. (f),(g) SEM (f) and NWRIM (g) images of an arbitrary ZJU pattern on an ${\mathrm{Al}}_2{\mathrm{O}}_3$ film. $\mathrm{NA}=0.85$.

Figure 5

NWRIM images of Blu-ray disks and integrated circuits. (a),(b) Conventional (a) and NWRIM (b) images of a Blu-ray disk. Boxed regions are magnified on the top insets. (c),(d) Conventional (c) and NWRIM (d) images of an integrated chip. Boxed regions are magnified in the left bottom insets, and the SEM and cross-sectional intensity distribution along the blue arrows are shown in (e) and (f). NWRIM and conventional illumination are abbreviated as NW and Conv., respectively. $\mathrm{NA}=0.85$.