Focusing and Scanning Microscopy with Propagating Surface Plasmons

Abstract

Here we demonstrate a novel surface plasmon polariton (SPP) microscope which is capable of imaging below the optical diffraction limit. A plasmonic lens, generated through phase-structured illumination, focuses SPPs down to their diffraction limit and scans the focus with steps as small as 10 nm. This plasmonic lens is implemented on a metallic nanostructure consisting of alternating hole array gratings and bare metal arenas. We use subwavelength scattering holes placed within the bare metal arenas to determine the resolution of our microscope. The resolution depends on the size of the scanning SPP focus. This novel technique has the potential for biomedical imaging microscopy, surface biology, and functionalization chemistry. DOI: 10.1103/PhysRevLett.110.266804 PACS numbers: 73.20.Mf, 68.37.Àd, 87.64.MÀ In conventional microscopy, features smaller than about half a wavelength cannot be resolved due to the diffraction limit of far-field optics. As the basic constituents of cells and nanotechnological devices are smaller than the wavelength of visible light, a variety of methods [1,2], including scanning near-field nanoprobes [3][4][5], have been developed for imaging below the diffraction limit. Current far-field methods exploit either the high refractive index of immersion media [6] or the photophysics of molecules [7][8][9][10]] to obtain high resolution images. Plasmonic structures are envisioned to enable surface-bound imaging with a resolution much below the optical wavelength, and indeed stationary hot spots [11][12][13] of subwavelength size can be generated from plasmons propagating in such structures. Subwavelength imaging with surface plasmons requires a freely scanned plasmonic focus, which has not been demonstrated up to now.The novel concept of plasmonic microscopy [14] via the excitation of evanescent waves on metallic nanostructures [15][16][17] offers not only evanescent out-of-plane resolution (as total internal reflection microscopy [18]) but also a large potential for in-plane superresolution: For a fixed light frequency the wavelength of surface plasmon polaritons (SPPs) is shorter than that of freely propagating photons [19][20][21]. The main barrier for plasmonic microscopy is the impossibility to use plasmon optics for detection: The readout is always optical. Wide-field plasmonic excitation (unfocused SPPs) yields an image that is limited in resolution by the detection optics.Enhancing the resolution beyond the limit imposed by the detection optics, or superresolution, is achievable provided that plasmon optics are used to provide tightly confined excitation (e.g., a SPP diffraction limited focus) in a scannable way. Recent theoretical [22][23][24] and experimental [25,26] developments have shown that plasmonic phase structuring might have the potential for 2D surface microscopy. Nevertheless, due to intrinsic problems of these techniques (the degree of complexity, resolution, field of view, or speed), superresolution plasmonic microscopy has yet to be implemented.We show here the fir...

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