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5-1-2007 Far-field ups erlens - Optical nanoscope Evgenii Narimanov Purdue University, [email protected]

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Narimanov, Evgenii, "Far-field - Optical nanoscope" (2007). Birck and NCN Publications. Paper 270. https://docs.lib.purdue.edu/nanopub/270

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FAR-FIELD SUPERLENS Optical Nanoscope

Breaking the diffraction limit for the resolution of conventional optical systems has long been the primary aim of optical imaging. The recently demonstrated far-fi eld optical superlens is paving the way to this elusive goal.

Evgenii E. Narimanov is at Birck Nanotechnology Center, Purdue z University, 1205 West State Street, West Lafayette, Detector Indiana 47907-2057. e-mail: [email protected] Zero Far field First order order nvented most probably by Galileo Galilei, named by Giovanni Faber, I improved and brought to the attention of biologists by Robert Hooke and Anton van Leeuwenhoek, and perfected Decay ×100 Enhancement objective by Ernst Abbe, the microscope FSL revolutionized life sciences and was Amplitude a key instrument to the advancement of knowledge in many areas. It is FSL perhaps the most important single Subwavelength object piece of scientific apparatus in the history of mankind. Even today, nearly Near field Specimen 400 years since its invention, the remains indispensable, as focused light is the only way to examine Figure 1 The schematics of the FSL. The evanescent are enhanced in the metal layer, owing to their living cells non-invasively. resonant coupling to surface plasmons, and then scattered into the propagating band by the subwavelength grating As first demonstrated by Abbe in at the top surface. The device provides an improvement on point-to-point imaging techniques, such as near-fi eld 18731, the resolution of a conventional scanning optical , but still requires close proximity between the lens and the object. Reproduced with optical microscope is constrained by the permission from ref. 8, copyright (2007) ACS. wavelength of light — something which is generally referred to as the ‘diffraction limit’. Although information about small (subwavelength) features of the object are However, the illusive goal of an example, the material losses) severely present in so-called scattered evanescent optical ‘nanoscope’ — a device capable limits the imaging distance, and the waves, these waves decay exponentially of direct imaging of subwavelength planar superlens appears ‘near-sighted’7. with distance and are only detectable structures (as opposed to point-by-point To deliver the subwavelength in the near field. In the far field, the scanning) — was suddenly brought to information recovered by the superlens, loss of these evanescent waves and the the realm of device engineering when this information fi rst needs to be information that they contain, precludes John Pendry proposed the concept imprinted onto the propagating (rather reconstructing an image of an object of the ‘superlens’5. Such a superlens than exponentially decaying) waves. It with a resolution better than half the is capable of enhancing evanescent is this critical ingredient for the creation incident wavelength. waves and thus leading to the recovery of a future optical nanoscope that was Although near-field scanning optical of the information they carry. This recently introduced by the researchers microscopy2 can detect evanescent enhancement originates from the from Berkeley in their ‘far-fi eld fields through an optical probe placed resonant coupling of the evanescent optical superlens’8. in the direct vicinity of the object, waves to surface plasmons, and when the The far-field superlens (FSL) the approach has limitations. The decay of evanescent waves in free space is proposed in ref. 8, is a slab superlens information is collected on a ‘point- compensated for, a nearly perfect image with periodic corrugations on its outer by-point’ basis, which is a slow process can be recovered6. surface (Fig. 1). The FSL used in the and does not allow direct projection of However, this balance between the experiments of ref. 8, consists of a real-time images. Similar issues limit decay of evanescent waves in free space 34-nm-thick silver slab surrounded the usefulness of other subwavelength and the resonant enhancement of surface by polymethyl methacrylate, beneath imaging methods, such as those based excitations is also the Achilles heel of the 2 layers of periodic silver corrugations, on nonlinear optics3 and stimulated superlens. Any limit on the efficiency of each with a height of 55 nm and a emission depletion4. the surface-excitation enhancements (for periodicity of 100 nm (one immediately

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on top of the slab surface, and the other above, shifted by half a period). The from the object first couples to the surface plasmons supported by the superlens, resulting in Linewidth:50 nm a substantial field enhancement — just Gap: 70 nm as in an ‘ordinary’ (planar) superlens. However, the enhanced evanescent components are then scattered by the surface corrugations. With the proper design of corrugation period, the enhanced evanescent components can Control be converted by the surface corrugations without FSL into a propagating that is able to reach the far field. To test the performance of their far- fi eld superlens, Zhaowei Liu and colleagues imaged several subwavelength patterns with diff erent geometries — from one- Control with dimensional arrays formed by chromium polarized (s-pol.) nanowires to pairs of lines. Figure 2a shows an example of a line pair — two lines, each 50 nm wide and separated by a 70-nm gap. As expected, optical imaging with a conventional optical microscope cannot resolve the line pair owing to FSL imaging the diff raction limit (Fig. 2b). Neither (p- and s-polarized) can imaging with s-polarized light as it does not couple to surface plasmons in the metal slab, and therefore is not enhanced (Fig. 2c). In contrast, under Figure 2 Far-fi eld imaging of a subwavelength pattern. a, Scanning electron microscope image of a pair of p-, the evanescent waves from nanowires with a 50-nm-wide slit separated by a 70-nm gap. When illuminated by 377-nm-wavelength light, the the object are strongly enhanced by the structure is not resolved by b, a conventional microscope or by c, the FSL operating under s-polarization. However excitation of surface plasmons in the silver d, using FSL with both p- and s-polarized light clearly resolves the object. The length of the scale bar is 200 nm. superlens, and are subsequently converted Reproduced with permission from ref. 8, copyright (2007) ACS. into measurable propagating waves in the far fi eld. Combining the evanescent components from p-polarized illumination with the propagating components from Of much greater importance is the subwavelength objects can now be optically s-polarized illumination, the pair of other limitation of the FSL: although the magnifi ed and projected into the far fi eld. lines of 50 nm width can be clearly image is projected into the far field, the Although the ultimate problem of optical imaged (Fig. 2d). lens itself must be in the near vicinity of imaging — the diffi culties in devising Th e essentially one-dimensional the object. But as opposed to scanning a nanoscope that can operate at a long pattern of surface corrugations in the FSL point-to-point techniques, such as distance from the object — is not yet fully demonstrated in ref. 8, can only lead to near-field scanning optical microscopy, overcome, Zhaowei Liu and colleagues have light scattering in one direction. As the the FSL has several critical advantages, brought us a step closer to that goal. scattering on the surface corrugations especially for applications in biomedical is critical for the performance of the sensing and imaging. Being essentially References FSL, Liu and colleagues focused on a projection lens, the FSL does not 1. Abbe, E. Archiv für Mikroskopische Anat. Entwicklungsmech. 9, imaging subwavelength one-dimensional require a time-consuming scanning 413–468 (1873). 2. Betzig, E., Trautman, J. K., Harris, T. D., Weiner, J. S. & structures. However, as they pointed process, enables fast and dynamic Kostelak, R. K. Science 251, 1468–1470 (1991). out, this is not a fundamental problem. imaging — especially for biologically 3. Hell, S. W. & Wichmann, J. Opt. Lett. 19, 780–782 (1994). With a more complicated (and essentially related applications — and can operate at 4. Hell, S. W. Nature Biotechnol. 21, 1347–1355 (2003). two-dimensional) pattern of surface low illumination intensities. 5. Pendry, J. B. Phys. Rev. Lett. 85, 3966–3969 (2000). 6. Fang, N., Lee, H. S., Sun, C. & Zhang, X. Science 308, corrugations, the scattering of the With their recent work, Liu and 534–537 (2005). evanescent to propagating waves can be colleagues established an important 7. Podolskiy, V. A. & Narimanov, E. E. Opt. Lett. 30, 75–77 (2005). achieved in all directions. milestone towards the optical nanoscope: 8. Liu, Z. et al. Nano Lett. 7, 403–408 (2007).

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