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Prof. Xiang Zhang's Laboratory at UC Berkeley |
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Optical Silver Superlensing Objective To experimentally demonstrate the sub-diffraction-limit by enhancing evanescent waves using a thin silver slab. Background material Conventional optical imaging systems such as optical microscopes have ultimate resolution limit called the diffraction limit; the finest feature size that can be resolved is on the order of incident wavelength. This is because far-field imaging systems cannot retrieve evanescent waves which contain sub-wavelength details of an object but decays within the near field. Utilizing the physics of surface plasmons, a thin silver slab, a superlens, is able to amplify and restore the evanescent field and image with sub-diffraction-limited resolution. Results We designed and fabricated a superlens structure for imaging experiment(Fig.1). A sub-wavelength grating object with 120nm period is inscribed in a Cr(50nm) layer by Focused Ion Beam milling( FEI Strata 201). Then a 40nm PMMA(Poly methyl metacrylate) spacing layer is planarized on which the superlens, a 35nm silver, is E-beam( SLOAN) evaporated. Conventional i-line(365nm) illumination is used for imaging by photolithography and negative photoresist(NFR105G JSR inc.) is directly spun on silver surface to record the reconstructed image. A control structure(Fig.1(b)) is also designed and fabricated where the silver layer is replaced by another PMMA layer.
The superlens imaging results show that the nano-wire object with 120nm period is clearly resolved (Fig.2(a)). The height modulation of the recorded image is easily observed in cross-section plot (Fig.2(b)) and the Fourier analysis showing sharp peaks of 126nm±7nm(Fig.2(c)) further confirms that the imaged period is indeed that of the object. These results are strong indications that the evanescent field of the object is restored and recorded as an image by superlensing. With our design, 60nm half-pitch object has been successfully imaged with 364nm incident wavelength, which is l/6 resolution, well beyond the diffraction limit.
The control experiment result evidently supports the role of silver as a superlens. Without the silver, no image contrast is recorded by photoresist (Fig.2(d)) regardless the optimization of the exposure and development processes. Its cross section (Fig.2(e)) and Fourier spectrum analysis(Fig.2(f)) also show no evidences of periodic structure imaged by the control sample.
In addition to the periodic gratings, an arbitrary object, " NANO" was also imaged. The image of the object (Fig.3(a)) captured by the superlens (Fig.3(b)) clearly shows far better resolution with line width of 89nm (Fig.3(d)) while that of the control experiment (Fig.3(c)) still resulted in diffraction limited image with line width of 360nm (Fig.3(e)), which is comparable to the exposure wavelength (365nm).
Reference Nicholas Fang, Hyesog Lee, Cheng Sun, Xiang Zhang, "Sub-Diffraction-Limited Optical Imaging with a Silver Superlens", Science, Vol 308, 2005,pp534-537 Far-field optical superlens Objective Background Results We experimentally demonstrated one-dimensional sub-diffraction-limited imaging by the FSL. We showed that a FSL could image a sub-wavelength object consisting of two 50nm wide lines separated by 70nm working at 377nm wavelength..
Fig. 2 Top: Schematic of the silver FSL. The computationally optimized geometry of the FSL is a=35 nm, b =d =55 nm, c=100 nm, e=45 nm, and f =105 nm. Bottom: Calculated OTF of the optimized FSL under p-polarized incident light with vacuum wavelength of 377 nm and grating wave-vector kL=2.5k0. The dashed red and blue curves represent the enhanced evanescent waves. Solid curves represent the propagating waves shifted from the evanescent waves. No wave-vector mixing occurs in the shaded range (2.8k0<|k|<4k0) that ensures unique imaging resolution up to 4k0. Fig. 3 Far-field imaging of a pair of nanowires. (a) Scanning electron microscopy image of an object nanowire pair with 50 nm wide slit and 70 nm gap inscribed by focused ion beam on a 40 nm thick Cr film on the quartz substrate. (b) Diffraction-limited image from a conventional optical microscope cannot resolve the two nanowires (NA=1.4, l0=377 nm). (c) FSL image that resolves the sub-diffraction objects due to strong evanescent enhancement via surface plasmon excitation at FSL. (d) The averaged cross-section image profiles from (b) and (c). Two-dimensional sub-diffraction-limited images can be theoretically reconstructed by rotating a new metamaterial far-field superlens. The metamaterial far-field superlens, composed of a metal-dielectric multilayer and a one-dimensional sub-wavelength grating, can work over a broad range of visible wavelengths intrinsically. We numerically demonstrated that the metamaterial far-field superlens could image object with feature size down to λ/4 at wavelength λ = 405nm, 500nm, and 600nm.
Fig. 4 Numerical demonstration of 2D imaging with sub-diffraction-limited resolution by the metamaterial FSL. The object is composed of eight circles with radius of 40 nm. H-field is 1 within each circle and is 0 anywhere else. The center-to-center distances between the eight circles range from 100 to 1059 nm. The working wavelength is 405nm. For a conventional optical microscope with NA=1.5, the sub-diffraction-limited features are not distinguishable; while the eight circular objects are clearly resolved in the reconstructed image utilizing metamaterial FSL. [1] Stéphane Durant, Zhaowei Liu, Jennifer M. Steele, and Xiang Zhang, "Theory of the transmission properties of an optical far-field superlens for imaging beyond the diffraction limit", J. Opt. Soc. Am. B, Vol. 23, No. 11, pp2383-, 2392, 2006 [2] Zhaowei Liu, Stéphane Durant, Hyesog Lee, Yuri Pikus, Nicolas Fang, Yi Xiong, Cheng Sun, and Xiang Zhang, "Far-field Optical Superlens", Nano Letters,7, 403, 2007 [3] Zhaowei Liu, Stéphane Durant, Hyesog Lee, Yuri Pikus, Yi Xiong, Cheng Sun and Xiang Zhang, "Experimental studies of far-field superlens for sub-diffractional optical imaging", Optics Express, 15, 6947, 2007 [4] Yi Xiong, Zhaowei Liu, Stéphane Durant, Hyesog Lee, Cheng Sun, and Xiang Zhang, "Tuning the far-field superlens: from UV to visible", Optics Express, 15, 7095, 2007 [5] Yi Xiong, Zhaowei Liu, Cheng Sun, and Xiang Zhang, "Two-Dimensional Imaging by Far-Field Superlens at Visible Wavelengths", Nano Letters, 7, 3360, 2007 [6] Hyesog Lee, Zhaowei Liu, Yi Xiong, Cheng Sun, Xiang Zhang, "Design, fabrication and characterization of a Far-field Superlens", Solid State Communication, 146, 202, 2008
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