Optical Silver Superlens Imaging Below the Diffraction Limit
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0919-J04-01
Optical Silver Superlens Imaging Below the Diffraction Limit Hyesog Lee1, Yi Xiong1, Nicholas Fang2, Werayut Srituravanich1, Stephane Durant1, Muralidhar Ambati1, Cheng Sun1, and Xiang Zhang1 1 UC Berkeley, Berkeley, 94720 2 University of Illinois at Urbana-Champaign, Urbana, 61801
ABSTRACT Conventional optical imaging systems cannot resolve the features smaller than approximately half the size of the working wavelength, called the diffraction limit. The superlens theory predicts that a flat lens made of an ideal material with negative permittivity and/or permeability is able to resolve features much smaller than working wavelength through the restoration of evanescent waves[1]. We experimentally demonstrated the superlens concept for the first time using a thin silver slab in a quasi-static regime; a 60nm half-pitch object was imaged with λ=365nm illumination wavelength, λ/6 resolution[3], and the imaging of 50nm half-pitch object under the same light source, λ/7, was also reported[4]. Here, we present mainly experimental studies of near-field optical superlens imaging. INTRODUCTION In conventional optical imaging systems such as microscopes, the propagating waves scattered by the object are collected in the far field to form an image. Such imaging method suffers from the fundamental resolution limit called the diffraction limit, about half of the working wavelength(λ/2), mainly due to the loss of evanescent waves, which carries subwavelength details of an object but decays away in the nearfield. In order to overcome such loss, contact mask imaging[12] is one of the early efforts to minimize the decay. Immersion microscopy improves the resolution by increasing the index(n) of surrounding medium(λ/2n) but there's a limited availability of high index materials in nature. Near-field scanning optical microscopy(NSOM) uses a tapered optical fiber in the vicinity of an object but suffers from slow serial scanning[13]. Recently, Pendry postulated the 'Perfect Lens' that can produce diffraction-free images[1]; a flat slab lens made of 'left-handed material'(LHM)[2], a material that exhibits negative permittivity and negative permeability simultaneously. Such perfect imaging is possible because an ideal LHM not only focuses propagating waves through negative refraction[2] but the evanescent waves are also amplified through the flat slab restoring every detail of subwavelength information of an object. However, no true optical LHM has been demonstrated sor far for two main reasons; simultaneous negativity of permittivity and permeability do not happen naturally – magnetic activity of natural materials diminish in high frequency and magnetic responses at Terahertz regime using metamaterials are only recently been reported[5,6] – and current nano-manufacturing technology is still not mature enough to produce metamaterials for optical frequency. For experimental demonstration of Perfect Lens concept, such obstacles can be circumvented by working in quasi-static regime with a thin metal slab that shows negative
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