Far-Field Subwavelength Resolution Imaging by Spatial Spectrum Sampling

Abstract

Imaging below the diffraction limit is always a public interest because of the restricted resolution of conventional imaging systems. To beat the limit, evanescent harmonics decaying in space must participate in the imaging process. Here, we introduce the method of spatial spectrum sampling, a novel far-field superresolution imaging method for microwave and terahertz regime. Strong dispersion and momentum conservation allow the spoof surface plasmon polaritons (SSP) structure to become a sensitive probe for spatial harmonics. This enables that the spatial information of the targets including both propagating and evanescent components, can be extracted by tuning and recording SSP in the far field. Then, the subwavelength resolution is constructed by the inversed Fourier transform of the sampled spatial spectrum. Using the modified subwavelength metallic grating as the probe, a far-field resolution of 0.17 is numerically and experimentally verified, and two-dimensional imaging ability is also fully discussed. The imaging ability and flexibility can be further optimizing the SSP structures.We are confident that our working mechanism will have great potentials in the superresolution imaging applications in the microwave and terahertz frequency range. IntroductionAchieving spatial resolution without limited by the working wavelength always attracts tremendous attention, due to the pervasive applications of imaging. However, the diffraction limit indicates the fine features smaller than half a wavelength are carried by the evanescent harmonics [1], whose amplitude exponentially decays with distance. The contribution of these harmonics is negligible when the imaging distance larger than a wavelength. To circumvent the inherent limitation, the optical elements should own ability of capturing the evanescent part of spatial information. By mechanically canning a sensitive probe, the near-field microscope (NFSM) [2-4] can well surpass the diffraction limit for thousand times, but the imaging operation is very time-wasting. To obtain superresolution images in a single shot, the concept of perfect lens is theoretically exploited by Pendry [1], utilizing the plasmonic effect in materials to enhance the evanescent waves [5,6]. Soon after this imaging method is experimentally verified at optical wavelength by plasmonic slabs, such as the noble metal [7,8] and dielectrics [9]. Although the imaging ability of such lenses can be further enhanced by other methods [10], the working distance is still subject to the near field [11,12]. A remarkable device, termed hyperlens, is capable of converting the evanescent harmonics into propagating waves through curved geometry [13][14][15][16] or gratings [17,18].In this way, superresolution pictures are formed in the far field. The hyperlens can be constructed by periodic arrangement of plasmonics materials and common dielectrics [11][12][13][14][15][16][17][18][19][20].When it comes to the longer wavelength, for example, the terahertz domain, which has great potential in non-destructive test...

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