Article Cite This: ACS Photonics 2018, 5, 834−843
Three-Dimensional Resolvable Plasmonic Concentric Compound Lens: Approaching the Axial Resolution from Microscale to Nanoscale Kai-Hao Chang,† Yen-Chun Chen,‡ Wen-Hao Chang,‡ and Po-Tsung Lee*,† †
Department of Photonics, College of Electrical and Computer Engineering, and ‡Department of Electrophysics, College of Science, National Chiao Tung University, Hsinchu 300, Taiwan S Supporting Information *
ABSTRACT: We propose the design and working principle of a plasmonic concentric compound lens (CCL) comprising inner circular nanoslits and outer circular nanogrooves. Dualwavelength operations have been achieved for 650 and 750 nm at nanoscale and microscale focal lengths along with their depth of focus (DOF). By tuning the arrangement of nanogrooves, the axial resolution can be modulated and the narrowest DOF is achieved by a design of gradually decreasing groove width. For the ultrahigh tunability of axial resolution, DOF over 400 nm for both working wavelengths is also achieved. We not only developed an approximate-perturbed-focus model for explaining the performance of DOF but also found an extraordinary way to improve the resolution. The enhanced resonance of central disk as nannoantenna in CCL also has great influence on nanofocusing with different deigns of outer nanogrooves. This work provides new sight of focusing ability governed by the general optical nanogrooves. The optimized CCL shows excellent focusing performance with a lateral resolution down to 0.32λ (λ = 650 nm), which is the best resolving ability achieved thus far in the near field region with a long focal length up to 500 nm. KEYWORDS: plasmonic lens, subwavelength focusing, axial resolution, depth of focus, dual-wavelength
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storage but also for biological engineering. Recently, the multilayered optical near-field recording disc has been proposed for achieving high-density image storages.11 The improvement in the axial resolution is thus essential to obtain well resolved target images, such that the optical signal can be extracted from the layer-by-layer recording pattern. In the context of biological imaging, the images of blood and cancer cells have been resolved with an ordinary lens, whose axial resolutions are limited within several micrometers.12 To develop novel biological engineering techniques, the subwavelength DOF of focal spot in axial systems should be small enough to resolve the Cajal body (0.3−1 μm), paraspeckle (0.2−1 μm), and the mitochondria (0.5−1.0 μm) inside a cell. The PALM techniques for three-dimensional subwavelength imaging have been developed with a resolution below 100 nm.13 Furthermore, a super powerful method of stimulated emission depletion can narrow down the fluorescence image by using two laser beams.14 However, the supporting objective lens suffers the diffraction limit (λ/2n sin(α) of lateral resolution, and λ/(n sin(α))2 of axial resolution, where α and n denote the aperture angle of lens and refractive index) since the mixed signal collected from dye molecules is indistinguishable for different depths at nanometer scales. Hence, the plasmonic lens
o explore science and structures at nanoscale, subwavelength imaging techniques surmounting the diffraction limit have been widely studied by many methods, such as, mass spectral imaging,1 photoactivated localization microscopy (PALM),2 near-field scanning nanoscopy,3 and techniques utilizing metamaterial lens systems.4 Among these methods, the PALM has been well developed and applied to many biological applications for detecting ultrasmall cells (
