Invited Paper
Contact microspherical nanoscopy: from fundamentals to biomedical applications V. N. Astratov1,3,a, A. V. Maslov2, A. Brettin1,3, K. F. Blanchette1, Y. E. Nesmelov1, N. I. Limberopoulos3, D. E. Walker Jr.3, and A. M. Urbas4 1
Department of Physics and Optical Science, Center for Optoelectronics and Optical Communication, University of North Carolina at Charlotte, Charlotte, NC 28223-0001, USA 2 Department of Radiophysics, University of Nizhny Novgorod, Nizhny Novgorod, Russia 3 Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, OH 45433 USA 4 Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OH 45433 USA ABSTRACT The mechanisms of super-resolution imaging by contact microspherical or microcylindrical nanoscopy remain an enigmatic question since these lenses neither have an ability to amplify the near-fields like in the case of far-field superlens, nor they have a hyperbolic dispersion similar to hyperlenses. In this work, we present results along two lines. First, we performed numerical modeling of super-resolution properties of two-dimensional (2-D) circular lens in the limit of wavelength-scale diameters, D 2, and relatively high indices of refraction, n=2. Our preliminary results on imaging point dipoles indicate that the resolution is generally close to λ/4; however on resonance with whispering gallery modes it may be slightly higher. Second, experimentally, we used actin protein filaments for the resolution quantification in microspherical nanoscopy. The critical feature of our approach is based on using arrayed cladding layer with strong localized surface plasmon resonances. This layer is used for enhancing plasmonic near-field illumination of our objects. In combination with the magnification of virtual image, this technique resulted in the lateral resolution of actin protein filaments on the order of /7. Keywords: Optical super-resolution; near-field microscopy; confocal microscopy
1. INTRODUCTION Optical super-resolution was a Holy Grail for photonics research in the past decade with the prominent examples of the optical far-field superlense [1-3], hyperlens [4, 5], Maxwell fish-eye, Eaton lenses [6-8], and superoscillatory lenses [9-11], plasmonic structured [12] and hyperstructured [13] illumination, as well as sparse imaging [14, 15]. The far-field superlenses amplify the objects’ optical near-fields and then convert them in the far-field due to gratings [1-3]. The hyperlenses provide an adiabatic compression of the wavevectors in metamaterials with hyperbolic dispersion until they become propagating waves that could project a magnified image into the far-field [4, 5]. The concepts of superlens and hyperlens became an inspiration for researchers working on super-resolution microscopy; however they have not resulted in many applications since these structures have significant losses and were very sensitive to the frequency and their electromagnetic environment. On the other hand, super-resolution imaging with dielectric microspheres emerged as a technique which can produce a profound impact on microscopy due to its simplicity, broad spectral range and a broad range of potential applications [16]. The most important recent advancements in this area include: i) proposal and demonstration of imaging through high-index (n>1.8, where n is the refractive index) liquidimmersed spheres [17-19] which made possible application of this technology for imaging biomedical structures and cells [20, 21], ii) integration with confocal microscopy [22], iii) rigorous resolution quantification methodology and demonstration of ~/7 resolution for long-period nanoplasmonic arrays [23-29], iv) developing readily attachable and movable slabs with embedded high-index spheres [23-25], ______________________________ a Email:
[email protected] Nanoscale Imaging, Sensing, and Actuation for Biomedical Applications XIV, edited by Alexander N. Cartwright, Dan V. Nicolau, Proc. of SPIE Vol. 10077, 100770S © 2017 SPIE · CCC code: 1605-7422/17/$18 · doi: 10.1117/12.2251869 Proc. of SPIE Vol. 10077 100770S-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/01/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
v) surface scanning functionality achieved by locomotion of microspheres [30] and by their integration with the tip of AFM probe [31]. Due to these developments, the practical applications of this technology are within a reach; however the enigmatic question is why the super-resolution takes place with these lenses since they neither have an ability to amplify the near-fields like in the case of far-field superlens [1-3], nor they have a hyperbolic dispersion similar to hyperlenses [4, 5]. Based on analogy with a solid immersion lenses [32, 33], it can be argued that the maximal resolution should be ~/(2NA), where is the illumination wavelength and NA=nosin is the numerical aperture determined by the index (no) of the object-space and by a half-angle () of the collection cone. This leads to a top limit of classical diffraction-limited resolution ~/4 assuming the maximal index of n=2. In this regard, the experimentally observed resolution of ~/7 [23-29] clearly points towards mechanisms going beyond the classical far-field optics. A particularly important issue is related to a possibility of coherent imaging determined by excitation of coupled modes in nanoscale objects. We theoretically showed that this regime can result in distorted images of subwavelength objects and that some features (nodes of antisymmetric modes) can be seen with an unprecedented resolution [34, 35]. Such coherent effects have been reported for imaging dielectric nanospheres [36]. Another example is represented by mapping of spatially localized photonic nanojets from phased diffraction gratings [37]. These effects can be especially misleading for the resolution quantification in subwavelength-periodic structures, such as Blue-ray disk [29]. In previous theoretical modeling work [38-40] the mechanisms of imaging incoherent pointdipoles have been examined for a cylindrical lens with the moderate indices 1.3
