Nanoscale PAPER Cite this: Nanoscale, 2017, 9, 14907 Received 23rd May 2017, Accepted 3rd September 2017 DOI: 10.1039/c7nr03654j rsc.li/nanoscale Localized plasmonic structured illumination microscopy with an optically trapped microlens† Anna Bezryadina, a Jinxing Li, b Junxiang Zhao, a Alefia Kothambawala, a Joseph Ponsetto, a Eric Huang, c Joseph Wang b and Zhaowei Liu* a Localized plasmonic structured illumination microscopy (LPSIM) is a recently developed super resolution technique that demonstrates immense potential via arrays of localized plasmonic antennas. Microlens microscopy represents another distinct approach for improving resolution by introducing a spherical lens with a large refractive index to boost the effective numerical aperture of the imaging system. In this paper, we bridge together the LPSIM and optically trapped spherical microlenses, for the first time, to demon- strate a new super resolution technique for surface imaging. By trapping and moving polystyrene and TiO 2 microspheres with optical tweezers on top of a LPSIM substrate, the new imaging system has achieved a higher NA and improved resolution. Introduction The development of the optical microscope revolutionized life and materials science; however, the resolution of conventional microscopes is limited to the half-wavelength scale (200–300 nm) due to the diffractive nature of the illuminating light. In modern medicine and biology there is a strong demand for surface imaging of various objects and processes at size scales far below the diffraction limit of visible light. Recent super resolution microscopy techniques, such as stimu- lated emission depletion microscopy (STED), 1–3 stochastic optical reconstruction microscopy (STORM), 4,5 and photo- activated localization microscopy (PALM) 6 have been demon- strated and attracted much attention. These methods have greatly improved the spatial resolution to sub-50 nm scales in all three dimensions, but at the cost of other imaging capabili- ties such as imaging speed, field of view and simplicity of the system, as well as phototoxicity. Structured illumination microscopy (SIM) 7–11 can achieve a reasonable speed and wide field of view simultaneously, but its resolution is limited to ∼2 times better than the diffraction limit, i.e. about 84 nm when in combination with total internal reflection fluorescence (TIRF) and the best available ultrahigh NA objective. 12 Recently, plasmonics has been introduced to the field of SIM to further improve its resolution. For instance, plasmonic structure illumination microscopy (PSIM) utilizes the surface plasmon interference to replace the traditional projected light pattern, so that the resolution improvement can surpass 2 times that of the diffraction limit. 13–16 Localized plasmon structured illumination microscopy (LPSIM) employs near- field excitation from the localized surface plasmons of fine periodic structures. 17,18 With this unique super resolution technique and 1.2NA objective, wide-field surface imaging with resolution down to 75 nm was demonstrated. This represents ∼3 times resolution improvement compared with conventional epi-fluorescence microscopy, while maintaining reasonable speed and compatibility with biological specimens. 18 Both PSIM and LPSIM also rely on the spatial frequency mixing between the illumination patterns and the object, therefore the super resolution image must be numeri- cally reconstructed using a SIM 19 or blind SIM reconstruction method. 14,18,20 The resonant plasmonic enhancement of the localized plasmonic fields provides strong excitation of targeted fluorescent labels, which allows shorter exposure times and thus faster imaging speeds. Currently, all SIM technologies are using commercial objec- tives for image collection, so that the objective NA becomes a major limiting factor. Combining dielectric microspheres with low NA objectives has been proven as a valuable scheme to improve the effective NA of the imaging system for resolving much finer structures. 21–28 By placing high-index dielectric microspheres close to the investigation surface, near-field coupling occurs and an extraordinary sharp focus (so-called † Electronic supplementary information (ESI) available: Video files showing trap- ping and moving 46 μm polystyrene spheres and 17 μm TiO 2 spheres with optical tweezers. See DOI: 10.1039/c7nr03654j a Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093, USA. E-mail: zhaowei@ucsd.edu b Department of Nanoengineering, University of California, San Diego, La Jolla, California 92093, USA c Department of Physics, University of California, San Diego, La Jolla, California 92093, USA This journal is © The Royal Society of Chemistry 2017 Nanoscale, 2017, 9, 14907–14912 | 14907 Published on 04 September 2017. Downloaded by University of California - San Diego on 31/10/2017 22:51:04. View Article Online View Journal | View Issue