A Novel Near-field Raman and White Light Imaging System for Nano Photonic and Plasmonic Studies
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1077-L11-06
A Novel Near-field Raman and White Light Imaging System for Nano Photonic and Plasmonic Studies Ze Xiang Shen, J. Kasim, Y. M. You, and C. L. Du Physics and Applied Physics, Nanyang Technological University, 1 Nanyang Walk, Blk 5 Level 3, Singapore, 637371, Singapore ABSTRACT We show the approaches in achieving high resolution Raman and white light imaging. In Raman imaging, a dielectric microsphere is trapped by the incoming laser, which was focused onto the sample by the microsphere. The microsphere was also used to collect the scattered Raman signals. We show the capability of this method in imaging various types of samples, such as Si devices and gold nanopattern. This method is comparatively easier to perform, better repeatability, and stronger signal than the normal near-field Raman techniques. Besides the Raman imaging, we also show a far-field confocal white light reflection imaging system that can be used for the fast imaging and characterization of nanostructures. This system uses a xenon (Xe) lamp as the incident light source and tunable aperture to enhance the spatial resolution. It has a spatial resolution of around 370 nm at a wavelength of 590 nm. With our system, we can clearly resolve images of 300 nm nanoparticles arranged in 2D honeycomb arrays with a period of 500 nm. Localized surface plasmons (LSPs) of isolated single and dimer gold nanospheres were also studied and the resonance energy difference between their LSPs was extracted. INTRODUCTION Raman spectroscopy measures molecular vibrations, which are determined by the structure and chemical bonding as well as the masses of the constituent atoms/ions. Raman spectra are unique in chemical and structural identifications. Conventional micro-Raman spectroscopy has a spatial resolution of about 500 nm, governed by the diffraction limit. To extend Raman imaging to the study of nano-materials, extensive efforts have been made to reduce the laser spot size below the diffraction limit by using scanning near-field optical microscopy (SNOM), which can be broadly divided into two approaches, laser delivered through an aperture [1–3] and tip-enhanced (apertureless) [4-6] near-field techniques. Here we report a new approach, which we believe is a disruptive approach to near-field Raman microscopy. In this method, the laser is focused to a spot smaller than diffraction limit by a dielectric microsphere. Besides being used as the excitation source for Raman spectroscopy, the incident laser beam is also used to hold the microsphere just above the sample surface, through the well-known optical tweezers mechanism [7,8]. Simulation studies on optical nanojet based on plane wave incident light have shown that sub-diffraction limited focusing can be achieved when the diameter of the dielectric microsphere is comparable to the wavelength of laser [9,10]. Optical tweezers controlled 10 µm solid immersion lens (SIL) was used by A. L. Birkbeck et al. to perform optical microscopy on chrome grating [11]. Here we show the capability of trapping a dielectric
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