Abstract
We present a novel concept to design apertureless plasmonic probes for near-field scanning optical microscopy (NSOM) with enhanced optical power throughput and near-field enhancement. Specifically, we combine unidirectional surface plasmon polariton (SPP) generation along the tip lateral walls with nanofocusing of SPPs through adiabatic propagation towards an apertureless tip. Three key design parameters are considered: the nanoslit width, the pitch period of nanogrooves for unidirectional plasmonic excitation and the pyramidal geometry of the NSOM probe for SPP focusing. Optimal design parameters are obtained with 2D analysis and two realistic probe geometries with patterned plasmonic surfaces are proposed using the optimized designs. The electromagnetic properties of the designed probes are characterized in the near-field and compared to those of a conventional single-aperture probe with same pyramidal shape. The optimized probes feature FWHM around 150nm, comparable with conventional NSOM designs, but over 3 orders of magnitude larger field enhancement, without degrading its spatial resolution. Our ideas effectively combine the resolution of apertureless probes with throughput levels much larger than those available even in aperture-based devices.
Highlights
The resolution of conventional optical microscopy is governed by Rayleigh diffraction and limited by the wavelength of operation [1,2]
Even though higher field enhancement and confinement may be achieved through the utilization of sharper tip geometries, in this study, we have focused on a 100nm probe tip, which is reliably attainable and reproducible with current nanofabrication technology [25]
Without using a conventional aperture geometry [12] or external illumination [10], we have numerically proven that simple nanopatterns, such as a slit and slit with an array of reflective grooves [15,16], may be used for near-field enhancement purposes, in combination with surface plasmon polariton (SPP) nanofocusing associated with the near-field scanning optical microscopy (NSOM) probe tip
Summary
The resolution of conventional optical microscopy is governed by Rayleigh diffraction and limited by the wavelength of operation [1,2]. Interesting plasmonic planar geometries that may focus and direct SPPs towards specific locations with sub-wavelength resolutions have been recently demonstrated both numerically and experimentally [15,16,17] Among these techniques, the ones that utilize SPP diffraction for directional excitation appears attractive for NSOM measurements, as it can provide higher electromagnetic energy focusing for the local probe geometry of interest. We first explore the twodimensional (2D) excitation of plasmonic surfaces by a metal slit in a planar geometry; we apply the optimized design parameters to 3D realistic probe designs to explore the effects of geometrical, unidirectional SPP nanofocusing for NSOM measurements Both proposed designs achieve electric field enhancement over 1000 times compared to a conventional single aperture probe, without compromising spatial resolution. The designed probes offer potential operations over a wide spectral range
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