Abstract
Label-free optical imaging of nanoscale objects faces fundamental challenges. Techniques based on propagating surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) have shown promises. However, challenges remain to achieve diffraction-limited resolution and better surface localization in SPR imaging. LSPR imaging with dark-field microscopy on metallic nanostructures suffers from low light throughput and insufficient imaging capacity. Here we show ultra-near-field index modulated PlAsmonic NanO-apeRture lAbel-free iMAging (PANORAMA) which uniquely relies on unscattered light to detect sub-100 nm dielectric nanoparticles. PANORAMA provides diffraction-limited resolution, higher surface sensitivity, and wide-field imaging with dense spatial sampling. Its system is identical to a standard bright-field microscope with a lamp and a camera – no laser or interferometry is needed. In a parallel fashion, PANORAMA can detect, count and size individual dielectric nanoparticles beyond 25 nm, and dynamically monitor their distance to the plasmonic surface at millisecond timescale.
Highlights
Label-free optical imaging of nanoscale objects faces fundamental challenges
We demonstrate ultra-near-field index modulated plasmonic nano-aperture label-free imaging (PANORAMA) that addresses existing issues for present Surface plasmon resonance imaging (SPRI) and LSPR imaging (LSPRI) techniques
The light scattering crosssection of a spherical particle is proportional to the 6th power of the particle diameter (d) given by σscatt 1⁄4 ð2π5d6n4med=3λ4incÞ*jðm[2] À 1Þ=ðm[2] þ 2Þj2, where nmed is the refractive index of the medium surrounding the particle, and a b c d e f g h i j k l
Summary
Label-free optical imaging of nanoscale objects faces fundamental challenges. Techniques based on propagating surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) have shown promises. We demonstrate the ability of PANORAMA to image dielectric nanoparticles as small as 25 nm by a standard transmission bright-field microscope with a tungsten-halogen lamp. When an imaging target resides outside the AGNIS’s longitudinal sensing range, i.e., too far away from the surface, it will show up as a standard light scattering object with its intensity reduced when transmitted through the AGNIS.
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