Nanophotonics with subwavelength apertures: theories and applications.
Date
2012-05-08
Authors
Pang, Yuanjie
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Abstract
This dissertation presents subwavelength optics with focus on the theory and applications
of subwavelength apertures in a metal film. Two main issues regarding the
optics with subwavelength apertures are investigated. As the first issue, the extraordinary
optical transmission (EOT) through a single hole in a metallic waveguide is
presented. A total transmission through a single subwavelength aperture is theoretically
predicted for a perfect electric conductor regardless of the aperture size, without
relying on aperture arrays and surface corrugations as presented in previous works.
The waveguide EOT is then applied to boost the optical throughput of an apertured
near-field scanning optical microscope (NSOM) probe. Using a new structure for
the apertured NSOM probe which allows for waveguide EOT, the optical throughput
and the damage threshold are boosted by 100× and 40× as compared to a conventional
structure, and the experimental findings are backed-up by comprehensive
finite-difference time-domain (FDTD) simulations. Single fluorescent molecules are
scanned using the EOT apertured NSOM probe, and a spatial resolution of 62 nm is
achieved. As the second issue, subwavelength apertures are found useful for optical trapping.
A small dielectric particle can significantly change the optical transmission through
an aperture by dielectric loading, and subsequently, a large optical force is induced which favors trapping. A self-induced back-action (SIBA) optical trap is designed
using a circular nanohole in a gold film. Trapping of 50 nm polystyrene particle
is experimentally achieved, which is not possible using a conventional single beam
optical tweezers. The circular nanohole SIBA trap works beyond the perturbative
regime, as proven by FDTD simulations and a Maxwell stress tensor analysis. We
further improve the nanohole trapping using a double-nanohole, which is more sensitive
for small dielectric changes due to the intense local field enhancement between
its two sharp tips. A single 12 nm silica sphere is experimentally trapped using the
double-nanohole, as the smallest trapped dielectric particle reported. We also achieve
the trapping of a single protein – a bovine serum albumin (BSA) protein with a hydrodynamic
radius of 3.4 nm in the folded form. The trapped BSA is also unfolded
by the large optical force, as confirmed by experiments with changing optical power
and changing pH. The high signal-to-noise ratio of 33 in monitoring single protein
trapping and unfolding shows a tremendous potential for using the double-nanohole
as a sensor for protein binding events at a single molecule level.
Description
Keywords
Nanophotonics, Aperture Theories, Surface Plasmon, Extraordinary Optical Transmission, Near-Field Optics, Optical Trapping, Surface-Enhanced Raman Spectroscopy, Novel Optical Device