Ph.D. Oral Defense(Arvind Sundaramurthy),June 14,1:15pm,CISX-Auditorium
arvisun at stanford.edu
Tue Jun 12 10:37:30 PDT 2007
Bowtie Nanoantennas: Optical Resonators at Visible Wavelengths
Ph.D. Oral Exam
Dept. of Electrical Engineering, Edward.L.Ginzton Lab, Stanford University.
Advisor: Prof. Gordon.S.Kino.
Date & Time: 14 June 07, 1:15pm(Refreshments served at 1:00pm).
Location: CIS-X Auditorium.
The resolving power of an optical system is fundamentally limited
by diffraction. According to the Rayleigh criterion, two points on a sample
cannot be resolved unless they are spaced apart by 0.61*lambda where lambda
is the wavelength of incident light in air. Antennas efficiently confine
and enhance the propagating fields incident on them into a region that is a
fraction of the incident wavelength. In this talk we present our work on
bowtie nanoantennas that resonate at optical frequencies. The bowtie
nanoantennas consisting of two opposing triangles facing each other
tip-to-tip, are fabricated by electron beam lithography(EBL) with triangle
lengths of 75nm and gap widths ranging from 16nm to 500nm. We
experimentally studied the scattering response of the bowtie antennas using
total internal reflection(TIR) microscopy. We observed that the resonant
scattering wavelength is a strong function of the gap width in the antennas.
We simultaneously studied the currents, field distribution and
scattering efficiencies in the Au bowties using finite difference time
domain(FDTD) simulations. The experimentally observed resonant wavelengths
in the antennas were in excellent agreement with the FDTD simulations. We
propose a model that predicts the change in resonant wavelength with gap
based on the current distribution in the antennas.
We then study the field enhancement in the bowties by measuring the
efficiency of two photon enhanced photoluminescence(TPPL) emitted by the Au
bowties. We experimentally determined the enhancement factor(|E|^2 >10^3)
in the antennas for the first time and these were in good agreement with
the FDTD computations of the enhancements.
We studied the light confinement and enhancement in the bowties by
two-photon polymerization(TPP) of SU-8 resist. We coated the bowties with
SU-8 optical resist and excited them with pulsed laser light focused to a
diffraction limited spot. We observed minimum resist features that were
<30nm exposed at low incident average powers(27 microwatts). The
experimental results compare very well with FDTD computations.
Finally we demonstrate a novel method for fabricating bowtie
antennas on an atomic force microscope (AFM) tip, to produce a scanning
probe optical microscope. We use the microscope for Raman spectroscopy and
imaging of an isolated single wall carbon nanotube (SWCNT) with an optical
resolution of ~24nm . The scanning probe microscope with the bowtie
nanoantenna has a superior light collection efficiency compared to aperture
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