Ph.D. Oral Defense(Arvind Sundaramurthy),June 14,1:15pm,CISX-Auditorium

[arvind sundaramurthy]

Bowtie Nanoantennas: Optical Resonators at Visible Wavelengths

Arvind Sundaramurthy

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.

Abstract:

        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 
based microscopes.
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