Plasmonic Antennas and Arrays for Optical Imaging and - TSpace

171 Pages·2013·3.23 MB·English

Plasmonic Antennas and Arrays for Optical Imaging and
Sensing Applications
by
Yan Wang
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Edward S. Rogers Sr. Department of Electrical
and Computer Engineering
University of Toronto
Copyright c 2013 by Yan Wang
(cid:13)
Abstract
Plasmonic Antennas and Arrays for Optical Imaging and Sensing Applications
Yan Wang
Doctor of Philosophy
Graduate Department of Edward S. Rogers Sr. Department of Electrical and Computer
Engineering
University of Toronto
2013
The optics and photonics development is currently driven towards nanometer scales.
However, diffraction imposes challenges for this development because it prevents confine-
ment of light below a physical limit, commonly known as the diffraction limit. Several
implications of the diffraction limit include that conventional optical microscopes are
unable to resolve objects smaller than 250nm, and photonic circuits have a physical
dimension on the order of the wavelength. Metals at optical frequencies display col-
lective electron oscillations when excited by photon energy, giving rise to the surface
plasmon modes with subdiffractional modal profile at metal-dielectric interfaces. There-
fore, metallo-dielectric structures are promising candidates for alleviating the obstacles
due to diffraction.
Thisthesisinvestigatesaparticularbranchofplasmonicstructures, namelyplasmonic
antennas, for the purpose of optical imaging and sensing applications. Plasmonic anten-
nas are known for their ability of dramatic near-field enhancement, as well as effective
coupling of free-space radiation with localized energy. Such properties are demonstrated
in this thesis through two particular applications. The first one is to utilize the interfer-
ence of evanescent waves from an array of antennas to achieve near-field subdiffraction
focusing, also known as superfocusing, in both one and two dimensions. Such designs
could alleviate the tradeoffs in the current near-field scanning optical microscopy by
ii
improving the signal throughput and extending the imaging distance. The second appli-
cation is to achieve more efficient radiation from single-emitters through coupling to a
highly directive leaky-wave antenna. In this case, the leaky-wave antenna demonstrates
the ability of enhancing the directivity over a very wide spectrum.
iii
Dedication
To my parents:
Lijun and Wenxia
iv
Acknowledgements
‘Knowledge is in the end based on acknowledgement.’
-Ludwig Wittgenstein
It was only through the guidance, help and support of the kind individuals and
organizations that this thesis is successful, to only some of whom it is possible to give
particular mention here.
First and foremost, I would like to express my sincere gratitude to my principal
supervisor, Prof. George V. Eleftheriades, who mentored me throughout my Master’s
andPh.Dstudies. Hisextraordinarypassionforresearch, tremendousinsightsforscience,
and unparalleled patience for teaching are some of the most admirable characters that
one can possess as a researcher and an advisor. It is my blessing to be mentored by
such an individual, from whom I have learned valuable lessons both academically and
personally. I would also like to thank my co-supervisor, Prof. Amr S. Helmy, who guided
me patiently in an area of research that I was very unfamiliar with at the beginning. I am
tremendously grateful for his vast knowledge in the practical aspect of optical science,
and his constant reminder of concerning engineering designs with the real world. His
insights and attitude have had significant influence on my approach to scientific research.
Next,IwouldliketothankProf.CostasD.Sarris,Prof.SeanV.Hum,Prof.JoycePoon,
and Prof. Wai Tung Ng from the University of Toronto, and Prof. Filippo Capolino from
the University of California, Irvine, for being on my supervisory and examination com-
mittees. Their feedback and questions have helped me gaining valuable understanding
and additional perspectives of my work.
Throughout the entire duration of my graduate study, I have had the great fortune
of meeting the most wonderful labmates, who not only offer stimulating discussions con-
cerning research problems, but also friendship and support that one often needs in a
relatively solitary research work. I am indebted to their aspiration of achieving excel-
v
lence, their passion for research as well as for beer. I cannot ask for a better group of
amazing individuals to share my interests, knowledge and perspectives of life.
I would also like to acknowledge the generous financial support provided by the On-
tario Graduate Scholarship, the Ontario Graduate in Scholarship in Science and Technol-
ogy, the Doctoral Completion Award of the University of Toronto, V. L. Henderson and
M. Bassett Research Fellowship in Electrical Engineering, Ewing Rae Graduate Schol-
arship in Electrical Engineering, and Frank Howard Guest Bursary in the Faculty of
Applied Science and Engineering, without which this work would not have been possible.
I would like to thank my mom, Lijun, for her unconditional love and support; as well
as my dad, Wenxia, for his encouragement and sharing his graduate school experience
with me. Last but not least, I would like to thank my boyfriend, Gerald, for sharing
the precious graduate school life experience together. It is a real blessing to have a like-
minded companion who constantly helps me to become a better version of myself in all
aspects of life.
Yan Wang
March, 2013
Toronto, Canada
vi
Contents
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Overview of plasmonic antennas . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1 Early developments and obstacles . . . . . . . . . . . . . . . . . . 3
1.2.2 Current developments . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Thesis objective, scope and outline . . . . . . . . . . . . . . . . . . . . . 6
2 Background 10
2.1 Metals at optical frequencies . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 Theoretical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.2 Empirical data and analysis . . . . . . . . . . . . . . . . . . . . . 13
2.2 Wavelength scaling for plasmonic antennas . . . . . . . . . . . . . . . . . 15
2.2.1 Theoretical analysis for dipole antennas . . . . . . . . . . . . . . . 16
2.2.2 Numerical analysis for dipole and slot antennas . . . . . . . . . . 18
3 Plasmonic Antennas for Near-Field Superfocusing 24
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.1 Fundamental trade-offs in NSOM technologies . . . . . . . . . . . 25
3.1.2 Other superresolution techniques . . . . . . . . . . . . . . . . . . 29
3.1.3 Engineering optical near-field with plasmonic antennas . . . . . . 30
3.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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3.2.1 Back-propagation method . . . . . . . . . . . . . . . . . . . . . . 33
3.2.2 Shifted-beam theory . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3.1 Background signal . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.3.2 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4 Plasmonic Antennas for Near-Field Superfocusing (2D) 65
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.1.1 2D monopole-array probe at microwave frequencies . . . . . . . . 66
4.1.2 Plasmonic monopole antennas at optical frequencies . . . . . . . . 68
4.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.3.1 Aperture probe with a single monopole antenna . . . . . . . . . . 79
4.3.2 Aperture probe with a monopole antenna array . . . . . . . . . . 80
5 Plasmonic Antennas for Far-Field Sensing 91
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.1.1 Nanoscale emitters . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.1.2 Nanoemitter radiation without antennas . . . . . . . . . . . . . . 94
5.1.3 Improving nanoemitter radiation with antennas . . . . . . . . . . 96
5.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.3 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.4.1 Plasmonic leaky-wave slot mode . . . . . . . . . . . . . . . . . . . 109
5.4.2 Antenna efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 113
6 Summary and Future Work 119
viii
A Metals at Optical Frequencies 124
A.1 Analytical models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
A.1.1 The Drude (free-electron) model . . . . . . . . . . . . . . . . . . . 124
A.1.2 The Lorentz (bound-electron) model . . . . . . . . . . . . . . . . 126
A.1.3 The Lorentz-Drude (LD) model . . . . . . . . . . . . . . . . . . . 127
A.2 Empirical data and parameterization . . . . . . . . . . . . . . . . . . . . 128
B Dispersion of the TM Surface-Mode for Rod-Antennas 132
0
C Parallel dipoles at air-dielectric interface 138
C.1 Radiation of an electric dipole . . . . . . . . . . . . . . . . . . . . . . . . 140
C.2 Radiation of a magnetic dipole . . . . . . . . . . . . . . . . . . . . . . . . 143
References 146
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List of Figures
2.1 Dispersion of the complex ε for Au, Ag and Al. . . . . . . . . . . . . . . 14
r
2.2 The geometry of a plasmonic nanorod antenna. . . . . . . . . . . . . . . 15
2.3 The effective wavelength dispersion of plasmonic rod antennas. . . . . . . 20
2.4 Resonance of a plasmonic rod antenna . . . . . . . . . . . . . . . . . . . 21
2.5 Resonance of a plasmonic slot antenna . . . . . . . . . . . . . . . . . . . 22
2.6 Comparison of the theoretical and simulated λ dispersion. . . . . . . . 23
eff
3.1 Scanning probe microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Examples of NSOM probes . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.3 Gaussian beam diffraction. . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4 Cutoff of the fiber mode(s) in a tapered probe . . . . . . . . . . . . . . . 28
3.5 Near-field plate schematics. . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.6 The schematic of the back-propagation system. . . . . . . . . . . . . . . 34
3.7 An example of the target function. . . . . . . . . . . . . . . . . . . . . . 35
3.8 The spatial and spectral distributions of the transmission function. . . . 37
3.9 Comparison of the numerical and analytical transmission functions. . . . 38
3.10 The transmission function for achieving various focal lengths. . . . . . . . 39
3.11 The transmission functions for various Gaussian target beamwidths. . . . 39
3.12 An example of metascreen based on the shifted-beam theory. . . . . . . . 41
3.13 Comparison of the target beam and the 3-slot array radiation. . . . . . . 44
3.14 Comparison of scattered near-field. . . . . . . . . . . . . . . . . . . . . . 45
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