Manipulating Optical Nonlinearities in Nanophotonic Systems via Plasmons and High-index Resonators Open Access

Abstract

Nanophotonic platforms allow light concentration and confinement at the subwavelength scales. This property is especially beneficial for the enhancement of nonlinear optical processes that are typically very weak in the bulk materials. Within this broad framework of nonlinear nanophotonics my research focuses on two main thrusts. The first research focus of my dissertation uses plasmonic nanostructures to manipulate and enhance the nonlinear optical effects in hybrid low-dimensional nanoscale systems. Introducing plasmonic nanostructures strongly confines the local electric field distribution and thus remarkably enhances the interaction strength between photons and adjacent materials. The nonlinear optical properties of these systems can be dramatically modified on a nanoscale benefiting from this subwavelength confinement of electromagnetic energy. The second thrust of my research focuses on using high-index dielectric resonators to enhance the light- matter interaction at the nanoscale. Lower optical loss and CMOS-compatibility make these dielectric resonators attractive new platforms for nanophotonics and nonlinear optics. The reported efficiency of nonlinear processes is orders of magnitude larger compared with plasmonic resonances.

In Chapter 1, an introduction of the background in nano-optics, nonlinear optics and ultrafast optics is given for reference.

In Chapter 2, the second harmonic generation (SHG) in the plasmon-WSe2 strong coupling system is discussed. We report the first experimental observation of Rabi splitting in the pump frequency dependence of the second-harmonic signal in such systems and this splitting phenomenon can be explained by a coupled-oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a nonperturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. This is a typical example to use plasmonic nanostructures in manipulating nonlinear optical effects in strong light-matter interaction systems.

In Chapter 3, an invertible nonlinearity of ITO thin films is investigated. ITO thin films are unique high-index materials which possess resonance modes in the epsilon-near-zero (ENZ) region and recently emerge as new platforms for enhancing the optical nonlinearity. We report a systematic theoretical and experimental study of the origin of ITO thin film nonlinearities contributed by the intraband transition in a non-parabolic conduction band and the interband transition via near infrared (NIR) and ultra-violet (UV) excitations. A new mechanism is found that the nonparobolicity of the band structure, which brings a larger effective mass in the intraband transition, and the Fermi energy, which determines the free carrier density, are two competing forces that jointly contribute to an invertible nonlinearity of ITOs in the NIR region. This corresponds to the second thrust of my dissertation, in which high-index resonators are implemented in enhancing nonlinear optical effects.

Table of Contents

1 Introduction 1

1.1 Nano-optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Optical Near-fields . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.2 Nanoscale Confinement of Matter . . . . . . . . . . . . . . . . 4

1.1.3 Nanofabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Nonlinear Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2.1 Nonlinear Susceptibility . . . . . . . . . . . . . . . . . . . . . 10

1.2.2 Second Harmonic Generation . . . . . . . . . . . . . . . . . . 15

1.2.3 Optical Kerr Effect . . . . . . . . . . . . . . . . . . . . . . . . 16

1.3 Ultrafast Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.3.1 Ultrafast Laser . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.3.2 Pump-probe experiments . . . . . . . . . . . . . . . . . . . . . 20

2 Second Harmonic Generation from a Single Plasmonic Nanorod Strongly

Coupled to a WSe2 Monolayer 22

2.1 Cavity QED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.1.1 Weak Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.1.2 Strong Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.2 Plasmon-exciton Strong Coupling . . . . . . . . . . . . . . . . . . . . 29

2.2.1 Plasmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.2 TMD materials . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.2.3 Nonlinearity of Plasmon-exciton Strong Coupling . . . . . . . 33

2.2.4 Nonlinear Two-oscillator Model . . . . . . . . . . . . . . . . . 34

2.3 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.3.1 Sample Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 39

2.3.2 Optical Characterization . . . . . . . . . . . . . . . . . . . . . 39

2.4 FDTD simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.5.1 Linear Optical Characterization Results . . . . . . . . . . . . 47

2.5.2 SHG Characterization Results . . . . . . . . . . . . . . . . . . 52

2.5.3 More Discussion on SHG . . . . . . . . . . . . . . . . . . . . . 56

2.5.4 More Discussion on THG . . . . . . . . . . . . . . . . . . . . . 59

2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3 Invertible Nonlinearities of ITO Thin Films 62

3.1 ENZ Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.1.1 Properties of ENZ Materials . . . . . . . . . . . . . . . . . . . 63

3.1.2 TCO thin films . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.2 Nonlinear Optical Effects . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.3 Intraband Transition and Interband Transition Model . . . . . . . . . 69

3.4 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

3.4.1 Sample Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 72

3.4.2 Optical Characterization . . . . . . . . . . . . . . . . . . . . . 72

3.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3.5.1 Linear Characterization Results . . . . . . . . . . . . . . . . . 73

3.5.2 Nonlinear Characterization Results . . . . . . . . . . . . . . . 75

3.5.3 More Discussion on Plasmon-ITO Coupling . . . . . . . . . . 84

3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Appendix A List of Publications 89

Bibliography 91

About this Dissertation

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School	Laney Graduate School
Department	Physics
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Physics, Optics Physics, Condensed Matter
Keyword	Strong Coupling Epsilon-near-zero Materials Nonlinear Optics Nanophotonics Ultrafast Optics
Committee Chair / Thesis Advisor	Harutyunyan, Hayk, Emory University
Committee Members	Burton, Justin, Emory University Cai, Wenshan, Georgia Institute of Technology Urazhdin, Sergei, Emory University Srivastava, Ajit, Emory University

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