Hot-Carrier Induced Nonlinear Optics in Plasmonic and Nanophotonic Systems 公开

Abstract

Nonlinear optics describes a wealth of exciting phenomena. This includes ultrafast optical responses, frequency conversion, multiphoton processes, and enhancement of ``forbidden" light-matter interactions. Some of the most novel of these effects are attributed to out-of-equilibrium or ``hot" carrier dynamics. The scope of this dissertation is to discuss hot-carrier roles in plasmonic and nanophotonic systems composed of metallic and semiconducting materials.

Chapter 1 presents an outline of the relevant physics for reference of the reader for later chapters.

Chapter 2 contains a study of nonlinear light emission from spatially confined plasmonic nanostructures. A geometry of Au nanowires separated from Au films by nanometric SiO2 layers controls the degree of spatial confinement of the resonant gap-mode plasmons, and thus probes the degree of momentum breakdown and its effect on photoluminescence (PL). Our results indicate that the PL signal from this nanoscale system has a nonlinear power law exhibiting two distinct linear regimes, differing from that of rough films, indicating that the physical mechanism of the nonlinear PL signal involving hot-carriers needs to be revisited.

Chapter 3 introduces a method for sputtering record-thin fully percolated Au films on an oxide substrate. We demonstrate wetting layer-free plasmonic gold films with thicknesses down to 3 nm obtained by deposition on substrates cooled to cryogenic temperatures. We systematically study the effect of substrate temperature on the properties of the deposited Au films, and show that substrate cooling suppresses the Vomer-Webber growth mode of Au, promoting early-stage formation of continuous Au films with improved surface morphology and enhanced optoelectronic properties.

Chapter 4 concludes with observations of negative extinction and sub-picosecond injected hot-carrier dynamics in an active amorphous Ge metasurface. We report pump-probe measurements performed on amorphous-Ge-based micro-resonator metasurfaces that exhibit strong resonant modes in the mid-infrared. We observe relative change in transmittance of dT/ T ~1 with subpicosecond (tau~0.5 ps) modulation speed, obtained with very low pump fluences of 50 uJ/cm^2. We attribute these observations to efficient free carrier promotion affecting light transmittance via high quality-factor optical resonances, followed by an increased electron-phonon scattering of free carriers due to the amorphous crystal structure of Ge.

Table of Contents

1 Introduction 1

1.1 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Electromagnetism Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Maxwell's Equations in Continuous Media . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Free Electron Model and the Plasma Frequency . . . . . . . . . . . . . . . . . . . . . . 3

1.2.3 Permittivity (Drude and Interband Model) . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.4 Kramers - Kronig Relations (epsilon' & epsilon'' Correspondence) . . . . . . . . . . . . . . . . . . 9

1.2.5 Nonlinear Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3 Nanoparticle Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3.1 Particle Plasmon Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3.2 Mie Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3.3 Gans Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.4 Quasi-Static and Electrodynamic Regimes . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.3.5 Q-factor and Microresonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.3.6 Coupling to Nearby Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.3.7 Multipole Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.3.8 Reciprocity Principle, Far- and Near-Field Coupling . . . . . . . . . . . . . . . . . . . 19

1.4 Plasmon Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.4.1 Lorentzian Lineshape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.4.2 Damping Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

1.4.3 Landau Damping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.4.4 Wave Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1.4.5 Angular Spectrum Representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1.5 Optical and Electronic Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.5.1 Absorption/Emission Mechanisms (Fermi Golden Rule) . . . . . . . . . . . . . . . . . 29

1.5.2 Band Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

1.5.3 Optical Transitions in Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

1.5.4 Optical Transitions in Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

1.6 Hot-Carrier Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

1.6.1 Hot-Carrier Production Timeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

1.6.2 Non-Fermi (Athermal) Electron Distribution . . . . . . . . . . . . . . . . . . . . . . . 35

1.6.3 Hot-Carrier (Thermal) Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

1.6.4 Thermalisation and Phonon Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2 Hot-Electron Induced Nonlinear Light Emission 39

2.1 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.3 Plasmonic Metal PL Literature and Anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.3.1 Smooth vs. Rough Film PL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.3.2 Ultrafast Hot-Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.4 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

2.4.1 Proposed Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

2.4.2 Nanowire Array Design (Gradient Gratings) . . . . . . . . . . . . . . . . . . . . . . . . 53

2.4.3 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.5 Fabrication Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

2.5.1 Gradient Nanowire Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

2.5.2 ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

2.5.3 Cryogenic Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.6.1 Gap Size Dependent NPL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.6.2 Plasmon Detuning NPL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

2.6.3 Effects of Surface Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

2.6.4 Low Fluence Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2.7 Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

2.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

3 Ultrathin Wetting Layer-Free Plasmonic Gold Films 71

3.1 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

3.2.1 Thin Film Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

3.3.1 Deposition of Ultrathin Au Films at Different Temperatures . . . . . . . . . . . . . . . 78

3.3.2 Optoelectronic Characteristics and Surface Morphologies of 5 nm Thick Au . . . . . . 79

3.3.3 Plasmonic 3-nm-thick Au Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

3.4 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

3.5 Conclusion and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4 Deep Optical Switching on Subpicosecond Timescales in an Amorphous Ge Metasurface 92

4.1 Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.2.1 Active Metamaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.2.2 Q-Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

4.2.3 All-Optical Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

4.3 Semiconductor Active Metasurfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

4.4 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.4.1 Overview and Sample Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.4.2 Pump-Probe Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

4.4.3 Negative Delay Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4.4.4 Positive Delay Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

4.5 Theoretical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

4.6 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

4.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Concluding Remarks 123

Published Work at Emory 126

Bibliography 127

About this Dissertation

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School	Laney Graduate School
Department	Physics
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Physics, Electricity and Magnetism Physics, Quantum Physics, Optics
关键词	Plasmonics
Committee Chair / Thesis Advisor	Hayk Harutyunyan, Emory University
Committee Members	Sidong Lei, Georgia State University Ajit Srivastava, Emory University Sergei Urazhdin, Emory University Connie Roth, Emory University

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