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Light Interaction with Strongly-Coupled Metamaterials and Chiral Media

  • SeoJooLee (대학원 물리학과 양자광학전공)

초록/요약

In this thesis, we study light interaction with strongly-coupled metamaterials and chiral media, and discuss the novel optical responses enabled by the interaction. In Chapter 2, we discuss the interaction with strongly-coupled metamaterial by taking into account the simplest structure, a double-slit resonator. Through rigorous coupled-mode theory and finite-difference time-domain (FDTD) numerical calculations, we reveal that resonant coupling between the two slits, mediated by circulating electromagnetic fields near the resonator, is possible. We show that the direction of light circulation is changed alternately by the resonance, which also causes a coincident change in the spatial distribution of the strongly-induced electric fields near the resonator. In Chapter 3, we discuss metamaterials in a more complicated form: periodic arrays of strongly-coupled closed ring resonators. From our analytic theory, we obtain closed-form representations for the transmission and reflection coefficients (T and R, respectively) of the metamaterials exhibiting resonant spectral behaviors. We show that the strong coupling of the ring resonator allows the resonance in T and R to be strongly dependent on the spacing between the resonators. We further apply effective medium theory, and show that the resonance in T and R is equivalent to the drastic metal-to-insulator phase transition in the effective permittivity of the metamaterial, implying that the effective phase of the metamaterial can be modified by controlling the spacing between the resonators. Having obtained the effective medium description of the metamaterial, we next apply our theory to the design of a practical optical device, a terahertz (THz) phase modulator enabled by the hyperbolic effective permittivity of the metamaterial. In addition, by utilizing the spacing-dependent nature of the effective permittivity, we numerically show that a single-sample metamaterial exhibiting the tunable phase modulation properties of both quarter waveplate (QWP) and half waveplate (HWP) is in-principle possible. On the basis of our theoretical results, we experimentally verify the QWP and the HWP made of the metamaterial. Because real-world realization of a single-sample phase modulator that operates as both a QWP and a HWP is shown to be challenging due to the mechanical tolerance limit of the sample, we instead demonstrate two tunable phase modulators operating separately as a QWP and a HWP. In Chapter 4, we study light interaction with chiral media, focusing on the circular dichroism (CD) effect. In particular, we propose an analytic study based on Poynting’s theorem to identify the microscopic origin of surface-enhanced CD in chiral molecule/nanostructure coupled systems, and show that CD signals have two distinct origins: inherent and induced. We find that the inherent CD of chiral molecules can be enhanced by locally induced electromagnetic fields and that the optical response of the nanostructure can itself become chiral in the presence of chiral molecules, forming an induced nanostructure CD. Further, in terms of a numerical analysis of CD from a medium having an intrinsically weak chiral response, we introduce a new meshing scheme, the so-called symmetry-preserved mesh, which enables numerical calculation with superior accuracy due to effective suppression of numerical artifacts.

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목차

ABSTRACT 1
List of Figures 6
Chapter 1 Introduction 13
References 15
Chapter 2 Double-slit resonator 19
2.1 Coupled mode theory 20
2.2 Near field enhancements of two-slit system 26
Conclusion 31
References 31
Appendix 33
Chapter 3 Strongly coupled metamaterial 35
3.1 Analytic theory on the transmission and reflection coefficients of the strongly- coupled metamaterial 36
3.2 Retrieval of effective indices of the metasurface from transmission and reflection coefficients. 43
3.3 Hyperbolic metamaterial as phase modulating devices 47
3.3.1 FDTD results of phase modulating properties 48
3.3.2 Terahertz experimental results 56
3.3.3 Flexible metamaterial film fabrications 57
Conclusions 69
References 70
Chapter 4 Interaction between metamaterials and chiral media 72
4.1 Poynting theorem of Bi-isotropic media 74
4.2 Induced circular dichroism in achiral metamaterial 76
4.2.1 microscopic analysis of CD signal with purely real- and imaginary-valued 80
4.2.2 microscopic analysis of CD signal with dispersive chiral molecule 83
Conclusion 85
References 85
Simulation technics 88
Chapter5 Numerical evaluation of circular dichroism from chiral medium/nanostructure coupled systems by using the finite-element method 92
5.1 Numerical CD artifacts from broken mirror-symmetric mesh 94
5.1.1 Hampered CD signals from chiral molecule/nanostructure coupled system by CDmesh 96
5.1.2 mesh convergence test of CD simulation 99
5.2 Implementation step of Bi-isotropic media in FEM 102
5.2.1 Implementation I 102
5.2.2 Implementation II 106
5.3 Verification for the implementation of bi-isotropic in FEM simulation 108
5.3.1 Optical responses of homogenous Bi-isotropic slab 108
5.3.2 Scattering properties of gold nanoparticle coated with chiral media 110
Conclusion 116
References 116
Chapter 6 Summary 120

Summery (written in Korean)
Publications
Acknowledgement

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