Novel Metamaterials and Their Applications in Subwavelength Waveguides, Imaging and Modulation

The development of metamaterials has opened the door for engineering electromagnetic properties by subwavelength artificial atoms , and hence accessing new properties and functionalities which cannot be found among naturally occurring materials. In particular, metamaterials enable the flexibility of independently controlling the permittivity and permeability to be almost any arbitrary value, which promises to achieve deep subwavelength confinement and focusing of electromagnetic waves in different spectrum regimes. The next stage of this technological revolution will be focused on the development of active and controllable metamaterials, where the properties of the metamaterials are expected to be tuned by external stimuli. In this sense, some natural materials are also promising to provide the tunable capability, particularly in the near infrared and terahertz domains either by applying a voltage or shining light on the materials. The objective of this dissertation is to investigate novel metamaterials and explore three important applications of them: subwavelength waveguiding, imaging and modulation. The first part of this dissertation covers the theory, design and fabrication of several different types of metamaterials, which includes artificially designed metamaterials and some naturally existing materials. The second part demonstrates metal gratings functioning as designer surface plasmonic waveguides support deep subwavelength surface propagation modes at microwave frequency. The third part proposes multilayered metal-insulator stack as indefinite metamaterial that converts evanescent waves to propagating waves, hence deep subwavelength image can be observed. The fourth part explores the tunability of several natural materials - gallium (Ga), indium tin oxide (ITO) and graphene, and demonstrates electro-optical (EO) modulators based on these materials can be achieved on nano-scale. The final part summarizes the work presented in this dissertation and also discusses some future work for photodetection, photovoltaics, and modulation

Graphene Based Waveguides

Graphene, which is well known as a one-atom thick carbon allotrope, has drawn lots of attention since its first announcement due to remarkable performance in mechanical, electrical, magnetic, thermal, and optical areas. In particular, unique properties of graphene such as low net absorption in broadband optical band, notably high nonlinear optical effects, and gate-variable optical conductivity make it an excellent candidate for high speed, high performance, and broadband electronic and photonics devices. Embedding graphene into optical devices longitudinally would enhance the light-graphene interaction, which shows great potential in photonic components. Since the carrier density of graphene could be tuned by external gate voltage, chemical doping, light excitation, graphene-based waveguide modulator could be designed to have high flexibility in controlling the absorption and modulation depth. Furthermore, graphene-based waveguides could take advantages in detection, sensing, polarizer, and so on

Multi-Physics Modeling of Terahertz and Millimeter-Wave Devices

In recent years, there have been substantial efforts to design and fabricate millimeter-wave and terahertz (THz) active and passive devices. Operation of microwave and photonic devices in THz range is limited due to limited maximum allowable electron velocity at semiconductor materials, and large dimensions of optical structures that prohibit their integration into nano-size packages, respectively. In order to address these issues, the application of surface plasmons (SPs) is mostly suggested to advance plasmonic devices and make this area comparable to photonics or electronics. In this research, the feasibility of implementing THz and millimeter-wave plasmonic devices inside different material platforms including: two-dimensional electron gas (2DEG) layers of hetero-structures, silicon wafers and graphene, are elaborated. To this end, an analytical model is developed to describe the propagation of two-dimensional plasmons along electron gas layers of biased hetero-structures. Using this analytical model, the existence of new plasmonic modes along the biased electron gas is reported for the first time. For an independent verification, a novel multi-physics simulator is developed to analyze active terahertz plasmonic structures. It is also anticipated that the solver can offer novel ideas for guiding the SPs inside the future plasmonic circuits. In a different approach to design plasmonic devices in a widely used material platform, silicon, a THz modulator is proposed. Using a full wave simulator, it is shown that plasmonic wave can propagate along an indented n-type doped silicon wafer (which is later covered with a metallic layer) with large attenuations. However, the signal losses can be prohibited by applying bias voltages onto the metal as the thickness of the depletion layer between the metal and silicon increases. At the end, an effective method to couple incident waves onto an infinitely thin graphene mono-layer is presented. As will be illustrated, the surface waves along a corrugated metal can efficiently transit into graphene and successfully launch plasmons

Chalcogenide Glass-on-Graphene Photonics

Two-dimensional (2-D) materials are of tremendous interest to integrated photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. In this paper, we present a new route for 2-D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides claiming improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators

바나듐 이산화물 기반 상변이 격자들을 이용한 능동 열광학 변조

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2019. 2. 이병호.지난 약 20년간 다양한 형태의 나노광학 소자들이 플라즈모닉스와 메타물질 기술들을 통해 개발되어 왔다. 이러한 소자들의 주된 목표는 금속 표면에 빛을 집속함으로써 기존 벌크 광학 소자들의 기능을 파장 한계 이하로 소형화된 스케일에서 구현하거나, 자연에 없는 특성을 보이는 광학적 물질을 개발하는 것이다. 이를 통하여 궁극적으로 차세대 초소형 고집적 광학 시스템을 만드는 것이 두 분야의 주요한 목표이다. 특히, 유전 함수 스펙트럼이 외부 자극에 의해 조절이 가능한 능동 물질과 나노광학 소자의 결합을 통해 능동적으로 외부 자극에 의해 기능이 변조되는 나노광학 소자들에 대한 연구들이 큰 주목을 받아왔다. 최근에 대표적인 절연체-금속 상변이 물질인 바나듐이산화물을 이용한 나노광학 소자 개발이 주목 받고 있다. 바나듐이산화물은 가시광선, 근적외선 및 적외선 파장 대역에서 온도에 따라 큰 폭의 유전 함수 변화가 일어난다는 장점이 있다. 본 박사학위 논문에서는 주기적으로 배열된 광 격자 구조들에 바나듐이산화물을 결합하여 입사광의 산란 양상을 큰 폭으로 조절하는 새로운 공학적 응용 방법들을 제안한다. 바나듐이산화물의 상변이를 이용하여 산란된 가시광선 및 근적외선의 운동량, 세기, 스펙트럼, 그리고 편광을 효율적으로 조절하는 새로운 방법들을 개발하였다. 각 장 별로 제안한 광소자의 목적 및 동작 대역에 따라서 격자의 주기, 구성 물질 및 구조를 설계하는 이론 및 전산모사 연구를 수행하고 이를 실험적으로 검증한다. 첫번째 장에서는, 능동 나노광학에 대한 간략한 소개와 함께 바나듐이산화물의 유전 함수 변화를 중심으로 동작 파장 대역에 따른 상변이 현상의 특성을 논한다. 다음으로는 금속 표면 위에 집속된 빛의 형태인 표면 플라즈몬 폴라리톤을 여기하고 그 방향을 뒤바꾸는 상변이 광 스위치를 제안한다. 열에 의해 뒤바뀌는 비대칭 나노안테나와 파장 이하 광학 격자에 의한 운동량 보상에 기반하여 표면 플라즈몬 폴라리톤의 방향성이 크게 변함을 보인다. 세번째 장에서는, 광대역에서 비공진 작동이 가능한 직진 광 투과율 조절기를 바나듐 이산화물 격자를 통해 구현한다. 근적외선 대역에서 나타나는 유전체-플라즈모닉 상변이 특성과 나노 도파로에 의한 광대역 초소형 간섭계 구조에 기반하여 고성능 능동 투과율 변조가 가능함을 보인다. 네번째 장에서는, 가시광선의 색 스펙트럼과 편광 방향을 효율적으로 조절하는 바나듐이산화물 메타필름 연구를 소개한다. 반사형 구조의 완전 흡수 현상과 유효 매질 이론을 통한 편광 의존적 유효 굴절률의 설계를 활용한, 가시광선의 효율적인 능동 변조 프레임워크를 제시한다. 마지막으로 결론에서는 연구 결과들의 요약과 그 가치에 대한 논의로 학위논문을 마무리 한다. 본 박사학위 논문의 결과는 능동적 변조가 요구되는 차세대 초소형 광 스위치, 초소형 디스플레이 픽셀, 공간 광 변조기 등에 기여할 수 있을 것으로 기대된다.For the last two decades, various nanophotonic devices have been developed in the fields of plasmonics and metamaterials. There are two main goals of the nanophotonic devices in the fields. One is to confine light on metallic surface and manipulate it in subwavelength scale beyond the diffraction limit. The second is to design artificial materials with extraordinary optical properties that do not exist in the nature. Nanophotonics community has been trying to achieve such phase goals for next-generation miniaturized and integrated optical systems. In particular, dynamically tunable nanophotonic devices have been in spotlight and demonstrated by combining dynamically reconfigurable optical materials and periodic optical grating structures. Optical responses of such devices can be tuned dynamically by application of external stimuli such as heat, voltage, current, and optical pulse. Recently, the representative insulator-to-metal phase-transition material responsive to thermal stimulus, vanadium dioxide, has been thoroughly studied for optical applications. Vanadium dioxide is advantageous in manipulating the visible, near-infrared, and infrared light by virtue of the large change of the dielectric function. This dissertation proposes the novel nanophotonic engineering methods of combining optical grating structures and vanadium dioxide for dynamic, extraordinary thermo-optic modulation of light scattering. Specifically, momentum, intensity, color spectrum, and polarization state of scattered light are modulated based on the transition characteristics of vanadium dioxide and periodic grating functionalities in the visible and near-infrared. Every main chapters present the theoretical and numerical studies involving effects of optical grating geometry for the certain intended modulation functions. In the first chapter, the concepts of dynamic nanophotonics are introduced. In addition, wavelength-dependent optical properties of vanadium dioxide are discussed with experimental studies. In the second chapter, nanophotonic phase-transition switch is proposed for asymmetric excitation and dynamic directivity switching of surface plasmon polaritons. The vanadium dioxide metagrating is demonstrated as the directional switch using thermally switchable asymmetric unit cell and grating period-assisted momentum matching of surface plasmon polaritons. The third chapter introduce the broadband efficient modulation method of transmissivity with vanadium dioxide diffraction grating. Rather than using subwavelength-spaced resonant metasurfaces, diffractive large period grating is designed for broadband operation. Based on such diffraction modulation, forward transmissivity is largely modulated with high efficiency over the broad bandwidth in the near-infrared. In the fourth chapter, the deep subwavelength-spaced gratings, reflective metafilms, are proposed by introducing mirror and effective medium approximation for efficient operation in the visible spectrum. For high contrast modulation of visible light color spectrum, near-unity absorption is designed in noble metal-vanadium dioxide metafilm and tuned owing to large anisotropy and boosted phase-transition effect. Moreover, dynamic polarization modulation is achieved by virtue of extremely anisotropic tunability of absorptions dips. At last, conclusion is presented with brief summary and discussions. The results of the dissertation would help developing and improving next-generation optical switch, ultracompact display pixel, and high performance spatial light modulators in the future.Abstract i Contents iv List of Tables vi List of Figures vii Chapter 1 Introduction 1 1.1 Overview of dynamic nanophotonics 1 1.2 VO2: a volatile phase-transition material 7 1.3 Overview of this dissertation 12 Chapter 2 Dynamic directional switching of surface plasmon polaritons with metal-VO2 metagrating 16 2.1 Introduction 16 2.2 Principles of metal-VO2 metagrating 20 2.2.1 Tunable near-field scattering near VO2 nanoantenna 20 2.2.2 Design of tunable asymmetric launching of surface plasmon polaritons 23 2.3 Experimental demonstration 32 2.3.1 Fabrication of metal-VO2 metagrating 32 2.3.2 Measurement 34 2.4 Summary and discussions 37 Chapter 3 Broadband efficient modulation of forward transmission with VO2 diffraction grating 39 3.1 Introduction 39 3.2 Principles of VO2 diffraction grating 43 3.2.1 Photonic modes in VO2 waveguide 43 3.2.2 Design of VO2 grating modulator 55 3.3 Experimental demonstration 61 3.3.1 Fabrication of VO2 grating modulator 61 3.3.2 Measurement 62 3.4 Summary and discussions 66 Chapter 4 Tunable multifunctional phase-transition effect with noble metal-VO2 metafilm in the visible 67 4.1 Introduction 67 4.2 Absorbing Ag-VO2 metafilm in the visible 69 4.2.1 Role of metallic mirror 69 4.2.2 Role of effective medium approximation for dynamic metafilm 75 4.2.3 Experimental demonstration 85 4.3 Summary and discussions 96 Chapter 5 Conclusion 97 Bibliography 100 Appendix 113 초 록 114Docto