Terahertz optoelectronic applications of nanomaterial network films
Terahertz optoelectronic applications of nanomaterial network films
- 주제(키워드) terahertz , optoelectronic , nanomaterial
- 발행기관 아주대학교
- 지도교수 안영환
- 발행년도 2017
- 학위수여년월 2017. 2
- 학위명 박사
- 학과 및 전공 일반대학원 에너지시스템학과
- 실제URI http://www.dcollection.net/handler/ajou/000000024422
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
In recent years, most plasmonic devices and metamaterials have been based on conventional metals present in nature with research underway to produce new structures and develop novel materials with new optical properties. However, the resonance behaviors of the devices are fixed, which is a limitation for their adoption in various applications. One solution for overcoming this problem is the use of nanomaterial-based network films instead of conventional metals. The conductivities and dielectric constants of nanomaterials can be successfully engineered by using post-treatment techniques such as a chemical treatment and optical illumination. Thus, nanomaterial-based network films have led to new areas of research such as terahertz (THz) plasmonic devices and metamaterials. This thesis presents the fabrication of plasmonic and metamaterial devices, operating in the THz frequency range by using nanomaterial network films. In order to determine the THz transmission properties of nanomaterial network films and devices, single-walled carbon nanotubes (SWNTs), reduced graphene oxide (rGO), zinc oxide (ZnO), and silver nanowire (AgNW) were investigated using THz time-domain spectroscopy. Firstly, we present THz wave modulation by using free-standing ZnO nanowire (NW) network films. The ZnO NW films are virtually transparent to THz waves without UV illumination. Conversely, the THz waves are attenuated under very low-intensity UV illumination, making the ZnO NW films a promising platform for low-loss, low-power, and all-optical THz modulators. Measurement of the complex dielectric constants reveal that the UV laser induces an enhancement in the AC conductivity while leaving the real part of the dielectric constant unchanged. The relatively slow time response implies that the UV-induced modulation is closely linked to surface trap states. Further, the THz attenuations shows clear saturation behavior with respect to the UV intensity, from which we extracted the ZnO NW surface trap density. Using THz time-domain spectroscopy, we measure rGO network films deposited onto quartz substrates from dispersion solutions using a spraying method. The rGO network films demonstrate a high conductivity of about 900 S/cm in the THz frequency range after a high-temperature reduction process. The frequency-dependent conductivities and refractive indices of the rGO films are obtained and analyzed with respect to the Drude free-electron model, which is characterized by a large scattering rate. Finally, we demonstrate that the THz conductivities can be manipulated by controlling the reduction process, which correlates well with the DC conductivity above the percolation limit. Furthermore, we demonstrate that thick single-walled nanotube films fabricated by a filtration method are capable of shielding against THZ waves. The shielding effectiveness can be engineered by controlling the film thickness and we achieved 38 dB for a 950-nm-thick film. In addition, we find that the films exhibit a dispersion in the dielectric constants obeying the Drude free-electron model, whereas the plasma frequency decreases with increasing film thickness. Using nanotube films with a thickness greater than the skin depth, we fabricated grid polarizers by a laser machining process that enables us to achieve a large polarization extinction ratio. Most importantly, we demonstrate the fabrication of plasmonic and metamaterials devices, operating in the THz frequency range, by using highly conductive, SWNT network films. We fabricate various patterns onto SWNT films using photolithography or laser machining techniques, the resonance behaviors of which are determined by geometric parameters such as the periodicity of the array patterns of the shapes of the individual elements. The excellent mechanical properties of SWNT films enable us to fabricate free-standing and highly flexible devices. In addition, using post-processing techniques, such as chemical treatments and nanoparticle coatings, we are able to engineer the dielectric constants of the SWNT films, including enhancing or degrading the conductive properties. As a result of the post-processing, the resonance peaks of the plasmonic devices are suppressed or retrieved, which is not achievable in conventional metal films. In particular, we are able to control the metamaterial resonances, implying the possibility of fabricating tunable optoelectronic devices without changing the device structures. Finally, we fabricated plasmonic devices capable of operating in the THz frequency range using AgNW network films. AgNW films exhibit a high conductivity and good transparency in the visible-light range with a figure of merit comparable to that of conventional transparent conducting oxide films. The THz conductivity of AgNW films can be engineered by post-treatment procedures such as welding using graphene oxide flakes and chemical vapor treatments. Using photolithography, we fabricate the plasmonic devices including the slot antenna arrays, the resonance behaviors of which are determined by geometric parameters such as the length of individual elements. We find that the AgNW plasmonic device works as an effective microbial sensor with a sensitivity as good as that of conventional metal films. More importantly, the plasmonic resonance varies with the sheet resistance of the film, enabling us to manipulate the quality factor and the peak position of the resonance by controlling the thickness of the films and by the post-procedures such as a chemical vapor treatment.
more목차
1 Introduction 1
2 THz Plasmonic and Metamaterial Devices Based on Conventional Metals 5
2.1 THz Plasmonic Devices 5
2.1.1 Introduction 5
2.1.2 Theory of THz Surface Plasmon Polaritons 6
2.1.3 Shape Resonance 8
2.2 THz Metamaterial Devices 10
2.2.1 Introduction 10
2.2.2 Engineering the Electromagnetic Response 12
2.2.3 Split-Ring Resonator 13
2.3 Active Control of THz Plasmonic and Metamaterial Devices 16
2.4 THz Applications of Nanomaterial-based Films 18
3 UV-induced THz wave Modulation in Free-standing ZnO Nanowire Films 20
3.1 Introduction 20
3.2 Fabrication of Free-standing ZnO Nanowire Films 22
3.3 UV-induced THz Transmission thorugh ZnO Films 23
3.4 Time Response of UV-induced Switching at THz range 27
3.5 THz Absorptance on UV intensity in ZnO NW Films 29
3.6 Conclusion 30
4 THz Conductivity of reduced Graphene Oxide Films 32
4.1 Introduction 32
4.2 Fabrication of rGO Network Films 34
4.3 THz Transmission through rGO Films 36
4.4 Complex Optical Constants of rGO Films 38
4.5 THz Conductivity Change with Reduction Temperatures 41
4.6 Conclusion 43
5 THz wave Polarizers of Single-walled Carbon Nanotube Films with High Shielding Effectivenss 44
5.1 Introduction 44
5.2 Fabrication of SWNT Films by Fitration Method 45
5.3 THz Transmission through SWNT Films 46
5.4 THz Polarizer using SWNT Network Film 50
5.5 Conclusion 52
6 Dielectric Constant Engineering of Single-Walled Carbon Nanotube Films for Metamaterials and Plasmonic Devices 54
6.1 Introduction 54
6.2 THz Transmission through Free-standing SWNT Films 56
6.3 THz Plasmonic Devices based on SWNT Films 58
6.4 Dielectric Constant Enineering using Chemical doping at Metamatrials and Plasmonic Devices 63
6.5 Conclusion 69
7 Chemical Control of Plasmonic Devices Fabricated on Silver Nanowire Network Films 71
7.1 Introduction 71
7.2 Fabrication of AgNW Network Films 73
7.3 THz Transmission thorugh AgNW Network Films 74
7.4 THz Plasmonic Devices using AgNW Network Films 78
7.5 Resonance Frequency Shift as Changing Film Conductivity 81
7.6 Conclusion 88
Conclusion 90
References 93
Publications List 120
