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Terahertz optoelectronic applications of nanomaterial network films
- Alternative Title
- Hong Jung Taek
- Author(s)
- 홍정택
- Alternative Author(s)
- Hong Jung Taek
- Advisor
- 안영환
- Department
- 일반대학원 에너지시스템학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2017-02
- Language
- eng
- Keyword
- terahertz; optoelectronic; nanomaterial
- Alternative Abstract
- 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.
- Fulltext
- Appears in Collections:
- Graduate School of Ajou University > Department of Energy Systems > 4. Theses(Ph.D)
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