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Study on the Active Phase Control in Graphene-Based Terahertz Devices

초록/요약

Graphene, possessing a zero bandgap with linear energy momentum dispersion relation near the Dirac point, is one-atom thick carbon layer of sp2 hybridization arranged in hexagonal lattice structure. Graphene has been recognized for outstanding chemical stability, highest electrical mobility and excellent mechanical strength of carriers. Such kind of characteristics enable graphene-based novel optoelectronic and photonic devices including an optical modulator operating with ultra-broadband width from visible to terahertz (THz) frequency. In the graphene-light interaction, inter-band transition dominants in visible range and near infrared range with its limited absorption of only 2.3%, whereas intra-band transition dominants in the THz range where field amplitude of the electro-magnetic wave could be significantly modulated. Due to unique conical band structure of graphene, the Fermi energy in the vicinity of the Dirac point can be effectively controlled by tuning the biased gate voltage. By utilizing such tunable graphene`s conductivity characteristic with gate voltage, actively controlled THz amplitude modulation and phase modulation are demonstrated in this research. By utilizing the powerful gating effect of ion gel in our device structure; ion gel adopted on the top of graphene transferred on quartz substrate, resulting in efficient tuning of the graphene's Fermi level. On the presence of gate voltage, charge accumulates within several nanometers in distances into the graphene/ion-gel interfaces. Based on the power-law dependency for mass less charge carrier in graphene, ⎹EF⎹ ∝⎹n⎹1/2, where n defines as charge carrier concentration and EF as graphene`s Fermi level. From this relation, with electrical gating, effective tuning of the graphene's conductivity is possible through the modification of the graphene's Fermi level. Synthesis of high-quality graphene over large area is essential for realization of the THz device exhibiting high performance. Among the availability of various graphene synthesis method, chemical vapor deposition (CVD) technique has great potential for mass production, uniform growth, layer control and large-area synthesis. By using copper-based catalytic subtract with <0.001% carbon solubility rate, growth of continuous and defect free single layer graphene is possible. However, various factors such as graphene patches at grain boundary, defects on the surface, under growth, non-uniform growth and wrinkles on the surface influence the CVD process. In this paper, I provide detailed analysis of growth factors related to CVD graphene growth process, such as pre-treatment process, carbon and hydrogen flow rate and its ratio, growth timing, pressure conditions, and effect of cooling rate. With optimizing best growth condition for all these factors, I successfully synthesized high quality monolayer graphene. After successfully synthesis of the high quality and large area graphene, I fabricate the graphene-based THz modulator operating by ion-gel gating method and analyze its THz response. In the device, according to the Tinkham formula, the conductivity tuning of the graphene sandwiched two different dielectric material (in our research, ion-gel layer and quartz substrate) can significantly modify the phase and amplitude of the reflected THz signal at specific direction. The response of the device for forward and backward THz propagation directions are properly studied to demonstrate actively tunable amplitude and phase modulation of the THz wave. Based on transmission and reflection measurement of the THz wave, we achieved the intensity modulation up to 97% with continuous 0 to π phase shift at reflected signal with the applied electric signal less than few volts. Broad operation range in spectrum without any resonant feature is another key finding of our simple THz modulator device structure. Mono-layer, bi-layer, triple-layer and quad-layer samples were prepared by randomly stacking technique of the CVD graphene. The proposed research is based on actively tunable THz phase and amplitude modulation benefits from low operational voltage, excellent phase modulation, low insertion loss, simple device structure and wide operational range, in contrast to previously reported graphene-based phase modulator exhibiting complex structure, narrow operation range and large insertion loss.

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목차

Chapter 1: Introduction 1
Chapter 2: Fabrication of high-quality graphene 4
2.1 Introduction of graphene. 4
2.2 Properties of graphene. 4
2.3 Graphene growth. 8
2.4 Graphene quality determining factors. 11
2.4.1 Pre-treatment method. 13
2.4.2 Carbon and hydrogen flow rate 16
2.4.3 Cooling rate 18
2.5 Optimum growth conditions 21

Chapter 3: Graphene THz modulator. 23
3.1 Motivation. 24
3.2 Device fabrication. 27
3.3 Operational principle 29
3.4 Numerical calculation. 32
3.5 Experimental measurement set-up 34
3.6 Control experiment. 36
3.7 Experimental result. 37
Chapter 4: Conclusion 51
References 53

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