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Highly-efficient terahertz pulse generation and its applications for terahertz nonlinear spectroscopy on graphene
- Alternative Title
- In Hyung Baek
- Author(s)
- 백인형
- Alternative Author(s)
- In Hyung Baek
- Advisor
- Fabian Rotermund
- Department
- 일반대학원 에너지시스템학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2014-02
- Language
- eng
- Keyword
- terahertz wave; graphene; nonlinear spectroscopy
- Alternative Abstract
- In this thesis, single-cycle terahertz (THz) pulses are considered in three respects: (i) efficient THz pulse generation via optical rectification, using non-collinear phase-matching in LiNbO3; (ii) novel methodologies for THz nonlinear spectroscopy using such intense THz pulses; and (iii) THz linear/nonlinear spectroscopy on mono- to multi-layer graphene, which is a material relevant to the THz region of the electromagnetic spectrum owing to the unique behavior of electrons within it. We begin by introducing a nearly diffraction-limited Gaussian THz beam with an electric field as strong as 350 kV/cm. Such a field is obtained by utilizing a tilted-pulse front-pumping (TPFP) method in order to compensate for huge index dispersion inside lithium niobate crystals. Pump-to-THz optical conversion efficiency of 1.36×10-3 is obtained with 2.4 mJ pumping. Its maximum average power is approximately 3.3 mW at a repetition rate of 1 kHz. Several technical methodologies based on this THz pulse generation system and with use in nonlinear THz-spectrum experiments are then discussed. These include THz nonlinear time-domain spectroscopy, THz nonlinear transmission/z-scan measurement, and THz-pump/THz-probe time-resolved spectroscopy. We discuss their design and use for observation of the presence of THz nonlinearities in both mono- and multi-layer graphene structures, which are synthesized through chemical vapor deposition (CVD) and stacked with an innovative graphene-transfer technique. THz linear absorption behavior of graphene is then theoretically and experimentally investigated. A modified Drude model, including Fabry-Perot interference, agrees well with the experimental results. We also report that the conductivity of our randomly-stacked graphene increases with the number of layers, without exhibiting the electronic degradation usually observed in multi-layer graphene constructed using similar techniques. This can be explained with the idea that the random orientations of neighboring graphene layers counteract the degradation of conductivity. These behaviors of graphene are also reflected in nonlinear THz-region experimental results. Graphene exhibits a much higher nonlinear absorption coefficient in the THz spectral region than it does in the visible or near-infrared spectral regions because of enhanced free-carrier absorption. Excited free-carriers emit many phonons, transferring their energy to the lattice. The carrier-phonon scattering process reduces carrier mobility and simultaneously enhances the THz absorption of graphene. We observed for the first time that the change of free-carrier-induced THz nonlinearity in graphene depends on the number of layers. Our observations of the phonon-coupled intraband free carrier relaxation dynamics for graphene are then reported. These measurements were taken by probing the transmission of weak THz pulses under another intense THz pumping. Hot electrons rapidly relax because of scattering with optical phonons within 3 ps, depending on the incident THz fluence. Subsequently, electronic temperatures relax to the substrate temperature, via scattering with acoustic phonons and thermal recombination etc.
- Fulltext
- Appears in Collections:
- Graduate School of Ajou University > Department of Energy Systems > 3. Theses(Master)
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