Construction of terahertz spectrometers and their use in characterization of conductivities and modulation behaviors of graphene layers /

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2017
Kaya, Emine
Aim of this thesis was to construct two new terahertz spectrometers, THz Time Domain Spectroscopy (THz-TDS) and Time Resolved THz Spectroscopy (TRTS) for characterization of the static and dynamic properties of novel materials. After successful construction and signal optimization, system characterizations of both systems were completed with well-known samples. Possible use of ionic liquid doped graphene devices on PVC and PE substrates were characterized by THz-TDS. Devices operated at low voltages and provided nearly 100 % modulation between 0.2 THz and 1.5 THz at ca. 3.5 V. High modulation depth over such a broad spectrum and simple device structure make these modulators promising candidates in THz and related technologies. THz sheet conductivities of large-area CVD grown graphene samples were extracted with THz-TDS measurements. Comparison of results with literature showed that THz-TDS is a noncontact and reliable technique that allows characterization of frequency dependent sheet conductivity of graphene. Carrier relaxation dynamics of graphene samples were investigated with TRTS. A decrease in photo induced conductivity is observed with single layer graphene sample with 800 nm excitation due to its highly doped nature. Thickness dependent THz dynamics of multi layer graphene samples with 800 nm showed that samples had similar initial responses which can be explained as hot carrier cooling via carrier-carrier scattering and carrier-phonon scattering. Existence of interlayer charge transfers were suggested with observation of slower decay times of thicker samples due to slower recombinations. 400 nm pump studies of samples resulted in comparably shorter decay times due to initial hot carrier cooling and fast recombination rates of charges stemming from existence of easier interlayer transfer of charges.
Citation Formats
E. Kaya, “Construction of terahertz spectrometers and their use in characterization of conductivities and modulation behaviors of graphene layers /,” Ph.D. - Doctoral Program, Middle East Technical University, 2017.