Graduate Thesis Or Dissertation


High-Field Terahertz Spectroscopy of Nanoscale Materials Public Deposited

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  • This dissertation covers my work relating to the application of strong terahertz (THz) radiation to study the nonlinear properties of nano-scale material. It presents experimental and numerical studies on the optical and electronic properties of various material system including single-layer graphene, multi-walled carbon nanotubes (MWCNTs), nanoantenna-patterned gallium arsenide (GaAs) and planar metamaterial on GaAs. The THz generation and detection system was developed by our group through several years both in hardware and software to employ strong and short THz pulses for time-domain pump-probe THz/optical spectroscopy. Due to the low photon energy (4 meV for 1THz) the broadband single-cycle THz pulses are useful for noncontact, non-invasive probing of the fundamental electronic properties of material such as conductivity, permittivity and photocarrier relaxation dynamics. The images of carrier conductivity of single-layer graphene samples were obtained by recording the THz transmission via raster scanning of the samples. The theory to extract the sheet conductivity of a single-layer graphene is based on the Drude model, analyzing the THz transmission with the thin-film Fresnel coefficients. Time-resolved measurements of photoexcited single-layer graphene were also performed. The time-resolved data from the pump-probe experiment reveal that both the strong THz field and photoexcitationenhance the THz transmission due to the increase of the carrier scattering rate, however, the relaxation of photocarriers shows opposite effects of the THz fields and photoexcitation. The photoexcitation increases the relaxation time via the reabsorption of optical phonon by photocarriers, while strong THz fields reduce the relaxation time because the fields redistribute the empty states in the conduction band and open up more phonon emission path. THz raster-scan imaging on aligned MWCNTs shows strong anisotropic linear and nonlinear THz responses. Strong nonlinear absorption was observed when THz field polarization was parallel to the MWCNTs axis, on the other hand, no nonlinear effect was observed for perpendicular polarization. The THz waveforms transmitted through the sample obtained by THz time-domain spectroscopy (THz-TDS) reveal phase shifts. Numerical analysis on the waveforms to extract the complex refractive index of material shows that strong THz field enhances the permittivity of the MWCNTs by inducing strong nonlinear electron dynamics. Time-resolved measurements on optically excited MWCNTs show that the photoexcitation induces interband transition in the sample. For the low and high THz regime (E_THz < 538 kV/cm, ETHz > 653 kV/cm) an increase of the carrier density by generating the hot carriers via interband transitions was observed. On the contrary in the moderate THz field strength (538 < E_THz < 653 kV/cm) the dominant sub-band scattering gives rise to more effective mass for the carriers, and it reduces their mobility. As a result, a switching feature in the THz field response of the optically excited sample was observed. THz plasmonic structures, consisting of a nanoantenna array fabricated on GaAs, have been tested for ultrafast optical switching by strong THz pulses. Photocarrier injection by optical excitation immediately turns off the plasmon resonance of the antenna, while the strong and short THz pulses instantly revive the antenna resonance. The strong THz fields drive intervalley scattering and interband tunneling of the photocarriers and consequently reduce the transient conductivity of the photoexcited GaAs leading to the revival of the antenna resonance. Time-resolved THz transmission of bare GaAs sample compared to that of the patterned GaAs shows that the carrier recovery rate is much faster in the patterned sample.Lastly, we studied a metamaterial structure fabricated on intrinsic GaAs to observe the dynamics of the plasmon induced transparency (PIT) when the sample is optically excited. The PIT structure shows a narrow spectral window of transparency due to the destructive interference of a super-radiant resonator (a central dipole antenna) to the sub-radiant resonator (split-ring resonator). We observed that the response of the PIT structure to the incident THz field does not show any nonlinear response. However, optical excitations resulted in the damping of the PIT resonance. Adjusting the timedelay between the excitation pulse and the THz incident field reveals a pulsation response in the transmitted THz pulses along with self-phase modulation and red-shift in the PIT response.
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