Ultrafast imaging of nonlinear terahertz dynamics in semiconductors

• Author / Creator

  Jensen, Charles E

• Ultrafast terahertz (THz) microscopy is an emerging field of research that leverages the imaging of picosecond electric field transients to explore free carrier responses from a near-field perspective. This thesis presents the first effort made to use an electro-optic imaging system to probe the subpicosecond changes in electron conductivity that can be induced by the intervalley scattering of conduction band electrons. Intervalley scattering will lower the overall conductivity in n-doped In0.53Ga0.47As thin films. The lowering of conductivity (and the subsequent enhancement of transmission) is herein referred to absorption bleaching. A near-field approach is necessary to properly understand intervalley scattering, since subpicosecond modulations in material conductivity give rise to a rectified THz pulse. Imaging in the near-field enables us to capture the rectified components of the transmitted electric fields before diffraction occurs. The presence of the rectified components is emphasized by looking to the time-domain evolution of the total electric field transmitted electric fields. We use a standard open aperture z-scan to characterize absorption bleaching in n-doped In0.53Ga0.47As thin films. This is the baseline from which a near-field electro-optic imaging z-scan is compared. Contrasting the baseline measurements to the results of the imaging z-scan, we find that the energies calculated from the ultrafast imaging z-scan follow nearly the same trend as the benchmark measurements. This marks the first evidence that a near-field electro-optic measurement of a transmitted THz electric field contains signatures which indicate THz-induced intervalley scattering is occurring in In0.53Ga0.47As thin films. In previous work, it was shown that the rectification of THz pulses can produce an asymmetric waveform, which, when integrated does not converge to zero. For the first time, we experimentally measure this on sub-picosecond time-scales by using a near-field electro-optic sampling system to measure THz waveforms in the near-field. Intense THz pulses passing through only a semi-insulating InP wafer are shown to possess no net asymmetry, whereas intense THz pulses passing through a negatively doped In0.53Ga0.47As epilayer grown on a lattice matched InP substrate are shown to diverge significantly. To interpret this result, we invoke a dynamic Drude model of conductivity that can be used to simulate nonlinear transmission. We find that the simulation is able to generate integrated THz electric fields that share similar features to those from experiment. This indicates that intervalley scattering is a material process that is capable of inducing subpicosecond changes in the transmitted electric field of intense THz pulses.

• Subjects / Keywords

  • Imaging
  • Terahertz
  • Ultrafast

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3SB3XC53

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.