Ultrafast Terahertz Dynamics in Semiconductors and Nanomaterials

  • Author / Creator
    Purschke, David N
  • Ultrafast terahertz (THz) spectroscopy is presently the most powerful tool available to materials scientists for directly probing ultrafast conductivity dynamics. Its low center frequency and phase-resolved measurement, which yields sub-picosecond temporal resolution, make it ideally suited for studying properties such as carrier mobility and relaxation dynamics. In new materials and nanomaterials, with rapid carrier lifetimes and difficulties with electronic contacts, THz spectroscopy can sometimes be the only technique able to study the detailed carrier transport properties. Recent sources of intense THz pulses, enabled by the tilted-pulse front technique, have also facilitated new spectroscopic techniques to investigate phenomena such as ultrafast high-field transport and hot-carrier physics. Moreover, with the development of novel sources of THz radiation the future of this exciting field promises to yield new insight into the nature of light-matter interaction.

    This thesis explores three areas of THz science and technology, beginning with THz pulse generation via optical rectification with tilted-pulse fronts. New perspectives are presented that highlight the role of higher-order angular dispersion (AD) in the spatiotemporal intensity of tilted-pulse fronts. A simple setup for controlling quadratic AD is proposed that could have impact in multiple areas of ultrafast science. Moreover, a novel tilted-pulse front THz source in GaN is proposed. Simulations are used to show that, due to the large GaN band gap and phonon frequency, this source could be capable of high-intensity THz pulse generation in the 5-12 THz gap where there is a current lack of ultrafast and high-spectral-intensity table-top sources. The development of tilted-pulse front sources over several years in the Ultrafast Nanotools lab is also detailed, culminating in the generation of THz pulses with peak electric fields in excess of 600 kV/cm. Second, the modulation of semiconductor photoluminescence (PL) with intense THz pulses, THz-ΔPL, is explored and shown to result from THz-control of hot carriers. To validate this conclusion, a diffusion-based model is developed that simultaneously captures both PL quenching near the band gap and PL enhancement in the high-energy tail with a single tunable parameter: the initial hot-carrier temperature. With minimal assumptions as to the nature of the detailed hot-carrier physics, the model shows excellent qualitative and even semi-quantitative agreement with data. Moreover, the diffusion model is shown to accurately describe the THz-ΔPL in multiple direct-gap semiconductors and, in principle, should be broadly applicable to the study of hot-carrier dynamics in a variety of semiconductors. Further experimental results prove that the simple diffusion model captures the scaling of THz-ΔPL over range of injection levels, however, the model breaks down at high fluence due to optical nonlinearities. This suggests that even more information about the hot-carrier physics can be found from this simple experiment and several extensions to the model are proposed that could provide clarity.

    Finally, time-resolved THz spectroscopy (TRTS) is applied to probe the transient photoconductivity of SnIP nanowires for the first time. SnIP, an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, our understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. New insight into the highly anisotropic electronic structure from quantum chemical calculations, along with measurements of the carrier scattering time from TRTS, reveals a carrier mobility as high as 280 cm^2V^−1s^−1 along the double-helix axis and a hole mobility of 238 cm^2V^−1s^−1 perpendicular to the double-helix axis. This high mobility is supported by a detailed comparison between the commonly used Drude-Smith model, plasmon model, and Bruggeman model for the optical conductivity that shows a similarly high mobility for each model. Notably, our results indicate that Drude-Smith model most effectively describes transport in these SnIP nanowires. Additionally, measurements of the THz infrared active vibrational spectrum reveals two vibrational resonances, which show excellent agreement with first-principles calculations. Interestingly, an ultrafast photoexcitation induced charge redistribution is observed that manifests as a reduction in amplitude of a twisting mode of the outer SnI helix on picosecond timescales. Finally, TRTS reveals that the carrier lifetime and mobility are limited by a trap density greater than 10^18 cm^−3. Most importantly, these results demonstrate a remarkably high carrier mobility for such a soft and flexible material, suggesting that it could be ideally suited for future flexible-electronics applications.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • 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.