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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2024)

Photoexcitation in materials generates nonequilibrium electrons, which take hundreds of femtoseconds to picoseconds to thermalize with phonons. Understanding thermal energy transport between electrons and phonons is crucial for various scientific and engineering applications, such as ultrafast demagnetization, nanophotonic and plasmonic phenomena. In my research, I utilized a pump/probe system to perform time-domain thermoreflectance (TDTR) measurements to investigate electron-phonon energy dynamics.In metals, absorption of light generates nonequilibrium electrons. Nonequilibrium electrons can transport heat over distances of tens to hundreds of nanometers before thermalizing with phonons. This diffusion causes a temperature rise in deeper layers of the film. At the metal/substrate interface, the resulting temperature difference induces picosecond acoustics, with the amplitude of the acoustics being proportional to the temperature rise. By measuring the change in acoustic amplitude along the film thickness, we determined the diffusion length of nonequilibrium electrons. To systematically study the role of nonequilibrium electrons in heat transport, we performed front/front and front/back measurements on thickness-gradient metals. These measurements provided insights into the spatial thermalization process within metals. We developed detailed heuristics to understand nonequilibrium electron energy transport. Additionally, we conducted similar experiments on bilayer samples, replacing the bottom ~20 nm metal with Fe or Pt, materials known for strong electron-phonon coupling g_ep. Due to the strong g_ep, there was few nonequilibrium electrons in the bilayer samples, leading to a different energy transport mechanism compared to monolayers. The results from bilayer were identical with monolayer, indicating that nonequilibrium electrons did not significantly affect energy transport. NbN films are promising superconducting materials widely used in various electronic devices, such as Josephson junction, hot electron bolometer and single-phonon detector. We conducted wavelength-dependent pump-probe measurements on different phases of NbN films to determine the electron-phonon coupling parameter g_ep. We studied the correlation between g_ep, density of states D(ε_f ) and transition temperature T_c and found a strong relationship between g_ep and T_c. In addition to characterization of nanomaterial with pump/probe system, I refined the process of nanofabrication. I successfully fabricated high-quality Au nanodisks and tuned their Local Surface Plasmon Resonance (LSPR) from 700 to 1000 nm.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    The development of approaches that can efficiently control the magnetization of magnetic materials is central to the creation of fast and low-power spintronic devices. Spin transfer torque can be used to electrically manipulate magnetic order in devices, but is typically limited to nanosecond timescales. Alternatively, spin–orbit torque can be employed, and switching with current pulses down to ~200 ps has been demonstrated. However, the upper limit to magnetization switching speed remains unestablished. Here, we show that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque. We probe the ultrafast magnetization dynamics due to spin–orbit torques with sub-picosecond resolution using the time-resolved magneto-optical Kerr effect (MOKE). We also estimate that the magnetization switching consumes less than 50 pJ in micrometre-sized devices.
      Cover page: Spin–orbit torque switching of a ferromagnet with picosecond electrical pulses
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