Determining hot-carrier transport dynamics from terahertz emission

Understanding the ultrafast excitation and transport dynamics of plasmon-driven hot carriers is critical to the development of optoelectronics, photochemistry, and solar-energy harvesting. However, the ultrashort time and length scales associated with the behavior of these highly out-of-equilibrium carriers have impaired experimental verification of ab initio quantum theories. Here, we present an approach to studying plasmonic hot-carrier dynamics that analyzes the temporal waveform of coherent terahertz bursts radiated by photo-ejected hot carriers from designer nano-antennas with a broken symmetry. For ballistic carriers ejected from gold antennas, we find an ~11-femtosecond timescale composed of the plasmon lifetime and ballistic transport time. Polarization- and phase-sensitive detection of terahertz fields further grant direct access to their ballistic transport trajectory. Our approach opens explorations of ultrafast carrier dynamics in optically excited nanostructures. When electrons are optically excited high into the conduction band, they usually thermalize rapidly, losing energy through scattering processes. The ability to efficiently extract hot-carriers in the upper conduction band would be useful but is hampered by the lack of understanding of the processes involved. Taghinejad et al . developed a technique based on analyzing the terahertz pulses emitted by the hot-carriers (see the Perspective by Bangle and Mikkelsen). Modeling allowed them to unravel the spatiotemporal dynamics of the hot-carrier transport, providing a clearer picture for developing applications such as energy harvesting and photochemistry that can exploit these high-energy hot-carriers. —Ian S. Osborne The spatiotemporal dynamics of hot-carriers can be determined from emitted terahertz pulses.