Characterizing Transient Dynamics of Hot Carriers via Terahertz Spectroscopies

Plasmonic hot electrons are high energy charges form as a natural outcome of the nonradiative plasmons decay in metallic nanostructures. Applications of hot carriers can be generally categorized in two subsets defined by the spatio-temporal characteristics of hot carriers. In the first set of applications, the confinement of hot carriers within the host plasmonic metal plays the key role. This regime of operation often relies on the electron and lattice heating, stemmed from the electron and electron-phonon thermalizations. Photothermal effects and the conventional Kerr-type optical nonlinearity fall into this category. The second set of applications, which has received a greater attention, relies on the interfacial transport of hot carriers over a Schottky junction that is formed at the interface of plasmonic metals and an adjacent dielectric material. Photovoltaics, photocatalysis, sub-picosecond nonlinear optics, and optoelectronics are among applications that immensely benefit from the transport of hot carriers. In these applications, understanding the transient dynamics of hot carriers from the generation to the transport and relaxation is the key to unlock the ultimate potentials this branch of plasmonics holds. Here, we study the dynamics of hot-carrier transport in terahertz (THz) regime by monitoring the time variation of the radiated THz fields following the interfacial transport of hot carriers. THz time domain spectroscopies enable us to exclusively monitor the transport dynamics regardless of other competing transient effects that occur with similar timescales and probabilities. The role of device symmetry, propagation vector of hot carriers, and electrical biasing will be discussed.