Time-Resolved THz Magnetospectroscopy: Techniques and Applications

Abstract

Ultrafast terahertz (THz) spectroscopy has been advanced by a research effort that includes technique development as a central focus. Multiple paradigm-shifting breakthroughs in experimental techniques have emerged in recent decades, including optical pump/THz probe spectroscopy, THz polaritonics, and THz magnetospectroscopy. Motivated by growing interest in integrating THz spectroscopy with an external magnetic field—THz magnetospectroscopy—the work of this thesis has been to develop THz spectroscopy techniques in the service of enabling THz magnetospectroscopy beyond the linear absorption regime. I have approached this goal via two primary routes.

The first is the development of a THz generation scheme in the context of the THz polaritonics platform (waveguide-based THz generation, interaction, and readout), aimed at reaching the THz amplitudes necessary for nonlinear waveguide-based THz magnetospectroscopy. We used a novel noncollinear velocity-matching scheme based on total-internal reflection of 800 nm pump beam, such that the in-plane velocities of the pump and THz are equal. Using this scheme we observe more than 10x enhancement of the THz spectral amplitude, and nearly 100x enhancement of the THz pulse energy. Finite-difference (FDTD) simulations are presented, which elucidate pump depletion mechanisms and indicate the possibility of further enhancement by using longer pump wavelengths.

The second is the development of a free-space optical pump/THz probe (OPTP) magnetospectrometer. This instrument includes kHz repetition rate single-shot detection, which decreases acquisition time and has enabled some of the first time-resolved measurements of carrier relaxation via cyclotron resonance experiments in bulk Si. In these measurements we observe directly the magnetic field dependence of the carrier relaxation time, which decreases with increasing magnetic field—largely a consequence of Landau-level degeneracy. We also present experimental evidence of a B-field induced metastable multiferroic state in an anomalous Y-type hexaferrite, along with time-resolved measurements of the spin reorientation transition in this material induced by laser heating.

Together, the methods developed represent significant steps towards enabling time-resolved studies of THz-frequency magnetic dynamics.