Time-domain terahertz spectroscopy and its application to antiferromagnetic resonances*

Antiferromagnets have shown promise for spintronics and quantum computing due to their high resonance frequencies in terahertz range. It is therefore important to have an experimental technique that can measure these resonances and their properties with high spectral resolution. Time-domain terahertz spectroscopy (TDTS) is a popular technique for measuring phonon and plasmon resonances, and can be extended to measuring magnons in a spectral range inaccessible to other methods such as FTIR or microwaves. While terahertz emission from magnetic heterostructures has been the primary focus of THz spintronics, terahertz absorption is promising for measuring antiferromagnetic magnon resonances. A common limiting factor for THz absorption spectroscopy is the Fabry-Pérot (FP) reflections that cause oscillations in the absorption spectra and thus reduce the resolution. Experimental and computational methods to remove these oscillations from the spectra are discussed for the antiferromagnetic insulator NiO. The 8 GHz resolution of the TDTS system resolves a magnon frequency shift that depends on the crystallographic orientation, which was previously unresolved due to the FP reflections. Furthermore, the temperature-dependence of the magnon resonances is investigated. Optical pump-terahertz probe spectroscopy is introduced and the time-resolved coupling mechanisms of resonance excitations is discussed.