Research Overview: https://wp.nyu.edu/katsumi/
"Revealing unique light-matter interaction of the Higgs mode in superconductors by terahertz multidimensional coherent spectroscopy."
Light-matter interaction in quantum materials is one of the critical aspects that elucidate their intriguing properties. In particular, the terahertz (THz) frequency range is of great interest as it allows us to access rich low-energy excitations in such materials due to the complex interplay between charge, spin, orbital, and lattice degrees of freedom. Recent advancements in generating an intense THz pulse enabled the investigations of nonlinear light-matter interaction, which can provide information unreachable by the linear light-matter coupling. More recently, THz multidimensional coherent spectroscopy (THz MDCS) has emerged as a new technique to disentangle different nonlinear optical processes of magnons, phonons, and plasmons. Yet, understanding the THz MDCS spectra is still in its infancy. In this seminar, I will present the recent results of THz MDCS in the case of a conventional superconductor NbN to elucidate the light-matter interaction of the Higgs mode, an amplitude collective excitation of the superconducting order parameter [K. Katsumi et al., arXiv:2311.16449 (2023)]. Using broad-band THz pulses as light sources, we observed a third-order nonlinear optical response whose power spectrum peaked at twice the superconducting gap energy 2∆. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency Ω and exhibited a resonant enhancement at temperature when Ω = 2∆. General theoretical considerations show that such resonance can only arise from a disorder-activated paramagnetic light-matter interaction. Numerical simulations reveal that even for a small amount of disorder, the Ω = 2∆ resonance is dominated by the superconducting amplitude mode over the entire investigated disorder range. This study demonstrated the ability of THz MDCS to investigate the physics of the Higgs mode in superconductors, which would open a new avenue to study them in other types of superconductors.
