Abstract-Nonlinear magnetization dynamics driven by strong terahertz fields

Matthias Hudl, Massimiliano d'Aquino, Matteo Pancaldi, See-Hun Yang, Mahesh G. Samant, Stuart S. P. Parkin, Hermann A. Dürr, Claudio Serpico, Matthias C. Hoffmann, Stefano Bonetti

https://arxiv.org/abs/1903.08395

We present a comprehensive experimental and numerical study of magnetization dynamics triggered in a thin metallic film by single-cycle terahertz pulses of ∼20 MV/m electric field amplitude and ∼1ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect (MOKE), and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing table-top terahertz sources.

Abstract-Anti-reflection coating design for metallic THz meta-materials

Matteo Pancaldi, Ryan Freeman,  Matthias Hudl,  Mattias C. Hoffmann,  Sergei Urazhdin,  Paolo Vavassori,   Stepfano Bonett

https://128.84.21.199/pdf/1711.05670.pdf


We developed a silicon-based, single-layer anti-reflection coating that suppresses the reflectivity of metals at near-infrared wavelengths, enabling optical probing of nano-scale structures embedded in highly reflective surroundings. Our design does not affect the interaction of terahertz (THz) radiation with these metallic structures that can be used to achieve THz near-field enhancement. We demonstrated the functionality of the design by calculating and measuring the reflectivity of both infrared and THz radiation from a silicon/gold double layer as a function of the silicon thickness. We also fabricated the unit cell of a THz meta-material, a dipole antenna comprising two 20-nm thick extended gold plates separated by a 2 µm gap, where the THz field is locally enhanced. We used the time-domain finite element method to demonstrate that such near-field enhancement is preserved in the presence of the anti-reflection coating. Finally, we performed magneto-optical Kerr effect on a single 3-nm thick, 1-µm wide magnetic wire placed in the gap of such a dipole antenna. The wire only occupies 2% of the area probed by the laser beam, but its magneto-optical response can be clearly detected. Our design paves the way for ultrafast time-resolved studies of strong THz-driven dynamics in nano-structures using table-top femtosecond near-infrared lasers. 

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