We demonstrate a control of spin-polarized currents and a nonlinear response to the excited state of magnetic films through measuring the terahertz waves generated by illuminating a W/CoFeB/Pt structure with a pair of femtosecond laser pulses. By changing the delay time of the two laser pulses, the waveforms and spectra of the terahertz waves are efficiently modulated. The terahertz wave generated by one pulse is obviously influenced by the other. The experimental results can be explained by a microscopic three-temperature model, which serves as proof for spin-flip scattering contributing to ultrafast demagnetization. Our experiment offers a new method for the investigation of ultrafast magnetism and terahertz spintronics.

The intrinsic chirality of natural materials is known to be weak, prompting extensive efforts to enhance chiral light-matter interactions on the metasurface platform. Chiral metasurfaces are typically created by manipulating the geometry of nanostructures, such as three-dimensional helical structures and slanted structures. However, these approaches are generally challenging to implement experimentally in optical frequency ranges. Here, we present the achievement of significant chirality on planar lithium niobate metasurfaces. We theoretically demonstrate that the birefringence of lithium niobate enables a strong coupling between two nearly degenerate resonant modes when rotating the optical axis. Despite the achiral geometric morphology of the lithium niobate structure, we show that these mixed modes exhibit chirality and can produce nearly full circular dichroism signals. Moreover, we experimentally validate our theoretical proposal using our advanced process technique for lithium niobate nanostructures and the measured circular dichroism signal reaches -0.53. These findings offer new possibilities for chiral metaphotonics.