Terahertz emission characterization of silicon based ferromagnetic heterostructures

Abstract

Since many applications of terahertz spectroscopy and imaging, the generation of broadband terahertz radiation is one of the most important challenges faced by terahertz scientists. Opto-spintronic terahertz emitters, composed of nanometer-thin magnetic multilayer, have the capability of producing high-quality, broad-band terahertz pulses. Integration of optospintronic terahertz emitters onto the silicon wafers is the first step towards their usage in modern photonic devices. In this work, Ta/CoFeB/Ir heterostructures were deposited on thermally oxidized silicon wafer by dc magnetron sputtering. Under illumination of a femtosecond laser pulse on the Ta/CoFeB/Ir trilayer heterostructure grown on silicon substrate, a spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is transported and injected into the neighboring non-magnetic metal layers of Ta and Ir. Consequently, the spin current can be converted into the charge current due to the strong spin-orbit coupling. The sub-picosecond transient charge current gives rise to the terahertz radiation into the free space. The terahertz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the heterostructure. The THz radiation from Ta/CoFeB/Ir heterostructure covers the 0.1 - 2.5 THz frequency range with the maximum around 0.64 THz. We also investigate the dependence of THz peak-to-peak values on the pump fluence. The THz emission was found to be saturated at pump fluences of ~0.73 mJ/cm<sup>2</sup>. Our results demonstrate the existence of the strong spinorbit coupling in the heavy metal Ir. Furthermore, we optimize the THz emission from the Ta/CoFeB/Ir heterostructure by change the thickness of Ir layer. According to the thickness dependence of THz emission from the heterostructure, the propagation lengths of the spin current at THz frequencies was extracted to be about 0.59±0.12 nm, which reveals a factor of 2 shorter as compared to GHz experimental measurement (~1.34 nm). Our experimental observation is consistent with that observed in the antiferromagnet IrMn layer, which may be attributed to different transport regimes. Finally, the optimized thicknesses were theoretically obtained as 2.4 nm and 1.1 nm for CoFeB and Ir layers, respectively.