Spintronic Terahertz Emission Spectroscopy Based on Ultrafast Terahertz Scattering Scanning Near-Field Optical Microscope

Spintronic terahertz (THz) emitters offer distinct advantages such as high efficiency, ultrabroadband capability, low cost, and easy integration. These emitters find applications not only in THz time-domain spectrometers driven by high-repetition-rate laser oscillators but also in the generation of intense THz electromagnetic pulses powered by high-energy femtosecond laser amplifiers. They have proven valuable in THz spectroscopy imaging and the exploration of strong-field THz physics. However, previous research on spintronic THz radiation mechanisms and device development relies primarily on far-field THz time-domain spectroscopy. The results of this approach present average THz emission information for the laser-pumped spot areas, which does not provide any insights into ultrafast spin currents and THz emission properties for the materials at micro-and nano-scales. In this study, we employ ultrafast THz scattering scanning near-field optical microscopy, driven by a femtosecond fiber laser oscillator, to investigate the spintronic terahertz emission properties of the ferromagnetic heterojunction material W/CoFeB/Pt at nanoscale. The utilization of this technology enables the detection of high signal-to-noise ratio spintronic THz emission at transverse scales as small as hundreds nanometers. This novel approach explores the generation, detection, and manipulation of ultrafast spin currents at THz frequencies with nano-spatial resolution. This study may inspire innovative ideas for the advancement of ultrafast THz spin optoelectronics.