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Femtosecond control of electron spin not only promises the capability of satisfying the ever-increasing demand of storage information and ultrafast manipulation of magnetization in mediums, but also delivering controllable, highlyefficient, cost-effective and compact terahertz sources. Femtosecond spin dynamics have been extensively investigated these years with the methods of ultrafast magnetic-optical Kerr effect, inverse Faraday effect, inverse spin Hall effect and so on. Recently emerged coherent terahertz emission spectroscopy can also be employed to study this ultrafast spin dynamics with its unique advantages. For example, terahertz emission spectroscopy is a coherent, time-resolved, contactless Ampere-meter, which can be used to deduce the spin-charge conversion. However, femtosecond laser interaction with magnetic mediums is a rather complex process, there are still lots of physical mechanisms waiting to be unveiled. Here, we systematically investigate the femtosecond spin dynamics in ferromagnetic materials via polarization-resolved terahertz emission spectroscopy. We obtain detectable electromagnetic field radiation with its polarization parallel to the external magnetic field direction, which was not observed in the same materials in previous work. Inverse spin-orbit torque tilting is responsible for the observed phenomenon. Based on this mechanism, the efficiency and polarization of the generated terahertz waves can be coherently controlled and manipulated not only by the external magnetic fields, but also by the sample structures and the pumping femtosecond laser pulses. Our work not only helps further deepen understanding of the physical mechanism of all-optical magnetization reversal, boosting future spin recording technology, but also offers a very promising way for developing novel and efficient terahertz functional sources and devices.
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