Contents I. Introduction
THANKS to the highly efficient spin-to-charge current conversion in ferromagnetic/heavy metal heterostructures, spintronic terahertz (THz) emitters can radiate ultrabroadband THz waves covering the whole frequency range from 0.1-30 THz [1]. It features many merits such as low cost, easy fabrication, large size and so on, which endows such kind of emitters also very promising for strong-field THz generation [2–6]. Although many pioneering studies and groundbreaking achievements have already been obtained, this interesting research topic is still in its infancy and needs further deep theoretical and experimental investigations, especially for the material fabrication conditions. Recent progress of systematical studies on sample roughness, crystal structure, interface intermixing has demonstrated that the performance of spintronic THz emitters has strong correlation to the sample fabrication parameters [7]. Furthermore, it has also been discovered that 1.5-time enhancement of THz efficiency has been realized through annealing [8]. However, when we systematically investigate this problem, we achieve 3 times higher efficiency after annealing W/CoFeB pumped femtosecond laser pulses, and we also further study the origin of this enhancement phenomenon.
