  To achieve high efficiency and good performance of spintronic terahertz
sources, we propose and corroborate a remnant magnetization method to radiate
continuously and stably terahertz pulses from W/CoFeB/Pt magnetic nanofilms
without carrying magnets on the transmitters driven by femtosecond laser
pulses. We systematically investigate the influences of the pumping central
wavelength and find out the optimal wavelength for a fixed sample thickness. We
also optimize the incidence angle of the pumping laser and find the emission
efficiency is enhanced under oblique incidence. Combing the aforementioned
optimizations, we finally obtain comparable radiation efficiency and broadband
spectra in W/CoFeB/Pt heterostructures compared with that from 1 mm thick ZnTe
nonlinear crystals via optical rectification under the same pumping conditions
of 100 fs pulse duration from a Ti:sapphire laser oscillator, which was not
previously demonstrated under such pulse duration. We believe our observations
not only benefit for a deep insight into the physics of femtosecond spin
dynamics, but also help develop novel and cost-effective ultrabroadband
spintronic terahertz emitters.
