  Flexible manipulation of terahertz-wave polarization during the generation
process is very important for terahertz applications, especially for the
next-generation on-chip functional terahertz sources. However, current
terahertz emitters could not satisfy such demand, hence calling for new
mechanism and conceptually new terahertz source. Here we demonstrate a
magnetic-field-controlled, highly-efficient, cost-effective, and broadband
terahertz source with flexible switch of terahertz polarization states in
ferromagnetic heterostructures driven by femtosecond laser pulses. We verify
that the chirality, azimuthal angle, and ellipticity of the generated
elliptical terahertz waves can be independently manipulated by delicately
engineering of the external applied magnetic fields via effectively
manipulating the photo-induced spin currents. Such an ultrafast photomagnetic
interaction-based, magnetic-field-controlled, and broadband tunable solid-state
terahertz source integrated with terahertz polarization tunability function not
only has the capability to reveal physical mechanisms of femtosecond spin
dynamics, but also demonstrates the feasibility to realize novel on-chip
terahertz functional devices, boosting the potential applications for
controlling elementary molecular rotations, phonon vibrations, spin
precessions, high-speed terahertz communication, and accelerating the
development of ultrafast terahertz opto-spintronics.
