Polarization shaped terahertz sources play a key factor in terahertz wireless communications, biological sensing, imaging, coherent control in fundamental sciences, and so on. Recently developed spintronic terahertz emitters have been considered as one of the next-generation promising high performance broadband terahertz sources. However, until now, polarization control, especially for twisting the circularly polarized terahertz waves at the spintronic terahertz source, has not yet been systematically explored and experimentally achieved. In this work, we not only demonstrate the generation of circularly polarized terahertz waves in cascade spintronic terahertz emitters via delicately engineering the amplitudes, applied magnetic field directions, and phase differences in two-stage terahertz beams but also implement the manipulation of the chirality, azimuthal angle, and ellipticity of the radiated broadband terahertz waves. We believe our work can help with further understanding of the ultrafast optical magnetic physics and may have valuable contributions for developing advance terahertz sources and optospintronic devices.

The laser terahertz emission microscopy (LTEM) technique, which breaks through the resolution limitation of terahertz waves from millimeter to micrometer scales, has been widely used in many real application circumstances, such as contactless chip nondestructive testing, biosensing, imaging, and so on. Recently developed spintronic terahertz emitters featuring many unique properties such as high efficiency, easy integration, low cost, large size and so on, may also have great applications in LTEM, which can be called spintronic terahertz emission microscopy (STEM). To achieve high efficiency and good performance in STEM, we propose and corroborate a remnant magnetization method to radiate continuous and stable terahertz pulses in W/CoFeB/Pt magnetic nanofilms without carrying magnets on the transmitters driven by nJ femtosecond laser pulses. We systematically optimize the incidence angle of the pumping laser and find the emission efficiency is enhanced under oblique incidence, and we finally obtain comparable radiation efficiency and broadband spectrum 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 long 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 broadband spintronic terahertz emitters for the applications in STEM.

operating at or below room temperature, Q is the order of meV, and this order corresponds to terahertz (THz) frequencies, which imply that future quantum devices working in THz frequency range will achieve low power consumption and high-speed performance. Electron spin, which is expected to break through the bottleneck of traditional microelectronic devices, has many excellent properties, [7,8] such as unique quantum properties, extremely low transmission energy consumption, high computing speed, high density storage, long spin relaxation time and fast response, nonvolatile, and so on. Spintronic devices can overcome the problems of heat dissipation and quantum effect caused by the miniaturization and integration of characteristic size based on the electronic charge properties. Along with the rapid progress of ultrafast laser technology and the increase of the device operating frequency, the development of ultrafast spintronic devices working in THz band is in full swing. [7,[9][10][11][12][13][14][15][16][17] When employing femtosecond laser pulses to illuminate magnetic materials, ultrafast spin currents can be produced. [18] High-efficiency conversion from spin currents to charge currents can be realized by using heterostructures such as topological materials or heavy metals. [19][20][21] THz emission spectroscopy can be used for contactless detect femtosecond Recently fashionable spintronic terahertz (THz) emission provides fresh pathways for contactless diagnosing femtosecond spin currents, opens the door for developing next-generation high-performance THz emitters, and accelerates the interdisciplinary of ultrafast THz optospintronics. However, one of the research highlights of ultrafast magnetism and the difficulty for further improving the spintronic THz emission productivity is how to efficiently reduce the energy consumption of all-optical magnetitic manipulation and improve the optical spin injection efficiency. Here, it is demonstrated, for the first time, gold nanorods (GNRs) plasmonic resonances can effectively increase the THz emission from W/CoFeB/Pt heterostructures by 140%. Systematic investigations of THz yield dependences on pumping laser incidence directionality, polarization, and the GNRs dimension parameter conclusively manifest the surface plasmon resonance validity. Theoretical interpretations combined with Drude-Lorentz model and numerical simulations semiquantitatively reproduce the experimental results. The observations prove that THz efficiency can be improved by nanophotonic technologies and may also spark inspiration for developing functional nano-THz optospintronic devices.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202101296.

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