The emerging two-dimensional ferromagnetic materials present atomic layer thickness and a perfect interface feature, which have become an attractive research direction in the field of spintronics for low power and deep nanoscale integration. However, it has been proven to be extremely challenging to achieve a room-temperature ferromagnetic candidate with well controlled dimensionality, large-scale production, and convenient heterogeneous integration. Here, we report the growth of wafer-scale two-dimensional Fe3GeTe2 integrated with a topological insulator of Bi2Te3 by molecular beam epitaxy, which shows a Curie temperature (T c ) up to 400 K with perpendicular magnetic anisotropy. Dimensionality-dependent magnetic and magnetotransport measurements find that T c increases with decreasing Fe3GeTe2 thickness in the heterostructures, indicating an interfacial engineering effect from Bi2Te3. The theoretical calculation further proves that the interfacial exchange coupling could significantly enhance the intralayer spin interaction in Fe3GeTe2, hence giving rise to a higher T c . Our results provide great potential for the implementation of high-performance spintronic devices based on two-dimensional ferromagnetic materials.

0.1-30 THz) have experienced an unprecedentedly rapid progress with potential in fundamental sciences and real applications, especially under the fast development of ultrafast laser technology. [1] Recently, terahertz technologies have also been successfully employed in laboratories as powerful tools for triggering other novel related researches and applications, such as electron accelerations, [2] strong-field nonlinear phononics, [3] nonlinear terahertz photonics, [4] nonionization imaging, [5] and so on. Significantly large potential of terahertz technology could be envisaged in its real applications such as nondestructive spectroscopy [6] and ionization-free imaging, highly sensitive sensing, [7] short-range wireless communication at terahertz bit rates, [8,9] and so on. However, hindering the development of this fascinating technology from real applications lies in the lack of highly efficient terahertz sources, versatile functional devices, and sensitive detectors, as well as compact and robust systems. Among them, terahertz sources are significantly important, especially for those integrated with flexible manipulation of terahertz polarization states. As well demonstrated, when carrying with optical spin and angular momentum, [10] it enables another freedom of utilizing terahertz waves and boosts Flexible manipulation of terahertz wave polarizations during the generation process is very important for terahertz applications, especially for the next-generation on-chip functional terahertz sources. However, current terahertz emitters cannot satisfy such demand, hence calling for new mechanisms and conceptually new terahertz sources. Here an efficient and broadband terahertz source with magnetic-field-controlled flexible switching for the polarizations between linear and elliptical states in ferromagnetic heterostructures driven by femtosecond laser pulses is demonstrated. More importantly, the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz wave can be precisely manipulated by harnessing external magnetic fields via effectively tailoring the photoinduced spin currents. Such an ultrafast photomagnetic interaction-based, magnetic-field-controlled, and broadband tunable solid-state terahertz source integrated with polarization tunability functions not only provides 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 communications, and accelerating the development of ultrafast terahertz optospintronics. Terahertz RadiationThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

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.

Label-free biosensor operating within the terahertz (THz) spectra have helped to unlock a myriad of potential terahertz applications, ranging from bio-material detection to point-of-care (PoC) diagnostics. However, the THz wave diffraction limit and the lack of emitter-integrated THz biosensor are hindering the proliferation of high resolution near-field label-free THz biosensing.Here, a monolithic THz emission biosensor is achieved for the first time by integrating asymmetric double-split ring resonator metamaterials with a ferromagnetic heterojunction spintronic THz emitter. This device exhibits an electromagnetically induced transparency window with resonance frequency of 1.02 THz and a spintronic THz radiation source with a bandwidth of 900 GHz, which are integrated on a fused silica substrate monolithically for the first time. It was observed that the Page 2 of 23 ACS Paragon Plus Environment ACS Applied Materials & Interfaces 3resonance frequency experienced a redshift behavior along with increasing the concentration of Hela cells and Pseudomonas due to the strong interaction between the spintronic THz radiation and the biological samples on the metamaterials. The spatial frequency redshift resolution is ~ 0.01 THz with a pseudomonas concentration increase from ~ 0.5×10 4 /mL to ~ 1×10 4 /mL. The monolithic THz biosensor is also sensitive to the sample concentration distribution with 15.68 sensitivity under spatial resolution of 500 μm, which is determined by the infrared pump light diffraction limit. This THz emission biosensor shows great potential for high resolution near-field biosensing applications of trace biological samples.

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.