Recent studies in spintronics have highlighted ultrathin magnetic metallic multilayers as a novel and very promising class of broadband terahertz radiation sources. Such spintronic multilayers consist of ferromagnetic (FM) and non-magnetic (NM) thin films. When triggered by ultrafast laser pulses, they generate pulsed THz radiation due to the inverse spin-Hall effect – a mechanism that converts optically driven spin currents from the magnetized FM layer into transient transverse charge currents in the NM layer, resulting in THz emission. As THz emitters, FM/NM multilayers have been intensively investigated so far only at 800-nm excitation wavelength using femtosecond Ti:sapphire lasers. In this work, we demonstrate that an optimized spintronic bilayer structure of 2-nm Fe and 3-nm Pt grown on 500 μm MgO substrate is just as effective as a THz radiation source when excited either at λ = 400 nm, λ = 800 nm or at λ = 1550 nm by ultrafast laser pulses (pulse width ~100 fs, repetition rate ~100 MHz). Even at low incident power levels, the Fe/Pt spintronic emitter exhibits efficient generation of THz radiation at all three excitation wavelengths. The efficient THz emitter operation at 1550 nm facilitates the integration of such spintronic emitters in THz systems driven by relatively low cost and compact fs fiber lasers without the need for frequency conversion.
The inverse spin Hall effect (ISHE) can be used to generate broadband terahertz (THz) radiation. This has been demonstrated recently [1 – 3]. We report on efficient generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The magnetic and nonmagnetic layers were epitaxially grown on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. Using the device in a counterintuitive orientation a hyperhemispherical Si lens was attached to increase the collection efficiency of the emitter. In this arrangement multiple reflections of the THz pulses from the substrate surfaces are avoided as the metallic layers act as an antireflection coating .
The experimentally determined dependence of the THz signal on the layer thicknesses was in qualitative agreement with simulations of the ISHE in the Fe-Pt bilayer. An optimum layer thicknesses of 2 nm and 3 nm were found for Fe and Pt, respectively. The optimized emitter provided a bandwidth of up to 8 THz for both the sapphire and MgO substrates which was mainly limited by the GaAs photoconductive antenna used as detector. The dynamic range reached 60 dB for the MgO substrate at a frequency of 1.5 THz. The pulse length was as short as 220 fs for a pump pulse length of the 800 nm pump laser of about 50 fs. In the case of MgO substrates strong THz absorption of MgO reduced the dynamic range above 3 THz considerably.
Average pump powers as low as 25 mW (at a repetition rate of 80 MHz) have been used for terahertz generation. This and the general performance makes the spintronic terahertz emitter compatible with established emitters using nonlinear generation methods.
 T. Seifert et al., Nature Photonics 10, 483 (2016)
 D. Yang, et al., Advanced Optical Materials. doi:10.1002/adom.201600270 (2016)
 Y. Wu, Adv. Mater. doi:10.1002/adma.201603031 (2016)
 J. Kröll et al., Optics Express 15, 6552 (2007)