Terahertz emission characteristics of a metasurface-enhanced spintronic terahertz emitter

An improved terahertz (THz) emission from a spintronic ferromagnetic/nonmagnetic bilayer film was demonstrated by coupling with a metasurface. The spintronic bilayer was grown by the e-beam deposition of nanometer-thick Ni/Pt films on an MgO substrate. On the opposite side of the MgO substrate, the metasurface consisting of a metal line array of 10 μm width, 410 μm periodicity and 110 nm thickness were fabricated via standard UV photolithography and resistive evaporation deposition of gold. At the lower frequency range, the increase in the THz power for the metasurface-enhanced sample was as much as two orders of magnitude compared to a bare Ni/Pt spintronic emitter. A broadband enhancement in the THz power spectra was observed from 0.05 to 2 THz. The THz power enhancement spectrum of the spintronic THz emitter (STE) with metasurface shows good agreement with the calculated surface plasmon resonance frequencies. The optical fluence excitation dependence of the THz emission showed enhancement over a wide range of fluence values. Moreover, the THz emission enhancement is compared with that of a photoconductive antenna employing an identical metal line array structure. The metasurface design results in higher enhancement factor in the STE, which may be attributed to a better matching of its frequency range with the calculated resonance frequencies. This proposed design improves the efficiency at which a metal bilayer STE is launched into free space; thereby improving the signal-to-noise ratio. These results offer a way of conveniently making the investigation of STE physics, more feasible.