Efficient Terahertz Generation Using Fe/Pt Spintronic Emitters Pumped at Different Wavelengths

Recent studies in spintronics (STs) have highlighted ultrathin magnetic metallic multilayers as a novel and very promising class of broadband terahertz (THz) radiation sources. Such ST 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 paper, we demonstrate that an optimized ST bilayer structure of 2 nm Fe and 3 nm Pt grown on 500
$\mu \text{m}$
MgO substrate is just as effective as a THz radiation source when excited either at
$\lambda = 800$
nm or at
$\lambda = 1550$
nm by ultrafast laser pulses from a femtosecond fiber laser (pulsewidth ~100 fs and repetition rate ~100 MHz). Even with low incident power levels, the Fe/Pt ST emitter exhibits efficient generation of THz radiation at both excitation wavelengths. The efficient THz emitter operation at 1550 nm facilitates the integration of such ST emitters in THz systems driven by relatively low cost and compact femtosecond fiber lasers without the need for frequency conversion.