Studies for enhancing THz emissions from optically excited spintronic metallic films
Metallic spintronic THz emitters, which consist of ferromagnetic (FM) and non-magnetic (NM) metallic thin film layers, are attracting much interest because of the unique emitting mechanism and characteristics. The THz emission from the metallic heterostructure originates either from the inverse-Hall effect or Rashba-Edelstein effect, both of which convert spin current to charge current through spin-orbit coupling. The spintronic THz emitters can be pumped by femtosecond laser pulses with a wide-range of wavelengths, and its emission can be very broadband (~30 THz). In addition, the ease with the device fabrication and high damage thresholds are strong advantages over other types of THz emitters, such as photoconductive antennas, even though the emission efficiency is relatively low. In this work, we report studies for enhancing the THz emissions from optically excited spintronic metallic films. Our model structure is Pt/Fe film on MgO substrate. The metallic thin films were deposited and shaped on MgO substrate (~500 m) by e-beam deposition and standard photolithographic technique, respectively. Pt/Co spintronic THz emitters on fused silica substrate were also fabricated and evaluated for comparison. Firstly, we have investigated the film-layer thickness dependences of these THz spintronic emitters and specified the optimized thickness; Pt = 3 nm and Fe = 2 nm for Pt/Fe emitter and Pt ≈ 5 nm and Co ≈ 5 nm for Pt/Co emitter. Secondly, we have investigated the pump wavelength dependences of Pt/Fe spintronic emitter in a wide range from 400 nm to 2,700 nm. We found there is no strong pump wavelength dependence of THz emission efficiency (per pump power) in the range from 400 to 2,200 nm. However, at longer wavelengths beyond 2,200 nm, we observed a sharp decrease of THz emission efficiency and its expected threshold at ~0.35 eV (~3.5 m). The existence of the threshold indicates a barrier at the Pt/Fe interface or a gap in the work function between the Fe and Pt layer. Lastly, we have investigated the influence of the antenna-shaped structure to the THz emission of spintronic films on THz emission. In this regard, we have fabricated circular-, rectangular-, and diabolo-shaped Pt/Fe THz emitters and found that such antenna-shaped structures enhanced the THz emission compared to un-shaped ones. The diabolo-shaped Pt/Fe emitter with a 100-nm contact thickness of Pt exhibited almost doubled efficiency compared with the un-shaped Pt/Fe emitter. The enhancement of the THz emission is attributed to the antenna effect of the shaped structures, which improves the out-coupling efficiency of THz radiation from the metallic film to the free space. In this conference presentation, we report these results in detail and discuss the influence of each factor to THz emission.