Identifying materials with an efficient spin-to-charge conversion is crucial for future spintronic applications. In this respect, the spin Hall effect is a central mechanism as it allows for the interconversion of spin and charge currents. Spintronic material research aims at maximizing its efficiency, quantified by the spin Hall angle and the spin-current relaxation length . We develop an all-optical contact-free method with large sample throughput that allows us to extract and . Employing terahertz spectroscopy and an analytical model, magnetic metallic heterostructures involving Pt, W and Cu80Ir20 are characterized in terms of their optical and spintronic properties. The validity of our analytical model is confirmed by the good agreement with literature DC values. For the samples considered here, we find indications that the interface plays a minor role for the spin-current transmission. Our findings establish terahertz emission spectroscopy as a reliable tool complementing the spintronics workbench. Figure 1. Schematic of the experiment. (a) Terahertz emission experiment. A femtosecond nearinfrared pump pulse excites electrons in both the ferromagnetic (FM, in-plane magnetization ) and non-magnetic (NM) metal layer. Due to the asymmetry of the heterostructure, a spin current is injected from the FM into the NM material where it is converted into an in-plane charge current by the inverse spin Hall effect (ISHE). The sub-picosecond charge-current burst leads to the emission of a terahertz (THz) pulse into the optical far-field. (b) Terahertz transmission experiment. A THz transient is incident onto either the bare substrate or onto the substrate coated by a thin metal film. By comparing the two transmitted waveforms and , the metal conductivity at THz NM THz pulse FM Femtosecond pump M j s j c ISHE (a) (b) Metal Substrate frequencies is determined.Figure 2. Typical THz emission raw data and sample characterization. (a) THz emission signal measured from a C40F40B20(3 nm)|Pt(3 nm) bilayer for two opposite orientations of the sample magnetization (± ). (b) Normalized pump-power dependence of the THz signal amplitude (RMS) for one orientation of the sample magnetization. (c) Pump-light absorptance, transmittance and reflectance as function of the Pt-layer thickness. (d) Frequency-dependent THz conductivities measured by THz transmission experiments (black and red dots) along with fits obtained by the Drude model (black and red solid lines).

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We demonstrate antenna-coupled spintronic terahertz (THz) emitters excited by 1550 nm, 90 fs laser pulses. Antennas are employed to optimize THz outcoupling and frequency coverage of ferromagnetic/nonmagnetic metallic spintronic structures. We directly compare the antenna-coupled devices to those without antennas. Using a 200 lm H-dipole antenna and an ErAs:InGaAs photoconductive receiver, we obtain a 2.42-fold larger THz peak-peak signal, a bandwidth of 4.5 THz, and an increase in the peak dynamic range (DNR) from 53 dB to 65 dB. A 25 lm slotline antenna offered 5 dB larger peak DNR and a bandwidth of 5 THz. For all measurements, we use a comparatively low laser power of 45 mW from a commercial fiber-coupled system that is frequently employed in table-top THz time-domain systems.