Abstract-Magnetically and optically tunable terahertz radiation from Ta/NiFe/Pt spintronic nanolayers generated by femtosecond laser pulses

Applied Physics Letters

Roman Adam,  Genyu Chen, Daniel E. Bürgler, Tianyu Shou, Ivan Komissarov, Sarah Heidtfeld,  Hilde Hardtdegen, Martin Mikulics, Claus M. Schneider, Roman Sobolewski,

(a) A normalized THz transient generated by a 100-fs-wide laser pulse impinging at a Ta/Py/Pt nanotrilayer through the MgO substrate (reverse illumination geometry). The top-left inset shows the trilayer stacking and the schematics of the THz-generation mechanism. The top-right inset presents a normalized THz power spectrum that corresponds to the pulse shown in the main panel. The spectrum exhibits a 3-dB cutoff at 0.85 THz and extends to 5 THz with exponentially decreasing intensity. (b) THz transient amplitude as a function of the incident laser beam power for both fundamental (λ = 800 nm; red solid circles) and frequency-doubled light (λ = 400 nm; blue solid circles). The inset shows the available power range for the 400 nm light and demonstrates a strongly increased efficiency of THz generation at the 400-nm wavelength.

https://aip.scitation.org/doi/abs/10.1063/1.5099201

We generate terahertz (THz) transients by illuminating a few-nanometer-thick Ta/NiFe/Pt nanolayers with a train of linearly polarized 100-fs-wide laser pulses. The transients are ∼1-ps-wide free-space propagating bursts of electromagnetic radiations with amplitudes that are magnetically and optically tunable. Their spectral frequency content extends up to 5 THz, and the 3-dB cutoff is at 0.85 THz. The observed transient electromagnetic signals originate from the NiFe/Pt bilayer, and their amplitude dependence on the external magnetic field, applied in the sample plane, very closely follows the static magnetization versus magnetic field dependence of the NiFe film. For the same laser power, excitation with highly energetic, blue light generates THz transients with amplitudes approximately three times larger than the ones resulting from excitation by infrared light. In both cases, the transients exhibit the same spectral characteristics and are linearly polarized in the perpendicular direction to the sample magnetization. The polarization direction can be tuned by rotation of the magnetic field around the laser light propagation axis. The characteristics of our THz spintronic emitter signals confirm that THz transient generation is due to the inverse spin Hall effect in the Pt layer and demonstrate that ferromagnet/metal nanolayers excited by femtosecond laser pulses can serve as efficient sources of magnetically and optically tunable, polarized transient THz radiation.