Maximizing the Electromagnetic Efficiency of Spintronic Terahertz Emitters

Optically pumped spintronic terahertz emitters (STEs) have, in less than a decade, strongly impacted terahertz (THz) source technology, by the combination of their Fourier-limited ultrafast response, their phononless emission spectrum and their wavelength-independent operation. However, the intrinsic strength of the inverse spin Hall effect governing these devices introduces a challenge: the optical-to-terahertz conversion efficiency is considerably lower than traditional sources. It is therefore primordial to maximize at least their electromagnetic efficiency independently of the spin dynamics at play. Using a rigorous time-domain treatment of the electromagnetic generation and extraction processes, an optimized design is presented and experimentally confirmed. With respect to the strongest reported spintronic THz emitters it achieves a 250% enhancement of the emitted THz field and therefore an 8 dB increase of emitted power. This experimental achievement brings STE close to the symbolic barrier of mW levels. The design strategy is generically applicable to any kind of ultrafast spin-to-charge conversion (S2C) system. On a broader level, our work highlights how a rigorous handling of the purely electromagnetic aspects of THz spintronic devices can uncover overlooked aspects of their operation and lead to substantial improvements.