Mathias Vanwolleghem
Spintronic Inverse Spin Hall Effect terahertz emitters (ISHE STE) have in a matter of just a few years burst onto the scene as a novel promising and highly efficient paradigm for pulsed terahertz generation. They compete with standard schemes based on nonlinear optical rectification or ultrafast semiconductor-based photoswitches, both in terms of emitted power and especially bandwidth. The ultrafast spin-to-charge (SCC) conversion mechanisms in ferromagnetic/non- magnetic 3d/5d metallic heterostructures, have led to demonstrations of phonon-gapless pulsed emitters with near uniform bandwidth close to the theoretical time-bandwidth product of the IR pumping pulses (up to 30 THz) [1]. Moreover, by optimizing the operation conditions the last generations of ISHE-STE have been successfully upscaled to achieve field strengths that approach the 1MV/cm regime [2] and emit over 10 μW of average power under simple standard Peltier cooling [3]. Still, THz spintronic generation has only been demonstrated in broadband pulsed emission. This is not a fundamental requirement, but was mainly motivated by the high pump power density when using pulsed femtosecond laser sources to overcome the limited SCC. In theory, the ultrabroadband transfer function of the superdiffusive spin transport underlying THz ISHE, should allow for harmonic spintronic photomixing over a bandwidth that is only limited by the spin-flip relaxation time of the spin-polarized photoexcited carriers. In other words, ISHE-STEs hold the promise of a new type of CW THz photomixers that can reach beatnote frequencies over 10THz without any notable rolloff and operable at room temperature.
In this talk, I will present our recent experimental and numerical findings and advances into this domain. The talk will review the current state-of-the art in CW THz generation [4], highlighting the unique position that CW ISHE photomixers would potentially take. The experimental challenges to successfully detect spintronic CW THz generation will be discussed. By developing an advanced stabilized THz heterodyne receiver and phase locking feedback of the photomixing lines, both narrow band subHz-level CW emission is obtained as well as successful spintronic downconversion of a IR frequency comb to a THz comb with 1GHz spacing. As predicted, no rolloff is observed over the entire receiver bandwidth (>1THz). This demonstration supported by FDTD simulations of the transport equations for spin-dependent diffusion in magnetic multilayers [5], hints at room- temperature photomixing up to 10THz and a near 5-octave wide THz comb operable at room temperature. A strategy based on photonic optimization of the spin pumping and the beat note extraction can improve the photomixing conversion efficiency by two orders of magnitude.
As an outlook, we will present future directions to further push the performance and demonstrations of this new paradigm of THz CW and comb generation. This includes among others heterodyne schemes beyond 2-3THz, upscaled pumping, cryo-cooled operation, and fibre tip integration.
References
[1] Seifert et al.: Nature Photonics, 10(7): 483–488, https://doi.org/10.1038/nphoton.2016.91, (2016).
[2] T. Seifert, S. Jaiswal, M. Sajadi, G. Jakob, S. Winnerl, M. Wolf, M. Kläui, T. Kampfrath; Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1 from a metallic spintronic emitter. Appl. Phys. Lett. 19 June 2017; 110 (25): 252402. https://doi.org/10.1063/1.4986755
[3] Tim Vogel, Alan Omar, Samira Mansourzadeh, Frank Wulf, Natalia Martín Sabanés, Melanie Müller, Tom S. Seifert, Alexander Weigel, Gerhard Jakob, Mathias Kläui, Ioachim Pupeza, Tobias
Kampfrath, and Clara J. Saraceno, "Average power scaling of THz spintronic emitters efficiently cooled in reflection geometry," Opt. Express 30, 20451-20468 (2022)
[4] S. Preu, G. H. Döhler, S. Malzer, L. J. Wang, A. C. Gossard; Tunable, continuous-wave Terahertz photomixer sources and applications. Journal of Applied Physics 15 March 2011; 109 (6): 061301. https://doi.org/10.1063/1.3552291