Abstract-Antenna-coupled spintronic terahertz emitters driven by a 1550 nm femtosecond laser oscillator

Applied Physics Letters

 U. Nandi, M. S. Abdelaziz, S. Jaiswal, G. Jakob, O. Gueckstock, S. M. Rouzegar, T. S. Seifert,   M. Kläui, T. Kampfrath,  S. Preu,

Schematic of THz emission from photoexcited FMNM bilayers, plain and microstructured. (a) A femtosecond laser pulse triggers ultrafast spin transport from the FM into the NM layer where the spin current js flowing along the z axis is converted into a charge current jc along the y direction, acting as a source of THz radiation. The direction of the in-plane magnetization of the FM layer is set along the x axis by an external magnetic field Bext. (b) Current distribution in an unstructured (plain) bilayer and (c) the STE bilayer embedded in the gap of an antenna. Note that THz current generation by the ISHE is independent of emitter type and antenna choice.

https://aip.scitation.org/doi/abs/10.1063/1.5089421

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 μm 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 μm 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.

Abstract-Impact of pump wavelength on terahertz emission of a cavity-enhanced spintronic trilayer

 

Applied Physics Letters

R. I. Herapath,  S. M. Hornett,   T. S. Seifert, G. Jakob,  M. Kläui,  J. Bertolotti, T. Kampfrath,  E. Hendry

Schematic of a spintronic trilayer with added dielectric cavity, grown on 0.5 mm of sapphire (Al2O3). The near-infrared pump pulse, incident through the substrate, is partially absorbed in the metallic layers, launching a spin current from the ferromagnetic (FM) layer into the nonmagnetic (NM) layers. The inverse spin Hall effect converts this ultrashort out-of-plane spin current into an in-plane charge current resulting in the emission of THz radiation into the optical far-field. A weak in-plane magnetic field (B) determines the magnetization direction and the linear polarization of the emitted THz field.

https://aip.scitation.org/doi/abs/10.1063/1.5048297

We systematically study the pump-wavelength dependence of terahertz pulse generation in thin-film spintronic THz emitters composed of a ferromagnetic CoFeB layer between adjacent nonmagnetic W and Pt layers. We find that the efficiency of THz generation is essentially flat for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, demonstrating that the spin current does not depend strongly on the pump photon energy. We show that the inclusion of dielectric overlayers of TiO2 and SiO2, designed for a particular excitation wavelength, can enhance the terahertz emission by a factor of up to two in field.

Abstract-Launching magnons at the terahertz speed of the spin Seebeck effect

T. Seifert, S. Jaiswal, J. Barker, I. Razdolski, J. Cramer, O. Gueckstock, S. Watanabe, C. Ciccarelli, A. Melnikov, G. Jakob, S.T.B. Goennenwein, G. Woltersdorf, P.W. Brouwer, M. Wolf, M. Kläui, T. Kampfrath

(Submitted on 3 Sep 2017)

https://arxiv.org/abs/1709.00768

Transport of spin angular momentum is an essential operation in spintronic devices. In magnetic insulators, spin currents are carried by magnons and can be launched straightforwardly by heating an adjacent metal layer. Here, we study the ultimate speed of this spin Seebeck effect with 10-fs time resolution in prototypical bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal by a laser pulse, the spin flow is measured using the inverse spin Hall effect and terahertz electrooptic sampling. The spin Seebeck current reaches its peak within ~200 fs, a hallmark of the photoexcited metal electrons approaching a Fermi-Dirac distribution. Analytical modeling shows the spin Seebeck response is virtually instantaneous because the ferrimagnetic spins react without inertia and the metal spins impinging on the interface have a correlation time of only ~4 fs. Novel applications for material characterization, interface probing, spin-noise detection and terahertz spin pumping emerge.

Abstract-Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

T. Seifert,

S. Jaiswal,

U. Martens,

J. Hannegan,

L. Braun,

P. Maldonado,

F. Freimuth,

A. Kronenberg,

J. Henrizi,

I. Radu,

E. Beaurepaire,

Y. Mokrousov,

P. M. Oppeneer,

M. Jourdan,

G. Jakob,

D. Turchinovich,

L. M. Hayden,

M. Wolf,

M. Münzenberg,

M. Kläui

& T. Kampfrath 

(some author names deleted from Labels at the bottom due to blogger word limit)

http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2016.91.html

Terahertz electromagnetic radiation is extremely useful for numerous applications, including imaging and spectroscopy. It is thus highly desirable to have an efficient table-top emitter covering the 1–30 THz window that is driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source that relies on three tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photoinduced spin currents, the inverse spin-Hall effect and a broadband Fabry–Pérot resonance. Guided by an analytical model, this spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1–30 THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz field amplitude, flexibility, scalability and cost.

Abstract-Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

T. Seifert, S. Jaiswal, U. Martens, J. Hannegan, L. Braun, P. Maldonado, F. Freimuth, A. Kronenberg, J. Henrizi, I. Radu, E. Beaurepaire, Y. Mokrousov, P. M. Oppeneer, M. Jourdan, G. Jakob,D. Turchinovich, L. M. Hayden, M. Wolf, M. Münzenberg, M. Kläui, T. Kampfrath

http://www.mathpubs.com/detail/1510.03729v1/Efficient-metallic-spintronic-emitters-of-ultrabroadband-terahertz-radiation

Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source based on tailored fundamental spintronic/photonic phenomena in magnetic metal multilayers: a spin-dependent generalization of the photo-Dember effect, the inverse spin-Hall effect and a broadband Fabry-P\'erot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort THz pulses fully covering the 1-to-30-THz range. Our novel source outperforms standard emitters such as ZnTe(110) crystals in terms of bandwidth, conversion efficiency, flexibility, scalability and cost.