Broadband Spintronic Terahertz Source with Peak Electric Fields Exceeding 1.5 MV/cm

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Broadband Spintronic Terahertz Source with Peak Electric Fields Exceeding 1.5 MV/cm

R. Rouzegar, A.L. Chekhov, Y. Behovits, B.R. Serrano, M.A. Syskaki, C.H. Lambert, D. Engel, U. Martens, M. Münzenberg, M. Wolf, G. Jakob, M. Kläui, T.S. Seifert, and T. Kampfrath

Phys. Rev. Applied 19, 034018 – Published 6 March 2023

Abstract

In this work, we improve the performance of an optically pumped spintronic terahertz emitter (STE) by a factor of up to 6 in field amplitude through an optimized photonic and thermal environment. Using high-energy pump pulses (energy 5 mJ, fluence >1 ${mJ / cm}^{2}$ , wavelength 800 nm, duration 80 fs), we routinely generate terahertz pulses with focal peak electric fields above 1.5 MV/cm, fluences of the order of 1 ${mJ / cm}^{2}$ , and a spectrum covering the range 0.1–11 THz. Remarkably, the field and fluence values are comparable to those obtained from a state-of-the-art terahertz table-top high-field source based on tilted-pulse-front optical rectification in ${Li Nb O}_{3}$ . The optimized STE inherits all attractive features of the standard STE design, for example, ease of use and the straightforward rotation of the terahertz polarization plane, without the typical 75% power loss found in ${Li Nb O}_{3}$ setups. It, thus, opens up a promising pathway to nonlinear terahertz spectroscopy. Using low-energy laser pulses (2 nJ, 0.2 ${mJ / cm}^{2}$ , 800 nm, 10 fs), the emitted terahertz pulse has a focal peak electric field of 100 V/cm, which corresponds to a 2-fold improvement, and covers the spectrum 0.3–30 THz.

Received 7 November 2022
Revised 23 December 2022
Accepted 24 January 2023

DOI:https://doi.org/10.1103/PhysRevApplied.19.034018

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Spintronics Ultrafast magnetic effects Ultrafast optics

Terahertz sources Terahertz spectroscopy Terahertz techniques

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

R. Rouzegar ^1,2,*,†, A.L. Chekhov ^1,2,†, Y. Behovits^1,2, B.R. Serrano^1,2, M.A. Syskaki^3,4, C.H. Lambert⁵, D. Engel ⁶, U. Martens⁷, M. Münzenberg⁷, M. Wolf², G. Jakob³, M. Kläui³, T.S. Seifert^1,2, and T. Kampfrath ^1,2

¹Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
²Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
³Institute of Physics, Johannes Gutenberg University Mainz, 55099 Mainz, Germany
⁴Singulus Technologies, 63796 Kahl am Main, Germany
⁵Department of Materials, ETH Zürich, CH-8093 Zürich, Switzerland
⁶Max-Born-Institut für nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany
⁷Institut für Physik, Universität Greifswald, 17489 Greifswald, Germany

^*m.rouzegar@fu-berlin.de
^†These authors contributed equally.

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Issue

Vol. 19, Iss. 3 — March 2023

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Images

Figure 1
Two photonic and thermal environments of the STE. (a) Standard STE configuration. An incident pump pulse (teal) deposits 50%–60% of its energy in a metallic STE stack (yellow), resulting in the emission of a terahertz electromagnetic pulse (red). The red solid line indicates the calculated terahertz electric-field amplitude. The backward-propagating terahertz field is strongly attenuated in the sapphire substrate but increases abruptly when traversing the interface to air. (b) The STE stack is a metallic trilayer, W(2 nm)| $CFB$ (1.8 nm)| $Pt$ (2 nm), in which the ferromagnetic $CFB$ layer has in-plane magnetization $M$ . Pump excitation induces a spin voltage that injects a spin current $j_{s}$ into the adjacent $Pt$ and $W$ paramagnetic layers. In these layers, the ISHE transforms $j_{s}$ into a transverse charge current $j_{c}$ that acts as a source of a terahertz electromagnetic pulse propagating forward and backward. (c) $Si$ -based STE ( $Si$ -STE) design with a >2 times larger forward-propagating electric field. The enhancement relies on >95% deposition of the pump pulse in the STE stack, negligible attenuation in the $Si$ , and the discontinuity of the forward-propagating terahertz electric field at the $Si /$ air interface. (d) $Si$ -STE design details. The STE is sandwiched between a dielectric mirror [ ${Si O}_{2}$ (165 nm)| ${Ti O}_{2}$ (94 nm)]₅ and a $Mg O$ layer, which minimize pump transmission into $Si$ and reflection into air. (e) Calculated fraction $A_{p}$ of the incident pump energy that is absorbed in the STE metal stack (purple curve) and measured pump-pulse intensity spectrum of the low-energy setup (green).
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Figure 2
Low-energy STE operation. (a) Schematic of the setup. A laser pulse [bandwidth-limited duration 10 fs, spectrum in Fig. 1] is focused into the STE (incident fluence 0.2 mJ/cm²). The resulting terahertz pulse is collimated with a parabolic mirror and focused into a $Zn Te (110)$ crystal (thickness 10 µm) for EOS by the probe pulse. (b) Terahertz electro-optic signal waveforms (probe ellipticity) obtained from a standard STE and a $Si$ -STE [Figs. 1 and 1]. (c) Fourier amplitude of the terahertz signals shown in panel (b). (d) Extracted electric field from the signals shown in panel (b). (e) Fourier amplitude of the extracted electric field.
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Figure 3
High-energy STE operation. (a),(b) Schematics of the setups. (a) The collimated pump-pulse beam (35 fs if bandwidth-limited, 800 nm, 5 mJ, 1.1 mJ/cm²) is partially transmitted by the STE on sapphire and removed by an ITO dichroic mirror and $Si$ wafer. (b) In contrast, the $Si$ -STE fully absorbs the pump beam. The generated terahertz beam is focused into a $Zn Te (110)$ crystal (thickness 10 µm) for EOS by the probe pulse. (c) Intensity distribution of the $Si$ -STE terahertz-beam focus as measured with a microbolometer array. (d) Electro-optic signal of STE and $Si$ -STE terahertz pulses and (e) their amplitude spectra. (f) Focal terahertz electric field of $Si$ -STE extracted from the electro-optic signals for three magnetization directions of the $Si$ -STE. The detector is insensitive to the terahertz electric field emitted at 90° magnetization orientation. (g) Measured terahertz power and peak electric field versus incident and, thus, absorbed pump fluence for the $Si$ -STE.
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Figure 4
Measured and modeled dynamics of the focal electric field from the $Si$ -STE. The blue curve shows the electric field $E (t)$ extracted from the electro-optical signal $S (t)$ . The red curve is a fit according to Eqs. (4) and (6) with $Γ_{es}^{- 1} = 107 \pm 5 fs$ and the pump-pulse duration of 84 fs (FWHM intensity) as fit parameters. The gray curve shows the resulting charge current inside the $Si$ -STE metal stack.
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Figure 5
Quantitative back-to-back comparison of nonlinear terahertz spectroscopy with terahertz pulses from an $Si$ -STE (blue solid lines) versus a ${Li Nb O}_{3}$ source without (red solid lines) and with polarizers (light-red solid lines) enabling terahertz field reversal from $E (t)$ to $- E (t)$ . (a) Electric fields obtained with a $Zn Te (110)$ detector (thickness 10 µm). The inset shows the Fourier amplitude of the electric fields. (b) Signal of terahertz Kerr effect $\propto E^{2}$ in a diamond window (thickness 200 µm). The resulting optical birefringence is probed by a pulse polarized at 45° with respect to the direction of $E$ .
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