Abstract-Modulating the polarization of broadband terahertz pulses from a spintronic emitter at rates up to 10 kHz

 

Oliver Gueckstock, Lukáš Nádvorník, Tom S. Seifert, Martin Borchert, Gerhard Jakob, Georg Schmidt, Georg Woltersdorf, Mathias Kläui, Martin Wolf,  Tobias Kampfrath, 

Polarization modulation of THz pulses. (a) In a spintronic THz emitter (STE), an incident femtosecond laser pulse drives spin transport from a ferromagnetic (F) into an adjacent nonmagnetic (N) layer. By the inverse spin Hall effect, the spin current 𝑗s$j_{\mathrm{s}}$ is converted into a transverse charge current 𝑗c$j_{\mathrm{c}}$ that emits THz radiation. The resulting THz field 𝑬$\mathit{E}$ is linearly polarized and perpendicular to the magnetization 𝑴$\mathit{M}$ of the F layer. We modulate the direction of 𝑴$\mathit{M}$ and, thus, 𝑬$\mathit{E}$ by an external in-plane magnetic field 𝑩ext${\mathit{B}}_{\mathrm{e}\mathrm{x}\mathrm{t}}$. The direction of 𝑴$\mathit{M}$ is monitored by means of the magneto-optic Kerr effect (MOKE). (b) In polarity-modulation (PM) mode, 𝑩ext=𝑩AC${\mathit{B}}_{\mathrm{e}\mathrm{x}\mathrm{t}}={\mathit{B}}_{\mathrm{A}\mathrm{C}}$ is a harmonic AC field 𝑩AC(𝑡)=𝒖𝑦𝐵AC(𝑡)${\mathit{B}}_{\mathrm{A}\mathrm{C}}(t)={\mathit{u}}_y{B}_{\mathrm{A}\mathrm{C}}(t)$ with 𝐵AC(𝑡)=𝐵AC0cos(2𝜋𝑓AC𝑡)$B_{\mathrm{A}\mathrm{C}}(t)=B_{\mathrm{A}\mathrm{C}0}\cos (2\pi f_{\mathrm{A}\mathrm{C}}t)$ from an electromagnet, thereby alternating the 𝑴$\mathit{M}$ direction between ±𝒖𝑦$\pm {\mathit{u}}_y$. (c) In the more general direction-modulation (DM) mode, 𝑩AC(𝑡)${\mathit{B}}_{\mathrm{A}\mathrm{C}}(t)$ is superimposed by a perpendicular DC magnetic field 𝑩DC=𝐵DC𝒖𝑥${\mathit{B}}_{\mathrm{D}\mathrm{C}}=B_{\mathrm{D}\mathrm{C}}{\mathit{u}}_x$ from a permanent magnet. The angle 𝜃(𝑡)$\theta (t)$ of the resulting 𝑩ext=𝑩AC+𝑩DC${\mathit{B}}_{\mathrm{e}\mathrm{x}\mathrm{t}}={\mathit{B}}_{\mathrm{A}\mathrm{C}}+{\mathit{B}}_{\mathrm{D}\mathrm{C}}$ and, thus, 𝑴$\mathit{M}$ oscillates continuously between values ±𝜃0$\pm {\theta }_0$.

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-8-7-1013&id=453389

Reliable modulation of terahertz electromagnetic waveforms is important for many applications. Here, we rapidly modulate the direction of the electric field of linearly polarized terahertz electromagnetic pulses with 1–30 THz bandwidth by applying time-dependent magnetic fields to a spintronic terahertz emitter. Polarity modulation of the terahertz field with more than 99% contrast at a rate of 10 kHz is achieved using a harmonic magnetic field. By adding a static magnetic field, we modulate the direction of the terahertz field between angles of, for instance, −53° and 53° at kilohertz rates. We believe our approach makes spintronic terahertz emitters a promising source for low-noise modulation spectroscopy and polarization-sensitive techniques such as ellipsometry at 1–30 THz.

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