## Monday, July 26, 2021

### Abstract-Polarization-sensitive terahertz spectroscopy of multilayer graphene-based films

Anatoly Kvitsinskiy

Efficient devices for control properties of electromagnetic waves are essential for the development of terahertz (THz) technologies. But despite the great progress achieved in a study of graphene, the influence of the number of graphene layers on its properties in the THz frequency range has not yet been sufficiently studied. In this work, we experimentally studied properties of multilayer graphene (MLG) films in the frequency range 0.2–0.8 THz, at a room temperature, and a relative humidity of 40%. Using our custom-made THz time-domain spectroscopic polarimetry system, we obtained spectra of the complex relative permittivity and the electrical conductance of the chemical vapor deposition graphene with ~14, ~40, and ~76 layers of graphene on glass substrates. It is shown that the conductance increases nonlinearly with an increase in the graphene layer number and reaches, for ~76 layers, 0.06 S for the real, and 0.03 S for the imaginary part, respectively.

## Sunday, July 25, 2021

### Abstract-Ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene–metal hybrid metasurface

Wenli Huang, Xiaoqing Luo, Yuanfu Lu, Fangrong Hu, and Guangyuan Li

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-60-22-6366

We propose an ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene–metal hybrid metasurface. The metasurface unit cell is composed of two metal stripes enclosed with a graphene rectangular ring. Results show that when the metasurface is normally illuminated by a terahertz wave polarized along the metal stripes, it can act as an ultra-broadband bandpass filter over the spectral range from 1.49 THz to 4.05 THz, corresponding to a fractional bandwidth of 92%. Remarkably, high transmittance above 90% covering the range from 1.98 THz to 3.95 THz can be achieved. By changing the Fermi level of graphene, we find that the attenuation within the passband can be dynamically tuned from 2% to 66%. We expect that the proposed ultra-broadband terahertz bandpass filter with tunable attenuation will find applications in terahertz communication and detection and sensing systems.

© 2021 Optical Society of America

## Saturday, July 24, 2021

### Abstract-Two-Color TeraHertz Radiation by a Multi-Pass FEL Oscillator

Michele Opromolla,  Vittoria Petrillo,

https://www.mdpi.com/2076-3417/11/14/6495

In this paper, we show that an electron beam produced by a super-conducting linac, driven in a sequence of two undulator modules of different periods, can generate two-color Terahertz radiation with wavelengths ranging from 100 m to 2 m. The generated pulses are synchronized, both MW-class, and highly coherent. Their specific properties and generation will be discussed in detail. Besides the single-spike pulse structure, usually observed in oscillators, we show that both the THz pump and probe can be modulated in a coherent comb of pulses, enabling periodic excitation and stroboscopic measurements.

## Friday, July 23, 2021

### 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$\mathbit{E}$ is linearly polarized and perpendicular to the magnetization 𝑴$\mathbit{M}$ of the F layer. We modulate the direction of 𝑴$\mathbit{M}$ and, thus, 𝑬$\mathbit{E}$ by an external in-plane magnetic field 𝑩ext${\mathbit{B}}_{\mathrm{e}\mathrm{x}\mathrm{t}}$. The direction of 𝑴$\mathbit{M}$ is monitored by means of the magneto-optic Kerr effect (MOKE). (b) In polarity-modulation (PM) mode, 𝑩ext=𝑩AC${\mathbit{B}}_{\mathrm{e}\mathrm{x}\mathrm{t}}={\mathbit{B}}_{\mathrm{A}\mathrm{C}}$ is a harmonic AC field 𝑩AC(𝑡)=𝒖𝑦𝐵AC(𝑡)${\mathbit{B}}_{\mathrm{A}\mathrm{C}}\left(t\right)={\mathbit{u}}_{y}{B}_{\mathrm{A}\mathrm{C}}\left(t\right)$ with 𝐵AC(𝑡)=𝐵AC0cos(2𝜋𝑓AC𝑡)${B}_{\mathrm{A}\mathrm{C}}\left(t\right)={B}_{\mathrm{A}\mathrm{C}0}\mathrm{cos}\left(2\pi {f}_{\mathrm{A}\mathrm{C}}t\right)$ from an electromagnet, thereby alternating the 𝑴$\mathbit{M}$ direction between ±𝒖𝑦$±{\mathbit{u}}_{y}$. (c) In the more general direction-modulation (DM) mode, 𝑩AC(𝑡)${\mathbit{B}}_{\mathrm{A}\mathrm{C}}\left(t\right)$ is superimposed by a perpendicular DC magnetic field 𝑩DC=𝐵DC𝒖𝑥${\mathbit{B}}_{\mathrm{D}\mathrm{C}}={B}_{\mathrm{D}\mathrm{C}}{\mathbit{u}}_{x}$ from a permanent magnet. The angle 𝜃(𝑡)$\theta \left(t\right)$ of the resulting 𝑩ext=𝑩AC+𝑩DC${\mathbit{B}}_{\mathrm{e}\mathrm{x}\mathrm{t}}={\mathbit{B}}_{\mathrm{A}\mathrm{C}}+{\mathbit{B}}_{\mathrm{D}\mathrm{C}}$ and, thus, 𝑴$\mathbit{M}$ oscillates continuously between values ±𝜃0$±{\theta }_{0}$.

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.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement