Pavel Yu. Avdeev

The rigid coupling between the emitted THz polarization and the magnetization orientation of a spintronic emitter enables polarization control, but conventional approaches require tuning an external magnetic field. Here, we demonstrate that in an ultrathin W/FeGa/Pt heterostructure deposited on a PMN-PT (011) substrate, the polarization plane can be smoothly rotated by 40° using a DC voltage alone, under a fixed, weak bias magnetic field (24 Oe). The key element is a 5-nm-thick Galfenol (FeGa) layer sandwiched between heavy metals with opposite signs of the spin Hall angle: it simultaneously serves as the source of the ultrafast spin current and as a magnetostrictive transducer that converts substrate strain into a magnetic response. THz spintronic magnetometry, which probes the structure under the actual conditions of THz generation, allows us to quantitatively correlate the polarization rotation angle with the strain-induced magnetoelastic anisotropy transferred to the film. The obtained results are directed toward the design of compact THz sources with electrically tunable polarization.

Problem Statement: there is a need to create spintronic terahertz radiation sources that go beyond traditional ferromagnet/heavy metal heterostructures. Two-dimensional transition metal dichalcogenides (TMDs) are promising as an interface for spin-charge conversion, but integrating TMDs obtained by liquid-phase exfoliation into THz emitters remains a technologically and methodologically challenging task. Objective: to fabricate and characterize spintronic THz emitters based on TMD/Co heterostructures obtained by liquid-phase exfoliation, assessing the THz signal amplitude, and evaluate the role of interfacial spin-orbit conversion. Results: spintronic THz emitters were fabricated based on heterostructures of three types of dichalcogenides (WS2, WSe2, MoSe2) synthesized by liquid-phase exfoliation. Morphological analysis (scanning electron microscopy, atomic force microscopy) revealed a high coverage density of TMD crystallites (60–80%) with a characteristic size of 5–10 μm and a height of 20–500 nm. Analysis of the phase characteristics of THz pulses shows that, with integral recording, the dominant generation mechanism is ultrafast laser-induced demagnetization of cobalt. Practical Relevance: this work demonstrates the potential of liquid-phase exfoliation as a method for producing active layers of spintronic THz sources. Replacing expensive heavy metals with two-dimensional semiconductors reduces costs and improves the scalability of the technology.

We demonstrate an efficient THz spintronic emitter using a Co/WSe2 heterostructure that enables direct control of the emitted THz polarization. The device is excited by femtosecond laser pulses and leverages spin-to-charge conversion at the Co/WSe2 interface to generate broadband THz pulses. We observe that the THz emission polarization can be continuously tuned via the magnetization orientation without significant loss in intensity. At higher pump fields, nonlinear optical effects in the WSe2 layer contribute to the THz output, indicating an emission enhancement beyond the conventional spintronic mechanism.

Представлен метод регистрации терагерцевых импульсов за счет использования тонких ферромагнитных пленок. В основе метода лежит сверхбыстрое изменение намагниченности ферромагнетика вследствие возникновения крутящего магнитного момента, индуцированного терагерцевым импульсом (Зеемановское вращение - Zeeman-torque effect). В качестве источника терагерцевого излучения использовался нелинейно-оптический кристалл ZnTe, позволяющий генерировать терагерцевые импульсы с пиковой напряженностью порядка 0.1 МВ/см. Регистрация вращения намагниченности осуществлялась с помощью магнитооптического метода Фарадея с временным разрешением. Экспериментально продемонстрирована возможность временного стробирования магнитного отклика в пленках Co(2 нм)/Pt(2 нм) и Co(10 нм) в частотном диапазоне вплоть до нескольких терагерц.

A method for detecting terahertz pulses using thin ferromagnetic films is presented. The method is based on the ultrafast change in magnetization of a ferromagnet due to the emergence of a torque-induced magnetic moment generated by the terahertz pulse (Zeeman-torque effect). A nonlinear optical ZnTe crystal was used as the source of terahertz radiation, producing pulses with a peak electric field strength of approximately 0.1 MV/cm. The magnetization rotation was recorded using the time-resolved magneto-optical Faraday effect. The experiment demonstrated the possibility of temporally strobing the magnetic response in Co(2 nm)/Pt(2 nm) and Co(10 nm) films in the frequency range up to several terahertz.

In this work, the optical parameters of a 550 μm thick deuterated triglycine sulfate (DTGS) crystal in the range of 0.3–1.5 THz were investigated in low and high fields by time-domain spectroscopy. The X and Z axes of the refractive index ellipsoid were identified, and the refractive indices and extinction coefficients were determined. In addition, it was shown that, in strong THz fields, a phonon A-mode with a frequency of about 40 cm⁻¹ can be excited in the DTGS crystal when the THz wave vector coincides with the polar axis of the crystal and the polarization of the THz radiation coincides with the X-axis of the crystal. The use of the observed effect and the determination of the mode excitation parameters can be twofold. First, they show the limits of the THz field for non-invasive action on the crystal. Second, they present the mode excitation conditions that can be used, for example, in controllable absorbers.

В данной работе методом спектроскопии во временной области исследованы оптические параметры кристалла DTGS толщиной 550 мкм в диапазоне 0,3–1,5 ТГц в слабых и сильных полях. Идентифицированы оси X и Z эллипсоида показателя преломления, определены показатели преломления и коэффициенты экстинкции. Кроме того, показано, что в сильных ТГц полях в кристалле DTGS может возбуждаться фононная A-мода с частотой около 40 см⁻¹ при совпадении волнового вектора ТГц с полярной осью кристалла, а поляризация ТГц излучения – с осью X кристалла. Использование наблюдаемого эффекта и определение параметров возбуждения мод может быть двояким. Во-первых, показаны пределы ТГц поля для неинвазивного воздействия на кристалл. Во-вторых, представлены условия возбуждения мод, которые могут быть использованы, например, в управляемых поглотителях.

We present a novel spin-valve THz emitter composed of FeCo/TbCo2/FeCo tri-layers separated by a copper spacer, designed to provide amplitude modulation of THz radiation. The distinct coercivities of the magnetic layers enable magnetic field control of the THz emission, with the final amplitude formed by the interference of THz waves from the individual tri-layers. Experimental studies using time-domain spectroscopy reveal that the spin-valve structure, optimized for in-plane magnetic anisotropy, effectively modulates the THz signal and maintains constant polarization. Although the current optical-to-THz conversion efficiency is insufficient for commercial use, the findings offer crucial insights for enhancing efficiency through structural optimization, highlighting the potential for advanced THz emitters and modulators.

We report on the design of a spintronic emitter based on the Pt(3 nm)/Co(3 nm) structure, which enables the control over terahertz radiation polarization. Utilizing the field-induced magnetization rotation that takes place at low magnetic fields of up to 250 Oe at room temperature, we have achieved the full range of terahertz polarization rotation from 0° to 360°. This rotation became possible due to the uniaxial magnetic anisotropy induced in the plane of the cobalt film during its fabrication. We evaluated the efficiency of the Co/Pt structure in generating terahertz radiation and found that the terahertz pulse energy flux reaches ∼160 nJ/cm2 at an excitation flux of 4 mJ/cm2.