Antiferromagnetic magnonic charge current generation via ultrafast optical excitation.

Néel spin-orbit torque allows a charge current pulse to efficiently manipulate the Néel vector in antiferromagnets, which offers a unique opportunity for ultrahigh density information storage with high speed. However, the reciprocal process of Néel spin-orbit torque, the generation of ultrafast charge current in antiferromagnets has not been demonstrated. Here, we show the experimental observation of charge current generation in antiferromagnetic metallic Mn2Au thin films using ultrafast optical excitation. The ultrafast laser pulse excites antiferromagnetic magnons, resulting in instantaneous non-equilibrium spin polarization at the antiferromagnetic spin sublattices with broken spatial symmetry. Then the charge current is generated directly via spin-orbit fields at the two sublattices, which is termed as the reciprocal phenomenon of Néel spin-orbit torque, and the associated THz emission can be detected at room temperature. Besides the fundamental significance on the Onsager reciprocity, the observed magnonic charge current generation in antiferromagnet would advance the development of antiferromagnetic THz emitter.

Defect-induced helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films.

Nonlinear transport enabled by symmetry breaking in quantum materials has aroused considerable interest in condensed matter physics and interdisciplinary electronics. However, achieving a nonlinear optical response in centrosymmetric Dirac semimetals via defect engineering has remained a challenge. Here, we observe the helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films via the circular photogalvanic effect under normal incidence. This is activated by a controllable out-of-plane Te-vacancy defect gradient, which we unambiguously evidence with electron ptychography. The defect gradient lowers the symmetry, which not only induces the band spin splitting but also generates the giant Berry curvature dipole responsible for the circular photogalvanic effect. We demonstrate that the THz emission can be manipulated by the Te-vacancy defect concentration. Furthermore, the temperature evolution of the THz emission features a minimum in the THz amplitude due to carrier compensation. Our work provides a universal strategy for symmetry breaking in centrosymmetric Dirac materials for efficient nonlinear transport.

Linear and phase controllable terahertz frequency conversion via ultrafast breaking the bond of a meta-molecule.

The metasurface platform with time-varying characteristics has emerged as a promising avenue for exploring exotic physics associated with Floquet materials and for designing photonic devices like linear frequency converters. However, the limited availability of materials with ultrafast responses hinders their applications in the terahertz range. Here we present a time-varying metasurface comprising an array of superconductor-metal hybrid meta-molecules. Each meta-molecule consists of two meta-atoms that are "bonded" together by double superconducting microbridges. Through experimental investigations, we demonstrate high-efficiency linear terahertz frequency conversion by rapidly breaking the bond using a coherent ultrashort terahertz pump pulse. The frequency and relative phase of the converted wave exhibit strong dependence on the pump-probe delay, indicating phase controllable wave conversion. The dynamics of the meta-molecules during the frequency conversion process are comprehensively understood using a time-varying coupled mode model. This research not only opens up new possibilities for developing innovative terahertz sources but also provides opportunities for exploring topological dynamics and Floquet physics within metasurfaces.

Low-Permittivity and Low-Temperature Cofired BaSO4-BaF2 Microwave Dielectric Ceramics for High-Reliability Packaged Electronics.

In developing low-temperature cofired ceramic (LTCC) technology for high-density packaging or advanced packaged electronics, matching the coefficient of thermal expansion (CTE) among the packaged components is a critical challenge to improve reliability. The CTEs of solders and organic laminates are usually larger than 16.0 ppm of °C1-, while most low-permittivity (εr) dielectric ceramics have CTEs of less than 10.0 ppm °C1-. Therefore, a good CTE match between organic laminates and dielectric ceramics is required for further LTCC applications. In this paper, we propose a high-CTE BaSO4-BaF2 LTCC as a potential solution for high-reliability packaged electronics. The BaSO4-BaF2 ceramics have the advantages of a wide low-temperature sintering range (650-850 °C), low loss, temperature stability, and Ag compatibility, ensuring excellent performance in LTCC technology. The 95 wt %BaSO4-5 wt %BaF2 ceramic has a εr of 9.1, a Q × f of 40,100 GHz @11.03 GHz (Q = 1/tan δ), a temperature coefficient of the resonant frequency of -11.2 ppm °C1-, a CTE of +21.8 ppm °C1-, and a thermal conductivity of 1.3 W mK-1 when sintered at 750 °C. Furthermore, a dielectric resonant antenna using BaSO4-BaF2 ceramics, a typically packaged component of LTCC and laminate, was designed and used to verify the excellent performance by a gain of 6.0 dBi at a central frequency of 8.97 GHz and a high radiation efficiency of 90% over a bandwidth of 760 MHz. Good match and low thermal stress were found in the packaged components of BaSO4-BaF2 ceramics, organic laminates, and Sn-based solders by finite element analysis, proving the potential of this LTCC for high-reliability packaged electronics.

Directional terahertz holography with thermally active Janus metasurface.

Dynamic manipulation of electromagnetic (EM) waves with multiple degrees of freedom plays an essential role in enhancing information processing. Currently, an enormous challenge is to realize directional terahertz (THz) holography. Recently, it was demonstrated that Janus metasurfaces could produce distinct responses to EM waves from two opposite incident directions, making multiplexed dynamic manipulation of THz waves possible. Herein, we show that thermally activated THz Janus metasurfaces integrating with phase change materials on the meta-atoms can produce asymmetric transmission with the designed phase delays. Such reconfigurable Janus metasurfaces can achieve asymmetric focusing of THz wave and directional THz holography with free-space image projections, and particularly the information can be manipulated via temperature and incident THz wave direction. This work not only offers a common strategy for realizing the reconfigurability of Janus metasurfaces, but also shows possible applications in THz optical information encryption, data storage, and smart windows.

Antireflection Coatings Based on PEDOT:PSS Conductive Polymer Using d-Sorbitol Additives for Terahertz Spectroscopy and Imaging.

Optical antireflection has been employed for a variety of applications in terahertz spectroscopy and detectors. However, current methods encounter challenges in terms of cost, bandwidth, structural complexity, and performance. In this study, a low-cost, broadband, and easily processed THz antireflection coating scheme based on the model of impedance-matching effect is proposed, using a 6 wt % d-sorbitol-doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (s-PEDOT:PSS) film. By adjusting the thickness of the s-PEDOT:PSS film, these biocompatible conductive polymers enable a significant reduction of Fresnel reflection and operate over a broad bandwidth between 0.2 and 2.2 THz. Applying the antireflective coating to the surface of the sample substrate and electro-optic probe crystal in THz spectroscopy and near-field imaging shows that the spectral resolution is significantly improved, and the devices exhibit more excellent intended performance. The findings of this study could aid in improving the measurement capability of various THz time-domain spectroscopy and imaging system.

Terahertz Spin Current Dynamics in Antiferromagnetic Hematite.

An important vision of modern magnetic research is to use antiferromagnets (AFMs) as controllable and active ultrafast components in spintronic devices. Hematite (α-Fe2 O3 ) is a promising model material in this respect because its pronounced Dzyaloshinskii-Moriya interaction leads to the coexistence of antiferromagnetism and weak ferromagnetism. Here, femtosecond laser pulses are used to drive terahertz (THz) spin currents from α-Fe2 O3 into an adjacent Pt layer. Two contributions to the generation of the spin current with distinctly different dynamics are found: the impulsive stimulated Raman scatting that relies on the AFM order and the ultrafast spin Seebeck effect that relies on the net magnetization. The total THz spin current dynamics can be manipulated by a medium-strength magnetic field below 1 T. The control of the THz spin current achieved in α-Fe2 O3 opens the pathway toward tailoring the exact spin current dynamics from ultrafast AFM spin sources.

Antiferromagnetic Inverse Spin Hall Effect.

The inverse spin Hall effect (ISHE) is one of the accessible and reliable methods to detect spin current. The magnetization-dependent inverse spin Hall effect has been observed in magnets, expanding the dimension for spin-to-charge conversion. However, antiferromagnetic Néel-vector-dependent ISHE, which has been long time highly pursued, is still elusive. Here, ISHE in Mn2 Au/[Co/Pd] heterostructures is investigated by terahertz emission and spin Seebeck effect measurements, where [Co/Pd] possesses perpendicular magnetic anisotropy for out-of-plane polarized spin current generation and Mn2 Au is a collinear antiferromagnet for the spin-to-charge conversion. The out-of-plane spin polarization (σz ) is rotated toward in-plane by the Néel vectors in Mn2 Au, then the spin current is converted into charge current at two staggered spin sublattices. The ISHE signal is much stronger when the converted charge current is parallel to the Néel vector compared with its orthogonal counterpart. The Néel vector and resultant ISHE signals, which is termed as antiferromagnetic inverse spin Hall effect, can be switched. The finding not only adds a new member to the Hall effect family, but also makes antiferromagnetic spintronics more flexible.

Magnetic-Pole Flip by Millimeter Wave.

In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, "focused-millimeter-wave-assisted magnetic recording (F-MIMR)," is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau-Lifshitz-Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.

3D porous graphene-assisted capsulized cholesteric liquid crystals for terahertz power visualization.

We demonstrate a high-efficiency visualized terahertz (THz) power meter based on the THz-photothermochromism of capsulized cholesteric liquid crystals (CCLCs) embedded in three-dimensional porous graphene (3DPG). The graphene is a broadband perfect absorber for THz radiation and transfers heat efficiently, and its black background is beneficial for color measurement. Quantitative visualization of THz intensity up to 2.8×102mW/cm2 is presented. The minimal detectable THz power is 0.009 mW. With multi-microcapsule analysis, the relationship between THz power and the average hue value of CCLCs achieves linearity. The device can convert THz radiation to visible light and is lightweight, cheap, and easy to use.

Reconfiguration of magnetic domain structures of ErFeO3 by intense terahertz free electron laser pulses.

Understanding the interaction between intense terahertz (THz) electromagnetic fields and spin systems has been gaining importance in modern spintronics research as a unique pathway to realize ultrafast macroscopic magnetization control. In this work, we used intense THz pulses with pulse energies in the order of 10 mJ/pulse generated from the terahertz free electron laser (THz-FEL) to irradiate the ferromagnetic domains of ErFeO3 single crystal. It was found that the domain shape can be locally reconfigured by irradiating the THz - FEL pulses near the domain boundary. Observed domain reconfiguration mechanism can be phenomenologically understood by the combination of depinning effect and the entropic force due to local thermal gradient exerted by terahertz irradiation. Our finding opens up a new possibility of realizing thermal-spin effects at THz frequency ranges by using THz-FEL pulses.

Rapid Faraday Rotation on ε-Iron Oxide Magnetic Nanoparticles by Visible and Terahertz Pulsed Light.

Light- or electromagnetic wave-responsive magnetism is an attractive issue in spin chemistry and optical materials science. Herein we show the magnetization reversal induced by visible-light pulsed laser and the ultrafast dynamic magnetooptical effect caused by terahertz (THz) pulsed laser irradiation onto chemically synthesized magnetic films based on gallium-titanium-cobalt-substituted ε-Fe2O3 (GTC-ε-Fe2O3) and ε-Fe2O3 nanoparticles. Visible-light pulsed laser irradiation switches the sign of the Faraday effect in GTC-ε-Fe2O3 films. On the other hand, irradiating the ε-Fe2O3 film with pulsed THz light induces an ultrafast Faraday rotation in an extremely short time of 400 fs. The time evolution dynamics of these ultrafast magnetooptical effects are theoretically demonstrated by stochastic Landau-Lifshitz-Gilbert calculations of a nanoparticle model that considers all motions of the individual spins. These ε-iron oxide magnetic nanomaterials are expected to contribute to high-density magnetic memory media or high-speed operation circuit magnetic devices.

Enhancing terahertz magnetic near field induced by a micro-split-ring resonator with a tapered waveguide.

Substantial enhancement of terahertz magnetic near field achieved by the combination of a tapered metallic waveguide and a micro-split-ring resonator is demonstrated. The magnetic near field is probed directly via the magneto-optic sampling with a Tb3Ga5O12 crystal. The incident terahertz wave with a half-cycle waveform is generated by using the pulse-front tilting method. The magnetic near field at the resonant frequency is enhanced by more than 30 times through the combination of the waveguide and the resonator. The peak amplitude of the magnetic field with a damped oscillation waveform in the time domain is up to 0.4 T. The resonant frequency can be tuned by adopting different resonator designs. The mechanism of the enhancement is analyzed by performing calculations based on the finite element method. The strong terahertz magnetic near field enables the excitation of large-amplitude spin dynamics and can be utilized for an ultrafast spin control.