Xiaoqiang Zhang

Terahertz (THz) technology exhibits enormous potential application in imaging and next‐generation communications. However, the lacking of compact broadband THz sources capable with focusing and wavefront manipulation capabilities remains a significant challenge for its practical application. To address this, we propose a design paradigm that combines a sparse‐aperture shared‐aperture architecture with a multispectral weighted intensity optimization method, enabling the direct construction of a Pt/W binary‐phase metasurface on a spintronic terahertz emitter. The device has an overall aperture of 2 cm and features three groups of centrosymmetric sector‐shaped sub‐apertures, each group individually optimized for specific sub‐bands. Numerical results demonstrate that the device achieves broadband quasi‐achromatic focusing over the continuous 1.0–1.5 THz band, characterized by overlapping depth‐of‐focus ranges and stabilized focal‐spot intensity at a common design plane. The spectral intensity fluctuation is suppressed to below 33%. Meanwhile, at all representative frequencies, the edge preservation index remains above 0.68, verifying the robust broadband focusing performance. This work establishes a new paradigm for active terahertz metasurface design and provides a viable pathway toward compact, efficient, and quasi‐achromatic integrated photonic systems for advanced imaging and communication applications.

Orbitronics devices has attracted significant attention due to the potential for magnetization manipulation. Recent studies have demonstrated orbital polarization optically excited by femtosecond lasers in cobalt-platinum alloy ferromagnetic layers, which subsequently radiate terahertz waves through orbital-charge conversion. In this work, we show the interplay of multiple terahertz emission mechanisms from cobalt-platinum alloy/tungsten/magnesium oxide. We performed an analysis of transfer matrix method on the thickness dependence of tungsten and cobalt-platinum alloy, and find that terahertz emission predominantly originates from the contribution from orbital currents. It is shown that the orbital currents are converted into charge current via inverse orbital Hall effect in tungsten and inverse orbital Rashba-Edelstein effect at the tungsten/magnesium oxide interface. This work provides a common framework for understanding the interplay of light-induced spin and orbital currents in ferromagnetic/non-magnetic/oxide heterostructures.

Magnetic skyrmions are topologically protected spin textures characterized by their whirling structure, high stability, and potential for next‐generation magnetic devices. However, their practical implementation has been hindered by the absence of methods for ultrafast and energy‐efficient skyrmion writing and erasure. We theoretically propose the generation of a picosecond‐scale photoinduced radial current that produces an azimuthally polarized spin–orbit torque in a heavy‐metal/ferromagnet heterostructure. This approach enables ultrafast (< 50 ps) generation and controlled transitions between four distinct types of topological skyrmionic quasiparticles. Our results establish a versatile platform for optically controlled topological magnetism and open pathways for high‐speed, low‐power spin‐based memory and logic technologies.

In the past decades, terahertz (THz) technology has made significant progress. However, multifunctional THz devices are still lacking, particularly low-cost and convenient ones. In this paper, we combine a multifunctional metasurface with the unique forward and backward emission characteristics of THz waves from a thin spintronic THz emitter (STEs), carefully designing two types of dual-band achromatic metasurfaces (DBAMs). In these two DBAMs, THz waves at 0.85 THz and 1 THz generated by the STE are focused on the same focal point. More importantly, one of the THz waves features a conventional focused wavefront, while the other exhibits a focused helical wavefront. Our design significantly expands the capabilities of metasurfaces and greatly advances the development of state-of-the-art multifunctional THz devices.

The magneto-optical Kerr effect (MOKE), as one of the magneto-optical effects, exhibits polarization change upon reflection that can be used to explore the internal information of magnetic materials with broad applications in modern information technology. However, typically, MOKE is quite weak due to the lower magneto-optical interaction. To tremendously enhance the MOKE, quasi-bound states in the continuum in a one-dimensional Ce- doped Y₃Fe₅O₁₂ (CeYIG) film photonic crystal slabs (PCS) are proposed to improve the magneto-optical interaction in this work. A giant enhancement in the rotation angle and ellipticity of the longitudinal MOKE, which is about 93.4 and 136.8 times stronger than a pure uniform CeYIG, can be realized. Almost circularly polarized reflected beams with different chiralities are obtained with the CeYIG film. By tuning the geometric parameters of the PCS and the applied external magnetic field, dynamic control of polarization states of the reflected beams with different wavelengths can be realized. This magneto-optical metasurface provides a convenient way for the implementation of magneto-optical devices such as information memory devices, sensors, polarizers, and chiral devices.

High-frequency coherent acoustic phonons hold immense value in characterizing the coupling between magnetic, lattice, and electronic properties, offering nanometer-scale spatial resolution within the ultrafast timescale. However, efficiently propagating intense sub-THz coherent acoustic phonons across diverse materials remains a formidable challenge. Here, we demonstrate that using vanadium dioxide (VO2) as a transducer can induce enhanced coherent acoustic pulses that propagate efficiently (∼90%) into TiO2 due to excellent acoustic impedance matching and minor lattice interface mismatch compared with traditional metals such as Pt, Au, and Cu. Employing time-resolved pump–probe reflectivity spectroscopy, we observe pronounced coherent phonon oscillations reaching up to 0.164 THz from the longitudinal acoustic mode along the c axis in VO2/TiO2. Furthermore, the temperature and pump fluence dependence of the coherent phonon oscillation signals suggest that the metallic state of VO2 responds to these large coherent acoustic phonons.

Laser-driven manipulation of magnetic skyrmions holds great promise for the development of next-generation information memory devices due to the flexibility of optical modulation. In this study, we theoretically present an approach for generating and manipulating stable topological spin textures using a light-induced radially polarized magnetic field, along with the ability to easily control quasiparticles by adjusting the intensity of the input beam. The generated topological spin textures can shrink into a stabilized quasiparticle with a size significantly smaller than the light-induced radially polarized magnetic field in few picoseconds. Through exploiting the relaxation mechanism of the spin texture at the center of the beam, we demonstrate the creation and stabilization of quasiparticles with higher ring numbers. Furthermore, we investigate the motion of these distinct quasiparticles and discover that the nπ quasiparticles (n > 1) exhibit significantly faster velocities than a traditional skyrmion, with the 4π quasiparticle being the fastest. Our method offers a promising avenue for constructing various topological spin textures with multiring profiles in ultrafast and high-density data-storage devices.