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MXene is a promising two-dimensional (2D) material that is widely used in electro-photonic devices due to its unique properties. In this contribution, V2CTx, a novel MXene, was employed as a saturable absorber (SA) for hybrid passively mode-locked fiber lasers. An ultra-stable and self-starting mode-locked laser system with low threshold can be achieved using V2CTx nanosheets and nonlinear polarization evolution (NPE). Signal to noise ratio increased 13 dB compared with using only NPE SA. A 72 fs pulse duration is easily achieved from this hybrid mode-locked fiber laser system. To the best of our knowledge, this is the shortest pulse duration generated from the Yb-doped mode-locked fiber lasers using a hybrid or 2D SAs. This study proves that MXene V2CTx nanosheets can be developed as suitable SAs and served as potential advanced ultrafast photonic devices in the future.

In this paper, a double-layer acoustic metasurface (DAM) composed of a fixed lower acoustic metasurface (LAM) and a rotatable upper acoustic metasurface (UAM) is proposed for the generation of mode-reconfigurable acoustic orbital angular momentum (OAM). The UAM and LAM are divided into multiple sections, in which the hybrid structures combining cascaded Helmholtz resonators and a straight pipe are adopted to achieve specific phase compensation. By rotating the UAM, the incident acoustic plane wave can be efficiently converted into the vortex acoustic waves of reconfigurable topological charges ranging from −5 to +5 with distinguishable purity. Furthermore, the influences of the parameters on the purity of the generated topological charges have been investigated and discussed, such as the distance between LAM and UAM, rotatable angle error, and operating frequency. With the capability of reconfigurable OAM modes, the proposed DAM can be used to efficiently increase capacity or to conveniently switch between different channels in underwater vortex acoustic communications. [ABSTRACT FROM AUTHOR]

The vortex electromagnetic wave, which carries the Orbital Angular Momentum (OAM), reflects a new degree of freedom in addition to the traditional degrees of freedom such as intensity, phase, frequency, and polarization. Theoretically, vortex electromagnetic wave, at any frequency, has an infinite number of orthogonal modes that do not interfere with each other, and in recent years, they have shown important potential applications in the fields of radar imaging, wireless communication and so on. Therefore, they have attracted considerable attention from scholars worldwide owing to their high research value and application prospects. Here, this paper mainly introduces the recent research advances on the antenna technology of vortex electromagnetic wave, including single microstrip patch antenna, array antenna, traveling wave antenna, and metasurface antenna structure. The single microstrip patch antenna is widely used owing to its simple structure and low manufacturing cost. The traveling wave antenna can generate multi-OAM mode vortex electromagnetic waves in a wide-frequency range. The array antenna is easy to design and controllably generate high-gain OAM electromagnetic waves with different modes. The metasurface antennas do not require complex feeding networks, which has the advantage of a lower profile of the antenna. Finally, we summarize these four common vortex antennas and further look forward to their future developments.

Spin-dependent nonlinear harmonic generations from nanostructures may provide a new class of design for future applications such as nonlinear optical switching and modulation and nonlinear information processing. Most reported works are based on the utilization of metasurfaces with specific symmetry (such as C3 and C4) and the application of the Pancharatnam–Berry phase approach, making them sensitive to local defects. In this work, we investigated a controllable second harmonic generation (SHG) based on the topological spin-dependent edge state, which is robust to a broad class of perturbations. Simulation results demonstrated that spin-dependent edge states with strong field localization can be obtained through spin–orbit coupling, resulting in SHG with increased efficiency. In addition, the propagation direction of the enhanced SHG could be dynamically controlled using an excitation source with left/right circular polarizations, exhibiting negligible backscattering and topological protection against local defects. The proposed approach may open a new avenue for manipulating light–light interactions. [ABSTRACT FROM AUTHOR]

In this work, using a Richards-Wolf formalism, we derive explicit analytical relationships to describe vectors of the major and minor axes of polarization ellipses centered in the focal plane when focusing a cylindrical vector beam of integer order n. In these beams, the major axis of a polarization ellipse is found to lie in the focal plane, with the minor axis being perpendicular to the focal plane. This means that the polarization ellipse is perpendicular to the focal plane, with its polarization vector rotating either clockwise or anticlockwise and forming "photonic wheels". Considering that the wave vector is also perpendicular to the focal plane, we conclude that the polarization ellipse and the wave vector are in the same plane, so that at some point these can coincide, which is uncharacteristic of transverse electromagnetic oscillations. In a cylindrical vector beam, the spin angular momentum vector lies in the focal plane, so when making a circle centered on the optical axis, at some sections, the handedness of the spin vector and circular motion are the same, being opposite elsewhere. This effect may be called an azimuthal transverse spin Hall effect, unlike the familiar longitudinal spin Hall effect found at the sharp focus. The longitudinal spin Hall effect occurs when opposite-sign longitudinal projections of the spin angular momentum vector are spatially separated in the focal plane. In this work, we show that for the latter, there are always an even number of spatially separated regions and that, when making an axis-centered circle, the major-axis vector of polarization ellipse forms a two-sided twisted surface with an even number of twists. [ABSTRACT FROM AUTHOR]