Configurable topological beam splitting via antichiral gyromagnetic photonic crystal

Antichiral gyromagnetic photonic crystal(GPC)in a honeycomb lattice with the two interpenetrating triangular sublattices A and B magnetically biased in opposite directions can realize antichiral one-way edge states propagating along the same direction at its two parallel edges.Here,we report the construction and observation of topological beam splitting with the easily adjustable right-to-left ratio in an antichiral GPC.The splitter is compact and configurable,has high trans-mission efficiency,and allows for multi-channel utilization,crosstalk-proof,and robust against defects and obstacles.This magnificent performance is attributed to the peculiar property that antichiral one-way edge states exist only at zigzag edge but not at armchair edge of antichiral GPC.When we combine two rectangular antichiral GPCs holding left-and right-propagating antichiral one-way edge states respectively,bidirectionally radiating one-way edge states at two paral-lel zigzag edges can be achieved.Our observations can enrich the understanding of fundamental physics and expand to-pological photonic applications.

Intensity correlation properties of x-ray beams split with Laue diffraction

Beam splitting is one of the main approaches to achieving x-ray ghost imaging, and the intensity correlation between diffraction beam and transmission beam will directly affect the imaging quality. In this paper, we investigate the intensity correlation between the split x-ray beams by Laue diffraction of stress-free crystal. The analysis based on the dynamical theory of x-ray diffraction indicates that the spatial resolution of diffraction image and transmission image are reduced due to the position shift of the exit beam. In the experimental setup, a stress-free crystal with a thickness of hundredmicrometers-level is used for beam splitting. The crystal is in a non-dispersive configuration equipped with a double-crystal monochromator to ensure that the dimension of the diffraction beam and transmission beam are consistent. A correlation coefficient of 0.92 is achieved experimentally and the high signal-to-noise ratio of the x-ray ghost imaging is anticipated.Results of this paper demonstrate that the developed beam splitter of Laue crystal has the potential in the efficient data acquisition of x-ray ghost imaging.

In situ frequency gating and beam splitting of vacuum- and extreme-ultraviolet pulses

Monochromatization of high-harmonic sources has opened fascinating perspectives regarding time-resolved photoemission from all phases of matter.Such studies have invariably involved the use of spectral filters or spectrally dispersive optical components that are inherently lossy and technically complex.Here we present a new technique for the spectral selection of near-threshold harmonics and their spatial separation from the driving beams without any optical elements.We discover the existence of a narrow phase-matching gate resulting from the combination of the non-collinear generation geometry in an extended medium,atomic resonances and absorption.Our technique offers a filter contrast of up to 104 for the selected harmonics against the adjacent ones and offers multiple temporally synchronized beamlets in a single unified scheme.We demonstrate the selective generation of 133,80 or 56 nm femtosecond pulses from a 400-nm driver,which is specific to the target gas.These results open new pathways towards phase-sensitive multi-pulse spectroscopy in the vacuum-and extreme-ultraviolet,and frequencyselective output coupling from enhancement cavities.

Wideband Channel Estimation for THz Massive MIMO

Terahertz(THz)communication is considered to be a promising technology for future 6G network.To overcome the severe attenuation and relieve the high power consumption,massive multipleinput multiple-output(MIMO)with hybrid precoding has been widely considered for THz communication.However,accurate wideband channel estimation,which is essential for hybrid precoding,is challenging in THz massive MIMO systems.The existing wideband channel estimation schemes based on the ideal assumption of common sparse channel support will suffer from a severe performance loss due to the beam split effect.In this paper,we propose a beam split pattern detection based channel estimation scheme to realize reliable wideband channel estimation in THz massive MIMO systems.Specifically,a comprehensive analysis on the angle-domain sparse structure of the wideband channel is provided by considering the beam split effect.Based on the analysis,we define a series of index sets called as beam split patterns,which are proved to have a one-to-one match to different physical channel directions.Inspired by this one-to-one match,we propose to estimate the physical channel direction by exploiting beam split patterns at first.Then,the sparse channel supports at different subcarriers can be obtained by utilizing a support detection window.This support detection window is generated by expanding the beam split pattern which is determined by the obtained physical channel direction.The above estimation procedure will be repeated path by path until all path components are estimated.Finally,the wideband channel can be recovered by calculating the elements on the total sparse channel support at all subcarriers.The proposed scheme exploits the wideband channel property implied by the beam split effect,i.e.,beam split pattern,which can significantly improve the channel estimation accuracy.Simulation results show that the proposed scheme is able to achieve higher accuracy than existing schemes.

Independent phase manipulation of co-and cross-polarizations with all-dielectric metasurface

Phase carried by two orthogonal polarizations can be manipulated independently by controlling both the geometric size and orientation of the dielectric nanopost.With this characteristic,we demonstrate a novel multifunctional metasurface,which converts part of the incident linearly polarized light into its cross-polarization and encodes the phase of the two orthogonal polarizations independently.A beam splitter and a bifocal metalens were realized in a single-layer dielectric metasurface by this approach.We fabricated the bifocal metalens and demonstrated that two focal spots in orthogonal polarizations can be separated transversely or longitudinally at will.The proposed approach shows a new route to design multifunctional metasurfaces with various applications in holography and three-dimensional display.