Terahertz wave generation from liquid nitrogen

We present the results of research carried out for the first time, to the best of our knowledge, on the generation of terahertz radiation under the action of "single-color" and "dual-color" high-power femtosecond laser pulses on liquefied gas–liquid nitrogen. Our experimental results supported by careful theoretical interpretation showed clearly that under femtosecond laser radiation, liquid and air emit terahertz waves in a very different way. We assumed that the mobility of ions and electrons in liquid can play an essential role, forming a quasi-static electric field by means of ambipolar diffusion mechanism.

Metasurfaces for manipulating terahertz waves

Terahertz(THz)science and technology have attracted significant attention based on their unique applications in non-destructive imaging,communications,spectroscopic detection,and sensing.However,traditional THz devices must be sufficiently thick to realise the desired wave-manipulating functions,which has hindered the development of THz integrated systems and applications.Metasurfaces,which are two-dimensional metamaterials consisting of predesigned meta-atoms,can accurately tailor the amplitudes,phases,and polarisations of electromagnetic waves at subwavelength resolutions,meaning they can provide a flexible platform for designing ultra-compact and high-performance THz components.This review focuses on recent advancements in metasurfaces for the wavefront manipulation of THz waves,including the planar metalens,holograms,arbitrary polarisation control,special beam generation,and active metasurface devices.Such ultra-compact devices with unique functionality make metasurface devices very attractive for applications such as imaging,encryption,information modulation,and THz communications.This progress report aims to highlight some novel approaches for designing ultra-compact THz devices and broaden the applications of metasurfaces in THz science.

Geometric metasurface for polarization synthesis and multidimensional multiplexing of terahertz converged vortices

The investigation of converged twisted beams with a helical phase structure has a remarkable impact on both fundamental physics and practical applications.Geometric metasurfaces consisting of individually orientated metal/dielectric meta-atoms provide an ultracompact platform for generating converged vortices.However,it is still challenging to simultaneously focus left-handed and right-handed circularly polarized incident beams with pure geometric phase modulation,which hinders the independent operation on topological charges between these two helical components.Here we propose and experimentally demonstrate an approach to design terahertz geometric metasurfaces that can generate helicity-independent converged vortices with homogeneous polarization states by the superposition of two orthogonal helical vortices with identical topological charges.Furthermore,the multiplexing of polarization-rotatable multiple vortices in multiple dimensions,i.e.,in both longitudinal and transverse directions,and a vortex with an extended focal depth is confirmed by embedding polarization modulation into the geometric metasurfaces.The demonstrated approach provides a new way to simultaneously manipulate orthogonal helical components and expand the design dimension,enabling new applications of geometric metasurface devices in polarization optics,twisted-beam related image and edge detection,high capacity optical communication,and quantum information processing,to name a few.