Generation of polarization-multiplexed terahertz orbital angular momentum combs via all-silicon metasurfaces

Electromagnetic waves carrying orbital angular momentum(OAM),namely OAM beams,are important in various fields including optics,communications,and quantum information.However,most current schemes can only generate single or several simple OAM modes.Multi-mode OAM beams are rarely seen.This paper proposes a scheme to design metasurfaces that can generate multiple polarization-multiplexed OAM modes with equal intervals and intensities(i.e.,OAM combs)working in the terahertz(THz)range.As a proof of concept,we first design a metasurface to generate a pair of polarization-multiplexed OAM combs with arbitrary mode numbers.Furthermore,another metasurface is proposed to realize a pair of polarization-multiplexed OAM combs with arbitrary locations and intervals in the OAM spectrum.Experimental results agree well with full-wave simulations,verifying a great performance of OAM combs generation.Our method may provide a new solution to designing high-capacity THz devices used in multi-mode communication systems.

Metasurfaces designed by a bidirectional deep neural network and iterative algorithm for generating quantitative field distributions

Metasurfaces,which are the two-dimensional counterparts of metamaterials,have demonstrated unprecedented capabilities to manipulate the wavefront of electromagnetic waves in a single flat device.Despite various advances in this field,the unique functionalities achieved by metasurfaces have come at the cost of the structural complexity,resulting in a time-consuming parameter sweep for the conventional metasurface design.Although artificial neural networks provide a flexible platform for significantly improving the design process,the current metasurface designs are restricted to generating qualitative field distributions.In this study,we demonstrate that by combining a tandem neural network and an iterative algorithm,the previous restriction of the design of metasurfaces can be overcome with quantitative field distributions.As proof-of-principle examples,metalenses predicted via the designed network architecture that possess multiple focal points with identical/orthogonal polarisation states,as well as accurate intensity ratios(quantitative field distributions),were numerically calculated and experimentally demonstrated.The unique and robust approach for the metasurface design will enable the acceleration of the development of devices with high-accuracy functionalities,which can be applied in imaging,detecting,and sensing.

Integrated terahertz vortex beam emitter for rotating target detection

We propose a terahertz(THz)vortex emitter that utilizes a high-resistance silicon resonator to generate vortex beams with various topological charges.Addressing the challenge of double circular polarization superposition resulting from the high refractive index contrast,we regulate the transverse spin state through a newly designed second-order grating partially etched on the waveguide’s top side.The reflected wave can be received directly by a linearly polarized antenna,simplifying the process.Benefiting from the tuning feature,a joint detection method involving positive and negative topological charges identifies and detects rotational Doppler effects amid robust micro-Doppler interference signals.This emitter can be used for the rotational velocity measurement of an on-axis spinning object,achieving an impressive maximum speed error rate of∼2%.This approach holds promise for the future development of THz vortex beam applications in radar target detection and countermeasure systems,given its low cost and potential for mass production.

Mode splitting transmission effect of surface wave excitation through a metal hole array

The resonant frequencies of the excited surface waves on a metal hole array with respect to the incident angle were studied in the terahertz region.The experimental and theoretical results demonstrate that the resonant peak of surface wave excitation splits into two when transmitted through a metal hole array off-normally.The high-order mode with resonant frequency above the cutoff frequency fc(plasma frequency effect)has a shorter attenuation length than that of the low-order mode whose resonant frequency is below fc.The reason is that the high-order mode is a coupled mode consisting of surface wave and hole modes,while the low-order mode is just an excited surface wave(which can be considered as the spoof surface plasmons).Our investigation may open a door to distinguish the spoof surface plasmons and the coupled modes of surface waves and hole modes.

Dual-layered metasurfaces for asymmetric focusing

Asymmetric transmission,defined as the difference between the forward and backward transmission,enables a plethora of applications for on-chip integration and telecommunications.However,the traditional method for asymmetric transmission is to control the propagation direction of the waves,hindering further applications.Metasurfaces,a kind of two-dimensional metamaterials,have shown an unprecedented ability to manipulate the propagation direction,phase,and polarization of electromagnetic waves.Here we propose and experimentally demonstrate a metasurface-based directional device consisting of a geometric metasurface with spatially rotated microrods and metallic gratings,which can simultaneously control the phase,polarization,and propagation direction of waves,resulting in asymmetric focusing in the terahertz region.These dual-layered metasurfaces for asymmetric focusing can work in a wide bandwidth ranging from 0.6 to 1.1 THz.The flexible and robust approach for designing broadband asymmetric focusing may open a new avenue for compact devices with potential applications in encryption,information processing,and communication.

Spin-decoupled metalens with intensity-tunable multiple focal points

The control of spin electromagnetic(EM)waves is of great significance in optical communications.Although geometric metasurfaces have shown unprecedented capability to manipulate the wavefronts of spin EM waves,it is still challenging to independently manipulate each spin state and intensity distribution,which inevitably degrades metasurface-based devices for further applications.Here we propose and experimentally demonstrate an approach to designing spin-decoupled metalenses based on pure geometric phase,i.e.,geometric metasurfaces with predesigned phase modulation possessing functionalities of both convex lenses and concave lenses.Under the illumination of left-/right-handed circularly polarized(LCP or RCP)terahertz(THz)waves,these metalenses can generate transversely/longitudinally distributed RCP/LCP multiple focal points.Since the helicity-dependent multiple focal points are locked to the polarization state of incident THz waves,the relative intensity between two orthogonal components can be controlled with different weights of LCP and RCP THz waves,leading to the intensity-tunable functionality.This robust approach for simultaneously manipulating orthogonal spin states and energy distributions of spin EM waves will open a new avenue for designing multifunctional devices and integrated communication systems.

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.

Terahertz out-of-plane coupler based on compact spot-size converter

Low-loss dielectric terahertz(THz) chips are efficient platforms for diverse THz applications. One of the key elements in the chip is the coupler. Most of the available THz couplers are in-plane and couple the THz wave from the metal waveguide to the dielectric waveguide. However, out-of-plane couplers are more suitable for wafer-scale testing and tolerant of alignment variation. In this work, we propose an out-of-plane THz coupler for coupling the antenna to the dielectric waveguide. The device is constructed using a grating and a compact spot-size converter. As the conventional optical spot-size converters that apply directly to THz chips are too large, we have designed a compact spot-size converter based on a tapered waveguide with a lens. The total device is 2.9 cm long and can couple a 7 mm diameter THz beam to a 500 μm wide waveguide. The device can scan the THz beam, radiate the input rectangular waveguide mode to free space, and drive the rotation angle of the fan beam through the scanning frequency. We fabricated the device using a single lithography step on a silicon wafer.The out-of-plane coupling efficiency was found to be ~5 d B at 194 GHz. The fan-beam steering range was found to be around40° in the frequency range of 170–220 GHz. The proposed out-of-plane coupling technique may provide an effective way for THz wafer-scale testing with a higher degree of freedom for on-chip integration. Also, the proposed technique being nonmechanical, beam steering using it, may therefore find applications in THz radar, communication, and sensing.

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.