Graphene-empowered dynamic metasurfaces and metadevices

Metasurfaces,with extremely exotic capabilities to manipulate electromagnetic(EM)waves,have derived a plethora of advanced metadevices with intriguing functionalities.Tremendous endeavors have been mainly devoted to the static metasurfaces and metadevices,where the functionalities cannot be actively tuned in situ post-fabrication.Due to the in-trinsic advantage of active tunability by external stimulus,graphene has been successively demonstrated as a favorable candidate to empower metasurfaces with remarkably dynamic tunability,and their recent advances are propelling the EM wave manipulations to a new height:from static to dynamic.Here,we review the recent progress on dynamic metasur-faces and metadevices enabled by graphene with the focus on electrically-controlled dynamic manipulation of the EM waves covering the mid-infrared,terahertz,and microwave regimes.The fundamentals of graphene,including basic ma-terial properties and plasmons,are first discussed.Then,graphene-empowered dynamic metasurfaces and met-adevices are divided into two categories,i.e.,metasurfaces with building blocks of structured graphene and hybrid metasurfaces integrated with graphene,and their recent advances in dynamic spectrum manipulation,wavefront shap-ing,polarization control,and frequency conversion in near/far fields and global/local ways are elaborated.In the end,we summarize the progress,outline the remaining challenges,and prospect the potential future developments.

Physical-Layer Key Generation Based on Multipath Channel Diversity Using Dynamic Metasurface Antennas

Physical layer key generation(PKG)technology leverages the reciprocal channel randomness to generate the shared secret keys.The low secret key capacity of the existing PKG schemes is due to the reduction in degree-of-freedom from multipath fading channels to multipath combined channels.To improve the wireless key generation rate,we propose a multipath channel diversity-based PKG scheme.Assisted by dynamic metasurface antennas(DMA),a two-stage multipath channel parameter estimation algorithm is proposed to efficiently realize super-resolution multipath parameter estimation.The proposed algorithm first estimates the angle of arrival(AOA)based on the reconfigurable radiation pattern of DMA,and then utilizes the results to design the training beamforming and receive beamforming to improve the estimation accuracy of the path gain.After multipath separation and parameter estimation,multi-dimensional independent path gains are utilized for generating secret keys.Finally,we analyze the security and complexity of the proposed scheme and give an upper bound on the secret key capacity in the high signal-to-noise ratio(SNR)region.The simulation results demonstrate that the proposed scheme can greatly improve the secret key capacity compared with the existing schemes.

Optically controlled dielectric metasurfaces for dynamic dual-mode modulation on terahertz waves

Dynamically controlling terahertz(THz)waves with an ultracompact device is highly desired,but previously realized tunable devices are bulky in size and/or exhibit limited light-tuning functionalities.Here,we experimentally demonstrate dynamic modulation on THz waves with a dielectric metasurface in modeselective or mode-unselective manners through pumping the system at different optical wavelengths.Quasi-normal-mode theory reveals that the physics is governed by the spatial overlap between wave functions of resonant modes and regions inside resonators perturbed by pump laser excitation at different wavelengths.We further design/fabricate a dielectric metasurface and experimentally demonstrate that it can dynamically control the polarization state of incident THz waves,dictated by the strength and wavelength of the pumping light.We finally numerically demonstrate pump wavelength-controlled optical information encryption based on a carefully designed dielectric metasurface.Our studies reveal that pump light wavelength can be a new external knob to dynamically control THz waves,which may inspire many tunable metadevices with diversified functionalities.

Effects of realistic reradiation models in digital reconfigurable intelligent surfaces

Reconfigurable intelligent surfaces(RISs)are a promising technology for wireless communication applications,but their performance is often optimized using simplified electromagnetic reradiation models.In this study,we explore the impact on the RIS performance of more realistic assumptions,including the(possibly imperfect)quantization of the reflection coefficients,subwavelength inter-element spacing,near-field location,and presence of electromagnetic interference.We find that design constraints can cause an RIS to reradiate power in unwanted directions.Therefore,it is important to optimize an RIS by considering the entire reradiation pattern.Overall,our study indicates that a 2-bit digitally controllable RIS with a nearly constant reflection amplitude and RIS elements with a size and inter-element spacing between(1/8)th and(1/4)th of the signal wavelength may offer a reasonable tradeoff between performance,complexity,and cost.