Metasurfaces enabling structured light manipulation:advances and perspectives [Invited]

Metasurfaces and structured light have rapidly advanced over the past few years, from being paradigms to forming functional devices and tailoring special light beams for wide emerging applications. Here, we focus on harnessing metasurfaces for structured light manipulation. We review recent advances in shaping structured light by metasurfaces on different platforms（metal, silica, silicon, and fiber）. Structured light manipulation based on plasmonic metasurfaces, reflection-enhanced plasmonic metasurfaces, metasurfaces on fiber facets, dielectric metasurfaces, and sub-wavelength structures on silicon are presented, showing impressive performance.Future trends, challenges, perspectives, and opportunities are also discussed.

Generation and characterization of customized Laguerre–Gaussian beams with arbitrary profiles

We experimentally demonstrate the generation of customized Laguerre–Gaussian(LG)beams whose intensity maxima are localized around any desired curves.The principle is to act with appropriate algebraic functions on the angular spectra of LG beams.We characterize the propagation properties of these beams and compare them with non-diffraction caustic beams possessing the same intensity profiles.The results manifest that the customized-LG beams can maintain their profiles during propagation and suffer less energy loss than the non-diffraction caustic beams,and hence are able to propagate a longer distance.Moreover,the customized-LG beam exhibits self-healing ability when parts of their bodies are blocked.This new structure beam has potential applications in areas such as optical communication,soliton routing and steering,and optical tweezing.

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.

Optical bound states in the continuum in periodic structures:mechanisms,effects,and applications

Optical bound states in the continuum(BICs)have recently stimulated a research boom,accompanied by demonstrations of abundant exotic phenomena and applications.With ultrahigh quality(Q)factors,optical BICs have powerful abilities to trap light in optical structures from the continuum of propagation waves in free space.Besides the high Q factors enabled by the confined properties,many hidden topological characteristics were discovered in optical BICs.Especially in periodic structures with well-defined wave vectors,optical BICs were discovered to carry topological charges in momentum space,underlying many unique physical properties.Both high Q factors and topological vortex configurations in momentum space enabled by BICs bring new degrees of freedom to modulate light.BICs have enabled many novel discoveries in light-matter interactions and spin-orbit interactions of light,and BIC applications in lasing and sensing have also been well explored with many advantages.In this paper,we review recent developments of optical BICs in periodic structures,including the physical mechanisms of BICs,explored effects enabled by BICs,and applications of BICs.In the outlook part,we provide a perspective on future developments for BICs.

Active tuning of Si metasurface with large area

All-dielectric metasurfaces are usually limited because of their static functionality and small scale.In this paper,we use an easy nanofabrication technique to fabricate all-dielectric metasurfaces with the advantages of having dynamic tunability and a large area.Using an anodized aluminum oxide(AAO)template as an evaporation mask,a large-area metasurface embedded in polydimethylsiloxane(PDMS)(>2 cm^(2))is fabricated.The metasurface exhibits remarkable electric dipole(ED)and magnetic dipole(MD)resonances.Based on the solvent-swelling effect of PDMS in 20%toluene,the ED/MD resonance peak shifts dynamically∼40 nm to red.So far,to the best of our knowledge,a large-area metasurface embedded in PDMS and achieved by using the AAO template method has not appeared.

Optimizing illumination's complex coherence state for overcoming Rayleigh's resolution limit

We suggest tailoring of the illumination's complex degree of coherence for imaging specific two-and three-point objects with resolution far exceeding the Rayleigh limit.We first derive a formula for the image intensity via the pseudo-mode decomposition and the fast Fourier transform valid for any partially coherent illumination(Schell-like,non-uniformly correlated,twisted)and then show how it can be used for numerical image manipulations.Further,for Schell-model sources,we show the improvement of the two-and three-point resolution to 20%and 40%of the classic Rayleigh distance,respectively.

Generalized high-order twisted partially coherent beams and their propagation characteristics

Twist phase is a nontrivial statistical phase that only exists in partially coherent fields,which makes the beam carry orbital angular momentum(OAM).In this paper,we introduce a new kind of partially coherent beams carrying high-order twist phase,named generalized high-order twisted partially coherent beams(GHTPCBs).The propagation dynamics such as the spectral density and OAM flux density propagating in free space are investigated numerically with the help of mode superposition and fast Fourier transform(FFT)algorithm.Our results show that the GHTPCBs are capable of self-focusing,and the beam spot during propagation exhibits teardrop-like or the diamond-like shape in some certain cases.Moreover,the influences of the twist order and the twist factor on the OAM flux density during propagation are also illustrated in detail.Finally,we experimentally synthesize the GHTPCBs with controllable twist phase by means of pseudo-mode superposition and measure their spectral density during propagation.The experimental results agree well with the theoretical predictions.Our studies may find applications in nonlinear optics and particle trapping.

Hyperbolic metamaterials: fusing artificial structures to natural 2D materials

Optical metamaterials have presented an innovative method of manipulating light.Hyperbolic metamaterials have an extremely high anisotropy with a hyperbolic dispersion relation.They are able to support high-k modes and exhibit a high density of states which produce distinctive properties that have been exploited in various applications,such as super-resolution imaging,negative refraction,and enhanced emission control.Here,state-of-the-art hyperbolic metamaterials are reviewed,starting from the fundamental principles to applications of artificially structured hyperbolic media to suggest ways to fuse natural two-dimensional hyperbolic materials.The review concludes by indicating the current challenges and our vision for future applications of hyperbolic metamaterials.