Hybrid bound states in the continuum in terahertz metasurfaces

Bound states in the continuum(BICs)have exhibited extraordinary properties in photonics for enhanced light-matter interactions that enable appealing applications in nonlinear optics,biosensors,and ultrafast optical switches.The most common strategy to apply BICs in a metasurface is by breaking symmetry of resonators in the uniform array that leaks the otherwise uncoupled mode to free space and exhibits an inverse quadratic relationship between quality factor(Q)and asymmetry.Here,we propose a scheme to further reduce scattering losses and improve the robustness of symmetry-protected BICs by decreasing the radiation density with a hybrid BIC lattice.We observe a significant increase of radiative Q in the hybrid lattice compared to the uniform lattice with a factor larger than 14.6.In the hybrid BIC lattice,modes are transferred toГpoint inherited from high symmetric X,Y,and M points in the Brillouin zone that reveal as multiple Fano resonances in the far field and would find applications in hyperspectral sensing.This work initiates a novel and generalized path toward reducing scattering losses and improving the robustness of BICs in terms of lattice engineering that would release the rigid requirements of fabrication accuracy and benefit applications of photonics and optoelectronic devices.

Active-Thermal-Tunable Terahertz Absorber with Temperature-Sensitive Material Thin Film

It is shown that active-tunable terahertz absorbers can be realized in a sandwich-structured system comprising an ultrathin dielectric film(polyimide) on a temperature-sensitive substrate(InSb) with a metal film on the back by utilizing the intrinsic carrier density(N) variation in InSb. When increasing the temperature from 250 to 320 K, N in InSb varied from ～5.50×1015 to ～2.98×1016 cm–3. Fixing the thickness of dielectric film with the value of 1.37 μm, the absorption peak shifted from 1.41 to 3.29 THz while keeping absorption higher than 99%. This active tunability can respond to even a slight temperature perturbation, and shows polarization insensitivity as well as high tolerance of incidence-angle(absorption peak can still exceed 90% even the incidence angle reaches 60°). Besides, the refractive index of polyimide(PI) has thermal stability at the terahertz range and the merit of good workability. These characteristics guarantee the stability of activetunable performance. The peculiarities and innovations of this proposal promise a wide range of high efficiency terahertz devices, such as thermal sensors, spatial light modulators(SLMs) and so on.

Recent progress in graphene terahertz modulators

Graphene has been recognized as a promising candidate in developing tunable terahertz(THz)functional devices due to its excellent optical and electronic properties,such as high carrier mobility and tunable conductivity.Here,we review graphene-based THz modulators we have recently developed.First,the optical properties of graphene are discussed.Then,graphene THz modulators realized by different methods,such as gate voltage,optical pump,and nonlinear response of graphene are presented.Finally,challenges and prospective of graphene THz modulators are also discussed.

Active terahertz beam steering based on mechanical deformation of liquid crystal elastomer metasurface

Active metasurfaces are emerging as the core of next-generation optical devices with their tunable optical responses and flat-compact topography.Especially for the terahertz band,active metasurfaces have been developed as fascinating devices for optical chopping and compressive sensing imaging.However,performance regulation by changing the dielectric parameters of the integrated functional materials exhibits severe limitations and parasitic losses.Here,we introduce a C-shape-split-ring-based phase discontinuity metasurface with liquid crystal elastomer as the substrate for infrared modulation of terahertz wavefront.Line-focused infrared light is applied to manipulate the deflection of the liquid crystal elastomer substrate,enabling controllable and broadband wavefront steering with a maximum output angle change of 22°at 0.68THz.Heating as another control method is also investigated and compared with infrared control.We further demonstrate the performance of liquid crystal elastomer metasurface as a beam steerer,frequency modulator,and tunable beam splitter,which are highly desired in terahertz wireless communication and imaging systems.The proposed scheme demonstrates the promising prospects of mechanically deformable metasurfaces,thereby paving the path for the development of reconfigurable metasurfaces.

Generating superposed terahertz perfect vortices via a spin-multiplexed all-dielectric metasurface

Perfect optical vortices(POVs),characterized as a ring radius independent of topological charge(TC),possess extensive application in particle manipulation and optical communication.At present,the complex and bulky optical device for generating POVs has been miniaturized by leveraging the metasurface,and either spindependent or spin-independent POV conversions have been further accomplished.Nevertheless,it is still challenging to generate superposed POVs for incidences with orthogonal circular polarization.Here,a spinmultiplexed all-dielectric metasurface method for generating superposed POVs in the terahertz frequency range is proposed and demonstrated.By using the multiple meta-atom comprised structure as the basic unit,the complex amplitude of two superposed POVs is modulated,decoupled,and subsequently encoded to left-and righthanded circular polarization incidences.Furthermore,two kinds of metasurfaces are fabricated and characterized to validate this controlling method.It is demonstrated that the measured intensity and phase distributions match well with the calculation of the Rayleigh–Sommerfeld diffraction integral,and the radius of superposed POVs is independent of TCs.This work provides promising opportunities for developing ultracompact terahertz functional devices applied to complex structured light generation and terahertz communication,and exploring sophisticated spin angular momentum and orbital angular momentum interactions like the photonic spin-Hall effect.

Meta-optics inspired surface plasmon devices

Surface plasmons(SPs)are electromagnetic surface waves that propagate at the interface between a conductor and a dielectric.Due to their unique ability to concentrate light on two-dimensional platforms and produce very high local-field intensity,SPs have rapidly fueled a variety of fundamental advances and practical applications.In parallel,the development of metamaterials and metasurfaces has rapidly revolutionized the design concepts of traditional optical devices,fostering the exciting field of meta-optics.This review focuses on recent progress of meta-optics inspired SP devices,which are implemented by the careful design of subwavelength structures and the arrangement of their spatial distributions.Devices of general interest,including coupling devices,on-chip tailoring devices,and decoupling devices,as well as nascent SP applications empowered by sophisticated usage of meta-optics,are introduced and discussed.

Polarization-independent all-silicon dielectric metasurfaces in the terahertz regime

Dielectric metasurfaces have achieved great success in realizing high-efficiency wavefront control in the optical and infrared ranges. Here, we experimentally demonstrate several efficient, polarization-independent, all-silicon dielectric metasurfaces in the terahertz regime. The metasurfaces are composed of cylindrical silicon pillars on a silicon substrate, which can be easily fabricated using etching technology for semiconductors. By locally tailoring the diameter of the pillars, full control over abrupt phase changes can be achieved. To show the controlling ability of the metasurfaces, an anomalous deflector, three Bessel beam generators, and three vortex beam generators are fabricated and characterized. We also show that the proposed metasurfaces can be easily combined to form composite devices with extended functionalities. The proposed controlling method has promising applications in developing low-loss, ultra-compact spatial terahertz modulation devices.

Terahertz surface plasmonic waves: a review

Terahertz science and technology promise many cutting-edge applications.Terahertz surface plasmonic waves that propagate at metal–dielectric interfaces deliver a potentially effective way to realize integrated terahertz devices and systems.Previous concerns regarding terahertz surface plasmonic waves have been based on their highly delocalized feature.However,recent advances in plasmonics indicate that the confinement of terahertz surface plasmonic waves,as well as their propagating behaviors,can be engineered by designing the surface environments,shapes,structures,materials,etc.,enabling a unique and fascinating regime of plasmonic waves.Together with the essential spectral property of terahertz radiation,as well as the increasingly developed materials,microfabrication,and time-domain spectroscopy technologies,devices and systems based on terahertz surface plasmonic waves may pave the way toward highly integrated platforms for multifunctional operation,implementation,and processing of terahertz waves in both fundamental science and practical applications.We present a review on terahertz surface plasmonic waves on various types of supports in a sequence of properties,excitation and detection,and applications.The current research trend and outlook of possible research directions for terahertz surface plasmonic waves are also outlined.

Terahertz bound states in the continuum with incident angle robustness induced by a dual period metagrating

Metasurface-empowered bound state in the continuum(BIC)provides a unique route for fascinating functional devices with infinitely high quality factors.This method is particularly attractive to the terahertz community because it may essentially solve the deficiencies in terahertz filters,sensors,lasers,and nonlinear sources.However,most BIC metasurfaces are limited to specified incident angles that seriously dim their application prospects.Here,we propose that a dual-period dielectric metagrating can support multiple families of BICs that originate from guided mode resonances in the dielectric grating and exhibit infinite quality factors at arbitrarily tilted incidence.This robustness was analyzed based on the Bloch theory and verified at tilted incident angles.We also demonstrate that inducing geometric asymmetry is an efficient way to manipulate the leakage and coupling of these BICs,which can mimic the electromagnetically induced transparency(EIT)effect in our dual-period metagrating.In this demonstration,a slow-light effect with a measured group delay of 117 ps was achieved.The incidence-insensitive BICs proposed here may greatly extend the application scenarios of the BIC effect.The high Q factor and outstanding slow-light effect in the metagrating show exciting prospects in realizing high-performance filters,sensors,and modulators for prompting terahertz applications.

Moiré-driven electromagnetic responses and magic angles in a sandwiched hyperbolic metasurface

Recent moiréconfigurations provide a new platform for tunable and sensitive photonic responses,as their enhanced light–matter interactions originate from the relative displacement or rotation angle in a stacking bilayer or multilayer periodic array.However,previous findings are mostly focused on atomically thin condensed matter,with limitations on the fabrication of multilayer structures and the control of rotation angles.Structured microwave moiréconfigurations are still difficult to realize.Here,we design a novel moiréstructure,which presents unprecedented capability in the manipulation of light–matter interactions.Based on the effective medium theory and S-parameter retrieval process,the rotation matrix is introduced into the dispersion relation to analyze the underlying physical mechanism,where the permittivity tensor transforms from a diagonal matrix to a fully populated one,whereas the permeability tensor evolves from a unit matrix to a diagonal one and finally becomes fully filled,so that the electromagnetic responses change drastically as a result of stacking and rotation.Besides,the experiment and simulation results reveal hybridization of eigenmodes,drastic manipulation of surface states,and magic angle properties by controlling the mutual rotation angles between two isolated layers.Here,not only a more precisely controllable bilayer hyperbolic metasurface is introduced to moiréphysics,the findings also open up a new avenue to realize flat bands at arbitrary frequencies,which shows great potential in active engineering of surface waves and designing multifunctional plasmonic devices.