Coherent Perfect Absorption in Higher Order Systems

We investigate the phenomenon of coherent perfect absorption in a high-order system with three passive resonators coupled to a super-surface to form this three-state coherent perfect absorber. The effective parity time (PT) symmetry in the passive system has received much attention, and in this open three-state PT symmetric system, the incident wave is used as the effective gain instead of balancing the material gain and loss. We analyze the variation of coherent perfect absorption of this system with the coupling coefficient of the system by simulation.

Solar energy full-spectrum perfect absorption and efficient photo-thermal generation

Designing and manufacturing cost-effective absorbers that can cover the full-spectrum of solar irradiation is still critically important for solar harvesting.Utilizing control of the lightwave reflection and transmission,metamaterials realize high absorption over a relatively wide bandwidth.Here,a truncated circular cone metasurface(TCCM)composed of alternating multiple layers of titanium(Ti)and silicon dioxide(SiO_(2))is presented.Enabled by the synergetic of surface plasmon resonances and Fabry-Pérot resonances,the TCCM simultaneously achieves high absorptivity(exceed 90%),and absorption broadband covers almost the entire solar irradiation spectrum.In addition,the novel absorber exhibits great photo-thermal property.By exploiting the ultrahigh melting point of Ti and SiO_(2),high-efficiency solar irradiation absorption and heat release have been achieved at 700℃when the solar concentration ratio is 500(i.e.,incident light intensity at 5×10^(5) W/m^(2)).It is worth noting that the photo-thermal efficiency is almost unchanged when the incident angle increases from 0°to 45°.The outstanding capacity for solar harvesting and light-to-heat reported in this paper suggests that TCCM has great potential in photothermal therapies,solar desalination,and radiative cooling,etc.

Metamaterial terahertz device with temperature regulation function that can achieve perfect absorption and complete reflection conversion of ultrawideband

The field of terahertz devices is important in terahertz technology.However,most of the current devices have limited functionality and poor performance.To improve device performance and achieve multifunctionality,we designed a terahertz device based on a combination of VO_(2)and metamaterials.This device can be tuned using the phase-transition characteristics of VO_(2),which is included in the triple-layer structure of the device,along with SiO_(2)and Au.The terahertz device exhibits various advantageous features,including broadband coverage,high absorption capability,dynamic tunability,simple structural design,polarization insensitivity,and incidentangle insensitivity.The simulation results showed that by controlling the temperature,the terahertz device achieved a thermal modulation range of spectral absorption from 0 to 0.99.At 313 K,the device exhibited complete reflection of terahertz waves.As the temperature increased,the absorption rate also increased.When the temperature reached 353 K,the device absorption rate exceeded 97.7%in the range of 5-8.55 THz.This study used the effective medium theory to elucidate the correlation between conductivity and temperature during the phase transition of VO_(2).Simultaneously,the variation in device performance was further elucidated by analyzing and depicting the intensity distribution of the electric field on the device surface at different temperatures.Furthermore,the impact of various structural parameters on device performance was examined,offering valuable insights and suggestions for selecting suitable parameter values in real-world applications.These characteristics render the device highly promising for applications in stealth technology,energy harvesting,modulation,and other related fields,thus showcasing its significant potential.

Topological bound state in the continuum induced unidirectional acoustic perfect absorption

The interaction between cavity field and atoms plays an important role in exploring the abundant non-Hermitian physics and constructing powerful wave function devices.In this work,we propose theoretically and realize experimentally unidirectional perfect absorption in a non-Hermitian acoustic system with the help of the topological bound state in the continuum(BIC),which is established by the hybrid interaction between one trivial BIC and another conventional resonant state.In the 2D parameter space spanned by frequency and distance between the two resonators,the topological scattering singularities appear in pairs and are associated with topological distinguished charges.Meanwhile,we reveal the origin of topological charges and their continuous evolution with the loss factor.At a specific loss factor,two topological charges just annihilate together,and acoustic perfect absorption induced by topological BIC is realized at the left incidence,while there is no phase singularity and near-total reflection is observed at the right incidence,hence the system presents extreme asymmetry.Our work bridges the gap between scattering characteristics of non-Hermitian acoustic systems and topological scattering singularities,which may contribute to the research of novel non-Hermitian physics and the practical applications of advanced absorbers and sensors.

Reconfigurable metamaterial for asymmetric and symmetric elastic wave absorption based on exceptional point in resonant bandgap

Elastic wave absorption at subwavelength scale is of significance in many engineering applications.Non-Hermitian metamaterials show the ability in high-efficiency wave absorption.However,the single functionality of metamaterials is an important limitation on their practical applications for lack of tunability and reconfigurability.Here,we propose a tunable and reconfigurable non-Hermitian piezoelectric metamaterial bar,in which piezoelectric bars connect with resonant circuits,to achieve asymmetric unidirectional perfect absorption(UPA)and symmetric bidirectional perfect absorption(PA)at low frequencies.The two functions can be arbitrarily switched by rearranging shunted circuits.Based on the reverberation-ray matrix(RRM)method,an approach is provided to achieve UPA by setting an exceptional point(EP)in the coupled resonant bandgap.By using the transfer matrix method(TMM)and the finite element method(FEM),it is observed that a non-Hermitian pseudo-band forms between two resonant bandgaps,and the EP appears at the bottom of the pseudo-band.In addition,the genetic algorithm is used to accurately and efficiently design the shunted circuits for desired low-frequency UPA and PA.The present work may provide new strategies for vibration suppression and guided waves manipulation in wide potential applications.

Angle-selective perfect absorption with two-dimensional materials

Two-dimensional(2D)materials have great potential in photonic and optoelectronic devices.However,the relatively weak light absorption in 2D materials hinders their application in practical devices.Here,we propose a general approach to achieve angleselective perfect light absorption in 2D materials.As a demonstration of the concept,we experimentally show giant light absorption by placing large-area single-layer graphene on a structure consisting of a chalcogenide layer atop a mirror and achieving a total absorption of 77.6%in the mid-infrared wavelength range(~13μm),where the graphene contributes a record-high 47.2%absorptivity of mid-infrared light.Construction of such an angle-selective thin optical element is important for solar and thermal energy harvesting,photo-detection and sensing applications.Our study points to a new opportunity to combine 2D materials with photonic structures to enable novel device applications.

Tunable coherent perfect absorption in three-dimensional Dirac semimetal films

In this article,we investigate the phenomenon of coherent perfect absorption(CPA)with bulk Dirac semimetal(BDS)thin film.CPA of BDS appears at the frequency of 43.89 THz with 0°phase modulation of two coherent input lights.Meanwhile,it shows that CPA can be realized under oblique incidence circumstances for both TM and TE polarizations.Moreover,the frequency of CPA can be adjusted by altering the thickness of BDS thin film,and the dynamic regulation of CPA can be realized by changing the Fermi energy.Finally,the peak coherent absorption frequency can be controlled by changing the degeneracy factor.

Dual-symmetry-perturbed all-dielectric resonant metasurfaces for high-Q perfect light absorption

We demonstrate a high-Q perfect light absorber based on all-dielectric doubly-resonant metasurface.Leveraging bound states in the continuum(BICs)protected by different symmetries,we manage to independently manipulate the Q factors of the two degenerate quasi-BICs through dual-symmetry perturbations,achieving precise matching of the radiative and nonradiative Q factors for degenerate critical coupling.We achieve a narrowband light absorption with a>600 Q factor and a>99%absorptance atλ_(0)=1550 nm on an asymmetric germanium metasurface with a 0.2λ_(0)thickness.Our work provides a new strategy for engineering multiresonant metasurfaces for narrowband light absorption and nonlinear applications.

Omnidirectional and compact Tamm phonon-polaritons enhanced mid-infrared absorber

Narrow band mid-infrared(MIR)absorption is highly desired in thermal emitter and sensing applications.We theoretically demonstrate that the perfect absorption at infrared frequencies can be achieved and controlled around the surface phonon resonance frequency of silicon carbide(SiC).The photonic heterostructure is composed of a distributed Bragg reflector(DBR)/germanium(Ge)cavity/SiC on top of a Ge substrate.Full-wave simulation results illustrate that the Tamm phonon-polaritons electric field can locally concentrate between the Ge cavity and the SiC film,contributed to the improved light-phonon interactions with an enhancement of light absorption.The structure has planar geometry and does not require nano-patterning to achieve perfect absorption of both polarizations of the incident light in a wide range of incident angles.Their absorption lines are tunable via engineering of the photon band-structure of the dielectric photonic nanostructures to achieve reversal of the geometrical phase across the interface with the plasmonic absorber.

All-optical dynamic focusing of light via coherent absorption in a plasmonic metasurface

Vision,microscopy,imaging,optical data projection and storage all depend on focusing of light.Dynamic focusing is conventionally achieved with mechanically reconfigurable lenses,spatial light modulators or microfluidics.Here we demonstrate that dynamic control of focusing can be achieved through coherent interaction of optical waves on a thin beam splitter.We use a nanostructured plasmonic metasurface of subwavelength thickness as the beam splitter,allowing operation in the regimes of coherent absorption and coherent transparency.Focusing of light resulting from illumination of the plasmonic metasurface with a Fresnel zone pattern is controlled by another patterned beam projected on the same metasurface.By altering the control pattern,its phase,or its intensity,we switch the lens function on and off,and alter the focal spot’s depth,diameter and intensity.Switching occurs as fast as the control beam is modulated and therefore tens of gigahertz modulation bandwidth is possible with electro-optical modulators,which is orders of magnitude faster than conventional dynamic focusing technologies.

Principles of electromagnetic waves in metasurfaces

Metasurfaces are artificially structured thin films with unusual properties on demand. Different from metamaterials, the metasurfaces change the electromagnetic waves mainly by exploiting the boundary conditions, rather than the constitutive parameters in three dimensional(3D) spaces. Despite the intrinsic similarities in the operational principles, there is not a universal theory available for the understanding and design of metasurface-based devices. In this article, we propose the concept of metasurface waves(M-waves) and provide a general theory to describe the principles of them. Most importantly, it is shown that the M-waves share some fundamental properties such as extremely short wavelength, abrupt phase change and strong chromatic dispersion, which make them different from traditional bulk waves. It is shown that these properties can enable many important applications such as subwavelength imaging and lithography, planar optical devices, broadband anti-reflection, absorption and polarization conversion. Our results demonstrated unambiguously that traditional laws of diffraction, refraction, reflection and absorption should be revised by using the novel properties of M-waves. The theory provided here may pave the way for the design of new electromagnetic devices and further improvement of metasurfaces. The exotic properties of metasurfaces may also form the foundations for two new sub-disciplines called "subwavelength surface electromagnetics" and "subwavelength electromagnetics".