Coherent Perfect Absorption Research Articles

Tunable and broadband coherent perfect absorption by ultrathin black phosphorus metasurfaces

Black phosphorus (BP), a relative new plasmonic two-dimensional (2D) material, offers unique photonic and electronic properties. In this work, we propose a new tunable and broadband ultrathin coherent perfect absorber (CPA) device operating in the terahertz (THz) frequency range. It is based on a bifacial metasurface made of BP patch periodic arrays separated by a thin dielectric layer. Broadband CPA bandwidth is realized due to the ultrathin thickness of the proposed device and the extraordinary properties of BP. In addition, a substantial modulation between CPA and complete transparency is achieved by adjusting the phase difference between the two counter-propagating incident waves. The CPA performance can be tuned by dynamically changing the electron doping level of BP. The CPA response under normal and oblique transverse magnetic (TM) and electric (TE) polarized incident waves is investigated. It is derived that CPA can be achieved under both incident polarizations and across a broad range of incident angles. The presented CPA device can be used in the design of tunable planar THz modulators, all-optical switches, detectors, and signal processors.

Controllable coherent perfect absorber made of liquid metal-based metasurface.

Coherent perfect absorber (CPA) is a novel strategy proposed and demonstrated for solving the challenge to attain efficient control of absorption by exploiting the inverse process of lasing. The operation condition of CPA results in narrow-band, which is the main limitation obstruct it from practical applications. Here, we demonstrate a CPA with tunable operation frequency employing the liquid metal made reconfigurable metasurface. The flow of liquid metal is restricted with a plastic pipe for realizing a controllable liquid metal cut-wire. The adjustable electric dipolar mode of the reconfigurable cur-wire metasurface ensures that the quasi-CPA point can be dynamically controlled; the measured CPA under proper phase modulation is in good agreement with the simulation results. The proposed CPA system involving liquid metal for dynamic control of operation frequency will have potential applications and may stimulate the exploitation of liquid based smart absorption control of optical waves.

Acoustic conjugate metamaterials

Compared to metamaterials based on regulation of real material parameters, artificial materials designed by controlling complex parameters exhibit richer physics and different functions. In this work, we present acoustic conjugate metamaterials, which possess complex mass densities and bulk compressibilities with opposite phases, to manipulate acoustic waves. By adjusting the phases of the complex material parameters, we can change the handedness and loss/gain of the material, and then, produce unique phenomena in acoustic fields. Theoretical analysis and numerical simulations demonstrate that placing a slab of an acoustic conjugate metamaterial in a traditional medium can create acoustic laser modes, perfect absorption or coherent perfect absorption. Moreover, even if total reflection arises at the interface, extraordinary transmission is observed, which differs from the phenomenon of traditional total reflection. Therefore, acoustic conjugate metamaterials exhibit unprecedented functions that have not been observed in the metamaterials on account of real parameter manipulation.

Experimental demonstration of an electrically tunable broadband coherent perfect absorber based on a graphene–electrolyte–graphene sandwich structure

We propose and experimentally demonstrate the operation of an electrically tunable, broadband coherent perfect absorption (CPA) at microwave frequencies by harnessing the CPA features of a graphene–electrolyte–graphene sandwich structure (GSS). Using both a simplified lumped circuit model and full-wave numerical simulation, it is found that the microwave coherent absorptivity of the GSS can be tuned dynamically from nearly 50% to 100% by changing the Fermi level of the graphene. Strikingly, our simplified lumped circuit model agrees very well with the full-wave numerical model, offering valuable insight into the CPA operation of the device. The angle dependency of coherent absorption in the GSS is further investigated, making suggestions for achieving CPA at wide angles up to 80°. To show the validity and accuracy of our theory and numerical simulations, a GSS prototype is fabricated and measured in a C-band waveguide system. The reasonably good agreement between the experimental and the simulated results confirms that the tunable coherent absorption in GSS can be electrically controlled by changing the Fermi level of the graphene.

Coherent Perfect Absorption Laser Points in One-Dimensional Anti-Parity–Time-Symmetric Photonic Crystals

We investigate the coherent perfect absorption laser points (CPA-LPs) in anti-parity–time-symmetric photonic crystals. CPA-LPs, which correspond to the poles of reflection and transmission, can be found in the parameter space composed of gain–loss factor and angular frequency. Discrete exceptional points (EPs) split as the gain–loss factor increases. The CPA-LPs sandwiched between the EPs are proved to be defective modes. The localization of light field and the bulk effect of gain/loss in materials induce a sharp change in phase of the reflection coefficient near the CPA-LPs. Consequently, a large spatial Goos–Hänchen shift, which is proportional to the slope of phase, can be achieved around the CPA-LPs. The study may find great applications in highly sensitive sensors.

CPA-laser effect and exceptional points in -symmetric multilayer structures

The simultaneous existence of coherent perfect absorption (CPA) and lasing is one of the most intriguing features of non-Hermitian photonics. However, the link between CPA lasing and symmetry breaking at the exceptional point (EP) needs clarification. In this paper, we study the manifestations of the CPA-laser effect in a -symmetric multilayer loss-gain structure using both the transfer-matrix method and numerical simulations of the Maxwell-Bloch equations. We show that the maximal contrast between absorption and amplification at different phase relations between the input waves is reached well below the EP and therefore is not connected to true lasing. In this regime, there is a good qualitative agreement between both computational approaches. Above the EP, the system demonstrates lasing regardless of the parameters of the input waves. Thus, the maximal contrast between the absorption and amplification rather corresponds to the CPA amplifier than to the CPA laser.

The Scattering Problem in PT‐Symmetric Periodic Structures of 1D Two‐Material Waveguide Networks

AbstractA PT‐symmetric periodic structure with two‐material waveguide networks is constructed. In this study, how changing the number of cells affects the transmission properties is investigated. The results show that the PT‐unbroken (broken) region of the system is only determined by the cell structure, regardless of the number of unit cells. This means that any system has the same exceptional points (EPs), regardless of the number of cells and as long as the cell structure is consistent. In addition, it is confirmed that the coherent perfect absorbers and lasers (CPA lasers) occur in our model. The transfer matrix method is used to derive a sufficient condition for achieving the CPA laser point. A simple, effective formula for predicting the CPA laser state in an N unit cell system is derived.

High-Sensitivity Refractive Index Sensors Using Coherent Perfect Absorption on Graphene in the Vis-NIR Region

Plasmonic structures with sophisticated nanofabrication have revolutionized the ability to trap light on the nanoscale and enable high-sensitivity refractive index sensing. Previous theoretical res...

Coherent perfect absorber with independently tunable frequency based on multilayer graphene

We propose two terahertz coherent perfect graphene-based absorbers that can simultaneously realize the tunable resonant frequency and absorption. The Fermi energy of graphene, which can be changed by applied electric fields affect the frequency of absorption peaks. By adjusting the phase difference between two interfering beams, the absorption intensity can be controlled. The simulated results indicate that the absorptivity can achieve nearly 100% change and the adjusting range of absorption frequency is up to 16 THz. With the flexible movement of four absorption peaks, the absorption bandwidth over 90% can be extended to 10 THz. What is more, this paper displays the application to photoswitch by phase modulation. In particular, the proposed absorbers are polarization-insensitive and almost omnidirectional. We hope that our results can provide substantial help for the potential applications in the future metamaterial devices.

Spectral singularities of a potential created by two coupled microring resonators.

Two microring resonators, one with gain and one with loss, coupled to each other and to a bus waveguide, create an effective non-Hermitian potential for light propagating in the waveguide. Due to geometry, the coupling for each microring resonator yields two counter-propagating modes with equal frequencies. We show that such a system enables implementation of many types of scattering peculiarities. The spectral singularities, which are either the second or fourth order, separate parameter regions where the spectrum is either purely real or composed of complex eigenvalues; hence, they represent the points of the phase transition. By modifying the gain-loss relation for the resonators, such an optical scatterer can act as a laser, as a coherent perfect absorber, be unidirectionally reflectionless or transparent, and support bound states either growing or decaying in time. These characteristics are observed for a discrete series of the incident-radiation wavelengths.

Bandwidth bounds of coherent perfect absorber in resonant metasurfaces.

Coherent perfect absorber (CPA), a resonator with critical losses that can perfectly absorb all incident light, has been observed at various frequency regimes (from microwave to visible light). Besides the functional frequency, the bandwidth is also an important parameter in characterizing the performance of CPA. Here, we explore the bandwidth of CPA in a kind of weakly-coupled-resonance metasurfaces with 4κ2-γs2<0, where κ is the near-field coupling between the radiative and non-radiative resonant modes, and γsis the scattering loss rate of the radiative resonant mode. Based on the coupled mode theory, we analytically derive the upper and lower bounds of the bandwidth, and show that they are determined by the dissipation loss rates of the composed modes. To narrow the bandwidth, it is better to increase the radiative loss rate when designing a weakly coupled resonator. We also show that CPA is associated with a robust phase singularity with a winding number of ± 1. The conclusions are numerically verified in a designed resonant metasurface and could perform as a guideline for designing CPA in various resonant systems.

Perfectly Absorbing Exceptional Points and Chiral Absorbers.

We identify a new kind of physically realizable exceptional point (EP) corresponding to degenerate coherent perfect absorption, in which two purely incoming solutions of the wave operator for electromagnetic or acoustic waves coalesce to a single state. Such non-Hermitian degeneracies can occur at a real-valued frequency without any associated noise or nonlinearity, in contrast to EPs in lasers. The absorption line shape for the eigenchannel near the EP is quartic in frequency around its maximum in any dimension. In general, for the parameters at which an operator EP occurs, the associated scattering matrix does not have an EP. However, in one dimension, when the S matrix does have a perfectly absorbing EP, it takes on a universal one-parameter form with degenerate values for all scattering coefficients. For absorbing disk resonators, these EPs give rise to chiral absorption: perfect absorption for only one sense of rotation of the input wave.

Random anti-lasing through coherent perfect absorption in a disordered medium.

Non-Hermitian wave engineering is a recent and fast-moving field that examines both fundamental and application-oriented phenomena1-7. One such phenomenon is coherent perfect absorption8-11-an effect commonly referred to as 'anti-lasing' because it corresponds to the time-reversed process of coherent emission of radiation at the lasing threshold (where all radiation losses are exactly balanced by the optical gain). Coherent perfect absorbers (CPAs) have been experimentally realized in several setups10-18, with the notable exception of a CPA in a disordered medium (a medium without engineered structure). Such a 'random CPA' would be the time-reverse of a 'random laser'19,20, in which light is resonantly enhanced by multiple scattering inside a disorder. Because of the complexity of this scattering process, the light field emitted by a random laser is also spatially complex and not focused like a regular laser beam. Realizing a random CPA (or 'random anti-laser') is therefore challenging because it requiresthe equivalent of time-reversing such a light field in all its degrees of freedom to create coherent radiation that is perfectly absorbed when impinging on a disordered medium. Here we use microwave technology to build a random anti-laser and demonstrate its ability to absorb suitably engineered incoming radiation fields with near-perfect efficiency. Because our approach to determining these field patterns is based solely on far-field measurements of the scattering properties of a disordered medium, it could be suitable for other applications in which waves need to be perfectly focused, routed or absorbed.

Extraordinary acoustic scattering in a periodic PT-symmetric zero-index metamaterials waveguide

We propose an acoustic waveguide consisting of periodic parity-time-symmetric zero-index metamaterials separated by layers of passive medium. Consistent analytical derivations and numerical simulations show that a rich variety of extraordinary scattering effects can be achieved. For example, unidirectional transparency is found to occur at an exceptional point. At singular points, the system behaves as an acoustic coherent perfect absorber (CPA) laser. Bidirectional transparency, induced by the Fabry-Pérot resonance or balanced dissipation and amplification, is also observed. In contrast to the previous scheme without structural periodicity, the proposed periodic structure allows a greater number of conditions under which the CPA laser behavior and bidirectional transparency can be achieved, which may facilitate the experimental realization of such extraordinary effects.

Quantum coherent absorption of squeezed light

We investigate coherent perfect absorption (CPA) in quantum optics, in particular when pairs of squeezed coherent states of light are superposed on an absorbing beam splitter. First, by employing quantum optical input-output relations, we derive the absorption coefficients for quantum coherence and for intensity, and reveal how these will differ for squeezed states. Secondly, we present the remarkable properties of a CPA-gate: two identical but otherwise arbitrary incoming squeezed coherent states can be completely stripped off their coherence, producing a pure entangled squeezed vacuum state that with its finite intensity escapes from an otherwise perfect absorber. Importantly, this output state of light is not entangled with the absorbing beam splitter by which it was produced. Its loss-enabled functionality makes the CPA gate an interesting new tool for continuous-variable quantum state preparation.

Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides

We present a nanoscale active plasmonic waveguide system consisting of an array of periodic slits that can exhibit exceptional points and spectral singularities leading to several novel functionalities. The proposed symmetric active system operates near its cut-off wavelength and behaves as an effective epsilon-near-zero (ENZ) medium. We demonstrate the formation of an exceptional point (EP) that is accessed with very low gain coefficient values, a unique feature of the proposed nanoscale symmetric plasmonic configuration. Reflectionless ENZ transmission and perfect loss-compensation are realized at the EP which coincides with the effective ENZ resonance wavelength of the proposed array of active plasmonic waveguides. When we further increase the gain coefficient of the dielectric material loaded in the slits, a spectral singularity occurs at the ENZ resonance leading to super scattering (lasing) response at both forward and backward directions. These interesting effects are achieved by materials characterized by very small gain coefficients with practical values and at subwavelength scales due to the strong and homogeneous field enhancement inside the active slits at the ENZ resonance leading to enhanced light-matter interaction. We theoretically analyze the obtained EP, as well as the divergent spectral singularity, using a transmission-line model and investigate the addition of a second incident wave and nonlinearities in the response of the proposed active ENZ plasmonic system. Our findings provide a novel route towards interesting nanophotonic applications, such as reflectionless active ENZ media, unidirectional coherent perfect absorbers, nanolasers, and strong optical bistability and all-optical switching nanodevices.

Higher-order exceptional point in a cavity magnonics system

We propose to realize the pseudo-Hermiticity in a cavity magnonics system consisting of the Kittel modes in two small yttrium-iron-garnet spheres coupled to a microwave cavity mode. The effective gain of the cavity can be achieved using the coherent perfect absorption of the two input fields fed into the cavity. With certain constraints of the parameters, the Hamiltonian of the system has the pseudo-Hermiticity and its eigenvalues can be either all real or one real and other two constituting a complex-conjugate pair. By varying the coupling strengths between the two Kittel modes and the cavity mode, we find the existence of the third-order exceptional point in the parameter space, in addition to the usual second-order exceptional point existing in the system with parity-time symmetry. Also, we show that these exceptional points can be demonstrated by measuring the output spectrum of the cavity.

A Zero‐Rank, Maximum Nullity Perfect Electromagnetic Wave Absorber

AbstractElectromagnetic wave absorbers formed from a metamaterial layer are demonstrated and near‐perfect absorption is realized across much of the spectrum. Alternatively, an unpatterned low‐loss dielectric layer forms an absorber of coherent light and shows near‐zero reflectance and high absorption. The latter is termed a coherent perfect absorber (CPA) and may be described as a time‐reversed laser. Here, a more fundamental system of dielectric metasurfaces is experimentally demonstrated, which may achieve either coherent or incoherent absorption. The absorber is viewed as two CPAs with eigenmodes of opposite symmetry, and may achieve incoherent absorption with a null scattering matrix, or coherent absorption with a rank‐1 scattering matrix. Metasurface absorbers are classified based on their scattering matrix rank and nullity. The demonstrated system and theory establish a path to realize a new class of absorbers useful for future novel applications, including hyperspectral imaging and energy harvesting.

Photonic-doped epsilon-near-zero media for coherent perfect absorption

Coherent perfect absorption increases light absorption to 100% via appropriate interference of waves and provides attractive opportunities for flexible control of light absorption. In this work, we demonstrate that the coherent perfect absorption can be realized using photonic doping of epsilon-near-zero media with both lossless and lossy defects. Based on photonic doping theory, the epsilon-near-zero medium with defects can be homogenized as an effective medium. We find that the lossless defects can tune the effective permeability to be zero, and the lossy defects can further tune the effective permeability to be a pure imaginary value to realize the coherent perfect absorption. Moreover, we show that the doping scheme enables many ways to control absorption, such as multiple channels and multiple doping defects, etc. Our work reveals a unique approach for advanced coherent perfect absorption with flexible control functionalities.

Spectral singularities of odd- PT -symmetric potentials

We describe one-dimensional stationary scattering of a two-component wave field by a non-Hermitian matrix potential which features odd-$PT$ symmetry, i.e., symmetry with $(PT)^2=-1$. The scattering is characterized by a $4\times 4$ transfer matrix. The main attention is focused on spectral singularities which are classified into two types. Weak spectral singularities are characterized by zero determinant of a diagonal $2\times 2$ block of the transfer matrix. This situation corresponds to the lasing or coherent perfect absorption of pairs of oppositely polarized modes. Strong spectral singularities are characterized by zero diagonal block of the transfer matrix. We show that in odd-$PT$-symmetric systems any spectral singularity is self-dual, i.e., lasing and coherent perfect absorption occur simultaneously. Detailed analysis is performed for a case example of a $PT$-symmetric coupler consisting of two waveguides, one with localized gain and another with localized absorption, which are coupled by a localized antisymmetric medium. For this coupler, we discuss weak self-dual spectral singularities and their splitting into complex-conjugate eigenvalues which represent bound states characterized by propagation constants with real parts lying in the continuum. A rather counterintuitive restoration of the unbroken odd-$PT$-symmetric phase subject to the increase of the gain-and-loss strength is also revealed. The comparison between odd- and even-$PT-$symmetric couplers, the latter characterized by $(PT)^2=1$, is also presented.