Perfect Absorption Condition Research Articles

Revision of Sabine's and Eyring's reverberation theories

Sabine's and Eyring's reverberation theories are still the most important theoretical frameworks for room acoustics. The important physical quantities derived from these theories are the average sound pressure level (sound energy density) and the reverberation time (sound energy decay). Eyring revised Sabine's theory to become the reverberation time zero in the perfect absorption condition. However, Eyring's theory has contradictions regarding the sound energy density in a steady state and the energy decay from the steady state. In this study, in order to resolve these contradictions, the theoretical framework for the consistent system is presented introducing the new concept of the average impulse response in a room, and the contradictions in the current room acoustic theory are clarified. Based on the theoretical framework, Sabine's reverberation theory is revised by a different way of Eyring's theory. The reverberation time of the revised theory is shorter than that of Sabine's theory and longer than that of Eyring's theory. Finally, ray-tracing computer simulations were performed to verify the revised theory. Results of sound ray tracing simulations were in better agreement with the values calculated using the revised theory rather than those calculated using Sabine's and Eyring's theories.

Dual-Band All-Optical Logic Gates by Coherent Absorption in an Amorphous Silicon Graphene Metasurface.

The dual-band polarization-independent all-optical logic gate by coherent absorption effect in an amorphous silicon (a-Si) graphene metasurface is investigated theoretically and numerically. Taking the substrate effect into consideration, the coherent perfect absorption condition of the a-Si graphene metasurface is derived on the basis of the Cartesian multipole method. The coherent nearly perfect absorption of the a-Si graphene metasurface is realized by the interference of multipole moments and the interband transition of monolayer graphene, achieving peak values of 91% and 92% at 894.5 nm and 991.5 nm, respectively. The polarization independence of the coherent absorption is revealed due to the center symmetry of the structure of the a-Si graphene metasurface. The dual-band polarization-independent all-optical XOR and OR logic gates are implemented at 894.5 nm and 991.5 nm by the a-Si graphene metasurface based on the coherent nearly perfect absorption, which has the opportunity to be utilized in all-optical computing, all-optical data processing, and future all-optical networks.

Large-area polycrystalline α-MoO3 thin films for IR photonics

In recent years, the excitation of surface phonon polaritons (SPhPs) in van der Waals materials received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide (α-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic material due to its ability to support SPhPs for three orthogonal directions at different wavelength bands (range 10–20 μm). Here, we report on the fabrication, structural, morphological, and optical IR characterization of large-area (over 1 cm2 size) α-MoO3 polycrystalline film deposited on fused silica substrates by pulsed laser deposition. Due to the random grain distribution, the thin film does not display any optical anisotropy at normal incidence. However, the proposed fabrication method allows us to achieve a single α-phase, preserving the typical strong dispersion related to the phononic response of α-MoO3 flakes. Remarkable spectral properties of interest for IR photonics applications are reported. For instance, a polarization-tunable reflection peak at 1006 cm−1 with a dynamic range of ΔR = 0.3 and a resonance Q-factor as high as 53 is observed at 45° angle of incidence. Additionally, we report the fulfillment of an impedance matching condition with the SiO2 substrate leading to a polarization-independent almost perfect absorption condition (R < 0.01) at 972 cm−1 which is maintained for a broad angle of incidence. In this framework our findings appear extremely promising for the further development of mid-IR lithography-free, scalable films, for efficient and large-scale sensors, filters, thermal emitters, and label-free biochemical sensing devices operating in the free space, using far-field detection setups.

Ultra-sparse near-perfect sound absorbers.

There is a trade-off between the sparseness of an absorber array and its sound absorption imposed by wave physics. Here, near-perfect absorption (99% absorption) is demonstrated when the spatial period of monopole-dipole resonators is close to one working wavelength (95% of the wavelength). The condition for perfect absorption is to render degenerate monopole-dipole resonators critically coupled. Frequency domain simulations, eigenfrequency simulations, and the coupled mode theory are utilized to demonstrate the acoustic performances and the underlying physics. The sparse-resonator-based sound absorber could greatly benefit noise control with air flow and this study could also have implications for electromagnetic wave absorbers.

Non-linear coherent perfect absorption in the proximity of exceptional points

Coherent perfect absorption is one of the possibilities to get high absorption but typically suffers from being a resonant phenomena, i.e., efficient absorption only in a local frequency range. Additionally, if applied in high power applications, the understanding of the interplay of non-linearities and coherent perfect absorption is crucial. Here we show experimentally and theoretically the formation of non-linear coherent perfect absorption in the proximity of exceptional point degeneracies of the zeros of the scattering function. Using a microwave platform, consisting of a lossy nonlinear resonator coupled to two interrogating antennas, we show that a coherent incident excitation can trigger a self-induced perfect absorption once its intensity exceeds a critical value. Note, that a (near) perfect absorption persists for a broad-band frequency range around the nonlinear coherent perfect absorption condition. Its origin is traced to a quartic behavior that the absorbance spectrum acquires in the proximity of the exceptional points of the nonlinear scattering operator.

Mechanically Tunable Terahertz Metamaterial Perfect Absorber

The development of a wide range of technologies based on terahertz (THz) electromagnetic radiation drives a strong demand for flexible optical elements. Metasurfaces based on metallic resonators offer a versatile toolkit that permits easy tuning over a wide spectral range by the geometric design. Herein, a mechanically tuned metasurface perfect absorber comprised of split‐ring resonators in combination with a metallic mirror in a microcavity arrangement, is demonstrated. By mechanically tuning the length of the microcavity in the range of 10 μm and above, precise control over the perfect absorption condition is exhibited. A maximum recorded extinction of 45.8 dB is obtained at the perfect absorption condition, corresponding to a suppression of the reflected radiation by almost five orders of magnitude. Experiments are performed in a reflection arrangement using a terahertz time‐domain spectrometer. Simulations of the experimental arrangement show that near‐field effects are weak and the enhancement of metamaterial perfect absorption is in agreement with purely interferometric effects.

Ultra-sparse near-perfect sound absorbers

Sparseness of an absorber array and its sound absorption performance is a trade-off, which has attracted much research interest. Here, we demonstrate near-perfect absorption (99% absorption) when the spatial period of monopole-dipole resonators is close to one working wavelength (95% of the wavelength). The condition for perfect absorption is to render degenerate monopole-dipole resonators critically coupled. Frequency domain simulations, Eigenfrequency simulations, and coupled mode theory are utilized to reveal the underlying physics and the acoustic performances, showing a good agreement with measurements. The sparse-resonator-based sound absorber could greatly benefit noise control while maintaining air flow and the results of this study could have implications for electromagnetic wave absorbers.

Reconfigurable spiral underwater sound-absorbing metasurfaces

A well-designed coiled metasurface can be applied to obtain extraordinary underwater sound isolation. In this letter, we propose a quasi-perfect sound-absorbing metasurface in spiral shape form. This spiral metasurface(SM) meets the two conditions for perfect absorption, which are no transmission at the outlet and no reflection at the inlet. The sound-absorbing characteristics of the SM are investigated through numerical simulation. The simulation results show that the peak frequency of prefect sound absorption can be tuned flexibly over the spectrum, which depends on the design of the parameters. The frequency-controllable SM has quasi-prefect sound absorption coefficient(up to almost 0.998).

Hybrid perfect metamaterial absorber for microwave spin rectification applications

Metamaterials provide compelling capabilities to manipulate electromagnetic waves beyond the natural materials and can dramatically enhance both their electric and magnetic fields. The enhanced magnetic fields, however, are far less utilized than the electric counterparts, despite their great potential in spintronics. In this work, we propose and experimentally demonstrate a hybrid perfect metamaterial absorbers which combine the artificial metal/insulator/metal (MIM) metamaterial with the natural ferromagnetic material permalloy (Py) and realize remarkably larger spin rectification effect. Magnetic hot spot of the MIM metamaterial improves considerably electromagnetic coupling with spins in the embedded Py stripes. With the whole hybridized structure being optimized based on coupled-mode theory, perfect absorption condition is approached and an approximately 190-fold enhancement of spin-rectifying photovoltage is experimentally demonstrated at the ferromagnetic resonance at 7.1 GHz. Our work provides an innovative solution to harvest microwave energy for spintronic applications, and opens the door to hybridized magnetism from artificial and natural magnetic materials for emergent applications such as efficient optospintronics, magnonic metamaterials and wireless energy transfer.

Complete multipolar description of reflection and transmission across a metasurface for perfect absorption of light.

Relating the electromagnetic scattering and absorption properties of an individual particle to the reflection and transmission coefficients of a two-dimensional material composed of these particles is a crucial concept that has driven both fundamental and applied physics. It is at the heart of both the characterization of material properties as well as the phase and amplitude engineering of a wave. Here we propose a multipolar description of the reflection and transmission coefficients across a monolayer of particles using a vector spherical harmonic decomposition. This enables us to provide a generalized condition for perfect absorption which occurs when both the so-called generalized Kerker condition is reached and when the superposition of odd and even multipoles reaches a critical value. Using these conditions, we are able to propose two very different designs of two-dimensional materials that perfectly absorb a plane electromagnetic wave under normal incidence. One is an infinite array of silica microspheres that operates at mid-infrared frequencies, while the other is an infinite array of germanium nano-clusters that operates at visible frequencies. Both systems operate in a deeply multipolar regime. Our findings are important to the metamaterials and metasurfaces communities who design materials mainly restricted to the dipolar behavior of individual resonators, as well as the self-assembly and nanochemistry communities who separate the individual particle synthesis from the materials assembly.

Numerical Study on the Absorption Characteristics of Subwavelength Metallic Gratings Covered with a Lossy Dielectric Layer

Optical absorbers have been a topic of intense research due to their importance in many applications. In particular, multi-band and perfect absorption features in a desired frequency range are essential in broadband applications. In this work, we numerically studied the absorption properties of subwavelength metallic gratings coated with a dielectric layer. Here, the structure is considered to be an integration between a resonant cavity and a subwavelength metallic grating. Two appropriately designed structures can exhibit multi-band absorption properties. In addition to the numerical simulation results, we elaborate on determining the appropriate structural parameters that yield the desired spectral absorption profile in the visible range. We also numerically identify critical coupling conditions for perfect absorption.

Subwavelength Interferometric Control of Absorption in Three-port Acoustic Network

Utilizing the effect of losses, we show that symmetric 3-port devices exhibit coherent perfect absorption of waves and we provide the corresponding conditions on the reflection and transmission coefficients. Infinite combinations of asymmetric inputs with different amplitudes and phase at each port as well as a completely symmetric input, are found to be perfectly absorbed. To illustrate the above we study an acoustic 3-port network operating in a subwavelength frequency both theoretically and experimentally. In addition we show how the output from a 3-port network is altered, when conditions of perfect absorption are met but the input waves phase and amplitude vary. In that regard, we propose optimized structures which feature both perfect absorption and perfect transmission at the same frequency by tuning the amplitudes and phases of the input waves.

Acoustic characterization of silica aerogel clamped plates for perfect absorption purpose

Silica aerogel has been widely studied as bulk material for its extremely low density and thermal conductivity. Plates or membranes made of this extremely soft materials exhibits interesting properties for sound absorption. A novel signal processing method for the characterization of an acoustic metamaterial made of silica aerogel clamped plates is presented. The acoustic impedance of a silica aerogel clamped plate is derived from the elastic theory for the flexural waves, while the transfer matrix method is used to model reflection and transmission coefficients of a single plate. Experimental results are obtained by using an acoustic impedance tube. The difference between the measured and modeled reflexion and transmission coefficients is minimized under constraints to recover the acoustic parameters of the silica aerogel plate. Once the properties of the silica aerogel plate are obtained, the perfect absorption condition is derived by studying the reflection coefficient of a aerogel plate rigidly backed...

Perfect and broadband acoustic absorption in deep sub-wavelength structures for the reflection and transmission problems

The mechanisms to achieve perfect acoustic absorption by sub-wavelength structures in both reflection and transmission problems are reported. While the mechanism consists in critically coupling a single resonance independently of its nature in the reflection problem, the mechanism becomes more complicated in the transmission problem. To tackle these issues, we use asymmetric interacting resonators, whose interaction leads to the perfect absorption condition. The analyzed system consists in a panel with a periodic distribution of thin slits, the upper wall of which being loaded by Helmholtz Resonators. The propagation in the slit is highly dispersive due to the presence of the resonators, producing slow sound conditions and down-shifting the slit resonance to low frequencies. By controlling the geometry of the resonators, the visco-thermal losses are tuned to compensate the leakage of the system and fulfill the perfect absorption condition. In the case of the reflection problem, a single resonator is enough to obtain the perfect absorption. However, in the case of transmission, using an array of identical Helmholtz resonators only quasi-perfect absorption can be obtained. A possible solution is the use of double interacting resonators, one acting as reflecting wall for the previous one. This procedure can be iteratively repeated and one can design perfect and broadband acoustic absorbers based on the rainbow trapping mechanism.

Theoretical T Circuit Modeling of Graphene-Based Metamaterial Broadband Absorber

We demonstrate a broadband light absorber with its absorption being able to reach as high as 90 % and above ranging from the ultraviolet to the visible regimes. A theoretical model is given for the purpose of analyzing the physical mechanism of the absorption. By applying the equivalent T circuit model of metamaterial layers to the analysis of our designed absorber, our calculated results are in good agreement to that of the theoretical model and satisfy the perfect-absorption condition very well.

Use of slow sound to design perfect and broadband passive sound absorbing materials.

Perfect (100%) absorption by thin structures consisting of a periodic arrangement of rectangular quarter-wavelength channels with side detuned quarter-wavelength resonators is demonstrated. The thickness of these structures is 13-17 times thinner than the acoustic wavelength. This low frequency absorption is due to a slow sound wave propagating in the main rectangular channel. A theoretical model is proposed to predict the complex wavenumber in this channel. It is shown that the speed of sound in the channel is much lower than in the air, almost independent of the frequency in the low frequency range, and it is dispersive inside the induced transparency band which is observed. The perfect absorption condition is found to be caused by a critical coupling between the rectangular channel (sub-wavelength resonators) and the incoming wave. It is shown that the width of a large absorption peak in the frequency spectrum can be broadened if several rectangular channels in the unit cell are detuned. The detuning is achieved by varying the length of the side resonators for each channel. The predicted absorption coefficients are validated experimentally. Two resonant cells were produced with stereolithography which enabled the authors to incorporate curved side resonators.

Tunable terahertz absorber based on complementary graphene meta-surface

Recently, metamaterials have attracted considerable attention because of their unique properties and potential applications in many areas, such as in bio-sensing, imaging, and communication. Among these researches, the metamaterial absorber has aroused much interest of researchers. The metamaterial absorber is important due to a broad range of potential application to solar energy, sensing, coatings for reducing the reflection, and selective thermal emitters. As a two-dimensional honeycomb structure composed of a single layer carbon atom, graphene is a promising candidate for tuning metamaterials and plasmonic structures due to its unique properties which differ substantially from those of metal and semiconductors. In this paper, we propose a tunable terahertz absorber based on graphene complementary metamaterial structure by removing periodic cut-wires on the graphene meta-surface. On the basis of the tunability of graphene conductivity, the absorber possesses a frequency tunable characteristic resulting from the change of graphene Femi level by altering the applied voltage. Here, we mainly study the influences of Fermi level of graphene and the size of the structure on the absorption characteristic of this metamaterial absorber. We finally obtain the corresponding Femi level and structural size under the perfect absorption condition. In addition, we utilize the multiple reflection theory to explore the physical mechanism, and verify the feasibility of the simulation method at the same time. The research indicates that the absorber can achieve 99.9% perfect absorption at 1.865 THz when the graphene Femi level is 0.6 eV, the thickness of substrate is 13 m, and the length and width of slit are 2.9 m and 0.1 m, respectively. When graphene Femi level increases from 0.4 eV to 0.9 eV, the resonance frequency of the absorber is blue-shifted from 1.596 THz to 2.168 THz. Meanwhile, the absorption rate increases from 84.68% at 0.4 eV to a maximum value of 99.9% at 0.6 eV, then gradually decreases to 86.63% at 0.9 eV. The maximum modulation of the absorption rate is 84.55% by varying the Femi level. When the thickness of substrate increases, the resonant frequency is red-shifted. The resonant frequency is blue-shifted when both the width and the length of the cut-wire on graphene increase. On the basis of the proposed graphene meta-surface absorber, one can gain different resonant frequencies by adjusting the structure geometric size and graphene Femi level. The graphene complementary structure can also be designed into different patterns to achieve the purpose of practical application.

Directional perfect absorption using deep subwavelength low-permittivity films

We experimentally demonstrate single beam directional perfect absorption (to within experimental accuracy) of p-polarized light in the near-infrared using unpatterned, deep subwavelength films of indium tin oxide (ITO) on Ag. The experimental perfect absorption occurs slightly above the epsilon-near-zero (ENZ) frequency of ITO where the permittivity is less than one. Remarkably, we obtain perfect absorption for films whose thickness is as low as ~1/50th of the operating free-space wavelength and whose single pass attenuation is only ~ 5%. We further derive simple analytical conditions for perfect absorption in the subwavelength-film regime that reveal the constraints that the ITO permittivity must satisfy if perfect absorption is to be achieved. Then, to get a physical insight on the perfect absorption properties, we analyze the eigenmodes of the layered structure by computing both the real-frequency/complex-wavenumber and the complex-frequency/real-wavenumber modal dispersion diagrams. These analyses allow us to attribute the experimental perfect absorption behavior to the crossover between bound and leaky behavior of one eigenmode of the layered structure. Both modal methods show that perfect absorption occurs at a frequency slightly larger than the ENZ frequency, in agreement with experimental results, and both methods predict a second perfect absorption condition at higher frequencies attributed to another crossover between bound and leaky behavior of the same eigenmode. Our results greatly expand the list of materials that can be considered for use as ultrathin perfect absorbers and also provide a methodology for the design of absorber systems at any desired frequency.

A parallel plate wet denuder for acidic gas measurement

AbstractA parallel plate wet denuder (PPWD) was designed and tested for acidic gas absorption in this study. The PPWD is composed of two frosted glass plates coated with nano‐TiO2 particles. The gap of the plates is 4 mm, the active surface is 7.5 cm in width and 15 cm in height. The DI water is flowing downward along the surfaces to form a uniform water film to absorb the gas flowing upward along the gap. The gas absorption efficiency and memory effect were tested using test gases of HF (3.57–3.80 ppb), HCl (0.48–15.63 ppb), and HNO3 (8.43–20.92 ppb) generated from permeation tubes at the flow rate of 5–10 L/min, scrubbing DI water flow rate of 1.0 mL/min/plate, and pH = 6.8 ± 0.3. The experiment was also conducted for HF and HCl with scrubbing water pH value of 3.08 and 4.10 to examine the effect of pH on the absorption efficiency. The experimental data at different flow rates match with the theoretical values very well assuming perfect absorption condition at the water film when the scrubbing water pH = 6.8 ± 0.3. The absorption efficiency of the three gases at the sampling flow rate of 5 L/min was nearly 100% when the scrubbing water pH value was 6.8 ± 0.3, but was found to decrease slightly when the pH value of scrubbing water was reduced from 6.8 to 3.08, which indicates slight modification of perfect absorption condition at the water film. The memory effect test showed that the rise time was 15 to 25 min and the fall time was 10 to 25 min for the three gases. Good uniformity of the water film was observed during the 9‐day ambient air sampling for HF, HCl, and HNO3, which indicates the applicability of the present PPWD for ambient air sampling. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Conditions of perfect absorption for a single‐layer electromagnetic‐wave absorber under oblique incidence

AbstractThe conditions for perfect absorption under oblique incidence in a single‐layer homogeneous absorber backed by a perfectly conducting plate are derived in this article. The cases corresponding to the two types of polarization of the incidence‐wave, perpendicular polarization (TE) and parallel polarization (TM), are discussed, respectively. Some conclusions and curves are given. © 1993 John Wiley &amp; sons, Inc.