The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

All-dielectric metamaterials

Within a time span of 15 years, acoustic metamaterials have emerged from academic curiosity to become an active field driven by scientific discoveries and diverse application potentials. This review traces the development of acoustic metamaterials from the initial findings of mass density and bulk modulus frequency dispersions in locally resonant structures to the diverse functionalities afforded by the perspective of negative constitutive parameter values, and their implications for acoustic wave behaviors. We survey the more recent developments, which include compact phase manipulation structures, superabsorption, and actively controllable metamaterials as well as the new directions on acoustic wave transport in moving fluid, elastic, and mechanical metamaterials, graphene-inspired metamaterials, and structures whose characteristics are best delineated by non-Hermitian Hamiltonians. Many of the novel acoustic metamaterial structures have transcended the original definition of metamaterials as arising from the collective manifestations of constituent resonating units, but they continue to extend wave manipulation functionalities beyond those found in nature.

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Acoustic metamaterials: From local resonances to broad horizons.

The recent advent of acoustic metamaterials has initiated a strong revival of interest on the subject of sound absorption. The present review is based on the physics perspective as the coherent basis of this diverse field. For conventional absorbers, viscous dissipation and heat conduction at the fluid-solid interface, when expressed through micro-geometric parameters, yield an effective medium description of porous media and micro-perforated panels as effectual sound absorbers. Local resonances and their geometric and symmetry constraints serve as the framework for surveying a variety of acoustic metamaterial absorbers that can realize previously unattainable absorption spectra with subwavelength-scale structures. These structures include decorated membrane resonators, degenerate resonators, hybrid resonators, and coiled Fabry-Perot and Helmholtz resonators. As the acoustic response of any structure or material must obey the causality principle, the implied constraint—which relates the absorption spectru...

Sound Absorption Structures: From Porous Media to Acoustic Metamaterials

Using the concepts of slow sound and critical coupling, an ultra-thin acoustic metamaterial panel for perfect and quasi-omnidirectional absorption is theoretically and experimentally conceived in this work. The system is made of a rigid panel with a periodic distribution of thin closed slits, the upper wall of which is loaded by Helmholtz Resonators (HRs). The presence of resonators produces a slow sound propagation shifting the resonance frequency of the slit to the deep sub-wavelength regime ( λ/88). By controlling the geometry of the slit and the HRs, the intrinsic visco-thermal losses can be tuned in order to exactly compensate the energy leakage of the system and fulfill the critical coupling condition to create the perfect absorption of sound in a large range of incidence angles due to the deep subwavelength behavior.

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Ultra-thin metamaterial for perfect and quasi-omnidirectional sound absorption

Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. The panels are composed of a periodic array of varying crosssection waveguides, each of them being loaded by Helmholtz resonators (HRs) with graded dimensions. The low cut-off frequency of the absorption band is fixed by the resonance frequency of the deepest HR, that reduces drastically the transmission. The preceding HR is designed with a slightly higher resonance frequency with a geometry that allows the impedance matching to the surrounding medium. Therefore, reflection vanishes and the structure is critically coupled. This results in perfect sound absorption at a single frequency. We report perfect absorption at 300 Hz for a structure whose thickness is 40 times smaller than the wavelength. Moreover, this process is repeated by adding HRs to the waveguide, each of them with a higher resonance frequency than the preceding one. Using this frequency cascade effect, we report quasi-perfect sound absorption over almost two frequency octaves ranging from 300 to 1000 Hz for a panel composed of 9 resonators with a total thickness of 11 cm, i.e., 10 times smaller than the wavelength at 300 Hz.

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Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels for transmission problems

We numerically demonstrate total absorption in graphene in the near-infrared and visible wavelength ranges by means of critical coupling with guided resonances of a photonic crystal slab. In this wavelength range, there is no plasmonic response in undoped graphene, so the critical coupling is entirely controlled by the properties of the photonic crystal resonance. We discuss the general theory and conditions for absorption enhancement and critical coupling in a thin film and give design rules for a totally absorbing system. We present examples in the near-infrared and visible, using both a lossless metallic mirror and a realistic multilayer dielectric mirror.

https://web.stanford.edu/group/fan/publication/Piper_ACS_2014.pdf

Total Absorption in a Graphene Monolayer in the Optical Regime by Critical Coupling with a Photonic Crystal Guided Resonance

We consider a mirror-symmetric resonator with two ports. We show that, when excited from a single port, complete absorption can be achieved through critical coupling to degenerate resonances with opposite symmetry. Moreover, any time two resonances with opposite symmetry are degenerate in frequency and absorption is always significantly enhanced. In contrast, when two resonances with the same symmetry are nearly degenerate, there is no absorption enhancement. We numerically demonstrate these effects using a graphene monolayer on top of a photonic crystal slab, illuminated from a single side in the near-infrared.

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Total absorption by degenerate critical coupling

We demonstrate experimentally close to total absorption in monolayer graphene based on critical coupling with guided resonances in transfer printed photonic crystal Fano resonance filters at near infrared. Measured peak absorptions of 35% and 85% were obtained from cavity coupled monolayer graphene for the structures without and with back reflectors, respectively. These measured values agree very well with the theoretical values predicted with the coupled mode theory based critical coupling design. Such strong light-matter interactions can lead to extremely compact and high performance photonic devices based on large area monolayer graphene and other two–dimensional materials.

/pdf/approaching-total-absorption-at-near-infrared-in-a-large-59v15oz8h8.pdf

Approaching total absorption at near infrared in a large area monolayer graphene by critical coupling

We numerically demonstrate broadband absorption enhancement in the visible in monolayer MoS2 placed on a photonic crystal slab, backed by a perfect electric conductor mirror. The absorption enhancement arises from interaction between the MoS2 and the guided resonances of the photonic crystal slab. Through consideration of light trapping theory and parametric studies of the design parameters, it is shown that the optimal photonic crystal slab has a grating period that is subwavelength to most of the frequency region of interest, has large air holes, and is thin. The optimal design features an average absorption of 51% at normal incidence in the visible above the band gap of MoS2 (400–690 nm) and is less than 100 nm thick. We also comment on the angular response of the optimal device.

https://pubs.acs.org/doi/pdf/10.1021/acsphotonics.5b00510

Broadband Absorption Enhancement in Solar Cells with an Atomically Thin Active Layer

A fundamental limitation of transparent conducting electrode design is thought to be the trade-off between photonic and electronic performances. The photonic transmission property of a transparent conducting electrode, however, is not intrinsic but depends critically on the electromagnetic environment where the electrode is located. We develop the concept of optical impedance transformation, and use this concept to design nanophotonic structures that provide broadband and omnidirectional reduction of optical loss in an ultrathin transparent conducting electrode, without compromising its electrical performance.

Jessica R. Piper

Papers

Total Absorption in a Graphene Monolayer in the Optical Regime by Critical Coupling with a Photonic Crystal Guided Resonance

Total absorption by degenerate critical coupling

Approaching total absorption at near infrared in a large area monolayer graphene by critical coupling

Broadband Absorption Enhancement in Solar Cells with an Atomically Thin Active Layer

Optical impedance transformer for transparent conducting electrodes.