Absorption of electromagnetic energy by a material is a phenomenon that underlies many applied problems, including molecular sensing, photocurrent generation and photodetection. Commonly, the incident energy is delivered to the system through a single channel, for example by a plane wave incident on one side of an absorber. However, absorption can be made much more efficient by exploiting wave interference. A coherent perfect absorber is a system in which complete absorption of electromagnetic radiation is achieved by controlling the interference of multiple incident waves. Here, we review recent advances in the design and applications of such devices. We present the theoretical principles underlying the phenomenon of coherent perfect absorption and give an overview of the photonic structures in which it can be realized, including planar and guided-mode structures, graphene-based systems, parity- and time-symmetric structures, 3D structures and quantum-mechanical systems. We then discuss possible applications of coherent perfect absorption in nanophotonics and, finally, we survey the perspectives for the future of this field.

Coherent perfect absorbers: linear control of light with light

Perfect absorption is an interdisciplinary topic with a large number of applications, the challenge of which consists of broadening its inherently narrow frequency-band performance. We experimentally and analytically report perfect and broadband absorption for audible sound, by the mechanism of critical coupling, with a sub-wavelength multi-resonant scatterer (SMRS) made of a plate-resonator/closed waveguide structure. In order to introduce the role of the key parameters, we first present the case of a single resonant scatterer (SRS) made of a Helmholtz resonator/closed waveguide structure. In both cases the controlled balance between the energy leakage of the several resonances and the inherent losses of the system leads to perfect absorption peaks. In the case of the SMRS we show that systems with large inherent losses can be critically coupled using resonances with large leakage. In particular, we show that in the SMRS system, with a thickness of λ/12 and diameter of λ/7, several perfect absorption peaks overlap to produce absorption bigger than 93% for frequencies that extend over a factor of 2 in audible frequencies. The reported concepts and methodology provide guidelines for the design of broadband perfect absorbers which could contribute to solve the major issue of noise reduction.

Perfect and broadband acoustic absorption by critically coupled sub-wavelength resonators

The absorption of electromagnetic energy by a material is a phenomenon that underlies many applications, including molecular sensing, photocurrent generation and photodetection. Typically, the incident energy is delivered to the system through a single channel, for example, by a plane wave incident on one side of an absorber. However, absorption can be made much more efficient by exploiting wave interference. A coherent perfect absorber is a system in which the complete absorption of electromagnetic radiation is achieved by controlling the interference of multiple incident waves. Here, we review recent advances in the design and applications of such devices. We present the theoretical principles underlying the phenomenon of coherent perfect absorption and give an overview of the photonic structures in which it can be realized, including planar and guided-mode structures, graphene-based systems, parity-symmetric and time-symmetric structures, 3D structures and quantum-mechanical systems. We then discuss possible applications of coherent perfect absorption in nanophotonics, and, finally, we survey the perspectives for the future of this field.

We investigate the effect of nonlinearity in novel parity-time (PT) symmetric potentials. We show that new types of nonlinear self-trapped modes can exist in optical PT synthetic lattices.

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Optical solitons in PT periodic potentials

In order to effectively realize and control the critical coupling, a graphene-based hyperbolic metamaterial has been proposed to replace the absorbing thin film in the critically coupled resonance structure. Our calculations demonstrate that the critical coupling effect (near-perfect light absorption) can be achieved at the near-infrared wavelength by using this layered structure, while the critical coupling frequency can be tuned by varying the Fermi energy level of graphene sheets via electrostatic biasing. Moreover, we show that the critical coupling frequency can be tuned by changing the thickness of the dielectric or layer number of the graphene sheets in the unit cell of the graphene-dielectric HMM. The optimization performance has also been indicated, which may offer an opportunity towards the experimental designs of high efficient graphene based critical coupling devices.

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Critical coupling with graphene-based hyperbolic metamaterials

We study coherent perfect absorption (CPA) of light in a Kerr nonlinear metal-dielectric composite medium, illuminated from the opposite ends. Elementary symmetry considerations reveal that equality of the incident light intensities is a prerequisite to ensure CPA in both linear and nonlinear systems for specific system parameters. We also derive the sufficient conditions for having CPA. We further show that, while CPA in a linear system is insensitive to the incident power level, in a nonlinear system CPA can be achieved only for discrete intensities with interesting hysteretic responses. Our unified formulation of CPA and waveguiding identifies them as opposite scattering phenomena. We further investigate light-induced CPA in on-resonant and off-resonant systems.

Light-controlled perfect absorption of light.

We study a critically coupled system [Opt. Lett. 32, 1483 (2007)] with a Kerr nonlinear spacer layer. Nonlinearity is shown to inhibit null scattering in a critically coupled system at low powers. However, a system detuned from critical coupling can exhibit near-complete suppression of scattering by means of nonlinearity-induced changes in refractive index. Our studies reveal clearly an important aspect of critical coupling as a delicate balance in both the amplitude and the phase relations, while a nonlinear resonance in dispersive bistability concerns only the phase.

Nonlinearity induced critical coupling

We study theoretically a parity-time (PT)-symmetric, saturable, balanced gain-loss system in a ring-cavity configuration. The saturable gain and loss are modeled by a two-level medium with or without population inversion. We show that the specifics of the spectral singularity can be fully controlled by the cavity and the atomic detuning parameters. The theory is based on the mean-field approximation, as in the standard theory of optical bistability. Further, in the linear regime we demonstrate the regularization of the singularity in detuned systems, while larger input power levels are shown to be adequate to limit the infinite growth in absence of detunings.

Cavity-controlled spectral singularity

In this Letter, we study the excitation of gap solitons under the conditions of coherent perfect absorption. Our system consists of a symmetric periodic structure with alternating Kerr nonlinear and linear layers illuminated from both ends. We show near-total transfer of incident light energy into the gap solitons resulting in null scattering. We also report on the nonlinear super-scattering states. Both the exact and the approximate results, which show good agreement based on nonlinear characteristic matrix methods, are presented.

Gap solitons with null-scattering

We study excitation of gap solitons under the conditions of coherent perfect absorption (CPA). Our system consists of a symmetric periodic structure with alternating Kerr nonlinear and linear layers, illuminated from both the ends. We show near-total transfer of incident light energy into the gap solitons resulting in null-scattering. We also report on the nonlinear super-scattering (SS) states. Both the CPA and the SS states are shown to be characterized by typical field distributions. Both the exact and the approximate results (based on nonlinear characteristic matrix method) are presented, which show good agreement.

K. Nireekshan Reddy

Papers

Light-controlled perfect absorption of light.

Nonlinearity induced critical coupling

Cavity-controlled spectral singularity

Gap solitons with null-scattering

Gap solitons with null-scattering