D. G. Baranov, A. Krasnok, T. Shegai, A. Alù, and Y. Chong, Coherent Perfect Absorbers: Linear Control of Light with Light, Nat. Rev. Mater. 2, 17064 (2017)

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.

D. G. Baranov, A. Krasnok, and A. Alù, Coherent Virtual Absorption Based on Complex Zero Excitation for Ideal Light Capturing, Optica 4, 1457 (2017)

Here, we introduce the concept of coherent virtual absorption, accessing these modes by temporally shaping the incident waveform. We show that engaging these complex zeros enables storing and releasing the electromagnetic energy at will within a lossless structure for arbitrary amounts of time, under the control of the impinging field. The effect is robust with respect to inevitable material dissipation and can be realized in systems with any number of input ports. The observed effect may have important implications for flexible control of light propagation and storage, low-energy memory, and optical modulation.

A. Krasnok, D. Baranov, H. Li, M.-A. Miri, F. Monticone, and A. Alú, Anomalies in Light Scattering, Adv. Opt. Photonics 11, 892 (2019)

Recently, a broad spectrum of exceptional scattering phenomena attainable in suitably engineered structures has been predicted and demonstrated. Examples include bound states in the continuum, exceptional points in parity–time (𝒫𝒯)-symmetrical non-Hermitian systems, coherent perfect absorption, virtual perfect absorption, nontrivial lasing, nonradiating sources, and others. In this paper, we establish a unified description of such exotic scattering phenomena and show that the origin of all these effects can be traced back to the properties of poles and zeros of the underlying scattering matrix. We provide insights on how managing these special points in the complex frequency plane provides a powerful approach to tailor unusual scattering regimes.

F. Monticone, D. Sounas, A. Krasnok, and A. Alù, Can a Nonradiating Mode Be Externally Excited? Nonscattering States versus Embedded Eigenstates, ACS Photonics 6, 3108 (2019)

In this Letter, we discuss the general problem of exciting radiationless field distributions in open cavities, with the goal of clarifying recent findings on this topic. We point out that the radiationless scattering states, like anapoles, considered in several recent studies, are not eigenmodes of an open cavity; therefore, their external excitation is neither surprising nor challenging (similar to the excitation of nonzero internal fields in a transparent, or cloaked, object). Even more, the radiationless anapole field distribution cannot be sustained without the actual presence of external incident fields. Conversely, we show that the Lorentz reciprocity theorem prevents the external excitation of radiationless optical eigenmodes, as in the case of embedded eigenstates and bound states in the continuum in open cavities, while there is no limit to how close one can approach these nonradiating states in the lossless limit. Our discussion clarifies the analogies and differences between invisible bodies, nonradiating sources, anapole scatterers and emitters, and embedded eigenstates, especially in relation to their external excitation.

Y. Ra’Di, A. Krasnok, and A. Alù, Virtual Critical Coupling, ACS Photonics 7, 1468 (2020).

Here, we extend the notion of critical coupling to high-Q lossless resonators, based on tailoring the temporal profile of the excitation wave. Utilizing coupled-mode theory, we demonstrate an effect analogous to critical coupling by mimicking loss with nonmonochromatic excitations at complex frequencies. Remarkably, we show that this approach enables unitary excitation efficiency in open systems, even in the limit of extreme quality factors in the regime of quasi-bound states in the continuum.

H. Li, A. Mekawy, A. Krasnok, and A. Alù, Virtual Parity-Time Symmetry, Phys. Rev. Lett. 124, 193901 (2020).

Here, we propose a gain-free route to PT symmetry by extending it to complex-frequency excitations that can mimic gain in passive systems. Based on the concept of virtual absorption, extended here to implement also virtual gain, we implement PT symmetry in the complex-frequency plane and realize its landmark effects, such as broken phase transitions, anisotropic transmission resonances, and laser-absorber pairs, in a fully passive, hence inherently stable, system. These results open a path to establish PT symmetry and non-Hermitian physics in passive platforms.

Scattering Anomalies

S. Lepeshov and A. Krasnok, Virtual Optical Pulling Force, Optica 7, 1024 (2020)

Herein, we revisit the issue of optical forces by their analytic continuation to the complex frequency plane and considering their behavior in the transient regime. We show that the exponential excitation at the complex frequency offers an intriguing ability to achieve a pulling force for a passive resonant object of any shape and composition, even in the paraxial approximation. The approach is elucidated on a dielectric Fabry–Perot cavity and a high-refractive-index dielectric nanoparticle, a fruitful platform for intracellular spectroscopy and lab-on-a-chip technologies, where the proposed technique may find unprecedented capabilities.

Z. Sakotic, A. Krasnok, N. Cselyuszka, N. Jankovic, and A. Alú, Berreman Embedded Eigenstates for Narrow-Band Absorption and Thermal Emission, Phys. Rev. Appl. 13, 064073 (2020).

In this work, we demonstrate a class of embedded eigenstates based on Berreman modes in epsilon-near-zero layered materials and propose realistic silicon carbide structures that support high-Q (10^3) resonances based on these principles. The proposed structures demonstrate strong absorption in a narrow spectral and angular range, giving rise to quasicoherent and highly directive thermal emission