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. We have thoroughly examined recent breakthroughs in designing and utilizing these devices. We have elucidated the theoretical foundations of coherent perfect absorption and surveyed various photonic structures capable of achieving it. These encompass planar and guided-mode structures, graphene-based systems, parity-symmetric and time-symmetric structures, 3D structures, and quantum-mechanical systems. Furthermore, we have explored potential applications in nanophotonics and provided an outlook on 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)

We have introduced coherent virtual absorption, accessing modes by shaping the incident waveform. Complex zeros enable energy storage and release in lossless structures, controlled by the impinging field. Robust against material dissipation, it applies to systems with any input ports. This effect has significant implications for flexible light control, memory with low energy, 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. Our  paper presents a unified description of exotic scattering phenomena, linking them to the properties of poles and zeros in the scattering matrix. By effectively manipulating these points in the complex frequency plane, we offer valuable insights for customizing unconventional 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 explore exciting radiationless field distributions in open cavities, aiming to clarify recent findings. We highlight that radiationless scattering states, such as anapoles, studied in recent research, are not eigenmodes of open cavities. Thus, their external excitation is unsurprising, similar to exciting nonzero internal fields in a transparent or cloaked object. Additionally, the sustained radiationless anapole field distribution requires the presence of external incident fields. Conversely, we demonstrate that the Lorentz reciprocity theorem prevents external excitation of radiationless optical eigenmodes, like embedded eigenstates and bound states in the continuum in open cavities. Remarkably, there is no limit to approaching these nonradiating states in the lossless scenario. Our discussion clarifies the analogies and distinctions between invisible bodies, nonradiating sources, anapole scatterers and emitters, and embedded eigenstates, particularly regarding their external excitation.

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

We have extended the concept of critical coupling to high-Q lossless resonators by manipulating the excitation wave's temporal profile. Using coupled-mode theory, we have successfully replicated critical coupling effects by employing nonmonochromatic excitations at complex frequencies to simulate loss. Significantly, our findings reveal that this method achieves optimal excitation efficiency in open systems, even when dealing with extremely high 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).

We have proposed a gain-free approach to achieving PT symmetry by incorporating complex-frequency excitations, simulating gain in passive systems. By introducing the concept of virtual absorption and virtual gain, we have successfully implemented PT symmetry in the complex-frequency plane. Our work demonstrates key effects like broken phase transitions, anisotropic transmission resonances, and laser-absorber pairs, within a fully passive and inherently stable system. These findings pave the way for establishing PT symmetry and non-Hermitian physics in passive platforms.

Scattering Anomalies

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

In our work, we analyze optical forces by extending them to the complex frequency plane and studying their behavior in the transient regime. We demonstrate that exponential excitation at complex frequencies enables a pulling force on resonant objects of any shape and composition, even in the paraxial approximation. We illustrate this approach using a dielectric Fabry–Perot cavity and a high-refractive-index dielectric nanoparticle, which are valuable for intracellular spectroscopy and lab-on-a-chip technologies, highlighting the potential of our technique.

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).

We have successfully showcased embedded eigenstates using Berreman modes in epsilon-near-zero layered materials. Our work presents realistic silicon carbide structures with high-Q (10^3) resonances. These structures exhibit exceptional absorption within a limited spectral and angular range, resulting in quasicoherent and highly directive thermal emission.