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arXiv:2306.14478 · Physical Review A

Virtual perfect absorption in resonant media and their PT -symmetric generalizations

Denis V. Novitsky, Alexander Sergeevich Shalin

Published Nov 27, 2023• 6 citations
arXivHTMLPDF

Abstract

Virtual perfect absorption (VPA) is an effect simulating real absorption of light by using excitation at a complex frequency corresponding to a scattering zero. We theoretically study VPA in resonantly absorbing and amplifying media irradiated by two counterpropagating waves with exponentially growing amplitudes. We show that VPA critically depends on the medium density (i.e., level of loss or gain) deteriorating in the high-density limit. In contrast, almost ideal VPA persists in the $\mathcal{P}T$-symmetric loss-gain bilayer. For high enough gain, powerful quasilasing pulses are observed at later times symmetrically (single amplifying layer) or asymmetrically ($\mathcal{P}T$-symmetric structure) generated in both propagation directions.

DOI
10.1103/physreva.108.053513

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Virtual perfect absorption (VPA) is an effect simulating real absorption of light by using excitation at a complex frequency corresponding to a scattering zero. We theoretically study VPA in resonantly absorbing and amplifying media irradiated by two counterpropagating waves with exponentially growing amplitudes. We show that VPA critically depends on the medium density (i.e., level of loss or gain) deteriorating in the high-density limit. In contrast, almost ideal VPA persists in the PT\mathcal{P}TPT-symmetric loss-gain bilayer. For high enough gain, powerful quasilasing pulses are observed at later times symmetrically (single amplifying layer) or asymmetrically (PT\mathcal{P}TPT-symmetric structure) generated in both propagation directions.
Virtual gain refers to the simulation of real light amplification using radiation with exponentially decaying amplitude so that its complex frequency corresponds to the scattering pole. We study theoretically virtual gain in a two-level resonant medium for different regimes of light-matter interaction depending on the radiation intensity. We show that virtual gain at the pole can be most clearly observed for low intensities, when the medium is absorbing, in contrast to the saturated medium at high intensities. The efficiency of virtual gain can be tuned with the light intensity and controlled dynamically through the population inversion of the medium. Our results show that resonantly absorbing media paradoxically mimic a gainlike response, which admits a number of related phenomena and methods to mold both optical signals and material properties without relying on instability-prone gain media.
Abstract Time-invariant photonic structures amplify or absorb light based on their intrinsic material gain or loss. The coherent interference of multiple beams in space, e.g., in a resonator, can be exploited to tailor the wave interaction with material gain or loss, respectively maximizing lasing or coherent perfect absorption. By contrast, a time-varying system is not bound to conserve energy, even in the absence of material gain or loss, and can support amplification or absorption of a probe wave through parametric phenomena. Here, we demonstrate theoretically and experimentally how a subwavelength film of indium tin oxide, whose bulk permittivity is homogeneously and periodically modulated via optical pumping, can be dynamically tuned to act both as a non-resonant amplifier and a perfect absorber, by manipulating the relative phase of two counterpropagating probe beams. This extends the concept of coherent perfect absorption to the temporal domain. We interpret this result as selective switching between the gain and loss modes present in the momentum bandgap of a periodically modulated medium. By tailoring the relative intensity of the two probes, high-contrast modulation can be achieved with up to 80% absorption and 400% amplification. Our results demonstrate control of gain and loss in time-varying media at optical frequencies and pave the way towards coherent manipulation of light in Floquet-engineered complex photonic systems.
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In the time-reversed counterpart to laser emission, incident coherent optical fields are perfectly absorbed within a resonator that contains a loss medium instead of a gain medium. The incident fields and frequency must coincide with those of the corresponding laser with gain. We demonstrated this effect for two counterpropagating incident fields in a silicon cavity, showing that scattering [corrected] can be modulated [corrected] by two orders of magnitude, the maximum predicted by theory for our experimental setup. In addition, we showed that absorption can be reduced substantially by varying the relative phase of the incident fields. The device, termed a "coherent perfect absorber," functions as an absorptive interferometer, with potential practical applications in integrated optics.
Self-formation of one or several soliton-like pulses from a nonsoliton powerful laser pulse during its propagation through a two-component resonant amplifying—absorbing medium with an energy dissipation channel is studied numerically. Both components of the medium are simulated by two-level particles. Homogeneous mixtures of these components and media with consecutive amplifying and absorbing layers are considered. A qualitative analysis of the results is based on a simplified model of the process of soliton-like pulse propagation.
In this thesis we address a series of new problems in non-hermitian optical scattering with increasing degrees of complexity. We develop the theory of reflectionless scattering modes, introducing a novel and broad class of impedance-matched eigenproblems: for a given structure, find the incident wavefronts and frequencies which are not partially reflected, but are instead transmitted through the scatterer, or dissipated within it. We analyze the relevant symmetry properties, and find that they support exceptional points (EPs) which can be directly probed with steady-state excitation. We study degenerate coherent perfect absorption (CPA EP), which is a specific example of the new kind of EP mentioned above. We show that the CPA EP is chiral in a ring resonator, and use this to design a resonator which predominantly absorbs or reflects light, depending on the direction of incidence. We extend CPA from cavities with a linear dielectric response to include the saturating nonlinearity and dispersion of a two-level absorbing medium. We show that the SALT algorithm in the single-mode regime can also be used to find the saturable CPA modes through a simple mapping. We also clarify the bad-cavity limit of dispersive but linear CPA, identifying new modes that are hybrids of the cavity and atomic degrees of freedom. We present and solve the general problem of scattering from an arbitrary cavity with a saturable two-level amplifying or absorbing medium, generalizing the known phenomenology of bistability that has mainly been studied in structures with little or no spatial complexity. We carefully analyze the validity of approximations used for isolated resonances, and find that the previously used inverse single-pole approximation requires some modification for the case of scattering.