Dynamically modulated perfect absorbers

We introduce the concept of multichannel dynamically modulated perfect absorbers (DMPAs), which are periodically modulated lossy interferometric traps that completely absorb incident monochromatic waves. The proposed DMPA protocols utilize a Floquet engineering approach which produces a variety of emerging phenomena and features: reconfigurability of perfect absorption (PA) for a broad range of frequencies of the incident wave; PA for infinitesimal local losses, and PA via critical coupling with high-$Q$ modes by inducing back-reflection dynamical mirrors.

Figure 1

Schematics of various physical setups that can be used for DMPA: (a) a single-mode fiber coupled to a time-periodic phase modulator, (b) a network of four oscillators (resonators) with modulated coupling constants, (c) two single-mode fibers with a time-modulated coupler, and (d) an acoustic cavity with two cantilevers where the in-between air domain is periodically modulated in time. Weakly lossy elements are indicated with green.

Figure 2

Parametric evolution of the zeros of the $\mathcal{S}$ matrix as the loss $\gamma$ at one resonator of a network of four coupled resonators increases [see Appendix pp1 for the corresponding Hamiltonian and Fig. 1 for a mechanical analog]. The driving frequency is $\omega =1$ and the amplitude is $\beta =0.9$. The positions of the zeros for $\gamma =0$ are indicated with solid circles. The two sets of trajectories correspond to strong $c\approx -1$ (red dashed lines) and weak $c\approx -0.2$ (black solid lines) coupling of the resonators with the leads. The crosses indicate the predictions of perturbation theory [Eq. (4)]. Right inset: The DMPA can be reconfigured to occur for an extremely small value of ${\gamma }_{\mathrm{DMPA}}\approx 2.76\times {10}^{-6}$ when the driving frequency is $\omega =1$ and the driving amplitude is $\beta =0.01\left(c=-0.5\right)$. Left inset: The Floquet network (with fixed topology and loss $\gamma =0.0322)$ can be reconfigured by changing the driving frequency $\omega$ and amplitude $\beta$ in order to perfectly absorb an incident wave with a dense set of frequencies ${\mathrm{\Omega }}_0$. We show the numerically evaluated ${\mathrm{\Omega }}_{\mathrm{DMPA}}$ versus ${\omega }_{\mathrm{DMPA}}$.

Figure 3

Parametric evolution (versus increasing driving amplitude $\beta )$ of the zeros of the $\mathcal{S}$ matrix associated with a network of four fully connected resonators, two of which are coupled to leads (see Appendix pp2). The other two resonators have losses characterized by a loss strength $\gamma =0.4749$ (fixed in these simulations). The driving frequency of the coupling is $\omega =5$ and the amplitude is increased from $\beta =0.15$ (indicated with a solid circle) to $\beta =0.3$. The DMPA occurs for $\beta \approx 2.3$. This Floquet driving induces a dynamical mirror which can be used for PA via critical coupling.