Reflectionless programmable signal routers

Abstract

We demonstrate experimentally that reflectionless scattering modes (RSMs), a generalized version of coherent perfect absorption, can be functionalized to perform reflectionless programmable signal routing. We achieve versatile programmability both in terms of operating frequencies and routing functionality with negligible reflection upon in-coupling, which avoids unwanted signal power echoes in radio frequency or photonic networks. We report in situ observations of routing functionalities like wavelength demultiplexing, including cases where multichannel excitation requires adapted coherent input wavefronts. All experiments are performed in the microwave domain based on the same irregularly shaped cavity with strong modal overlap that is massively parametrized by a 304–element-programmable metasurface. RSMs in our highly overdamped multiresonance transport problem are fundamentally intriguing because the simple critical coupling picture for reflectionless excitation of isolated resonances fails spectacularly. We show in simulation that the distribution of damping rates of scattering singularities broadens under strong absorption so that weakly damped zeros can be tuned toward functionalized RSMs.

Covert Scattering Control in Metamaterials with Non‐Locally Encoded Hidden Symmetry

“…In parallel with the present work, the reduced-basis representation is now also emerging to efficiently model massively parametrized complex scattering systems such as "smart" radio environments. [55,56] Analogous to Section 2.2, we conclude that if n−v Ã0 = Ã0 n−v and W  m = n−v W  , then m S = S m . These conditions are compatible with absorption mechanisms that preserve the effective wave operator's parity, for instance, homogeneous absorption.…”

“…Non-locality is the norm rather than an exception in electromagnetism and more generally, wave engineering, but it was often neglected or treated as a nuisance. More recently, however, non-local interactions between meta-atoms have been embraced to tailor the dispersion relations of diverse metamaterial platforms [1][2][3][4] and to achieve new functionalities for applications including analog signal processing, [5][6][7][8] space compression, [9][10][11] and the multifunctionality of metasurfaces. [12][13][14] Usually, non-locality is discussed in the canonical basis, for example, in terms of direct interactions of far-apart meta-atoms.…”

“…[ 20 ] In general, the continuous tunability of a system parameter is necessary to meet this condition, [ 20 ] and if additional constraints on the specific frequency or routing functionalities exist, more tunable degrees of freedom are needed. [ 7,8,21 ] If, however, the system has parity‐time ($$\mathcal {PT}$$) symmetry, then the system's scattering matrix is guaranteed to have zero eigenvalues at some frequencies without any tuning. [ 20,22–25 ] Moreover, a single symmetry‐preserving continuous tuning parameter is sufficient in that case to achieve the coalescence of two zero eigenvalues and their eigenvectors, giving rise to a reflectionless EP with a broader lineshape.…”

Backpropagation-free training of deep physical neural networks

“…3B and further detailed in the supplementary text, section 2.4. Although the setup resembles that recently used to implement with high fidelity and in situ reprogrammability desired linear transfer functions for signal differentiation ( 53 ) and routing ( 54 ), in this case we sought a nonlinear mapping. Hence, we defined the metasurface configuration as the input and the transfer function as the output of our mathematical operation.…”

RIS-Based Radio Localization in Rich Scattering Environments: Harnessing Multi-Path with ANN Decoders

“…The field is still rapidly evolving. Importantly, most of the theory and algorithms covered in this chapter apply more generally to any massively parametrized complex medium (MPCM) [8]. RIS-parametrized richscattering radio environments are currently the most prominent example of MPCMs, but emerging dynamic metasurface antennas (DMAs) [9,10,11] and wave-based signal processors [12,13] equally rely on MPCMs [8].…”