Realization of active metamaterials with odd micropolar elasticity

Materials made from active, living, or robotic components can display emergent properties arising from local sensing and computation. Here, we realize a freestanding active metabeam with piezoelectric elements and electronic feed-forward control that gives rise to an odd micropolar elasticity absent in energy-conserving media. The non-reciprocal odd modulus enables bending and shearing cycles that convert electrical energy into mechanical work, and vice versa. The sign of this elastic modulus is linked to a non-Hermitian topological index that determines the localization of vibrational modes to sample boundaries. At finite frequency, we can also tune the phase angle of the active modulus to produce a direction-dependent bending modulus and control non-Hermitian vibrational properties. Our continuum approach, built on symmetries and conservation laws, could be exploited to design others systems such as synthetic biofilaments and membranes with feed-forward control loops.

Non-Foster acoustic radiation from an active piezoelectric transducer

The quality factor of a passive, linear, small acoustic radiator is fundamentally limited by its volume normalized to the emitted wavelength, imposing severe constraints on the bandwidth and efficiency of compact acoustic sources and of metamaterials composed of arrangements of small acoustic resonators. We demonstrate that these bounds can be overcome by loading a piezoelectric transducer with a non-Foster active circuit, showing that its radiation bandwidth and efficiency can be largely extended beyond what is possible in passive radiators, fundamentally limited only by stability considerations. Based on these principles, we experimentally observe a threefold bandwidth enhancement compared to its passive counterpart, paving the way toward non-Foster acoustic radiation and more broadly active metamaterials that overcome the bandwidth constraints hindering passive systems.

Acoustic Willis meta-atom beyond the bounds of passivity and reciprocity

Willis metamaterial enables exotic manipulations of acoustic waves with a precise combination of bulk modulus, mass density, and Willis parameters. While the realization of unrestricted and completely decoupled constitutive parameters would extend the horizon of future applications, the restriction of passivity and reciprocity dictate a hard bound in the values of achievable polarizabilities and correlations between them. Here, we break the bound of passivity and reciprocity by instituting a basis and independent kernel for each constitutive polarization in a virtualized metamaterial platform, active metamaterials realizing artificial polarization with the digital convolution. We demonstrate decoupled control of all four constitutive parameters in a nonreciprocal regime, at the same time achieving values of polarizabilities beyond the passivity limit. Broadband, flat-response nonreciprocal Willis coupling is also demonstrated with analytically designed causal frequency dispersion. Our approach will be useful for nonreciprocal wave manipulation and communication for broadband operation.

Use of Active Metamaterial as a Phase Shifter Integrated into the Waveguide

Active metamaterials usage is one of the most promising ways to control the characteristics of antennas, waveguides, and other microwave devices. This article proposes the controlled metamaterial design in the form of an electromagnetic crystal with switches located at the nodes of the crystal lattice. This metamaterial application for changing the fundamental mode phase of the WR-137 waveguide is investigated. Controlling the characteristics of the metamaterial is performed by switching pin diodes at the nodes of the lattice, so this control method allows you to achieve a high speed system, as well as to switch only certain pin diodes. Electrodynamic modeling was carried out, on the basis of which the characteristics of the waveguide were obtained for different metamaterial closed nodes combination, which changes the the electromagnetic wave phase.

Nanoporous gold nanoleaf as tunable metamaterial

We have studied optical properties of single-layer and multi-fold nanoporous gold leaf (NPGL) metamaterials and observed highly unusual transmission spectra composed of two well-resolved peaks. We explain this phenomenon in terms of a surface plasmon absorption band positioned on the top of a broader transmission band, the latter being characteristic of both homogeneous “solid” and inhomogeneous “diluted” Au films. The transmission spectra of NPGL metamaterials were shown to be controlled by external dielectric environments, e.g. water and applied voltage in an electrochemical cell. This paves the road to numerous functionalities of the studied tunable and active metamaterials, including control of spontaneous emission, energy transfer and many others.

Non-reciprocal robotic metamaterials

Non-reciprocal transmission of motion is potentially highly beneficial to a wide range of applications, ranging from wave guiding to shock and vibration damping and energy harvesting. To date, large levels of non-reciprocity have been realized using broken spatial or temporal symmetries, yet mostly in the vicinity of resonances, bandgaps or using nonlinearities, thereby non-reciprocal transmission remains limited to narrow ranges of frequencies or input magnitudes and sensitive to attenuation. Here, we create a robotic mechanical metamaterials wherein we use local control loops to break reciprocity at the level of the interactions between the unit cells. We show theoretically and experimentally that first-of-their-kind spatially asymmetric standing waves at all frequencies and unidirectionally amplified propagating waves emerge. These findings realize the mechanical analogue of the non-Hermitian skin effect. They significantly advance the field of active metamaterials for non hermitian physics and open avenues to channel mechanical energy in unprecedented ways.