Ultra-compact wave devices based on deep subwavelength spatially dispersive effects

Devices exploiting the interaction of matter with waves, be it of electromagnetic nature (as light), acoustic (as sound) or mechanical (as vibrations), are at the basis of many modern technological applications, including telecommunication systems, information processing technologies, energy management techniques and consumer electronics. Understanding how to tame the relation between a wave's temporal dynamics (frequency) and its characteristic spatial scale (wavelength), also known as the dispersion relation, is a pivotal task in the engineering of disruptive new wave technologies. Two different kinds of physical effects can be exploited to tailor the dispersive properties of a system: inertia and multiple scattering. Inertia leads to temporal dispersion, i.e. a dependency of the material properties on frequency. Multiple scattering leads to spatial dispersion (SD), and describes the fact that the response of a medium at a given point may also depend on excitation at a different point. Altogether, these two phenomena capture the vast majority of wave matter interactions processes occurring in natural and artificial materials.While temporal dispersion has been exploited for decades in wave engineering, for instance in the design of microwave filters, research about SD in wave systems is still in its infancy. Artificially engineered media such as metamaterials (MTMs) or photonic crystals (PCs) can support giant SD, leading to novel properties such as large optical activity, negative refraction, or photonic band gaps. Yet, all these new concepts share a drastic limitation: to trigger significant SD, they must be composed of building blocks whose sizes or separating distances must be comparable to the wavelength. This intrinsic granularity leads to diffraction limited, bulky systems, which is a very vexing problem especially at low frequencies, where compact, deep subwavelength devices with strong SD are not available.In this five-year effort, we aim at solving this issue and develop a new class of systems that exhibit giant spatial dispersion at the subwavelength scale. We will focus on applying these concepts in the relevant fields of microwaves and acoustics, in which deep subwavelength wave systems with strong wave-matter interaction potential are crucially missing, due to the large size of the wavelength involved. Our program has two bold and cohesive objectives: (i) we will explore theoretically and experimentally different physical mechanisms to induce strong spatially dispersive effects at the subwavelength scale and (ii) we will conceive, design, and realize spatially dispersive compact devices with completely new functionalities, demonstrating their relevance from wave generation (compact sources), to transmission (media and waveguide) and manipulation (devices). To achieve this, we will depart from the usual paradigms of Bragg interferences (PCs) or low-order spatially dispersive passive resonators (MTM) that are inherently wavelength-scaled. Instead, we will explore Fano-resonance-induced multiple scattering or actively controlled resonator ensembles. We will demonstrate ultra-subwavelength, highly resonant EM cavities with high Purcell factors at microwave frequencies, and create the first low-power integrated MASER source functioning at room temperature. Our findings will also enable ultracompact and low-weight waveguides and filters, which are much-sought functionalities in embedded technology applications such as satellites communications. In acoustics, we will develop active systems with strong second order SD, explore the associated new physics, and apply our findings to low-frequency sound management and compressed sensing lenses for ultrasonic imaging. In the following, we describe in details the proposed synergistic research that we will unveil during this five-year effort.