Spatial coherence in 2D holography.

Holography is a long-established technique to encode an object's spatial information into a lower-dimensional representation. We investigate the role of the illumination's spatial coherence properties in the success of such an imaging system through point spread function and Fourier domain analysis. Incoherent illumination is shown to result in more robust imaging performance free of diffraction artifacts at the cost of incurring background noise and sacrificing phase retrieval. Numerical studies confirm that this background noise reduces image sensitivity as the image size increases, in agreement with other similar systems. Following this analysis, we demonstrate a 2D holographic imaging system realized with lensless, 1D measurements of microwave fields generated by dynamic metasurface apertures.

Learned Integrated Sensing Pipeline: Reconfigurable Metasurface Transceivers as Trainable Physical Layer in an Artificial Neural Network.

The rapid proliferation of intelligent systems (e.g., fully autonomous vehicles) in today's society relies on sensors with low latency and computational effort. Yet current sensing systems ignore most available a priori knowledge, notably in the design of the hardware level, such that they fail to extract as much task-relevant information per measurement as possible. Here, a "learned integrated sensing pipeline" (LISP), including in an end-to-end fashion both physical and processing layers, is shown to enable joint learning of optimal measurement strategies and a matching processing algorithm, making use of a priori knowledge on task, scene, and measurement constraints. Numerical results demonstrate accuracy improvements around 15% for object recognition tasks with limited numbers of measurements, using dynamic metasurface apertures capable of transceiving programmable microwave patterns. Moreover, it is concluded that the optimal learned microwave patterns are nonintuitive, underlining the importance of the LISP paradigm in current sensorization trends.

Single-frequency 3D synthetic aperture imaging with dynamic metasurface antennas.

Through aperture synthesis, an electrically small antenna can be used to form a high-resolution imaging system capable of reconstructing three-dimensional (3D) scenes. However, the large spectral bandwidth typically required in synthetic aperture radar systems to resolve objects in range often requires costly and complex RF components. We present here an alternative approach based on a hybrid imaging system that combines a dynamically reconfigurable aperture with synthetic aperture techniques, demonstrating the capability to resolve objects in three dimensions (3D), with measurements taken at a single frequency. At the core of our imaging system are two metasurface apertures, both of which consist of a linear array of metamaterial irises that couple to a common waveguide feed. Each metamaterial iris has integrated within it a diode that can be biased so as to switch the element on (radiating) or off (non-radiating), such that the metasurface antenna can produce distinct radiation profiles corresponding to different on/off patterns of the metamaterial element array. The electrically large size of the metasurface apertures enables resolution in range and one cross-range dimension, while aperture synthesis provides resolution in the other cross-range dimension. The demonstrated imaging capabilities of this system represent a step forward in the development of low-cost, high-performance 3D microwave imaging systems.

Phaseless computational ghost imaging at microwave frequencies using a dynamic metasurface aperture.

We demonstrate a dynamic metasurface aperture as a unique tool for computational ghost imaging at microwave frequencies. The aperture consists of a microstrip waveguide loaded with an array of metamaterial elements, each of which couples energy from the waveguide mode to the radiation field. With a tuning mechanism introduced into each independently addressable metamaterial element, the aperture can produce diverse radiation patterns that vary as a function of tuning state. Here, we show that fields from such an aperture approximately obey speckle statistics in the radiative near field. Inspired by the analogy with optical correlation imaging, we use the dynamic aperture as a means of illuminating a scene with structured microwave radiation, receiving the backscattered intensity with a simple waveguide probe. By correlating the magnitude of the received signal with the structured intensity patterns, we demonstrate high-fidelity, phaseless imaging of sparse targets. The dynamic metasurface aperture as a novel ghost imaging structure can find application in security screening, through-wall imaging, as well as biomedical diagnostics.

Oxide-Free Actuation of Gallium Liquid Metal Alloys Enabled by Novel Acidified Siloxane Oils.

Electrowetting and electrocapillarity of liquid metals have a long history, and a recent explosion of renewed interest. Liquid metals have electromagnetic properties and surface tensions (>500 mN/m) that enable new forms of reconfigurable devices. However, the only nontoxic option, gallium alloys, suffer from immediate formation of a semirigid surface oxide. Although acids or electrochemical reduction can remove this oxide, these approaches surround the gallium alloy in a fluid that is also electrically conducting, diminishing electromagnetic effectiveness and precluding electrowetting actuation. Reported here are acidified siloxanes that remove and prevent oxide formation. Importantly, the siloxane oil associatively incorporates hydrochloric or hydrobromic acids, is electrically insulating, is chemically stable, removes etching byproducts (including water), and allows robust electrowetting. This work opens up new opportunities for liquid metal reconfiguration, and is of fundamental interest due to the unexpected chemical stability of the acidified siloxanes and their application to other materials and surfaces.