Extremely Tunable Meta-Systems: Design Considerations and Universal Model-Based Wave-Control Framework

Designing and optimizing a reconfigurable wave system (e.g., a reconfigurable intelligent surface, a dynamic metasurface antenna, a wave-domain physical neural network) for a desired functionality is generally challenging because the effects of the tunable degrees of freedom are intertwined. First, I will argue that stronger coupling between the tunable entities boosts the control over the reconfigurable wave systems's transfer function. Intuitively, the longer a wave bounces around between the tunable entities within the system, the more sensitivity it accumulates to their configuration. I will present analytical, numerical, and experimental results to substantiate this observation, considering for concreteness a dynamic metasurface antenna as reconfigurable wave system. Second, I will discuss our recent efforts to formulate and calibrate physics-based models of reconfigurable wave systems that faithfully capture the inter-element coupling, irrespective of the complexity of the system. I will present closed-form and gradient-descent-based methods to calibrate the parameters of such models so that they can serve as digital twins for a priori unknown experimental reconfigurable wave systems, including a calibration method compatible with intensity-only detection. I will carefully explore the role of ambiguities in these physics-based digital twins, clarifying in what scenarios ambiguities need to be lifted and how that can be achieved. By comparison with physics-agnostic digital twins, I will highlight the favorable inductive bias of physics-based models. Once calibrated, these physics-based digital twins enable efficient in-software optimization of the reconfigurable wave system’s configuration for a desired functionality without further access to the experimental system. I will share insights into efficient optimization algorithms, notably for systems with few-bit programmable degrees of freedom. Third, I will apply this technique to characterize reconfigurable wave systems in the realm of wireless sensing. Specifically, I will report experimental results on remotely estimating the full scattering matrix of a multi-port device under test without any ambiguity. In this scheme, the wireless propagation environment is interpreted as an "over-the-air fixture" that can be treated as reconfigurable wave system. Hence, it is amenable to being characterized based on the aforementioned technique. Subsequently, the over-the-air fixture can be de-embedded to obtain the scattering properties of the device under test. The concept is relevant to RFID and wireless bioelectronics.