Metacrystals: inversely-designed 3D-printed intelligent panels for 6G communications

Article https://doi.org/10.1038/s41467-026-73019-x Metacrystals: inversely-designed 3D-printed intelligent panels for 6G communications Received: 25 January 2025 Accepted: 22 April 2026 1234567890():,; 1234567890():,; Check for updates Mohammad M. Asgari 1 , Peter B. Catrysse 2, Shuai S. A. Yuan1, Haiwen Wang 3, Shanhui Fan 2,3 & Viktar Asadchy 1 Metasurfaces represent a promising platform for improving coverage in future communication systems. Passive designs are especially attractive because they need no power supply and can be manufactured at low cost. However, most passive metasurfaces work well only for one polarization, frequency band, or incidence angle, which limits their practical use. Here we propose passive intelligent panels, termed metacrystals, that overcome these limitations by enabling highly complex multiplexed responses to multiple incident waves simultaneously and independently. This capability is enabled by a compact volumetric architecture that goes beyond conventional metasurfaces by exploiting a finite, yet still modest, thickness to unlock substantially more degrees of freedom. Through simulations and experiments, we demonstrate all-dielectric metacrystals capable of simultaneously controlling anomalous reflection and absorption, both in transmission and reflection regimes. Designed using inverse topology optimization, these metacrystals combine structural integrity, straightforward scalability, and compatibility with lowcost 3D printing for operation up to 100 GHz. The advent of sixth-generation (6G) and future wireless technologies will transform communications by offering higher data rates, improved energy efficiency, and lower latency1. However, the realization of high data rates necessitates the exploration of new frequency bands, such as millimeter (mm) waves and sub-THz bands2,3. While these frequencies offer vast amounts of bandwidth, they also present considerable challenges due to their high atmospheric attenuation, free-space path loss, and harsher scattering effects when encountering obstacles4. Therefore, reliance on traditional multipath propagation is no longer feasible, and directional beams must be used for communication1. Moreover, higher-frequency signals are often blocked by obstacles, such as walls, requiring a denser network of base stations and relays. Recently, metasurfaces, also referred to as intelligent surfaces, have been proposed to mitigate these challenges by efficiently redirecting communication signals in free space to bypass obstacles5–8. These artificial surfaces, strategically positioned on walls, ceilings, and even windows, can substantially enhance both indoor and outdoor signal coverage through anomalous reflection or refraction9, requiring minimal to no energy for their operation. Most of the existing studies on intelligent surfaces focused on achieving reconfigurable responses8. Programmable metasurfaces are capable of dynamically manipulating several wave characteristics, including wave vector, polarization, frequency, and wavefront, within a unified structure10,11. However, they have proven to be too expensive for widespread adoption in the communication industry. This is primarily due to their requirement to operate at high frequencies (above 30–50 GHz), their large physical footprint (approximately one square meter) even for incorporating a single communication channel, and the need for highly tunable constituent elements9. Consequently, their non-reconfigurable (completely passive) counterparts have recently gained great attention due to their significantly lower manufacturing and maintenance costs9. In fact, in many real-world scenarios, reconfigurability is not necessary because the positions of the receivers and transmitters are fixed or weakly varying. For instance, in industrial 1 Aalto University, Department of Electronics and Nanoengineering, Espoo, Finland. 2Stanford University, E. L. Ginzton Laboratory, Department of Electrical Engineering, Stanford, CA, USA. 3Stanford University, E. L. Ginzton Laboratory, Department of Applied Physics, Stanford, CA, USA. e-mail: mohammadmahdi.asgari@aalto.fi; viktar.asadchy@aalto.fi Nature Communications | (2026)17:4912 1 Article settings, machinery and sensors are usually installed in fixed locations; the infrastructure and major pathways in large public hubs remain constant; and in office environments, the locations of access points are typically fixed. While various pathways for the analytic design of passive intelligent surfaces were proposed (e.g., anomalous reflectors12–14, smart skins15, metagratings16–18, and aperiodic gratings19), all of them lack the sufficient versatility for realistic applications. Indeed, in most practical scenarios, it is necessary for the intelligent surface to operate effectively across both signal polarizations, multiple frequency bands, various angles of arrival, and even all at once. Realizing such versatile surfaces with current analytical or semi-analytical design techniques remains very challenging, as these techniques rely on specific homogenization models (e.g., based on polarizability20, susceptibility21, or surface impedance tensors22). Factors, such as frequency dispersion, nonlocality, and anisotropy make the implementation of the unit cells with required material parameters hardly possible. Recent work on multifunctional metasurfaces at microwave and sub-THz frequencies falls into two main classes: multi-incidence and multi-dimensional. Multi-incidence designs operate under multiple incidence angles or wave vectors; examples include angle-dependent/independent metasurfaces23–26, directional Janus metasurfaces27, and schemes multiplexing guided and space waves28. Multi-dimensional designs simultaneously control several wave properties (polarization p, propagation direction/wavefront angle θ, phase ϕ, and amplitude A) typically for a single incident wave. Demonstrations include concurrent control of polarization and direction29–33, wave-vector modulation across frequencies using multi-band metasurfaces34–37, and co-modulation of polarization and wavefront38,39. These passive metasurfaces demonstrated to date still realize only one or two such multi-incidence/dimensional functions at a time. In this paper, we introduce the concept of metacrystals, which are all-dielectric binarized composites capable of performing multidimensional functionalities for multiple predefined incident waves concurrently and independently. This versatile responses are achieved by transitioning from the traditional sub-wavelength singlelayer metasurface design to a bulk multilayer topology with a larger number of degrees of freedom. We use the terminology metacrystals due to their similarity to both photonic crystals (supporting multiple diffraction orders) and metamaterials (with deeply sub-wavelength building blocks). To design the metacrystals, we employ an inverse design method using adjoint-based topology optimization40 that was recently us (...truncated)