Juan Sebastian Gomez Diaz

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally adopted in optical material modeling. The growing interest in plasmonic, polaritonic, and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials—metamaterials and metasurfaces—can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In metasurfaces, in particular, nonlocality engineering has emerged as a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials and metamaterials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward—toward new scientific discoveries and technological advancements.

We present a study of the radiation pattern of photoconductive antennas (PCA) subject to substrate chip defects, aiming at characterizing fabrication tolerances. We show that the asymmetries observed over the cross-polar radiation patterns may be exploited to numerically estimate the most prominent trends of the substrate chip geometrical imperfections. A Figure of Merit (FoM) has been defined, characterized and validated through simulations and experimental measures, proving it is sensitive to both the magnitude and location of the substrate irregularities. The study has been focused on two kinds of substrate defects, the angled dicing and the off-centered antenna gap, and it has considered three planar antenna geometries: dipole, bow-tie and Sierpinski triangle dipole. A numerical antenna factor related to the planar antenna geometry imprinted over the substrate is included in the FoM expression for fair comparison of substrate irregularities among PCAs with different metallic patterns. A simple setup for experimentally obtaining the value of the FoM through collimated beam raster scanning measure of the cross-polar radiation pattern has been proved useful to validate the practical relevance of the FoM. These values asses the substrate chip fabrication quality and help to identify the position and magnitude of the substrate defects that have the greatest impact on the PCA performance.

The next generation of infrared (IR) sensors may enable unprecedented applications in fields like spectroscopy, health monitoring, and communication systems. For instance, metasurface-enhanced micro-electromechanical system (MEMS)-based IR sensors have demonstrated excellent performance in terms of responsivity and spectral-selectivity. However, it is burdensome to experimentally determine the performance limits of this and other IR sensing technologies as it requires time-consuming and expensive systematic testing not always feasible in research settings. To address this challenge, an automated solution for characterizing miniaturized sensing devices is presented in this paper and applied to experimentally determine the performance limits of MEMS-based IR sensors. The system offers low-cost, rapid, and automated characterization of on-chip IR sensors, determining key performance metrics such as noise, responsivity, and noise-equivalent power. The platform is flexible, easily adapted to different types of devices, chip layouts, and light sources, and is designed to test a large number of sensors within the same wafer −spending ~20 seconds per device− using a combination of optical and radiofrequency interrogation techniques. The system has been applied to test over 1500 MEMS-based IR sensors. Collected data revealed hidden trade-offs between responsivity and noise with respect to the device thickness and allowed a statistical analysis of sensing response versus device geometrical dimensions. The best-performing devices exhibit a quality factor, responsivity, fluctuation induced noise, and noise-equivalent power of 2391, 164 Hz/nW, 0.257 Hz/ $\sqrt {\mathbf {Hz}}$ , and 5.01 pW/ $\sqrt {\mathbf {Hz}}$ respectively. The proposed automated platform provides an efficient and cost-effective solution for characterizing the next generation of IR sensing devices.

In this Tutorial, we overview recent developments to break and manipulate electromagnetic nonreciprocity in two-dimensional (2D) materials without relying on magnetic fields. To this purpose, we provide a general conductivity model to describe gyrotropic metasurfaces that exhibit nonreciprocity through different physical mechanisms enabled by 2D materials, including optical pumping, drifting electrons, ferromagnetic monolayers, mechanical strain, and spatiotemporal modulation. We describe in detail the resulting systems, focusing on near-field phenomena, associated to nonreciprocal bulk and edge surface plasmon propagation, and on far-field responses, related to Faraday/Kerr rotation and optical dichroism of waves propagating in free-space. Additionally, we review and contextualize recent advancements in magnet-free nonreciprocal devices based on 2D materials, ranging from Faraday rotators and optical dichrosim, to plasmonic and photonic isolators, hyperlenses, and tunable optical traps. We conclude by providing our outlook for the future development of this technology and its potential applications in areas such as communications, sensing, wave generation, and spectroscopy, among others.

We demonstrate that Cherenkov radiation can be manipulated in terms of operation frequency, bandwidth, and efficiency by simultaneously controlling the properties of drifting electrons and the photonic states supported by their surrounding media. We analytically show that the radiation rate strongly depends on the momentum of the excited photonic state, in terms of magnitude, frequency dispersion, and its variation vs the properties of the drifting carriers. This approach is applied to design and realize miniaturized, broadband, tunable, and efficient terahertz and far-infrared sources by manipulating and boosting the coupling between drifting electrons and engineered hyperbolic modes in graphene-based nanostructures. The broadband, dispersive, and confined nature of hyperbolic modes relax momentum matching issues, avoid using electron beams, and drastically enhance the radiation rate—allowing that over 90% of drifting electrons emit photons. Our findings open an exciting paradigm for the development of solid-state terahertz and infrared sources.

We propose engineering optical traps over plasmonic surfaces and precisely controlling the trap position with an external bias by inducing in-plane nonreciprocity on the surface. The platform employs an incident Gaussian beam to polarize targeted nanoparticles, and exploits the interplay between nonreciprocal and spin-orbit lateral recoil forces to construct stable optical traps and manipulate their position within the surface. To model this process, we develop a theoretical framework based on the Lorentz force combined with nonreciprocal Green’s functions and apply it to calculate the trapping potential. Rooted on this formalism, we explore the exciting possibilities offered by graphene to engineer stable optical traps using low-power laser beams in the mid-IR and to manipulate the trap position in a continuous manner by applying a longitudinal drift bias. Nonreciprocal metasurfaces may open new possibilities to trap, assemble and manipulate nanoparticles and overcome many challenges faced by conventional optical tweezers while dealing with nanoscale objects.

We demonstrate that Cherenkov radiation can be manipulated in terms of operation frequency, bandwidth, and efficiency by simultaneously controlling the properties of drifting electrons and the photonic states supported by their surrounding media. We analytically show that the radiation rate strongly depends on the momentum of the excited photonic state, in terms of magnitude, frequency dispersion, and its variation versus the properties of the drifting carriers. This approach is applied to design and realize miniaturized, broadband, tunable, and efficient terahertz and far-infrared sources by manipulating and boosting the coupling between drifting electrons and engineered hyperbolic modes in graphene-based nanostructures. The broadband, dispersive, and confined nature of hyperbolic modes relax momentum matching issues, avoid using electron beams, and drastically enhance the radiation rate - allowing that over 90% of drifting electrons emit photons. Our findings open a new paradigm for the development of solid-state terahertz and infrared sources.