This project aims at the theoretical and numerical study of electromagnetic 'line waves' (LWs) and their perspective applications. These recently discovered wave objects channel the energy flow along a 1-D track and can be supported by suitable planar discontinuities of the surface impedance in artificial (metasurfaces) and natural (e.g., graphene) low-dimensional materials.
Their possible applications are abundant and potentially disruptive, yet they are largely unexplored. In flat-optics scenarios, LWs could effectively route light signals in a precise, versatile, and potentially reconfigurable fashion, while maintaining robustness in the presence of bends and possible imperfections. Moreover, they exhibit inherent chiral-coupling properties that can be of interest in quantum-computing scenarios, and very high field enhancements that can find interesting applications in nonlinear optics and chemical/biological sensing. Finally, their radiative regimes can lead to novel types of microwave and THz antennas of interest for future communication systems.
Although LWs can be viewed as the 1-D counterparts of surface waves, their theoretical and experimental study is considerably more complex, and requires a completely new arsenal of tools. Specifically, there is a crucial need for physically incisive parameterizations in order to identify suitable material platforms and operating frequencies from microwaves to optics.
The objective of the project is to provide novel approaches to theoretical and numerical modeling of LW-based devices operating at microwave and THz frequencies, including the unexplored scenarios of reconfigurable platforms and radiative regimes. The expected outcomes are important at both the fundamental-science and technological levels, as they will provide a more complete understanding of the physics underlying LWs, their range of existence, and potential impact in enabling technologies ranging from sensing to wireless communications.
This project addresses key modeling and design issues related to LWs, a novel kind of linear waveguide with potentially disruptive technological breakthroughs in areas ranging from sensing to wireless communications and THz systems (Fig. 7).
Confining light at subwavelength scales is of fundamental importance in order to enhance its interactions with matter, which is crucial in many applications, including chemical and biological sensing [2], nonlinear [3], and quantum [4] electromagnetics. By comparison with conventional SWs [1], LWs bring about an additional dimension and degrees of freedom, enabling wave-energy localization and transport along 1-D tracks. These properties are inherently suited for flat optics [35], which promises to replace the bulky and expensive components (lenses, alignment systems) of conventional refractive optics with planar, easy to fabricate, CMOS-compatible MTSs. This is one of the expanding frontiers in the fields of metamaterials, which is witnessing an exponential growth in terms of scientific interest (over 4000 papers published in 2019) and an increasing impact on the economy and society at large [36].
The strong confinement and topological-like protection [11] render LW propagation robust with respect to imperfections and fabrication tolerances. Their inherent spin-momentum locking properties [10,16] enable polarization-dependent propagation effects that can be of interest for quantum optics and computing [14]. Their peculiar near-field structure can give rise to anomalous optical forces [18] of interest for microfluidics and micro-optomechanical systems. Moreover, just like SWs, LWs are inherently suited for chemical and biological sensing. Indeed, sensors based on surface-plasmon resonance have recently shown promising potentials for rapid virus detection and COVID-19 diagnosis [37], and LWs may provide the additional benefits of stronger field localization and enhancement.
As recently shown [16], the LW concept has also intriguing ramifications in the emerging field of non-Hermitian optics [17], which elevates losses from detrimental, second-order effects to instrumentally interplaying with gain. The field of non-Hermitian flat optics, with the MTS constitutive parameters spanning the entire complex plane, looks very promising and is mostly uncharted territory.
Advancement potential
In spite of its great interest and potentials, the field of LWs remains still in its infancy, with many of its possibilities as largely unexplored.
On one hand, the modeling tools developed in this project are expected to overcome the limitations of the existing approaches to the dispersion analysis of LWs; this will have a direct impact on the applications, since an accurate modal characterization of LWGs is the basis for the design of any guided-wave or radiating device derived thereof.
On the other hand, the study of leaky regimes has not been addressed so far in the literature, with the only exception of [16]. It will provide the groundwork for the design of extremely thin, flat, or even conformal antennas and arrays based on uniform or periodic LWGs operating in leaky regimes. Such LLWAs would be characterized by a number of attractive features, such as simple feeding structures, radiated beams frequency scannable in elevation, and high versatility in beam shaping via longitudinal adiabatic modulation of the waveguide parameters [32]. Such modulation, in particular, may be facilitated by the large number of physical and geometrical parameters characterizing the various types of MTSs used as LWG constituents. LLWAs would thus allow for synthesizing different classes of radiation patterns, with applications ranging from local area networks to wireless power transfer while combining practical benefits of low profile and lightweight to the user.
35. F. Capasso, Nanophotonics 7, 953, 2018
36. Metamaterial Market by Material Type, Application, Vertical and Geography, https://tinyurl.com/y5sl6xy7
37. G. Qiu et al., ACS Nano 14, 5268, 2020