Time-Varying Materials for Extreme Control of Electromagnetic Waves (Invited)

Conventional electromagnetic structures are typically engineered in three-dimensional space. While effective, they face inherent limitations, including electromagnetic reciprocity and energy conservation, which pose significant challenges to their functionalities. Overcoming these constraints demands more than traditional material shape or structural engineering. Recent interest in time-varying media has garnered substantial attention in the electromagnetic and optical community. The introduction of time as an additional degree of freedom in metamaterials engineering has yielded remarkable advancements, especially in metasurfaces. This breakthrough enables the bypassing of many fundamental constraints associated with conventional time-invariant materials. In this presentation, we will share the recent progress achieved in our group in the field of time-varying materials. Firstly, we will discuss the conditions necessary to break reciprocity through uniform modulation of material properties in time. We have established that nonreciprocal wave propagation is attainable only when the material itself exhibits bianisotropy. Secondly, we will demonstrate that in a passive and time-invariant multiport system, delivering power from each port to a single output port with full efficiency is traditionally forbidden by energy conservation laws. To circumvent this, we introduce time-modulation to the system. With time-varying properties, it becomes feasible to efficiently direct power from each direction to the output port, achieving perfect power combining of waves irrespective of incident phases. Finally, I will introduce the concept of two-dimensional photonic time crystals, leveraging a time-varying metasurface platform. This system manifests a momentum bandgap in k-space, where surface wave energy experiences exponential growth. Additionally, I will discuss how a resonant metasurface can be employed to achieve an infinitely wide momentum bandgap.