Photonics of Time-Varying Media (2111.08640v2)

Abstract: Time-varying media have recently emerged as a new paradigm for wave manipulation, thanks to thesynergy between the discovery of novel, highly nonlinear materials, such as epsilon-near-zero materials, and the questfor novel wave applications, such as magnet-free nonreciprocity, multi-mode light shaping, and ultrafast switching. Inthis review we provide a comprehensive discussion of the recent progress achieved with photonic metamaterials whoseproperties stem from their modulation in time. We review the basic concepts underpinning temporal switching and itsrelation with spatial scattering, and deploy the resulting insight to review photonic time-crystals and their emergentresearch avenues such as topological and non-Hermitian physics. We then extend our discussion to account for spa-tiotemporal modulation and its applications to nonreciprocity, synthetic motion, giant anisotropy, amplification andother effects. Finally, we conclude with a review of the most attractive experimental avenues recently demonstrated,and provide a few perspectives on emerging trends for future implementations of time-modulation in photonics.

• The paper introduces ultrafast switching and nonreciprocal wave propagation through temporal modulation of photonic metamaterials.
• It details the creation and implications of photonic time-crystals, including momentum band-gaps and robust topological edge states.
• The research demonstrates experimental advances using metasurfaces and nanostructures, paving the way for tunable, magnet-free optical devices.

Overview of "Photonics of Time-Varying Media"

The paper "Photonics of Time-Varying Media" provides a comprehensive review of recent advancements in the field of photonic metamaterials that exhibit temporal modulation of their properties. This work explores the fundamental concepts and applications of time-varying media, offering insights into nonreciprocal wave propagation, photonic time-crystals, and the implications of spatiotemporal modulation.

Fundamental Concepts and Temporal Modulation

The authors begin by discussing the basic principles underpinning electromagnetic time-switching and its relationship with spatial scattering. Temporal modulation involves changing the properties of a material over time, leading to novel wave manipulation techniques such as ultrafast switching and light shaping. The research explores epsilon-near-zero materials and other nonlinear materials that enable significant modulation of wave propagation, highlighting their effectiveness in achieving magnet-free nonreciprocity and other advanced optical phenomena.

Photonic Time-Crystals

The exploration of photonic time-crystals (PTCs) is a central theme of the paper. These are systems periodically modulated in time, leading to phenomena analogous to energy band-gaps in spatially periodic structures. Temporal modulation results in the creation of k-gaps (momentum band-gaps), where waves are amplified due to parametric pumping. The paper analyses mechanisms of parametric amplification in detail, outlining how this can be harnessed for energy localization and amplification applications.

Topological properties in PTCs are also examined, showcasing how they can support edge states at temporal boundaries, analogous to spatial topological insulators. This implies potential for robust, defect-tolerant edge modes in temporal frameworks, a prospect with significant implications for future photonic devices.

Spatiotemporal Modulation and Nonreciprocal Devices

The interplay of spatial and temporal modulation introduces a range of new phenomena. The paper discusses how spatiotemporal crystals, characterized by traveling-wave-like perturbations, enable the design of nonreciprocal devices without the need for magnetic bias. This is crucial for developing devices such as optical isolators and circulators, which are pivotal in advanced communication systems.

The authors further consider synthetic motion and its application to graphene drifts and electromagnetic drag effects, which mimic phenomena observed in moving media. The homogenization theory provided in this context lays a foundation for understanding complex bianisotropic responses arising from simultaneous spatial and temporal modulations.

Experimental Realizations and Challenges

In exploring practical implementation, the paper reviews various experimental platforms, including metasurfaces and nanostructured materials, that have been developed to test the theories of time-varying photonics. The challenges of modulating materials at optical frequencies and achieving sufficient modulation depths are noted, along with emerging materials such as epsilon-near-zero media that show promise for modulating refractive index changes efficiently under ultrafast excitation.

Implications and Future Directions

The research showcases the breadth of applications enabled by time-varying photonics, from enhanced energy manipulation to the development of tunable and reconfigurable photonic devices. The theoretical framework established opens avenues for further exploration in optical computing, communication technologies, and developing topological insulators in time dimensions. Prospective advances in materials science, such as developing new high-mobility materials with fast carrier dynamics, are anticipated to drive future breakthroughs in this field.

In summary, this paper provides an expansive, detail-rich exploration of time-modulated photonics, offering both a robust theoretical foundation and insights into practical applications. It encourages further exploration into this rapidly evolving domain, with potential implications across various technological and scientific fields.