Spectral Causality and the Scattering of Waves

Abstract

Causality - the principle stating that the output of a system cannot temporally precede the input - is a universal property of nature. Here, we show that analogous input-output relations can also be realized in the spectral domain by leveraging the peculiar properties of time-modulated non-Hermitian photonic systems. Specifically, we uncover the existence of a broad class of complex time-modulated metamaterials which obey the time-domain equivalent of the well-established frequency-domain Kramers-Kronig relations (a direct consequence of causality). We find that, in the scattering response of such time-modulated systems, the output frequencies are inherently prohibited from spectrally preceding the input frequencies, hence we refer to these systems as 'spectrally causal'. We explore the consequences of this newly introduced concept for several relevant applications, including broadband perfect absorption, temporal cloaking of an 'event', and truly unidirectional propagation along a synthetic dimension. By emulating the concept of causality in the spectral domain and providing new tools to extend the field of temporally modulated metamaterials ("chrono-metamaterials") into the complex realm, our findings may open unexplored opportunities and enable relevant technological advances in various areas of photonics and, more broadly, of wave physics and engineering.

arXiv comment21 pages, 4 figures and 1 supplementary figure

References in corpus (20)

• Topological Photonics
• Synthetic Dimension in Photonics
• Topological quantum matter in synthetic dimensions
• Synthetic dimensions in integrated photonics: From optical isolation to 4D quantum Hall physics
• Observation of arbitrary topological windings of a non-Hermitian band
• Low-loss composite photonic platform based on 2D semiconductor monolayers
• Fresnel drag in space-time modulated metamaterials
• Broadening the Bandwidth of Metamaterial Cloaks with Non-Foster Metasurfaces
• Non-Reciprocal gain in non-Hermitian time-Floquet Systems
• Optical time reversal from time-dependent Epsilon-Near-Zero media
• Optical Isolation based on Space-time Engineered Asymmetric Photonic Bandgaps
• Parametric amplification and bidirectional invisibility in PT-symmetric time-Floquet systems
• Wood Anomalies and Surface-Wave Excitation with a Time-Grating
• Time-Varying Materials in Presence of Dispersion: Plane-Wave Propagation in a Lorentzian Medium with Temporal Discontinuity
• On the electromagnetic properties of active media
• A Generalization of the Kramers-Kronig Relations for Linear Time-Varying Media
• Temporal Switching to Extend the Bandwidth of Thin Absorbers
• Bidirectional invisibility in Kramers-Kronig optical media
• Capturing Broadband Light in a Compact Bound State in the Continuum
• Optical Supersymmetry in the Time Domain

Cited by in corpus (10)

• Observation of Temporal Reflections and Broadband Frequency Translations at Photonic Time-Interfaces
• Tutorial: synthetic frequency dimensions in dynamically modulated ring resonators
• $\hslash \omega$ versus $\hslash \mathbf{k}$: Dispersion and Energy Constraints on Time-Varying Photonic Materials and Time Crystals
• An Archimedes' Screw for Light
• Tapered Photonic Switching
• Multiple actions of time-resolved short-pulsed metamaterials
• Sum Rule Bounds Beyond Rozanov Criterion in Linear and Time-Invariant Thin Absorbers
• Temporal Parity-Time Symmetry for Extreme Energy Transformations
• Acoustic Blackbody Absorption: Transcending Causality Limits through Instability-Induced Softness
• Broadband Low-loss Unidirectional Reflection On-chip with Asymmetric Dielectric Metasurface