Achieving bi-anisotropic coupling through uniform temporal modulations without inversion symmetry disruption.

Temporal modulations provide a new approach for realizing metamaterials. In this study, through the imposition of uniform temporal modulations, we achieve two types of reciprocal bi-anisotropic metamaterials. Notably, these achievements do not rely on any spatial modulation, preserving inversion symmetry at any instantaneous time. This stands in sharp contrast to the scenario of traditional bi-anisotropic metamaterials, where the disruption of inversion symmetry by spatial arrangements is necessary. Conditions for realizing nonzero bi-anisotropic coupling are discussed and verified through full-wave simulations. Our work will stimulate research in the field of temporal bi-anisotropic metamaterials, as well as the application of temporal modulations in manipulating photonic spin angular momentum.

Nonlocal effective medium theory for phononic temporal metamaterials.

We have developed a nonlocal effective medium theory (EMT) for phononic temporal metamaterials using the multiscale technique. Our EMT yields closed-form expressions for effective constitutive parameters and reveals these materials as reciprocal media with symmetric band dispersion. Even without spatial symmetry breaking, nonzero Willis coupling coefficients emerge with time modulation and broken time-reversal symmetry, when the nonlocal effect is taken into account. Compared to the local EMT, our nonlocal version is more accurate for calculating the bulk band at high wavenumbers and essential for understanding nonlocal effects at temporal boundaries. This nonlocal EMT can be a valuable tool for studying and designing phononic temporal metamaterials beyond the long-wavelength limit.

Lorentzian dispersive antireflection temporal coatings with multiple time durations.

As the temporal counterparts of traditional antireflection coatings, antireflection temporal coatings (ATCs) provide a novel approach to eliminate reflections by employing two-step temporal modulations. The interval between these two temporal modulation steps is called the time duration of the ATC. In this Letter, we explore ATCs utilizing Lorentzian dispersive media through an extended temporal transfer matrix method, and we discover that they exhibit diverse time durations and offer the potential for enhanced transmission. On one hand, the Lorentzian dispersive ATC can function as a temporal quarter-wave impedance transformer, similar to nondispersive ATCs. In this scenario, the time durations are consistently shorter than those of nondispersive ATCs, gradually converging to a constant value as the dielectric constant of the output layer approaches infinity. On the other hand, by finely tuning the parameters of the Lorentzian dispersive temporal coating, reflections can also be accidentally eliminated, which is not achievable with nondispersive temporal coatings. Consequently, Lorentzian dispersive ATCs offer additional time durations compared with nondispersive ATCs. Furthermore, Lorentzian dispersive ATCs with different time durations lead to distinct transmission characteristics. In certain cases, they can even enhance transmissions, a feat unattainable for nondispersive ATCs. These Lorentzian dispersive ATCs are feasible in the gigahertz and even terahertz regimes.