Photonic Time-Crystals (PTCs) are materials in which the refractive index varies periodically and abruptly in time. This medium exhibits unusual properties such as momentum bands separated by gaps within which waves can be amplified exponentially, extracting energy from the modulation. This article provides a brief review on the concepts underlying PTCs, formulates the vision and discusses the challenges.
Interactions of large-amplitude relativistic plasma waves were investigated experimentally by propagating two synchronized ultraintense femtosecond laser pulses in plasma at oblique crossing angles to each other. The electrostatic and electromagnetic fields of the colliding waves acted to preaccelerate and trap electrons via previously predicted, but untested injection mechanisms of ponderomotive drift and wake-wake interference. High-quality energetic electron beams were produced, also revealing valuable new information about plasma-wave dynamics.
Recent advances in ultrafast, large-modulation photonic materials have opened the door to many new areas of research. One specific example is the exciting prospect of photonic time crystals. In this perspective, we outline the most recent material advances that are promising candidates for photonic time crystals. We discuss their merit in terms of modulation speed and depth. We also investigate the challenges yet to be faced and provide our estimation on possible roads to success.
Epsilon-near-zero (ENZ) materials that operate in the spectral region where the real part of the permittivity crosses zero have recently emerged as a promising platform for all-optical switching because of the large, optically induced reflectance and transmittance modulation they offer at ultrafast speeds. To gain insights into the ENZ modulation, this study focuses on the reflectance and transmittance modulation of commonly used ENZ switching schemes and applies an analytical framework both for intraband and interband pumping. We consider the effects of the wavelength, the angle, and the probe polarization on the modulation amplitude for different configurations, specifically highlighting the locations of the maximum reflectance/ transmittance modulation and the maximum refractive index modulation, which often occur at different wavelengths around the ENZ point. We find that the maximum modulation, while proximal to the ENZ point, can occur away from the ENZ point and even slight deviations can result in seemingly anomalous modulation behavior. The occurrence of resonances at the ENZ region for ultrathin films further increases the modulation strength. This work paves the path for practical and effective all-optical modulation approaches employing ENZ materials, and will help design the best experimental configurations for future material studies and nonlinear optical experiments employing ENZ materials.
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