One-dimensional photonic crystals are commonly used as high-reflectivity mirrors, Bragg gratings, or for constructing vertical-cavity surface-emitting lasers (VCSELs). By extending periodicity to the spatiotemporal domain, spatiotemporal photonic crystals (STPCs) have enabled new methods of wave manipulation, supporting spatially decaying and temporally growing eigenstates. However, their scattering characteristics and precise gain control remain underexplored. Additionally, these non-Hermitian systems, which rely on parametric processes, are expected to exhibit distinct propagation phenomena. In this study, we constructed STPCs by applying sinusoidal perturbations in both space and time. Our findings reveal that introducing temporal perturbations can simultaneously induce the transitions of photonic bandgap-type and spontaneous PT-symmetry breaking. Moreover, the coherent effects between spatial and temporal perturbations can manipulate gain, enabling tunable devices such as cloaking-amplifiers.
Recent breakthroughs in the field of non-Hermitian physics present unprecedented opportunities, from fundamental theories to cutting-edge applications such as multimode lasers, unconventional wave transport, and high-performance sensors. The exceptional point, a spectral singularity widely existing in non-Hermitian systems, provides an indispensable route to enhance the sensitivity of optical detection. However, the exceptional point of the forementioned systems is set once the system is built or fabricated, and machining errors make it hard to reach such a state precisely. To this end, we develop a highly tunable and reconfigurable exceptional point system, i.e., a single spoof plasmonic resonator suspended above a substrate and coupled with two freestanding Rayleigh scatterers. Our design offers great flexibility to control exceptional point states, enabling us to dynamically reconfigure the exceptional point formed by various multipolar modes across a broadband frequency range. Specifically, we experimentally implement five distinct exceptional points by precisely manipulating the positions of two movable Rayleigh scatterers. In addition, the enhanced perturbation strength offers remarkable sensitivity enhancement for detecting deep-subwavelength particles with the minimum dimension down to 0.001λ (with λ to be the free-space wavelength).
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