Defect-Symmetry Engineered Gyroidal Photonic Crystals Achieve Monotonic Modulation of Bandgap Width With a Stable Mid-Gap.

Three-dimensional (3D) photonic crystals with a photonic bandgap (PBG) facilitate strong control of light propagation. In many applications, PBG width needs to be systematically manipulated while the system needs to be operated at predetermined wavelengths. Conventional methods for such situations require the coordinated modulation of multiple factors, which complicates practical implementation. Achieving monotonic tuning of the PBG width while maintaining a stable central frequency through single-parameter modulation remains challenging. Here, inspired by the twin grain boundary discovered in natural gyroidal networks, we construct a series of gyroid-derived tubular networks incorporating twin mirror symmetry as a periodic design element with different gap distances between the twin mirror planes. Both calculations and experimental validation demonstrated that these structures exhibit monotonic tunability in PBG width with increasing gap distance between the twin mirror planes, while the central frequency of the PBG maintains stable. Electric field analysis indicates that the introduced mirror symmetry with different gap distances changes the distribution of electrical field from discrete mode into continuous mode gradually, contributing to the PBG monotonic phenomenon. This work opens a route for manipulating the spectral position and width of PBG in 3D photonic crystals separately and highlights the potential of structural defect engineering for photonic crystal design and fabrication.