Switchable 3D Photonic\nCrystals Based on the Insulator-to-Metal\nTransition in VO<sub>2</sub>

Photonic crystals (PhCs) are optical structures characterized\nby\nthe spatial modulation of the dielectric function, which results in\nthe formation of a photonic band gap (PBG) in the frequency spectrum.\nThis PBG blocks the propagation of light, enabling filtering, confinement,\nand manipulation of light. Most of the research in this field has\nconcentrated on static PhCs, which have fixed structural and material\nparameters, leading to a constant PBG. However, the growing demand\nfor adaptive photonic devices has led to an increased interest in\nswitchable PhCs, where the PBG can be reversibly activated or shifted.\nVanadium dioxide (VO<sub>2</sub>) is particularly notable for its\nnear-room-temperature insulator-to-metal transition (IMT), which is\naccompanied by significant changes in its optical properties. Here,\nwe demonstrate a fabrication strategy for switchable three-dimensional\n(3D) PhCs, involving sacrificial templates and a VO<sub>2</sub> atomic\nlayer deposition (ALD) process in combination with an accurately controlled\nannealing procedure. The resulting VO<sub>2</sub> inverse opal (IO)\nPhC achieves substantial control over PBG in the near-infrared (NIR)\nregion. Specifically, the synthesized VO<sub>2</sub> IO PhC exhibits\nPBGs near 1.49 and 1.03 μm in the dielectric and metallic states\nof the VO<sub>2</sub> material, respectively, which can be reversibly\nswitched by adjusting the external temperature. Furthermore, a temperature-dependent\nswitch from a narrow-band NIR reflector to a broad-band absorber is\nrevealed. This work highlights the potential of integrating VO<sub>2</sub> into 3D templates in the development of switchable photonics\nwith complex 3D structures, offering a promising avenue for the advancement\nof photonic devices with adaptable functionalities.