Spectroscopic characterization of 3D photonic crystals

We present a series of optical experiments on 3D photonic crystals suitable for advanced physics laboratory courses or senior capstone projects. The crystals are fabricated from a water solution of monodisperse polystyrene microspheres, with diameters of 200, 220, 370, and 430 nm, using the vertical deposition method. The microspheres self-assemble into a face-centered cubic structure, resulting in a pseudo-bandgap along the [111] direction. We study this bandgap through reflection and angle-resolved transmission spectroscopy using a fiber-optic-coupled USB spectrometer. Our experimental results align closely with predictions based on photonic band diagrams and the Bragg–Snell law, demonstrating the scale invariance of Maxwell's equations. These experiments provide an accessible and engaging platform for students to explore the physics of light interaction with periodic structures.