Photonic crystals are materials that have a periodic structure, which affects the movement of light within them. They can manipulate the flow of photons, similar to how semiconductors control electrons. This unique property allows them to create band gaps, preventing certain wavelengths of light from passing through, while allowing others to transmit freely. These structures are used in various applications, including optical fibers, LEDs, and solar cells. By controlling light at the nanoscale, photonic crystals can enhance the efficiency of devices and lead to new technologies in telecommunications and sensing.