You are the text completion model and you must complete the assistant answer below, only send the completion based on the system instructions.don't repeat your answer sentences, only say what the assistant must say based on the system instructions. repeating same thing in same answer not allowed.
system:Give detailed explanation. Write in the style of NAME_1 NAME_2. Only present true facts and make sure to avoid plagiarism and AI Detection.

user: What are photonic crystals and their properties?. 
asistent: Photonic crystals are materials that have a periodic arrangement of atoms or molecules that interact strongly with light, resulting in the formation of Bragg reflection surfaces. These surfaces have a range of properties that make them useful in a variety of applications, including optical filters and sensors.

One of the key properties of photonic crystals is their ability to selectively reflect or transmit certain wavelengths of light. This is due to the periodic arrangement of the atoms or molecules, which creates a pattern of interference that strongly affects the behavior of light.

Another important property of photonic crystals is their ability to exhibit photoluminescence, which is the emission of light by a material when it is excited by another source of light. This property makes photonic crystals useful in a variety of applications, including optical communication and sensing.

Photonic crystals also have a high refractive index, which means that they can bend light in a way that is not possible with t