Optoelectronic devices and displays use advanced materials to emit, reflect, modulate, or refract light. Instead of looking at the material properties, the focus will be how the geometry plays an important role on the optical properties. The layer thickness, the material anisotropy or the periodicity of a structure can determine the emission, absorption or reflection characteristics. Optimizing the position of emitting molecules in the OLED stack can increase the external quantum efficiency. Periodic liquid crystal structures can generate diffraction with efficiency close to 100%. Chiral liquid crystal self-assembles into a periodic structure, with high reflectivity in the photonic band gap. This effect can be used to build laser cavities when a light emitting layer is placed between two mirrors. Chiral liquid crystal layers are also used to couple light in and out of waveguiding semi-transparent glasses for augmented reality displays.