Photonic cavities are an essential part of many modern optical devices, from laser pointers to microwave ovens. Just as we can store water in a tank and create standing waves on the surface of the water, we can confine light in a photonic resonator whose walls are strongly reflective. Just as water surface waves depend on the geometry of the tank (shape, depth), specific optical modes can be created in a photonic cavity whose properties (colour and spatial distribution of intensity) can be tuned by changing the dimensions of the cavity. When the size of the cavity is very small—much smaller than the wavelength of the light confining it (nano-cavity in the case of visible light)—an intensification effect of the light is produced that is so strong that it influences the electrons on the walls of the cavity. A mixture between photons and electrons is then produced, giving rise to hybrid modes between light and matter known as plasmons.