In order to get lasing inside our bacteria we constructed an optical cavity inside and around the bacteria. An optical cavity is an arrangement of optical components which traps the light inside in a closed path (standing waves), where the light can resonate. For most normal lasers this is accomplished by two mirrors (figure 1A). In our project PHB granules are formed inside cells and by total internal reflection the light should be trapped inside them. Another way to form an optical cavity is by encapsulation of the bacteria in a material with a higher refractive index than the inside of the cell. This can be achieved by the enzyme silicatein, which polymerizes monomers such as silicic acid and tin dioxide, creating the desired reflective layer (figure 1B). In order to trap the light inside these cavities it is necessary that the light ‘fits’ inside. Therefore standing waves have to be formed within the cavity. Here we will determine what the minimal required size of these cavities is to fit exactly one wavelength inside. This will not give us a definite answer whether we will get lasing, but a very strict lower bound of the required cavity size.

In both methods of capturing light (PHB granules and covering the cell with silica), the light will become trapped by whispering gallery mode (WGM) resonance (Humar et al, 2015).