The cavity 102 defines an empty volume formed in the insulator 108 has its walls defined by the insulator 108 and may extend through either (or both) the first electrode 106 or the second electrode 104, in which case the first electrode and/or second electrode also define the walls of the cavity 102. The cavity 102 is preferably cylindrical and has a diameter of 0.1 μm-1 mm. More preferably, the diameter ranges from 0.1 μm-500 μm, 1 μm-100 μm, or 100 μm-500 μm. The cavity 102 will be filled with a gas that contacts the cavity walls, fills the entire cavity 102 and is selected for its breakdown voltage or light emission properties at breakdown. Light is produced when the voltage difference between the first electrode 106 and the second electrode 104 creates an electric field sufficiently large to electrically break down the gas (nominally about 104 V-cm). This light escapes from the microcavity 102 through at least one end of the cavity 102.