Another form of a pillar cavity appears very similar to that of Figure 1. However, here the mode propagates up and down in the pillar, and the resonant wavelength is always below the cutoff wavelength in the pillar waveguide [12, 13]. The mirror at the pillar top is formed by the silver contact. (a) In one case, the mirror at the bottom of the pillar is formed by a thin silver film. (b) In another case, the bottom mirror is formed by a Bragg reflector, similar to a traditional VCSEL. The red indicates the active region.
