  We present a scheme for achieving thermal bistability based on the selective
coupling of three optical resonances. This approach requires one of the
resonant frequencies to be temperature dependent, which can occur in materials
exhibiting strong thermo-optic effects. For illustration, we explore thermal
bistability in two different passive systems, involving either a periodic array
of Si ring resonators or parallel GaAs thin films separated by vacuum and
exchanging heat in the near field. Such a scheme could prove useful for thermal
memory devices operating with transition times $\lesssim$ hundreds of
milliseconds.
