To illustrate the functionality of polaritonic devices, consider the hypothetical circuit in Fig. 2 (right). The optical excitation pulses that generate phonon-polaritons, in the top left and bottom right of the crystal, enter normal to the crystal face (into the page). The resulting phonon-polaritons will travel laterally away from the excitation regions. Entrance into the waveguides is facilitated by reflective and focusing structures. Phonon-polaritons are guided through the circuit by terahertz waveguides carved into the crystal. Circuit functionality resides in the interferometer structure at the top and the coupled waveguide structure at the bottom of the circuit. The latter employs a photonic bandgap structure with a defect (yellow) that could provide bistability for the coupled waveguide.