We present a new structure that combines a metal–dielectric–metal sandwich with a periodic structure to form a plasmon polariton photonic crystal. Three-dimensional finite-difference time-domain simulations show a clear bandgap in the terahertz regime. We exploited this property by adding a defect to the crystal, which produces a cavity with a quality factor of 23.3 at a wavelength of $3.45\mu \mathrm{m}$ . Despite the small Q factor, the ultrasmall sensing volume of 15 zeptoliters produces an extremely large Purcell constant of $4.8\times {10}^6$ . Compared to photonic crystals with similar Purcell constant, the bandwidth is several orders of magnitude larger, or about $7\mathrm{THz}$ , ensuring high tolerances to manufacturing parameters, and environmental changes, as well as a high specificity owing to the possibility of broadband spectral fingerprint detection.