Nanocavity plasmonic device for ultrabroadband single molecule sensing

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 μm. Despite the small Q factor, the ultrasmall sensing volume of 15 zeptoliters produces an extremely large Purcell constant of 4.8 × 10 ⁶. Compared to photonic crystals with similar Purcell constant, the bandwidth is several orders of magnitude larger, or about 7 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.