Observation of Surface Plasmon Polaritons and Bloch Surface Waves in a Metal–Dielectric Photonic Crystal

The excitation of two different electromagnetic surface waves—surface plasmon polaritons (SPPs) and Bloch surface waves (BSWs)—is demonstrated in a 1-D metal–dielectric photonic crystal with numerical and experimental studies. The discussed structure consists of an Ag–TiO2 thin-film stack forming a metal–insulator–metal–insulator device. The thickness of the TiO2 layer placed between the metals is tested for two different values (50 and 300 nm), which also allows the excitation of guided-mode resonances. It is observed that BSWs in this metal–dielectric structure behave similar to the case of all-dielectric photonic crystals, whereas the SPP modes display similar properties to those excited in metal–insulator–metal cavities. The sensitivity of these surface states to variations in the refractive index (RI) of the external dielectric is characterized. For the case of the plasmonic modes, a maximum sensitivity of
${(}{7}.{2} \pm {0}.{3}{)} \times {10}^{{3}}$
nm/RIU was measured, while for the BSW the maximum sensitivity was
${(}{1}.{20} \pm {0}.{05}{)} \times {10}^{{2}}$
nm/RIU. Due to the large field enhancement and penetration on external media, these surface states display exceptional properties for application in optical sensors, and the presented results provide interesting possibilities in the design of novel sensing structures with a flexible selection of surface states for interrogation.