Resonance shifts in SPR curves of nonabsorbing, weakly absorbing, and strongly absorbing dielectrics

The resonance shifts (i.e., the resonance angle and the reflectance minimum) in surface plasmon resonance (SPR) curves due to the complex refractive index and/or thickness variations of dielectric films were investigated. For both, nonabsorbing and absorbing dielectrics, the resonance angle shifts linearly with the refractive index and/or thickness variations. The reflectance minimum of the nonabsorbing dielectric does not change as the resonance angle shifts. For an absorbing dielectric, the direction of the reflectance change depends strongly on the magnitude of the absorption and thickness of the metal film. The reflectance minimum of the sensor with a thin metal film decreases before increasing while that of the sensor with a thick metal film continuously increases as the absorption of the dielectric film increases. The phenomena were theoretically explained based on the SPR-generated evanescent field at the metal/dielectric interface associated with the optical properties of the sensor architecture.