Spectral, amplitude and phase sensitivity of a plasmonic gas sensor in a metallic photonic crystal slab geometry: Comparison of the near and far field phase detection strategies

Abstract

Using finite-difference time-domain method, we investigate phase and amplitude reflective properties of a metallic photonic crystal slab comprising a two-dimensional array of gold disks placed on top of a thin gold film resting on a dielectric substrate. Photonic crystal slab overcladding is a gaseous analyte. Throughout the paper we discuss prospectives of application of such photonic crystal slabs to sensing of changes in the gas refractive index. We start by studying the field distributions and spectral positions of two types of surface plasmons supported by such PhCs as a function of the photonic crystal lattice period, the gold film thickness, and the gold disk size. First, we find that the spectral positions of plasmon peaks depend almost linearly on the photonic crystal lattice constant, which is a manifestation of the delocalized nature of a plasmon extending over the PhC lattice. This is possible due to connectivity between metallic disks via an underlying thin metallic film. Second, we find that the width of plasmonic peaks is highly sensitive to the relative size of the metallic disks compared to the lattice period. This can be well explained via interaction between the localized plasmons situated at the disc edges in the adjacent unit cells. We therefore find that plasmons in the metallic PhC slabs featuring metallic discs connected by a metallic film exhibit both strong local and non-local properties. Third, we find that, generally, there are two types of plasmons supported by such metallic PhCs. One type is a plasmon with a large fraction of its field located in the gas overcladding, which is most suitable for sensing. In contrast, another plasmon type has its field concentrated at the interface between a thin metal film and a substrate; such plasmons are only weakly sensitive to the refractive index of the sensor overcladding, and, therefore, can serve as convenient references for the sensor measurements. Finally, we report sensor sensitivities to changes in the real part of the gas refractive index when using amplitude and phase-based detection strategies. We start by demonstrating that when measured in the sensor far field, the phase sensitive detection provides much lower detection limit ( 1.8 × 1 0 − 6 RIU) compared to that of the amplitude-based detection ( 1.5 × 1 0 − 4 RIU). We then find that sensor phase and amplitude sensitivities can be considerably enhanced when performing measurements using point detectors placed in the near field of a sensor. Particularly, we have established that both phase and amplitude sensitivities are maximal when measured in the sensor near field along the normal to the sensor surface going through the point of the highest symmetry of a photonic crystal unit cell ( Γ point). Sensor resolution as high as 2.2 × 1 0 − 7 RIU for the phase-based detection and 7.4 × 1 0 − 5 RIU) for the amplitude-based detection are found, with the most dramatic enhancement observed for the phase detection approach. We believe that experimental verification of the sensor sensitivity enhancement using phase detection in the sensor near field can be accomplished by using scanning near-field optical microscopy.