(235c) Novel Optical Sensors Utilizing Surface Plasmon Enhanced Transmission Via Diffraction Gratings with Controlled Nanostructured Topology to Probe Non-Absorbing and Absorbing Adsorbates

The interaction of light with nanostructures, particularly those containing noble metals, can produce a variety of unique and useful optical phenomena. Optical nanostructured sensors based on SPR have several advantages in comparison to those based on fluorescence-labeled agents in detection assays. Examples of benefits from SPR-based sensors include high sensitivity, exclusion of expensive and complex labeling processes, and in-situ real time monitoring of adsorption processes at the interface. Enhanced transmission through nanostructured gratings is one method SPR can be applied for sensing.

Grating-based optical sensors utilizing surface plasmon enhanced transmission have gained increased research interest in comparison to traditional prism-coupled configurations. The collinearly simple yet precise setup of spectrophotometer systems for grating-based sensors have superior potential for developing portable devices; traditional prism-coupled configurations require more complicated alignments. In this work, we demonstrate the enhanced transmission SPR via well-defined and controlled nanostructures of diffraction gratings. Not only were observed lager enhanced transmissions, but various matching conditions of SPR can be fine tuned by simple rotation of samples with respect to incident p-polarized light.

We also developed a novel gold-coated chirped grating in which various topologies are incorporated into a single diffraction grating. Notably, this grating demonstrates the impact of various local topologies, i.e. combinations of pitch and amplitude, on the responses of enhanced transmissions. In addition, we demonstrate the capabilities of this novel chirped grating as a substrate for high-performance optical sensing.

Since many adsorbates absorb light, a direct method to probe the existence and thickness of absorbing films utilizing SPR is necessary. Although SPR-based sensors mainly probe the thickness of non-absorbing adsorbates, there is interest in exploring the effect of molecular resonances (MR) in absorbing adsorbates on the SPR response. We also demonstrate the ability of SPR-based grating sensors to probe the interaction between MR and SPR. Notably, an enhanced transmission peak near the proximity of the Q band of a chromophore was observed. In other words, this enhanced transmission peak implies “transparent metal induced by MR and SPR”. In summary, this work demonstrates the utility and versatility of grating-based SPR sensing techniques.