Dielectric Multilayer Supporting Bloch Surface Waves for Quantitative Optical Sensing Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date5 Dec 2022


Modern experimental demands in analytical science have fueled renewed interest in improving spectroscopy's qualitative and quantitative capabilities. Also, the emergence of plasmonics to detect and manipulate the signal at optical frequencies in the nanoscale has opened new strategies and opportunities in optical sensing. In this regard, we introduce materials and experimental approaches to advance spectroscopy's quantitative detection.

In this study, we propose using all-dielectric multilayer structure material supporting Bloch surface wave (BSWs) and our experimental approach in advancing quantitative Raman and ellipsometry optical sensing techniques. However, an all-dielectric multilayer structure that supports Bloch Surface Waves (BSWs) offers intriguing alternatives for optical sensing without ohmic losses inherent in the conventional SPR technique. This fascinating advantage gives it an edge over other materials for Surface-Enhanced Raman Scattering (SERS) and label-free sensing applications. We adopted the prism coupling based on the Total internal reflection (TIR) technique for the BSWs excitation, which provides evanescent field matching and enables optimal signal collection close to the prism surface.

The optical characterization and electric field intensity distribution from the BSWs excitation were studied with MD 2000 Spectroscopic Ellipsometry (SE) and Finite Difference Time Domain (FDTD), respectively. In the optical characterization from the air side of the prism, the optical constants of material components in the dielectric multilayer show purely non-absorbing and transparent over the visible and infrared regions, as expected. Similarly, the model used in studying the BSWs shows the optimum incidence angle for BSWs excitation was achieved at ~46.61 degrees with bandwidth dependence. Additionally, the BSWs can only be excited in the s-polarization state of the collimated light source. On the other hand, the FDTD simulation shows the BSWs give an enhancement factor of ~ 103. However, to improve the enhancement factor, the BSW was successfully supported with Localized Surface Plasmons (LSPs) using gold nanoparticles as a good strategy for enhancing optical signals in Raman scattering. The LSPs excitation by BSWs provides a more significant enhancement factor of ~108 with the conductive Ta2O5 as the top layer of the multilayer structure.

Prototype spectroscopy was developed to perform a combined detection where calibration of the setup with resolution down to ~10-3 degrees was achieved, limited by 100 𝜇m pixel size of the CCD. After that, dual sensing spectroscopy, combining Localized surface plasmons (LSPs) excitation over an excitation area of ~ 10 mm2 with BSWs resonance for qualitative and quantitative Raman information, respectively, was successfully demonstrated. This dual spectroscopy uses the enhanced optical field from LSPs excitation for Surface Enhanced Raman Spectroscopy (SERS) which serves as specificity detection, while BSWs provided the quantitative SERS information. The proposed detection scheme was primarily applied to detect the organic dye methylene blue (MB), providing a sensitivity of ~10-2 °/nM from BSWs, while a SERS Limit of Detection (LOD) of ~1.50 × 10-2 nM was achieved from the LSPs excitation. The combined spectroscopy enables real-time, quantitative SERS information, potentially opening up alternative analytical strategies for quantitative Single Molecule SERS.

Additionally, we advance our detection strategy to Localized Surface Plasmon-Spectroscopic Ellipsometry (LSP-SE). They provide detection from enhanced phase (∆) and amplitude (Ψ) ellipsometric signals leading to BSWs excitation from a collective scattering of gold nanoparticles. In combination with phase measurement, they provide an alternative platform for label-free detection and increased sensitivity of plasmonic-based SE detection. The optical responses from the gold nanoparticles due to LSPs scattering effects provided ~4x enhanced phase detection sensitivity compared to the detection without LSPs effect. This sensing approach offers non-destructive, label-free SE-LSP detection spectroscopy as a simple but powerful strategy with potential applications in the biomedical field.

In conclusion, the study shows the potential of our all-dielectric multilayer material supporting Bloch surface waves to provide a promising approach for qualitative and quantitative detection, thereby opening more opportunities in Raman and ellipsometry.