Anomalous scattering of polystyrene microparticles revealed by evanescent wave coupled cavity ringdown spectroscopy.

Forward scattering is an essential tool for investigating the colloidal suspension of polystyrene microspheres (PSMs). Evanescent wave coupled cavity ringdown spectroscopy (EW-CRDS) shows the anomalous extinction behavior in the limit of PSM particles that is much larger than the wavelength. EW-CRDS is a highly sensitive technique that improves weak absorption signals by enhancing the absorption path length, allowing for probing a range of processes at the solid/liquid interface by assessing the extinction properties. Additionally, it possesses the ability to sense a minimum absorbance of 1.2 × 10-6. EW-CRDS provides sufficient accuracy to detect correlation effects for PSMs in water at the interfacial region and their influence on forward scattering or extinction. In this work, we discuss the impact of volume fraction on the extinction of scatterers composed of microparticles in aqueous media. The findings of this study will contribute to a deeper understanding of the scattering dynamics in colloidal suspensions, with potential applications in various fields, including biology and metrology.

Real-Time Probing of Melamine-Induced Gold Nanoparticle Aggregation Kinetics via Evanescent-Wave Coupled Cavity Ring-down Spectroscopy.

Insight into the aggregation kinetics of gold nanoparticles (GNPs) is critical for developing a colorimetric assay extensively used in chemical and biomolecular sensing. The aggregation of NPs plays a significant role in many natural and industrial processes, demanding comprehensive perceptions of the aggregation kinetics at a solid-liquid interface. However, the direct observation of the melamine-induced aggregation process of GNPs in the time-domain still remains a challenge. There is little to no information on the fundamental mechanisms of such kinetics using evanescent waves. Total internal reflection (TIR) has been applied to generate the evanescent field (EF), exploring aggregation kinetics near the solid-liquid interface. Here, we utilized a precise optical cavity-based method, an evanescent-wave coupled cavity ring-down spectroscopy (EW-CRDS), that can probe the melamine-induced aggregation kinetics of GNPs. The key feature of the present method is that the evanescent field generated by TIR illumination harnesses the power of CRDS to study 2D fractals via the collision and attachment of the GNPs and their melamine-induced aggregates at the interfacial region in real-time. This kinetic study reveals a critical point for diffusion-limited aggregation and provides insights into the design and optimization of colorimetric sensors that exploit the aggregation of GNPs. Furthermore, the EW-CRDS is a unique analytical approach that helps to deepen our understanding in probing the real-time aggregation process, detecting the presence of aggregator as compared to UV-vis and dynamic light scattering (DLS) spectroscopy.

Polarization-Multiplexed Incoherent Broadband Surface Plasmon Resonance: A New Analytical Strategy for Plasmonic Sensing.

Surface plasmon resonance (SPR) is an interfacial phenomenon, and the plasmonic sensors are based on the optical excitation of the collective oscillations of free electrons at a metal-dielectric interface. Here, we present the new development of an incoherent broadband (IBB)-SPR probe combining the wavelength interrogation technique with polarization-multiplexing (PM). The performance characteristics of the so-called PMIBB-SPR strategy was validated for the detection of nonenzymatic aqueous urea samples as a representative example for plasmonic sensing with an excellent wavelength and phase sensitivities of 0.1363 nm/mM and 10.34597 mM/deg, respectively. We further explored the missing link between plasmonic polariton resonance (PPR) and polarization modulation via the measurements of the Stokes parameters of the reflected light. This deepens our understanding of the fundamentals of polarization-multiplexed SPR phenomenon at the interface. This study thus paves the way to develop a new-generation analytical technique with the aim of tracking various real-time chemical and biological molecular interactions occurring at the interfaces.

Polarization mediated coherent and incoherent Bessel-moiré generation.

We demonstrate an efficient approach for the investigation of polarization states of a Bessel beam along the propagation direction. Furthermore, we propose a method to generate Bessel-moiré using a birefringent lens and Wollaston prism. The analysis showed that an experimentally generated incoherent moiré pattern is analogous to a theoretically simulated pattern. Moreover, we verified that inhomogeneous polarization states of a Bessel beam are still present in Bessel-moiré along its propagation direction. Our observation of Bessel-moiré due to mechanical vibration depicts the fact that incoherent moiré is stable, whereas the coherent moiré is sensitive to external vibration only along its propagation direction.

Moiré pattern generation by dual shearing and its modulation by surface plasmon resonance.

A novel surface plasmon resonance (SPR)-mediated Moiré fringe generation is theoretically and experimentally investigated using a birefringent lens (BL) and a Wollaston prism. Birefringent lens having two longitudinally shifted focal points provides radial shear, whereas Wollaston prism introduces lateral shear between the two radially sheared patterns for mutual orthogonal polarizations. Thus Moiré-like pattern is produced at the overlapping region of those patterns. Brief theory and experimental demonstration of coherent and incoherent Moiré fringe generation by such dual shearing is presented. The appearance and disappearance of Moiré pattern with the rotation of the analyzer axis has also been demonstrated. A modified experimental setup has also been developed to observe the influence of plasmonic excitation on such patterns which can be further utilized for photonic applications such as sensing and imaging.