This work aims to thoroughly present recent advancements in soft matter analysis using integrated photonics and quasi-surfacic resonant techniques. Specifically, it highlights the versatility of the designed resonant structure, made of organic micro-resonators, demonstrating their application as a “Sedimentation Rate Meter,” “Optical Viscometer,” and “Resonant Optical Zetameter” for analyzing transparent, dark, and opaque solutions.

The industrial interest of this research is significant in assessing the dynamic behavior of colloidal dispersions prior to large-scale product production. Stable colloidal dispersions rely on equilibrium between electrostatic forces between dispersed particles, maintaining a stable particle cloud. Perturbation in this equilibrium, arising from changes in particle charge or environmental conditions, can lead to aggregation, sedimentation, and phase changes that compromise product longevity. Identifying stable versus unstable products is critical in pre-industrial applications, such as latex rubber, paints or agro alimentary products. Conventional commercial devices often struggle to analyze dark or opaque substances because their measurement principle is about volumetric light propagation, which is hindered by strong absorption. In contrast, the light circulating within micro-resonators in our approach probes the environment over hundreds of micrometers, even in optically challenging substances.

A basic configuration, where the fabrication processes and the geometry is depicted in Fig. 1, consists of unidirectional coupling between a racetrack micro-resonator (MR) and a bus waveguide. Resonance occurs when the round-trip phase condition equals 2mπ (where m is an integer), resulting in resonant wavelengths given by. Here, is the effective refractive index of the propagating mode, P the resonator's geometric parameter, and m the mode number. Changes in the upper cladding environment, such as migration, sedimentation, or densification, affect, providing insights into soft matter behavior. The spectral difference between two successive resonant wavelengths, known as the Free Spectral Range (FSR), is monitored and is defined by: where is the excited wavelength, and the group refractive index of the structure incorporating modal dispersion.

The resonant structures, fabricated from organic resin (UV210) via deep-UV photolithography on oxidized silicon, feature micrometer-scale patterns with 400-nanometer gaps between the access waveguide and MRs. A Superlum diode emitting at 795 nm excites the resonances, with a broad emission spectrum (40 nm) generating multiple resonant peaks. These peaks enable dynamic observation of soft matter processes. Data acquisition is performed with an Ocean Optics spectrometer, controlled via MATLAB, which also performs real-time Fast Fourier Treatment (FFT) to calculate dynamically the FSR evolution. This integrated platform enables dynamic soft matter investigations, specifically colloid dispersion migration and stability assessment, using compact and cost-effective devices.

Initial studies were conducted on transparent silica nanoparticles (NPs) migration into water. A water tank was placed in direct contact with the photonic chip, creating a three-layer waveguide structure with the core (UV210) sandwiched between the lower cladding (SiO2) and the upper cladding (water). Introducing silica NPs altered the upper cladding's refractive index, impacting the global effective refractive index of the structure. The mode's evanescent tail acted as a probe, detecting sedimentation and migration processes. An increase in FSR is a specific signature of the sedimentation process [1]. The moment the FSR reaches its peak value marks the end of sedimentation, enabling precise comparison of the experimentally measured sedimentation rate with the theoretical value predicted by the classical Stokes model (Fig. 2) [2].

This study was extended to dark solutions by replacing silica NPs with black carbon nano-powder. Sedimentation was clearly observed for various concentrations, demonstrating the platform's capability to analyze dark substances (Fig. 3). Furthermore, replacing the water with an anionic surfactant solution, the sodium dodecyl sulfate (SDS), stabilized the colloid through equilibrium between repulsive and attractive forces, maintaining a constant FSR (Fig. 3) [3]. This confirmed the capability to discriminate between stable and unstable colloids dynamically and those results are corroborated by zeta potential measurements (Fig. 3). The water and SDS solution successfully surpasses the stability threshold of |30 mV|, a characteristic indicator of colloidal dispersion stability. These results demonstrate its suitability for testing industrial pre-products. 

Such optronic device developed in cleaning room and incorporated into an optical test platform enable investigation of pre-product stability from industrial. Its ability to probe dark and opaque substances and its capacity for real-time FSR monitoring highlight its potential for cost-effective and efficient stability analyses in various industries.

[1] B. Bêche, H. Lhermite, V. Vié, L. Garnier, ‘Method for determining a sedimentation or creaming rate', CNRS/Université Rennes, international extension PCT n° PCT/EP2019/051103, + United States, Patent n° : U.S. Application Number n° 16/966,416 (2020).

[2] L. Garnier, J. Gastebois, H. Lhermite, V. Vié, A. St Jalmes, H. Cormerais, E. Gaviot, B. Bêche, ‘On the detection of nanoparticle cloud migration by a resonant photonic surface signal towards sedimentation velocity measurements', 2023, Results In Optics, 12, 100430.1-13. https://doi.org/10.1016/j.rio.2023.100430

[3] J. Gastebois, A. Szymczyk, G. Paboeuf, F. Scholkopf, V. Vié, A. St Jalmes, H. Lhermite, H. Cormerais, F. Gauffre, B. Bêche, ‘Exploring colloidal stability and migration dynamics through integrated photonic into aqueous black carbon dispersion', SPIE Edition - The international Society for Optical Engineering, Sensors + Imaging : Remote Sensing for Agriculture, Ecosystems, and Hydrology, 2024, 13191-64, 1-10. https://doi.org/10.1117/12.3030910