Quantitative diffusion and swelling kinetic measurements using large-angle interferometric refractometry

Optical Response of PDMS Surface Diffraction Gratings under Exposure to Volatile Organic Compounds

Monitoring volatile organic compounds (VOCs) in indoor air is significantly gaining importance due to their adverse effects on human health. Among the diverse detection methods is optical sensing, which employs materials sensitive to the presence of gases in the environment. In this work, we investigate polydimethylsiloxane (PDMS), one of the materials utilized for gas sensing, in a novel transducer: a surface relief diffraction grating. Upon adsorption of the volatile analyte, the PDMS grating swells, and its refractive index changes; both effects lead to increased diffraction efficiency in the first diffraction order. Hence, the possibility of VOC detection emerges from the measurement of the optical power transmitted or diffracted by the grating. Here, we investigated responses of PDMS gratings with varying surface profile properties upon exposure to VOCs with different polarities, i.e., ethanol, n-butanol, toluene, chloroform, and m-xylene, and compared their response in the context of the Hansen theory of solubility. We also studied the response of the grating with a 530 nm deep surface profile to different concentrations of m-xylene, showing a sensitivity and limit of detection of 0.017 μW/ppm and 186 ppm, respectively. Structures in the PDMS were obtained as copies of sinusoidal surface gratings fabricated holographically in acrylamide photopolymer and revealed good sensing repeatability, reversibility, and a fast response time. The proposed sensing technique can be directly adopted as a simple method for VOC detection or can be further improved by implementing a functional coating to significantly enhance the sensitivity and selectivity of the device.

High-Q lasing via all-dielectric Bloch-surface-wave platform

Controlling the propagation and emission of light via Bloch surface waves (BSWs) has held promise in the field of on-chip nanophotonics. BSW-based optical devices are being widely investigated to develop on-chip integration systems. However, a coherent light source that is based on the stimulated emission of a BSW mode has yet to be developed. Here, we demonstrate lasers based on a guided BSW mode sustained by a gain-medium guiding structure microfabricated on the top of a BSW platform. A long-range propagation length of the BSW mode and a high-quality lasing emission of the BSW mode are achieved. The BSW lasers possess a lasing threshold of 6.7 μJ/mm2 and a very narrow linewidth reaching a full width at half maximum as small as 0.019 nm. Moreover, the proposed lasing scheme exhibits high sensitivity to environmental changes suggesting the applicability of the proposed BSW lasers in ultra-sensitive devices.

Revealing molecular diffusion dynamics in polymer microspheres by optical resonances

Understanding the diffusion of small molecules in polymer microsystems is of great interest in diverse fundamental and industrial research. Despite the rapidly advancing optical imaging and spectroscopic techniques, entities under investigation are usually limited to flat films or bulky samples. We demonstrate a route to in situ detection of diffusion dynamics in polymer micro-objects by means of optical whispering-gallery mode resonances. Through mode tracking, interactions between solvent molecules and polymer microspheres, including sorption, diffusion, and swelling can be quantitatively analyzed. A turning point of mode response is observed, while the diffusion exceeds the sub-wavelength-thick outermost layer as the radial extent of resonances and starts penetrating the inner core. The estimated solubility in the glassy polymer is consistent with the predicted value using Flory-Huggins theory. Besides, the non-Fickian contribution is analyzed in such a glassy polymer-penetrant system. Our work represents a high-precision and label-free approach to describing characteristics in diffusion dynamics.

Interferometric Probing of Physical and Chemical Properties of Solutions: Noncontact Investigation of Liquids

Interferometry is a highly versatile tool for probing physical and chemical phenomena. In addition to the benefit of noncontact investigations, even spatially resolved information can be obtained by choosing a suitable setup. This review presents the evolution of the various setups that have evolved since the first interferometers were developed in the mid-nineteenth century and highlights the benefits, limitations, and typical areas of application. This review focuses on interferometry based on electromagnetic waves in the near-infrared and visible range applied to liquid samples, categorizes the chemical/physical properties (e.g., pressure, temperature, composition) and phenomena (e.g., evaporation, crystal growth, diffusion, thermophoresis) that can be assessed, and presents a comprehensive literature review of specific existing applications. Finally, it discusses some fundamental open questions with respect to geometric considerations and overlapping effects. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

All-polymer whispering gallery mode resonators for gas sensing

Sensitivity of polymeric microdisks is evaluated for selected compounds in their vapor phase such as humidity, isopropanol, toluene, limonene, 1-butanol, and pentanoic acid (valeric acid). Among these compounds, pentanoic acid exhibits the highest sensitivity (23 pm/ppm) with a limit of detection estimated to be around 0.6 ppm. We are interested in the contribution of the geometry deformation due to polymer swelling on the sensitivity as it may be engineered to improve performance of gas sensing devices. Experimental observations show a trend where sensitivity to humidity increased with the ratio of the undercut over the radius of the microcavity.

Two distinct mechanisms upon absorption of volatile organic compounds into siloxane polymers

The response of polysiloxane materials to volatile organic compounds (VOCs) including benzene, toluene, ethylbenzene, and toluene (BTEX), as well as cyclohexane, acetone, methanol and isopropanol is studied using thin film large-angle refractometry. Refractive index and thickness changes are measured to quantify the diffusion rate and partition coefficients associated with the absorption and desorption of VOC vapours into polydimethylsiloxane (PDMS) and polydiphenylsiloxane (PDPS) - PDMS copolymer films. Absorption of volatile solvent vapours into siloxane polymers is found to follow two distinct mechanisms with different absorption rates. These mechanisms are also associated with different excess volumes of mixing and may be accompanied by a polymer restructuring step.

Temperature-calibrated high-precision refractometer using a tilted fiber Bragg grating

We present a refractometer with main- and vernier-scale to measure the refractive index (RI) of liquids with high precision by using the fine spectrum structure of a tilted fiber Bragg grating (TFBG). The absolute RI values are determined by the accurate wavelength of cut-off mode resonances. The main- and vernier-scale are calibrated by measuring large groups of fine spectra at different cut-off mode resonances in a small RI range, and the use of vernier-scale certainly reduces the RI measurement uncertainty resulted from the discrete cladding mode resonances. The performance of the TFBG-based vernier refractometer is experimentally verified by exploring the temperature dependence of RI of anhydrous ethanol in a near infrared region, showing an enhanced accuracy to the order of 10^-4, high repeatability and temperature self-calibration capability.

Refractive indices of common solvents and solutions at 1550 nm

Using a large-angle refractometer, the refractive indices of water, methanol, acetonitrile, acetone, ethanol, formaldehyde solution, 2-propanol, hexanes, 1-propanol, 2-butanol, tetrahydrofuran, dichloromethane, 1,4-dioxane, cyclohexane, N,N-dimethylformamide, ethylene glycol, N,N-dimethylacetamide, chloroform, glycerol, trichloroethylene, dimethylsulfoxide, p-xylene, ethylbenzene, toluene, m-xylene, benzene, and o-xylene were measured at 1550 nm and compared to literature values. In addition, the refractive indices of 48 aqueous sucrose solutions, 34 aqueous sodium chloride solutions, 40 DMSO-water, 40 ethylene glycol-water, and 40 glycerol-water mixtures were also measured at different concentrations and fit with third-order polynomial expressions. This study provides near-infrared refractive indices that were not previously available and may help with the calibration of refractive index sensors that operate at or near 1550 nm.