Abstract
An optical resonance method for the determination of the strain- and stress-optical coefficients of optically transparent polymers is presented and exemplified for monodisperse and bidisperse molecular weight polystyrene (PS). This method employs whispering gallery modes (WGMs) resonation inside a spheroid polymeric cavity, suspended on an optical fiber taper waist, which, in turn, is used for subjecting the polymeric resonator to controlled strain conditions. The wavelength shifts of equal order transverse electric and transverse magnetic polarization WGMs are measured, as well as their relative birefringence versus applied strain. For monodisperse PS microspheroids (2 and 50 kDa) the stress-optical coefficient is negative, contrary to the results for bulk PS in the glassy state indicating different phenyl group orientation of the PS monomer with respect to the strain direction. In the bidisperse (2 and 50 kDa) spheroid with a symmetric monomer composition, local structural irregularities are probably responsible for the observed coupling between WGMs. The method possesses metrological capabilities for probing the molecular orientation of polymer-based resonators.
Highlights
Whispering gallery mode (WGM) light localization into spherical symmetry resonators is a powerful tool for the development of photonic devices for sensing,[1] lasing,[2] and spectroscopic[3] applications
The light localization by WGMs through an iterative round-trip resonation into a closed spherical cavity renders the spectral behavior of this system impressively sensitive to spatially confined modifications of its physical properties, as long as these modifications overlap with the modal volume of the WGMs
We introduced a new method for the precise measurement of the photoelastic properties of glassy polymers harnessing microspheroid resonators attached on tapered optical fibers
Summary
Whispering gallery mode (WGM) light localization into spherical symmetry resonators is a powerful tool for the development of photonic devices for sensing,[1] lasing,[2] and spectroscopic[3] applications. The light localization by WGMs through an iterative round-trip resonation into a closed spherical cavity renders the spectral behavior of this system impressively sensitive to spatially confined modifications of its physical properties, as long as these modifications overlap with the modal volume of the WGMs. The light localization by WGMs through an iterative round-trip resonation into a closed spherical cavity renders the spectral behavior of this system impressively sensitive to spatially confined modifications of its physical properties, as long as these modifications overlap with the modal volume of the WGMs Their confinement through total internal reflection occurs along the interface of the cavity surface with the outer environment that enhances the surface sensitivity of the WGMs. There are several examples wherein WGM light localization has been utilized for developing light localization, sensing, propulsion, and optomechanical oscillation devices, whereas resonant cavities are constituted from hard (glasses, crystals)[4] or soft (polymer) optical materials.[5] Specific stimulation schemes have been applied for modifying the optogeometrical characteristics of WGM spheroidal cavities by means of mechanical actuation (compression and stretching6) or application of electrical field[7] for tuning the shape and/or the refractive index of the resonator. All of this work is aimed at understanding the WGM resonance manipulation utilizing external stimula-
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