A Study of Wave Confinement and Optical Force in Polydimethlysiloxane–Arylazopyrazole Composite for Photonic Applications

Abstract

A refractive index of dielectrics was modified by several methods and was known to have direct influence on optical forces in nanophotonic structures. The present contribution shows that isomerization of photoswitching molecules can be used to regulate refractive index of dielectrics in-situ. In particular, spectroscopic study of a polydimethylsiloxane–arylazopyrazole (PDMS–AAP) composite revealed that refractive index of the composite shifts from 2.0 to 1.65 in trans and cis states, respectively, of the embedded AAP. Based on this, a proposition is made for a waveguide structure, in which external UV/Vis source reversibly regulates the conformation of the PDMS–AAP core. Computational study is performed using Maxwell’s equations on buried waveguide structure. The simulation, implemented in PYTHON, sequentially utilizes empirical refractive indices of the composite in the isomeric states in lieu of regulation by a source. The simulation revealed highly confined wave propagations for injected signals of 340 and 450 nm wavelengths. It is observed that the cis state suppresses higher order mode when propagating UV wavelength but allows it for visible light. This modal tuning demonstrated that single mode can be selectively excited with appropriate waveguide dimensions. Further impact of the tuning is seen in the optical force between waveguide pair where the forces shift between attractive and repulsive in relation to the isomeric state of the PDMS–AAP core. These effects which stem from the adjustment of refractive index by photoisomerization suggests that in-situ regulation of index is achievable by successful integration of photoswitching molecules in host materials, and the current PDMS–AAP composites investigated in this study can potentially enhance nanophotonic and opto-mechanical platforms.

Numerical analysis of the propagation modes of photo-switching PDMS-arylazopyrazole optical waveguide and thin-film spectroscopic characterization

“…Despite their remarkable success, azobenzenes often suffer from two performance issues: incomplete switching or low thermal stability of the metastable cis-isomer [2,3,4,5,6]. Arylazopyrazoles (AAPs) have been recently introduced as superior analogues of azobenzenes characterized by near quantitative conversion between two states and superior thermal half-life times of the metastable state [7,8,9,10]. AAPs undergo efficient reversible trans-to-cis photoisomerization upon alternating irradiation with UV or green light to achieve a conformational change.…”

“…AAPs undergo efficient reversible trans-to-cis photoisomerization upon alternating irradiation with UV or green light to achieve a conformational change. The facile synthetic access and the superior optical properties of AAPs make them a favorable choice in the fabrication of guided optics, optoelectronics, and optofluidic platforms [9,10].…”

“…Therefore, due to the important role of the refractive index in polymeric material, photo-chemically induced index modification in photonic devices gained considerable attention [11]. Tuning the propagation modes of the waveguide modifies the strength of the interaction force between the waveguide channels [10,12]. One way to inspect this tuning is through the embedding of photochromic organic molecules into the polymeric waveguides.…”

“…One way to inspect this tuning is through the embedding of photochromic organic molecules into the polymeric waveguides. However, while there are many examples of photoresponsive waveguides formed by an azobenzene based polymers that can achieve refractive index modulation upon illumination with specific wavelength of light [1,2], to the best of our knowledge, there are only few reported studies of polymeric waveguides based on the novel AAP molecular switches [10,11]. Hence, the present study aims to exploit the novel properties of the AAP molecular switches for the development of new photoswitchable light responsive polymeric waveguides that would enable the reversible photomodulation of the refractive index of azo polymer composite films.…”

“…We have previously demonstrated the fabrication of PDMS-AAP composite thin films and studied the refractive index changes through the trans-to-cis isomerization of the AAP unit embedded in the polymer matrix [6,9,10]. The PDMS-AAP matrix exhibits 100% light-triggered reversible efficiency, with maxima refractive index change during isomerization process [10,13,14].…”