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

Abstract

A new light-responsive arylazopyrazole (AAP) containing a polymer matrix thin film is fabricated by spin-coating different concentrations of the AAP azo dye into the polydimethylsiloxane (PDMS) polymer at 150°C. The new AAP molecular switch was also used to fabricate a solid-state PDMS-AAP waveguide by contact lithography and soft replica modeling methods in the micrometer scale. The refractive index of the spin-coated photoswitchable material can be modulated via the reversible trans-to-cis photoisomerization behavior of the AAP unit using different concentrations. When 0.01 M solution of the AAP unit was used, the refractive of the composite was 2.32 in the trans state and dropped to 1.85 in the cis state in the operating wavelength of 340 nm. At higher concentrations of 0.020 and 0.03 M, a wide refractive index tuning is achieved under the same wavelength. In 0.030 M, the refractive index was 2.65 for the trans state and 2.0 for the cis state. The results suggest that the increase in refractive index tuning is related to the concentration of the AAP unit of the composite film. Theoretically, the spectral properties of the composite film are also simulated with two methods: (1) the Maxwell equations and (2) the frequency dependent finite element, showing excellent agreement for the different propagation modes of the proposed waveguide for regulated signals of 365/525 nm wavelengths. Furthermore, the photoisomerization of the PDMS-AAP thin film is analyzed with UV–vis spectroscopy to demonstrate the isomerization responses of the AAP moiety in the solid state. In addition, preliminary photomechanical actuation properties of the composite film have been investigated. The PDMS-AAP waveguide described provides a new approach for optically tunable photonics applications in the UV–visible region.