Investigation of Polymer-based Responsive Photonic Structures Fabricated by Two Photon Polymerisation

Abstract

In this thesis, various 2D and 3D submicron polymer-based photonic structures are investigated for structural colour generation. 2D diffractive gratings have been designed and theoretically investigated by using Finite-Difference Time-Domain (FDTD) and simple scalar Kirchhoff diffraction theory (SKDT). Two photon lithography (TPL) is used to fabricate the designed gratings with varying geometric parameters based on rigid IPL-780 photoresist. The experimental transmittance spectra are in good agreement with FDTD simulation results, confirming the accuracy of TPL in the fabrication of submicron 2D gratings. Acrylic acid based pH responsive gratings have been fabricated by TPL. The pH-responsive gratings exhibit different experimental 0th order transmittance in air, in base and acid solutions, demonstrating their responsiveness over a wide pH range (2.5-13). The structure stability of pH-responsive gratings in solutions has been improved by dual photoresists design. The results presented provide proof of concept for PH-responsive gratings. Furthermore, vapour responsive 2D photonic structures fabricated using acrylamide based photoresist have been investigated. The response of 0th order transmittance of the multiline photonic structures to water vapour, ethanol vapour, isopropanol vapour and acetone vapour is quantitatively analysed. The transmittance spectra of multiline photonic structures exhibit increasing redshift with increasing vapour concentration and increasing solvent polarity (water > ethanol > isopropanol > acetone). The AFM and SEM characterisation for the structural properties coupled with FDTD numerical simulations have revealed the expansion of the photonic structure after exposure to vapours dominates the spectral red shift. The multiline photonic structures have also been successfully applied for information encryption, and by exploiting the change of the transmittance spectra arising from the modified refractive index of the ambient environment. The optical properties of submicron TPL-printed 3D photonic structures incorporating silica nanoparticles with a diameter of approximately 150 nm mixed with PEGDA- and PEGPEA-based photoresists have also been investigated. The effects of volume fraction of silica nanoparticles, sample geometric parameter and fabrication parameter setting (slicing) on the structural colour has been elucidated. Blue shifts in the scattering spectra are obtained by increasing silica volume fraction, decreasing slicing setting or decreasing sample cube dimension in xy. Focused ion beam microscopy (FIB) and FDTD simulations have revealed the mechanism of structural colour change via the interparticle distance and interlayer distance variation. Finally, further research in this field is discussed in the outlook chapter. Several ongoing promising multidimensional photonic structures are briefly presented.