Microscopic-characterization of photo-induced birefringence of azo-polymer thin film by focused surface plasmon
Introduction
On light irradiation azo-polymers experience remarkable structural modifications [1], [2], [3], [4], [5]. Owing to the structural changes, azo-polymers allow remote control of both the bulk and the surface properties in easy and flexible way [6], [7], [8]. Photo-induced isomerization of the azo-units [9] can produce the subsequent processes which induce re-orientation of the bulk leading to surface modification [10]. The reversibility of the photoinduced process pertaining to these materials presents another interesting property: the reconfigurability. In other words, the likelihood to reinitialize them through optical or heat treatment. Such properties empower dynamical control of azo-polymer based devices [11].
Rigorous quantitative analysis of anisotropic properties of azo-polymers in the form of thin films (∼100 nm) is sparse in scientific literature [12], while the form of thin film potentially contributes to integrate the dynamical property (to trigger specific surface morpho-physical properties in the presence of living cells) [13] into the azo-polymer based devices. This can be attributed to the fact that the phase difference produced by such thin films could not be measured accurately due to the sensitivity constraints of the adopted measurement methods in which propagating wave transmitted through the thin film was employed. Such measurements can be performed, by developing highly sensitive measurement method.
In this paper, we report a technique to completely characterize local birefringent properties of the photo-addressable azo-polymer fabricated in the form of nano-metric thick films. The fabricated azo-polymer acts as a uniaxial anisotropic medium when irradiated with suitable wavelength of preferred polarization after derivative isomerization exhibiting photo-orientation. We conduct this characterization by a method which involves the generation of focused surface plasmon (FSP). Since the FSP produces enhanced electric field in the region corresponding to the optical diffraction limit and characterizes the sample with the decaying evanescent field, it is rather most suitable for conducting this study. The use of FSP further aids in conveying the lateral heterogeneity of the azo - polymer thin films at microscopic sites. In the characterization, we excite FSP on the surface of the metal facing the sample and collect the reflected light through a high numerical aperture microscope objective [14]. In order to obtain birefringence [15] of the sample, we analyze the reflected spatial frequency distribution (i.e., the change in shape of the absorption pattern). In the process, we find that the distortion in the shape of the absorption pattern (eccentricity) is a direct measure of magnitude of birefringence while the rotation of the absorption pattern is a measure of the orientation of its fast axis. We use PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene-sulfonamide]-1,2-ethanediyl sodium salt]) thin films in Kretschmann configuration as a sample and extracted its anisotropic properties by spatial frequency response of FSP. Further, we also analyze the effect of various states of polarization of the pump beam on the anisotropic medium.
Section snippets
Numerical modelling and analysis
Fig. 1 shows a model to calculate the response of FSP against an anisotropic sample. We assume a substrate in Kretschmann configuration consisting of a coverslip with multi-layered coating and a uniaxial anisotropic layer on top of the assembly whose optic axis is parallel to the interface. Radially polarized light is converged on the surface of the coverslip to excite FSP, and the reflected light at the exit pupil of the objective is calculated to examine the reflected spatial frequency
Experimental setup and sample preparation
The experimental setup is depicted in Fig. 5(a). A collimated laser beam (wavelength: 632.8 nm) is passed through a radial polarization converter before impinging on a high NA (1.65) objective lens. The FSP was generated on the metal surface of the sample side. The light reflected from the metal film was collected by the same objective lens and the spatial frequency distribution appeared at the exit pupil of the microscope objective lens. The reflected spatial frequency distribution was
Conclusion
To the best of authors knowledge, we report for the first time, a technique to completely evaluate the microscopic photo-induced birefringence of azo-polymers in the form of nano-metric thin films on isotropic substrate. We conducted microscopic characterization of anisotropic samples by FSP technique, which has the volume of < 10 al. Magnitude of birefringence (∼0.01) and the orientation of the fast axis of the sample (azo-polymer thin films with ∼ 100 nm thickness) have been successfully
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The first author would like to acknowledge Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan for the scholarship. The authors would also like to acknowledge Prof. Shigeki Miyanaga and Assist. Prof. Tsutomu Sato of Muroran Institute of Technology for variable discussions and opinions regarding the fabrication of azo-polymer thin films.
Funding.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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