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    Surface Plasmon Resonance for In-Plane Birefringence Measurement of Anisotropic Thin Organic Film

    Year: 2021

    Journal: Plasmonics, Volume 16, AUG, page 1023–1028

    Authors: Kumar, Amrit; Gupta, Raj Kumar; Manjuladevi, V; Joshi, Ashutosh

    Organizations: DST India [IDP/SEN/06/2015, CRG/2018/000755]

    Keywords: Surface plasmon resonance; Kretschmann configuration; In-plane birefringence; Langmuir-Blodgett film; FDTD simulation

    The measurement of in-plane birefringence (Delta n) of ultrathin film is challenging due to a significant deviation of physical properties of materials in ultrathin regime as compared to that in bulk state. Surface plasmon resonance (SPR) phenomenon can be employed to measure change in refractive index of ultrathin film at a very high resolution. This article discusses simulation of SPR phenomenon in Kretschmann configuration for the measurement of Delta n in organic thin film exhibiting nematic-like ordering on the two-dimensional gold surface. The distribution of plasmonic field on the gold surface was found to be anisotropic. This suggested that the coupling plasmonic field with that of organic thin film exhibiting nematic-like ordering on the gold surface will be non-isotropic. Therefore, a nonzero difference in resonance angle (RA) was obtained from SPR measurement performed along the optic-axis (OA) and orthogonal to OA of the in-plane nematic ordering (Delta theta). A calibration surface showing the variation of (Delta theta) as a function of Delta n and thickness of thin organic film consisting of shape anisotropic tilted molecules exhibiting nematic-like ordering on gold surface was obtained. This calibration surface was employed for the measurement of Delta n of single layer of Langmuir-Blodgett films of cadmium stearate (CdSA) and 4'-octyl-4-biphenylcarbonitrile (8CB) deposited on SPR chips. The thickness of the LB films was estimated using X-ray reflectivity measurement, and Delta theta was measured using a home built SPR instrument. The Delta n values were found to be 0.012 and 0.022 for ultrathin films of CdSA and 8CB molecules, respectively.
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