An Accurate Method for Refractive Index Measurement of PDMS Using a DOE Beam Splitter

Polydimethylsiloxane (PDMS) is widely used in optical devices due to its high transparency, biocompatibility, chemical inertness and easy fabrication. Accurate measurement of the PDMS refractive index (RI) is critical for optical applications. In this study, a simple, accurate and reliable method is proposed for measuring the RI of PDMS, which leverages diffractive optical element (DOE) to determine the RI of PDMS by analyzing the uniformity of DOE filled with PDMS. The uniformity of the DOE beam splitter directly depends on the RI difference between the DOE material and filling material. The $3\times 3$ , $3\times 5$ , $3\times 7$ , and $5\times 5$ DOE beam splitters are designed using the Iterative Fourier Transform Algorithm (IFTA) and fabricated through one-step fabrication for RI measurement of PDMS. The RI of PDMS measured using the proposed method shows a mean n_PDMS of 1.3826 at 1550 nm with a standard deviation of 0.001, which represents a 12-fold improvement compared to the standard deviation of 0.012 from a mean n_PDMS of 1.3934 measured using spectroscopic ellipsometry (SE). This method demonstrates high accuracy and reliability, which can measure the RI of any unknown transparent material compatible with spin-coated or poured on a fabricated DOE beam splitter.