Creating Inexpensive Tunable Filters
from Birefringent Polymer Films

Ikaasa Suri, Melia Bonomo, Martin G. Cohen and John Noé

Laser Teaching Center
Department of Physics and Astronomy
Stony Brook University

Birefringent filters utilize changes in the state of polarized light in anisotropic materials to transmit different intensities of light at certain wavelengths. Such filters are employed in display and color filtering technology, as well as wavelength division multiplexing systems for optical communications. Birefringent materials are typically considered to be extremely intricate and expensive, requiring multiple layers of quartz or other crystals. In contrast, polymer films also exhibit birefringence and can be used in place of these more expensive materials.

The purpose of my project was to exploit the birefringent properties of low-cost polymer films to create tuneable filters that effectively transmit various types of polarized light at particular wavelengths. The material used was "High Performance" Scotch brand clear cellophane packaging tape, whose birefringent properties are acquired in its manufacturing process when the polymer molecules are aligned in the same direction. We made samples with 2, 4, 6, 8, and 10 parallel layers of tape and placed these between crossed and parallel polarizers. We illuminated the samples with a halogen bulb and recorded the transmitted light with a ThorLabs CSS100 Spectrometer. The observed spectra contain periodic oscillations in which the minima (for parallel polarizers) or maxima (for crossed polarizers) correspond to the sample being a half-wave retarder. We were able to determine the order of retardance for these minima by plotting the inverse of wavelength versus odd integer multiples of pi. The ten-layer sample was determined to be an eighth-order retarder at 645 nm. The figures below show the transmitted light spectrum for the ten-layer stack between parallel polarizers and the corresponding inverse wavelength plot.

Now that the birefringent properties of the cellophane tape are known, we can use Jones calculus and Mathematica to design specialized wavelength-selective filters and other polarization elements such as quarter- and half-wave plates. The trial-and-error design process involves calculating the effect of placing various numbers of tape strips at various angles relative to each other.

This work is supported by the Simons Foundation and the Laser Teaching Center.