Measurement of change in refractive index in polymeric flexible substrates using wide field interferometry and digital fringe analysis

Indium tin oxide coated polyethylene terephthalate (PET) polymeric films are widely used as substrates for future optoelectronic devices, such as organic LEDs, organic thin film transistors, and organic solar cells. These PET substrates are thin, flexible, and rugged. But residual stresses are trapped in polymeric substrates due to their manufacturing process, and this leads to the birefringence in flexible displays. In this paper we report the measurement of the change in refractive index of PET substrates using Mach-Zehnder interferometry and the Fourier transform fringe analysis technique. Change in refractive index was observed by means of bending the PET substrate. This change in birefringence varies the optical path difference between the two arms of the interferometer, leading to the fringe shift. From the fringe shift the phase change was extracted as a function of bending, and the change in the refractive index was determined experimentally for two wavelengths, i.e., red and green color lasers. We found that the value of change in the refractive index of these substrates increases on bending of the substrates. c 2012 Optical Society of America