The refractive index of polypropylene in the far infrared (FIR) is measured by means of a suitably modified laser of a FIR spectrometer. When thin polypropylene films of 12.7-μm nominal thickness are introduced in the optical cavity of a laser at the Brewster angle, the radiation ceases because of the change in the optical path of the laser beam. This change is measured from the displacement of one of the laser mirrors, which is necessary to restore the laser resonance. The refractive index of polypropylene is deduced from this measurement and from the film thickness, as obtained from an independent measurement based in pycnometry. The value obtained for the refractive index is 1.492(15) for the wavelengths between 118.834 and 251.140 μm, for a polypropylene film of 12.71(2)-μm thickness and 0.9049(7) g/cm<sup>3</sup> density.
© 2003 Optical Society of America
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(310.6860) Thin films : Thin films, optical properties
Jesús Flores-Mijangos and Virgilio Beltrán-López, "Far-Infrared Laser Measurement of the Refractive Index of Polypropylene," Appl. Opt. 42, 592-596 (2003)