Anisotropy of a terahertz electromagnetic response of filament microstructures of a composite based on polypropylene with carbon nanofibers

Linear dichroism and birefringence are detected in composite microfilaments consisting of polypropylene with carbon nanofibers in the terahertz spectral range. The magnitude of linear dichroism is a nonmonotonic function of the frequency, reaches values about 60% at low frequencies of 0.2–0.8 THz, then changes sign, and reaches -50% at a frequency of 1.8 THz, at which the maximum of the absorption band is observed in the microstructures under study. Birefrengence (the difference between the effective refractive indices of the ordinary and extraordinary waves) also changes sign at a frequency of about 1.5 THz, which is apparently due to the manifestation of resonant absorption in the studied microstructures near 1.8 THz. The observed birefringence is due to carbon nanofibers that are incorporated into the polypropylene filaments and are aligned in a certain manner because of the high-temperature stretching of filaments.