Swamping of circular Bragg phenomenon revealed by durations and average speeds of videopulses transmitted through chiral sculptured thin films

Abstract

We studied trends in three measures of both duration and average speed of optical videopulses transmitted through chiral sculptured thin films (STFs). The films, a class of nanoengineered materials which consist of parallel helical nanowires grown on a substrate, were taken to be linear or nonlinear with an intensity-dependent refractive index. We used a finite-difference algorithm to compute the evolution of the pulse shapes in the time domain. The durations of videopulses transmitted through chiral STFs tended to decrease with increasing carrier wavelength, while the average speeds tended to increase or remain roughly constant with increasing carrier wavelength. The durations and average speeds were similar irrespective of whether the incident pulse possessed a left or right circularly polarized carrier plane wave. That is, the circular Bragg phenomenon−due to a circular polarization dependent photonic bandgap exhibited by chiral STFs over a bandwidth called the Bragg regime−did not affect the results to any meaningful extent, in contrast to previous work. We attribute this finding to the wide bandwidth of the incident pulses swamping the Bragg regime.