Circularly polarized photoluminescence from the resonance region of vitrified chiral-nematic films

Information technologies rely heavily on the use of polarized light for optical information processing, display, and storage. Light-emitting chiral-nematic films represent a technological advance, because they can combine a light source, polarizer, and color filter in one optical element. Along these lines, light emission from vitrified chiral-nematic films is highly desirable. In addition, light emission from the resonance region of a chiral-nematic film is expected to produce almost pure circular polarization, presenting an intellectual challenge with potentially significant technological impacts. Sporadic attempts have been made in the past to uncover peculiar emission characteristics and to probe the chiral-nematic, structure as a medium for distributed feedback lasers. However, polarization analysis of the emitted light has not been carried out and the experimental evidence is inconclusive. My studies, therefore, have focused on photoluminescence within and without the resonance region of chiral-nematic films. Both steady-state and time-resolved data were acquired. Key results and observations are summarized in the following: (1)A rod-like achiral laser dye was dispersed in a series of chiral-nematic hosts with pitch lengths much longer than the emission wavelength. The experimentally observed dissymmetry factor was found to agree with a recent theory with all the input parameters determined a priori. Furthermore, circular polarization of linearly polarized photoluminescence originating in each quasinematic layer was found to be the predominant factor in the observed circularly polarized emission. (2)A constant sign and a relatively small value are characteristic of the dissymmetry factor across the emission spectrum for emission outside the resonance region. In sharp contrast, a sign reversal and values approaching pure circular polarization are characteristic of the dissymmetry factor for emission inside the resonance region. In addition, the emission lifetime was found to be independent of where emission took place relative to the resonance region. (3)A chiral-nematic film possessing a pitch gradient across its thickness was prepared with a selective reflection band that fully covered the emission spectrum. The dissymmetry factor underwent sign reversal across the emission spectrum for a given film orientation. Furthermore, the sign of the dissymmetry factor was found to be dependent on the pitch gradient.