Polarization-asymmetric bidirectional random laser emission from a twisted nematic liquid crystal

Abstract

This paper reports on the observation and detailed investigation of the bidirectional random lasing emitted from an active twisted nematic liquid crystal, of which the polarization states are asymmetric (non-parallel to each other). In such a laser, the liquid crystal acts as a random distributed feedback cavity with an inherently built-in polarization rotator. While propagating in the anisotropic cavity, the polarization of the dye-emitted light rotates with the gently twisted optic axis. The output polarization states are, therefore, parallel to each of the surface alignment directions. The mode stability and electrical switchability of the laser were also examined. Moreover, correlations between the lasing threshold, alignment direction, dye concentration, and film thickness were established. The findings not only suggest an optimum concentration for low-threshold random lasing operation but also disclose the threshold inversion behavior of nematic random lasers.