From Single to Multi Mode Lasing: The role of materials revealed in optical simulations
Comparative study of thin film Cholesteric Liquid Crystal (CLC) lasers made from different materials and optically pumped by external solid state laser reveals a striking dependence of lasing behaviour (ranging from single mode at the edge of the selective reflection band to multimode lasing) on the morphology and microstructure of CLC films. The materials studied belong to two groups: low molar mass liquid crystals and polymers. It is shown that the orientation of individual chiral domains and fluctuations in helical pitch contribute significantly to the type of lasing displayed by the material. Different ways of preparing CLC cells that lead to predominantly one type of lasing are discussed. The importance of variations of the helical pitch and domain orientation in producing single and multimode lasing is justified by optical simulations (4x4 matrix method) of lasing in multi-layered samples.