(616f) Circularly Polarized Lasers from Solid Films Comprising Chiral Conjugated Oligomers Doped with Functionalized Oligofluorenes

Since the first successful demonstration of circularly polarized lasers using cholesteric liquid crystal films three decades ago, intensive efforts have been made to overcome hurdles to potential applications as medical diagnostics, quantum cryptography, optical interconnects, optical switches and modulators. The orientational order parameter governing the laser dye's emission dipole is critical to spectral purity. The most commonly used laser dye, DCM, is capable of a very modest orientational order parameter less than 0.40. In addition to ensuring spectral purity, a high orientation order parameter is expected to increase lasing efficiency and to decrease threshold. Extrinsic factors that may disturb the host film's helical stack or laser dye's orientational order would cause adverse effects on lasing. Optical pumping and lasing could result in heating, causing the stop-band to undergo a red or blue shift depending on the type of cholesteric liquid crystal used as the host. A rise in temperature would also reduce dye molecules' orientational order parameter. Optical torque on the cholesteric liquid crystal structure could lead to a deterioration of laser performance. Yet another potential problem is laser-induced fluid flow disturbing the stop-band. In short, cholesteric liquid crystal fluids as the hosts are vulnerable to any or all of these adverse effects, which can be avoided by using solid cholesteric liquid crystals in principle. Prior attempts to use solid cholesteric liquid crystal films, however, suffered from spectral impurity of lasing apparently because of the difficulty preparing highly ordered, disclination-free films.

Our research was motivated to identify potential laser dyes characterized by a high fluorescence quantum yield and a high orientational order parameter governing emission dipoles, and to impart device stability and robustness by using solid cholesteric liquid crystals as the hosts. A high fluorescnece quantum yield is essential for a high lasing efficiency. Furthermore, an emission dipole parallel to the long molecular axis is imperative to achieving a high orientational order parameter. Monodisperse oligofluorenes developed in our laboratory for green and red light emission have been demonstrated to be superior laser dyes in terms of fluorescence quantum yield and orientational order parameter. To ensure device stability and robustness, these promising laser dyes are doped in low-molar-mass cholesteric glassy liquid crystals (GLCs) comprising chiral conjugated oligomers also developed in our laboratory. The order parameter is evaluated by ellipsometry as well as absorption and emission dichroism, and the effects of orientational order on lasing efficiency, threshold, and spectral purity are elucidated within the framework of recent theories.