Optical Gain and Photostability of Different Laser Dyes, Quantum Dots and Quantum Rods for Liquid Crystal Micro Lasers

Since the discovery of dye lasers, constant efforts are made towards developing optical gain materials with improved performance. Choosing an optical gain material for application in high-performance cholesteric liquid crystal micro-lasers requires a dedicated screening method. We employ Amplified Spontaneous Emission (ASE), and time-dependent fluorescence under pulsed excitation to study the optical gain parameter and photo-stability of laser dyes pyrromethene 567, pyrromethene 580, pyrromethene 597, pyrromethene 650, DCM, and Nile Red, dissolved in a nematic liquid crystal 5CB. Optical gain of CdSe/ZnS coreshell quantum dots dispersed in dodecene and CdSe/CdS coreshell quantum rods, dispersed in toluene was also measured and found to be much lower compared to the gain of fluorescent dyes. Pump energy and polarization dependence of ASE from dye molecules incorporated in the planar-aligned liquid crystal matrix is addressed. We determine the optical gain coefficient for different laser dyes by using the variable laser stripe illumination method using the small-signal gain model. The polarization dependence of ASE is determined in various geometries. We depict photostability from the half-life of emission intensity decay over a large number (10⁶-10⁷) of repeated laser pulses for fluorescent dyes, quantum rods and quantum dots. We find that quantum rods and quantum dots are much more stable compared to fluorescent dyes. Based on the photophysical characterization, we select pyrromethene 567 and pyrromethene 597 as efficient laser dyes for nematic liquid crystal micro-lasers.