Abstract
The topic of cholesteric-liquid-crystal lasers is a rapidly expanding research area in the field of soft-matter photonics. The increasing interest in this field is due to the high versatility that these lasers may possibly present and the prospects of giving rise to new miniaturized devices. However, further improvements in their operation capabilities are still required for potential applications. In this paper, we critically analyze the main strategies proposed up to now to optimize their performance. We show theoretically and experimentally that possible innovations in the device structure cannot produce lasers with threshold energies below a certain limit. This limit is determined by the light scattering and absorption losses inside the liquid crystal. Even assuming the case of samples free of defects and perfectly non-absorbing, an intrinsic light scattering, typical of mesogens, still remains. Numerical estimates of the thresholds indicate that these lasers could hardly be driven by compact light sources such as current electroluminescent or light-emitting diodes. Since the improvement possibilities regarding cell architecture seem to be exhausted, the advance must come from the use of new dye molecules. These molecules should show enhanced emission cross-sections and be efficiently integrable within the mesogenic solvent. In addition, the fluorescent systems must present very small quantum yields to triplet states if continuous-wave lasing is sought. In this respect, quantum dots are an alternative to explore for further investigations.
Highlights
In cholesteric liquid crystals (CLC), molecules are self-assembled forming a helical structure.When the pitch of the CLC helix is of the order of magnitude of visible light wavelength, the material can exhibit photonic properties
When CLCs are doped with fluorescent dyes, mirrorless laser generation can be obtained at the edges of the photonic band gap (PBG)
Have analyzed the main strategies the literature to optimize the performance of CLC
Summary
In cholesteric liquid crystals (CLC), molecules are self-assembled forming a helical structure. The DML presents in general a lower threshold than that of the edge mode (EM) of the undistorted structure In this respect Schmidtke et al [42] reported a reduction of about one order of magnitude in the laser threshold by stacking two layers of cholesteric polymer films with a phase jump of π/2. In order to avoid the laborious construction of polymeric structures and further simplify the construction of complex cells, Zhou et al [49] stacked different low-molecular CLC mixtures by using standard glass cells In this case, a remarkable improvement in the quality of the laser cavity was found. It is interesting to find a tool to compare the quality of different cell architectures in a systematic way This is important to determine the most efficient strategies to obtain low threshold CLC lasers. We will draw some conclusions about the possibilities and limitations of the CLC lasers in general
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