| Laser has been widely applied in various field such as telecommunication, biology, medicine, defense, and military, etc. and has made revolutionary change to the human beings’ life since it was developed in 1960. Currently, the most commonly used lasers mainly include solid state lasers, gas lasers, dye lasers and semi-conductor lasers. All these lasers are stable, but their fatal flaw is that their volume is too bulky, which can not meet the requirement of the development of integrated optics. The development of the integrated optics needs micro lasers. Therefore, micro lasers have been extensively investigated in recent years.Currently, the micro lasers that have been extensively studied mainly include Fabry-Perot lasers, distributed feedback lasers, micro-cavity lasers, and disc lasers, etc. All these lasers exhibit advantages such as low threshold, small volume, narrow line-width, and high modulation rate, and so on. However, their fabrication process is very complicated. Some of them even need high-precision coating film procedure, whose cost is extremely high.A cholesteric liquid crystal (CLC) can form Bragg reflection band because of its periodic helical structure and high birefringence, which is similar to a one-dimensional photonic crystal. Compared to a conventional photonic crystal, CLC exhibits many unique properties. First of all, the central wavelength and bandwidth of CLC’s reflection band can be easily adjusted by the external factors such as electrical field, magnetic field, temperature and pressure, etc. Therefore, it can be used for tunable devices such as tunable optical filter. Secondly, the fabrication process of CLC devices is very easy. A CLC can be obtained by doping some chiral dopants into the liquid crystal (LC) host and then filling into a LC cell. Because of these unique properties, CLCs have attracted great attention for flexible displays, optical switches and tunable optical filters.By doping some laser dye into the CLC, laser emission can be obtained from it. Because of its simple fabrication process, CLC laser has attracted great interest and has been extensively investigated in recent years. All these studies can be concluded into two aspects. One is tunable CLC laser and the other one is laser efficiency enhancement. Although various methods have been developed to enhance the laser efficiency, CLC laser efficiency is still too low to be applied in practical applications. In order to develop CLC lasers for practical applications, the laser efficiency must be further enhanced, which is the target of this thesis.In order to improve CLC laser efficiency, we have studied the effect of molecular arrangement, structure and thickness of the CLC on the laser efficiency, and analyzed the physical mechanism in this thesis. The main research contents are concluded as follows:1. The CLC’s structure was improved by rubbing on the cell surface after switching on and off the electric field applied on it and the physical mechanism was analyzed, It is observed that before the rubbing treatment, a thicker CLC sample has more defects and lower transmittance The CLC structure and transmittance can be significantly improved by the rubbing treatment. For example, before the rubbing treatment, a CLC sample≥25um is very blurred and the transmittance is close to 0 due to its focal conic structure, After the rubbing treatment, it becomes transparent and the transmittance is dramatically increased to-80%. The physical mechanism is analyzed.2. The influence of CLC’s structure on the laser efficiency was studied and the physical mechanism was analyzed. The experimental results show that the laser threshold is decreased and the laser efficiency is increased after the rubbing treatment. Moreover, the laser efficiency is enhanced more in a thicker dye-doped CLC sample after rubbing treatment. For example, the laser efficiency from a 5μm sample can only be enhanced by 1.6 times, while the laser efficiency from a 50μm sample can be enhanced by more than 10 times after the rubbing treatment. The physical mechanism has been analyzed.3. The effect of the CLC reflector on the laser efficiency was studied and the physical mechanism was analyzed. A CLC with the same handedness as the dye-doped CLC and with the bandwidth covering the laser wavelength from the dye-doped CLC was used as a reflector that placed on either side of the dye-doped CLC. The laser efficiency can be further enhanced with the use of the CLC reflector. In this case, however, the laser efficiency enhancement is decreased with the increase of the thickness. The physical mechanism has been analyzed. It is worth mentioning that, after the rubbing treatment and by added a reflector, we obtain laser efficiency up to 10%in 10μm laser samples, which is by far the maximum laser efficiency achieved in cholesteric liquid crystals. |
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