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Research on Influences of Dye and Reactive Mesogen Concentration on Tunable Laser from Cholesteric Liquid Crystals

Author: WangZhongYang
Tutor: ChenZuoJian
School: Xiamen University
Course: Optical Engineering
Keywords: Cholesteric liquid crystal (CLC) Laser Tunable Laser characteristics
CLC: O753.2
Type: Master's thesis
Year: 2014
Downloads: 4
Quote: 0
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Abstract


Cholesteric liquid crystal (CLC) can self-assemble to form a periodic helical structure with periodicity in the range of optical wavelengths. It can be considered as a one-dimensional photonic crystal (1D PC). Like a semiconductor, the PC has a band gap, so it can be used to manufacture the compact, mirror-less and spectrally tunable laser devices. It has attracted great attentions from the applications in laser display and other fields. Therefore, we study the effect of dye-concentration and reactive mesogen concentration on CLC laser performances, the main contents as follows:1. The reflection spectrum of sample was used to study how the chiral agent concentration impacts on the photonic band gap, which can help to determine the photonic band gap suitable for gain medium of PM597.2. The temperature and angle of pump light on CLC laser performance were studied and optimized pumping conditions were obtained. With the increase of operating temperature, the threshold increases gradually, but the slope efficiency reaches the largest value at30.5℃. Yet, the threshold increases and the slope efficiency decreases with the increase of incident angle. The highest slope efficiency of4%can be achieved with the incident of10°at30.5℃.3. Both the intensity of left-hand and right-hand circularly polarized fluorescence increase when the increase of dye concentration. Only the peak in the left-hand spectrum shifts to short wavelength. The intensity of non-polarized fluorescence increases with the increase of dye-concentration. Meanwhile, the shape of the spectrum changes from double peak to one peak.4. The laser behavior and spectral changes occurring in cholesteric liquid crystals by varying dye concentrations were investigated when pumped at532nm. It was found that the long-wavelength band edge and the laser line both exhibit a blue shift over21nm with increasing dye concentration. The dependence of the slope efficiency and threshold energy on the dye concentration was studied. And the highest slope efficiency of4%can be achieved with the dye concentration of0.5wt%. 5. Cano-wedge method and Abbe refractometer was used to measure the pitch and refractive index of different dye concentration of samples. It was found the blue shift of laser line can be ascribed to the decrease in average refractive index and pitch of the dye-doped cholesteric liquid crystals.6. Reactive mesogen RM257and small amount of initiator IRGACURE(?)2959were used to evaluate polymer stabilization effect. The change of center wavelength of photonic band gap and bandwidth with the temperature was analyzed. With the increasing of temperature, the center wavelength blue shifts, along with the narrowing of the bandwidth. The sample with initiator before/after irradiation, whose shift of center wavelength is slower than the sample without initiator, may due to the improvement of solubility of chiral S811in CLC caused by photoinitiator2959.7. The laser performance of DCM doped CLC with different concentration of reactive mesogen was studied. The increase of reactive mesogen concentration leads to the red shift of reflection band and laser wavelength. The threshold also linearly increases. But efficiency increases after the initial drop in the sample with the concentration of5wt%. The highest slope efficiency of2.72%can be achieved with the concentration of7wt%, meanwhile the threshold is13.1μJ.8. Cano-wedge method is utilized to measure the pitch of samples with variable reactive mesogen concentrations. The experiments reveal that the pitch increased with the increase of polymer concentration. And by using refractive index calculation method for mixed liquid crystal, we found that the increase of reactive mesogen concentration leads to the decrease of average refractive index and birefringence. But the increased product of refractive index and pitch can explain the red-shift of photonic band gap and laser wavelength.

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CLC: > Mathematical sciences and chemical > Crystallography > Amorphous and class crystalline > Class crystalline > Liquid crystal
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