Study On The Waveguide Spectrum And Laser Characteristics Of Dye Doped Nematic Liquid Crystals

Dye doped liquid crystals have the potential of building compact, all-organic mirrorless lasers with low threshold, tunability, and high sensitivity to various external factors such as light, pressure, electric and magnetic fields, chemical species, etc. Based on the work of solid-state dye lasers, in order to expand the research on tunable dye lasers, this article studied the waveguide spectrum and laser characteristics of dye doped nematic liquid crystals.Firstly, this article reviews recent research on dye doped liquid crystals both domestic and overseas. Then, this article presents the basic theory of lasers based on distributed feedback structure (DFB). Then, the materials, principle and procedures needed to fabricate a liquid crystals cell, and how to insert the mixtures of dye and liquid crystals into the cell is introduced. After that, some optical experiments are presented, including fluorescence, absorption spectrum, amplified spontaneous emission (ASE), narrow linewidth laser spectrum, and preparation of cholesteric liquid crystals was tried.The absorption spectrum exhibit some characteristic of polarization, i.e. the absorption of the polarized light paralleled to the director is stronger, due to the uniaxial optical proterties of liquid crystal moleculars.ASE was measured both under line pump and spot pump. Systematic work for ASE of PM650 doped liquid crystals under line pump was carried out, and the following results were found. The threshold voltage due to Frederiks effect was 630mV, only those voltage above this value can cause the reorientation of liquid crystals moleculars. Saturation voltage also existed. The ASE tuning range was a little more than 20nm for all different concentration. Compared to lower concentration, the ASE tuning range of higher concentration had a red shift about several nanometers. ASE intensity versus central wavelength and pump energy were also measured, and simulation were also carried out, experiments results and simulation agreed well. Under spot pump, the ASE was different from that under line pump. ASE under spot pump had a smaller tuning range and a wider linewidth. And the ASE tuning range of 0.5wt% PM650 doped liquid crystals was almost zero, but still 23nm under line pump.For PM567 doped liquid crystals, by using holographic excitation, narrow linewidth laser was realized at 571nm, the linewidth was narrower than 0.1nm, limited by the resolution of the spectrometer. Simulation on the relationship between the narrow linewidth laser and the half angle between two pump light was carried out by using MATLAB, indicating that the narrow linewidth laser at 571nm corresponded to a half angle between 45 and 46, which agreed well with the experiment.An attempt to prepare cholesteric liquid crystals was carried out. Experiments on the transmittance spectrum were presented.