| Laser is a device that can emit strong and coherent light in a straight direction. It was first developed in1960. Afterwards, various types of lasers have been developed, including semiconductor laser, solid state lasers, gas lasers, liquid lasers, pulsed lasers, and so on. The development of integrated optics requires miniature or even micro lasers, and the abovementioned lasers are too bulky to be used. Therefore, miniature lasers have attracted great attention in recent years. At present, there are mainly two types of miniature laser. One is distributed feedback laser (FDB). The other one is Fabry-Perot (F-P) cavity laser. Both of them must be fabricated by film coating technology, whose fabrication process is very complicated and the cost is very high. A new technology must be developed to overcome this issue for practical applications.By doping laser dye into cholesteteric liquid crystals (CLCs), lasing emission has been obtained under the excitation of a pumping laser. Since the fabrication of CLCs is very simple, which can be fabricated simply by doping some chiral dopant into the liquid crystal (LC) host, and also LC molecules are very cheap, it is a very good candidate for miniature laser. Therefore, CLC based lasers have attracted great attention in recent years.CLC exhibits periodic helical structure in which LC molecules are rotated along the helical axis. Since the LC molecule is a highly birefringent material, the periodic helical structure provides a periodic modulation of the refractive index. As a result, a photonic band gap is formed due to the Bragg reflection which is similar to a one-dimensional photonic crystal. The central wavelength of the photonic band gap λ0and bandwidth△λ are determined by Ao=(n)p and△λ=△An·p, which <n>、 An and p represent the average refractive index, birefringence and helical pitch of the LC. Different from the conventional photonic crystal, only the circularly polarized light with the same handedness as the CLC and within its reflection band gap will be reflected, while the circurlarly polarized light with the opposite handedness will be transmitted without any obstacle. Because of the characteristics, by doping laser dye or other highly fluorescent materials into a CLC, laser emission can be obtained under the excitation of a pumping laser.Compared with the abovementioned two types of miniature lasers, CLC based laser has some advantages, such as the simple fabrication process, operational flexibility and a good ability to regulate. CLCs can be simply obtained by doping a certain amount of chiral dopant in the nematic LC. Moreoever, the bandwidth and center wavelength of the reflection band can be changed by external factors, such as the chiral molecular concentration, temperature, electromagnetic field, illumination light and pressure etc. So far, however, the laser efficiency of CLC lasers is still low, the photochemical stability is not good, and the lifetime is short. All these shortcomings make it impossible for practical applications.In this paper, in order to enhance its laser efficiency, photochemical stability, and lifetime for practical applications, we first investigate the effect of the cell gap on the lasing efficiency of the dye-doped CLC then the optical properties of the quantum dot CdSe/ZnS and the factors effecting its fluorescence efficiency. The main contents include the following three parts:1) The effect of the cell gap on the lasing efficiency of the dye-doped CLC was investigated. It is observed that the cell gap has dramatic effect on the lasing efficiency. When the cell gap is thin, the lasing efficiency is dramatically increased with the increase of the cell gap. When the cell gap is thick enough, the increase of the lasing efficiency with the increase of the cell gap is not so dramatic. For example, the lasing efficiency is dramatically increased by63times when the cell gap is creased from5to10μn, while the lasing efficiency is only increased by3times when the cell gap is increased from10to15μm. The physical mechanism has been analyzed.2) The optical properties of the quantum dots CdSe/ZnS’s was investigated by studying the effect of its solution concentration on its fluorescence. The absorption and fluorescence spectra of different sizes of quantum dots in different solution concentration, ranging from0.8μmol/L to8pmol/L, were measured. It was observed that the variation of the fluorescence with the solution concentration of the quantum dots is different for different sizes of quantum dots. For the quantum dot in smaller size(≤4.0nm), the fluorescence intensity is increased with the increase of the quantum dot’s concentration. While for the quantum dot in larger sizes (>4nm), the fluorescence intensity is first increased and then decreased with increase of the quantum dot’s concentration and the optimal concentration is1.3μrriol/L. The reason and physical mechanism have been analyzed.3) The effect of the hexylthiophene (P3HT) on the quantum dot CdSe/ZnS’s fluorescence was investigated. The effect of the solution concentration of the P3HT on its fluorescence was first tested. Then we investigated the effect of P3HT on the quantum dot CdSe/ZnS’s fluorescence by mixing P3Ht and the quantum dot CdSe/ZnS’s at different ratio,1:1,1:2,1:4and1:8ratio respectively, for several different sizes of quantum dots. It was observed that with the increase of P3HT, the fluorescence intensity of the quantum dots is decreased, which indicates that the P3HT can dramatically reduce the fluorescence of the pure quantum dot. |
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