液晶/聚合物光栅激光器的制备研究

	液晶/聚合物光栅激光器的制备研究
	黄文彬
学位类型	博士
导师	宣丽
	2014-07
学位授予单位	中国科学院大学
学位专业	光学
摘要	液晶/聚合物光栅是通过定域聚合引发的相分离而形成的折射率周期变化结构：液晶与光敏单体的均匀混合物在全息光场中曝光，在干涉场的亮区，光敏单体会优先发生聚合，导致不反应的液晶分子在干涉场的暗区域相分离出现，最终形成对应干涉场亮区/暗区的聚合物/液晶层交替结构。这种周期在可见光波长范围的微结构制造成本低廉，成栅速度快，拥有电场调谐以及独特调控光场的能力。如果在制备过程中将染料混合到预聚物中，曝光完毕后染料会直接掺杂到液晶/聚合物光栅中。由于染料的增益作用以及光栅的选模特性，可以制备出有机分布反馈激光器，该激光器有着成本低，阈值低，线宽窄，出射波长调谐范围宽的特点，因而有巨大的潜力应用在高分辨率光谱仪领域。以往对于液晶/聚合物光栅的研究主要集中在从化学角度降低其驱动电压及提高其衍射性质上面。由于缺少对该光栅内部物理参数的理解，无法将光栅性质和激光器性能联系到一起，所以也就没有办法系统严密的从谐振腔角度来理解分布反馈激光器工作机理并相应的提高激光器性能。另一方面由于染料存在很强的浓度淬灭现象，必须以低浓度掺杂到液晶/聚合物光栅中来作为增益材料，使得导芯层厚度过厚，对光吸收效率低下，激光只能出侧面出光。所以现在的液晶/聚合物分布反馈激光器存在阈值较高，模式多，激光光斑发散角很大且无法控制，转化效率极低等缺点，严重阻碍了器件的商业化进展。针对以上的这些问题，本论文进行了以下几方面的研究工作。首先对液晶/聚合物光栅形貌进行了深入的研究，发现在低曝光强度时候，相分离出来的液晶在光栅中形成均匀层而不是以液滴形状出现，这样光栅散射损失小于4%，十分适合作为激光器谐振腔；然后利用光栅形成过程中实时的不同偏振光的衍射特性，确定了相分离的液晶分子最终沿着光栅矢量方向排列。最后以上面两个特性作为依据，设计了双折射测量实验来定量液晶相分离度，并且在不用任何拟合参数情况下，使用各向异性耦合波理论解释了光栅衍射特性。进一步的以该测量方法为依据，系统研究了照射光偏振，光强，光栅周期对光栅形貌及相分离度的影响，获得了一些制备理想谐振腔的普适规律，为接下来激光器反馈机理研究及性能提高打下了基础。针对液晶自取向沿着光栅矢量带来反馈光界面折射率十分小的问题，提出了清亮点制备光栅的解决方法，令液晶分子在光栅中混乱取向，使得激光能量效率从0.4%上升到了0.9%。然后对染料掺杂的液晶/聚合物光栅激光器的选模机理做了深入的研究，发现这种波导性DFB激光器所支持的模式由两个机理决定，首先是模式必须是以HPDLC层为导芯层的波导模式，这样光就不会从基板方向泄露出去，其次模式波长要满足光栅的布拉格条件，这样光在行进中才可以得到有效反馈。基于此理论对激光器的模式进行了调控和优化：由于液晶分子沿着光栅矢量排列，施加电场后，液晶分子最多可以有90度的转动，TM模式的光在光栅中的折射率可以有较大的变化。据此原理，对激光模式进行了8nm的连续调谐，为以后应用于高分辨率光谱仪打下了基础；针对激光器多模工作的问题，通过仔细调控光栅介质与玻璃基板间的折射率差，控制光子振荡周期，得到了单模工作的激光器。最后将有机半导体薄膜作为增益介质引入到液晶/聚合物激光器中，由于其无法和预聚物互溶，所以设计了外反馈DFB腔。液晶/聚合物光栅单独制备在有机半导体薄膜上面，两层通过疏式波进行相互作用。实验表明，增益薄膜厚度在75nm左右时候，器件可以在增益以及反馈两个物理过程之间得到平衡，从而得到最佳的激光性能。与基于染料的激光器相比，这种激光器的性能有大幅度的提高：单模出射，TE线偏振，阈值降到原来十分之一（21μJ/cm2），能量效率上升了六倍（5.9%）。进一步研究了泵浦偏振对器件性能的影响：s偏振泵浦下激光器性能最佳，工作阈值下降到18μJ/cm2，能量效率上升至6.5%。最后采用周期为390nm的液晶/聚合物光栅制备了二级DFB激光器，工作阈值进一步降至13μJ/cm2。并且利用光栅的耦合作用，使得激光垂直于基板表面发射，在垂直于光栅槽方向发散角近于衍射极限，大大提高了对激光能量的利用率。本论文系统研究了液晶/聚合物光栅形貌，液晶分子取向以及液晶相分离度，并给予了其简单直观的物理图像，使得与激光器性能息息相关的参数可以直接测量，在此基础上深入研究了DFB波导激光器的模式选择机理，优化和调控了激光器模式，最后将有机半导体材料和液晶/聚合物光栅很好的结合在一起，得到了大幅度的性能提高，为以后商业化实现低成本，高转化效率，低发散角，可调谐的有机DFB激光器打下了坚实的基础。
其他摘要	Holographic polymer dispersed liquid crystal (HPDLC) gratings are formed through photo-polymerization induced anisotropic phase separation of liquid crystal (LC) from the polymer matrix. Due to the advantages of easy processing, low-cost, and ability to be electrically switched, they have drawn particular research attention. It was not until in 2002 that HPDLC gratings were found to be able to provide efficient feedback and can be used as laser cavities. The optical feedback is based on coherent Bragg scattering from the periodic structure and is referred to as distributed feedback (DFB) cavity. These organic lasers can provide low-threshold, narrow linewidth and wavelength tunability lasing and are promising in using as spectroscopic sources. However, the current output lasing is multi-mode with a large divergence. The reported lasing threshold is high and the slope efficiency is ultra-low. In order to address these problems, the correlations between parameters of HPDLC gratings and laser device working performance have to be considered and the studies into enhancing lasing properties through optimization of HPDLC cavities should be put forward. Besides, novel gain medium should be introduced to replace the laser dye, as it suffers severely from concentration quenching and provides low absorption efficiency. The main researches of this dissertation are listed as follows:1. We successfully fabricated polymer scaffolding morphologic HPDLC transmission gratings from common acrylate monomers. The phase separated LCs form homogeneous layers instead of spherical domains. Transmittance measurement shows the grating shows little scattering which is less than 4%. We also show that due to the anchoring effect of polymer filaments across the pure LC layer, these phase separated LC molecules are well aligned along the grating vector. We further designed a birefringence experiment to measure the LC phase separate degree in HPDLC gratings, so all structural parameters can be obtained. Based on above understandings of the grating structure, we then analyze the effect of fabrication conditions, such as polarization state of the writing beams, exposure intensity and grating period on LC phase separation degree and some fabrication rules for efficient HPDLC grating are obtained. At last, the phase separated LC director configuration is altered by recording the grating at a high temperature. In this way, the refractive index modulation for feedback propagating along the grating vector can be increased and the slope efficiency for output lasing is enhanced from 0.4% to 0.9%.2. We investigated the mode selection mechanism and found it was based on two principles. First, the resonant modes should fulfill the waveguide conditions in the core layer which is the grating layer in dye-doped HPDLC DFB lasers. So the laser modes could not leak out through the two glass substrates, and light could be effectively confined inside the grating layer. Second, the resonant mode should fulfill the Bragg condition of the grating, so positive optical feedback can be obtained through coherent Bragg scattering at each corrugation. The mode could then be amplified through stimulated emission when it is travelling around. The mode emits lasing when the pump energy reaches the working threshold. Based on the mode selection mechanism, we modulate or optimize the output lasing modes accordingly. We show that the lasing wavelength could be continuously tuned over 8 nm by applying an electrical field. The homogenously aligned LC molecules along the grating vector could be reoriented to the cell normal, thereby increasing the effective refractive index experiencing by the laser mode. The electrical tunability of lasing wavelength from HPDLC based DFB lasers is an attracting advantage and can be used in optical spectroscopy. The DFB working structure is optimized from the perspective of refractive index difference between the cladding layer and the core layer to allow only one lasing mode being resonated.3. Organic semiconductor has been introduced as gain medium to HPDLC based DFB lasers. Unlike the dye molecules, organic semiconductor is immiscible with HPDLC prepolymer syrup. So we have designed a new DFB working structure in which the grating layer is fabricated separately on top of the gain medium layer. The two layers interact with each other by evanescent wave spread into the cladding layer. We show that the thickness of the organic semiconductor layer has to be around 75 nm to balance optical feedback and light amplification in order to obtain a high performance lasing output. The output lasing shows single-mode, linear polarization performance, and can be tuned in the gain spectrum of the gain medium. The working threshold is 21 μJ/cm2 (one tenth that of dye-doped HPDLC lasers) and the slope efficiency is increased to 5.9% (six times that of dye doped HPDLC lasers). We further show the effect of pump polarization on working performance and find s-polarization guarantees best working performance. We then made vertically surface-emitting organic lasers by employing second-order Bragg scattering. The period of the HPDLC grating is just 394 nm and emitted lasing is coupled out through first-order scattering. Thanks to more efficient light coupling in second-order Bragg scattering, the laser threshold of this surface-emitting laser is reduced to 13μJ/cm2. We also investigate the effect of pump length on laser threshold, and shows the refractive index modulation HPDLC grating is sufficient for efficient feedback as long as the pump length is longer than 0.03cm.
语种	中文
文献类型	学位论文
条目标识符	http://ir.ciomp.ac.cn/handle/181722/41414
专题	中科院长春光机所知识产出
推荐引用方式 GB/T 7714	黄文彬. 液晶/聚合物光栅激光器的制备研究[D]. 中国科学院大学,2014.

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