Defect‐Mode Lasing with Lowered Threshold in a Three‐Layered Hetero‐Cholesteric Liquid‐Crystal Structure
Defect‐Mode Lasing with Lowered Threshold in a Three‐Layered Hetero‐Cholesteric Liquid‐Crystal Structure
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DOI：
10.1002/adma.200501438
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发表时间：
2006-01
影响因子：
29.4
通讯作者：
M. Song;N. Y. Ha;Kazuhiro Amemiya;Byoungchoo Park;Y. Takanishi;K. Ishikawa;J. Wu;S. Nishimura;T. Toyooka;H. Takezoe
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文献类型：
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作者：
M. Song;N. Y. Ha;Kazuhiro Amemiya;Byoungchoo Park;Y. Takanishi;K. Ishikawa;J. Wu;S. Nishimura;T. Toyooka;H. Takezoe
胆甾相液晶(CLCs)被认为是自组装的一维光子晶体。自发形成螺旋结构以提供称为选择性反射的独特性质；即，具有与CLC螺旋相同的利手性的圆偏振(CP)光不能在对应于光子带隙(PBG)的频率范围内传播通过CLC。这种光学性质为我们提供了很有前途的光子器件。最广泛研究的性质是在光子带隙边缘的激光，在那里光子群速度接近于零。在低分子量液晶和聚合物胆甾相液晶(PCLC)方面已经取得了许多成果。在实际应用中，需要连续波激光。为此，降低激光阈值是必不可少的，已有广泛的研究致力于实现这一目标。本文报道了一种用于低阈值缺陷模激光的新型腔体结构。使用的细胞由三个CLC层组成，中间含有激光染料，具有相反的左手螺旋和比两侧的层更窄的PBG。研究发现，当缺陷模与边缘模重合时，缺陷态出现，光子态密度(DOS)得到共振增强。发现这种增强模式的激光表现出较低的阈值。为了降低激光阈值，必须提高发射速率。根据费米黄金法则，i个光本征模的发射率xi由下式表示
Cholesteric liquid crystals (CLCs) are regarded as self-assembling one-dimensional (1D) photonic crystals (PCs). A helical structure is spontaneously formed to give a unique property called selective reflection; i.e., circularly polarized (CP) light having the same handedness as the CLC helix cannot propagate through the CLCs in a frequency range corresponding to the photonic bandgap (PBG). This optical property provides us with promising photonic devices. The most extensively studied property has been lasing at the PBG edge, where the photon group velocity approaches zero. Many achievements have been reported in low-molecularweight CLCs and polymeric cholesteric liquid crystals (PCLCs). For practical applications, continuous-wave lasing is desired. For this purpose, lowering the lasing threshold is indispensable, and extensive studies have been devoted to achieving this goal. In this paper, we report a novel cavity structure for defect-mode lasing with a lowered threshold. The cell used consists of three CLC layers, the middle of which contains laser dyes and has the opposite handedness helix and a narrower PBG than those layers on either side. It was found that the defect states emerge, and the photonic density of state (DOS) is resonantly enhanced when the defect mode coincides with the edge mode. The lasing from this enhanced mode was found to show a lower threshold value. In order to reduce the lasing threshold, the emission rate must be enhanced. According to Fermi’s golden rule, the emission rate xi for i optical eigenmodes is expressed by