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在正交偏光显微镜下观察样品的侧面和端面,如图2所示。光子晶体光纤的纤芯空气孔直径D约为12.53 µm,包层的直径
$D_{\rm{0}}$ 约为62.44 µm,纤芯周围紧密规则地排列着空气孔,其直径d约为3.37 µm,空气孔间的距离a约为3.71 µm。有些微孔显示了暗色,说明没有充分注入液晶混合物。在正交偏光显微镜下,如果微孔完全没有充入液晶则显示纯黑色。图 2 偏光显微镜下样品照片。 (a) 侧面;(b) 端面
Figure 2. Photographs of sample under a polarizing microscope. (a) Side face; (b) End face
样品白光透射谱如图3所示。由光纤端面入射,另一端面测量。图3(a)中插图为白光光源的光谱图。由于光纤的吸收、散射损耗等,通过光纤的光谱形貌发生了变化,约602 nm和673 nm处有一个凹陷。设定光纤探头的方向与PCF的轴向方向的夹角为
$\theta $ ,设定$\theta {\rm{ = 0}}^\circ $ 为两个方向相互平行。采集了θ=0°、10°、20°、30°的透射谱。可以看出光谱形貌几乎相同,只是透过光强度在减小。$\theta {\rm{ = 0}}^\circ $ 时的透过光强最强。填充液晶混合物后PCF样品的透射谱如图3(b)所示。由于激光染料和液晶的吸收、散射损耗等透射光强度明显降低。图中阴影部分对应随机激光产生的波段580~610 nm。构建PCF的模型,选取激光辐射谱的中心波长590 nm,取液晶平均折射率值$n_1 = 1.607 = (n_{{e}} + n_{{o}})/2$ ,$n_e = 1.692$ ,$n_o = 1.522$ ,光纤包层的等效折射率$n_{clad} = 1.571$ ($n_{clad} = An_1 + Bn_2$ ,A为空气孔部分占包层面积的百分比,B为石英材料薄层面积的百分比,$n_1$ 为填充液晶折射率,$n_{\rm{2}}$ 为石英材料折射率),得到PCF中光场分布如图3(c)所示。由图可以看出,光波仍可以在基模传输。图 3 PCF白光透射谱。(a) 探测角度
$\theta $ 不同时无填充的PCF样品;(b) 无填充与填充液晶混合物后的PCF样品,$\theta {\rm{ = 0}}^\circ $ ;(c) PCF样品中光场分布Figure 3. White light transmission spectrum of PCF. (a) Unfilled PCF with different detection angles θ; (b) Unfilled PCF and liquid crystal mixture filled PCF,
$\theta {\rm{ = 0}}^\circ $ ; (c) Light field distribution in PCF -
在PCF样品侧面泵浦,泵浦光垂直光纤轴,并在另一侧面测得的激光辐射谱如图4所示。图4(a)中,随机激光峰出现在590~605 nm范围内。随着泵浦能量增大,随机激光峰强度明显增大,半高全宽(full width at half maximum,FWHM)变窄。当泵浦能量为210 μJ时FWHM约为0.3 nm。图4(b)中,在不同泵浦能量下的光谱数据通过线性拟合得到样品的侧面激光辐射阈值能量为95.27 μJ/pulse。
图 4 侧面出射随机激光辐射谱。 (a) 不同泵浦能量; (b) 阈值特性曲线;(c) 不同探测角度
Figure 4. Laser emission spectra on side face. (a) At different pump energies and; (b) Threshold characteristic curves; (c) At different detecting angles
从Anderson局域化出发,相干随机激光的发生机制可通过满足
$l_{tr}/\lambda < 1$ 条件表示,其中$${l_{tr}}{\rm{ = }}\frac{{{n_{av}}{\rm{ }}{P^2}{{\sin }^2}\theta l[n - {\lambda _0}({\rm d}n/{\rm d}\lambda )]}}{{{\lambda _{\rm{0}}}^{\rm{2}}}}$$ (1) 为染料掺杂手性向列相液晶的传输平均自由程[14]。式中:
$\lambda_{\rm{0}}$ 是其中一种模式的辐射波长,${n_{av}} = $ $ {[({n_e}^2 + {n_o}^2)/2]^{1/2}}$ 是手性向列相液晶的平均折射率,其中$n_e$ ,$n_o$ 分别为非寻常光折射率和寻常光折射率;$n$ 为$\lambda_{\rm{0}}$ 处折射率;P为手性向列相液晶的螺距;$\theta $ 为入射光与有序分子层的夹角;$l$ 为谐振腔的长度;${\rm d}n/{\rm d}\lambda$ 为折射率色散。而光子在随机介质中的扩散常数(D)可用下列关系式表示:$$D = vl_{tr}/3$$ (2) 式中:
$l_{tr}$ 为光子传输平均自由程;$v$ 为传输的平均速度。随着泵浦能量增大,随机激光峰值强度明显增大,这是因为随着泵浦能量的增大,使更多液晶分子呈现热涨落现象,进而使光子的传输平均自由程
$(l_{tr})$ 减小,扩散常数(D)减小,散射强度增强,随机激光峰值强度随之增强。发射谱中的尖峰数随着泵浦能量的增强而增加,这是因为随着泵浦能量的增强,光子的传输平均自由程$(l_{tr})$ 的减小,会形成更多的谐振微腔,进而会出现更多的尖峰。观察发现与液晶盒载体中的随机激光辐射谱有所不同,液晶盒载体中,随着泵浦能量的增大,随机激光峰强度伴随荧光辐射强度增大而增大。荧光辐射峰的FWHM约为几nm。随机激光峰分布在荧光辐射峰的鼓包上面[15],随机激光峰和荧光辐射峰强度差较小,说明由泵浦光激发样品产生的荧光辐射增强而出现随机激光。而PCF载体中,填充有液晶混合物的微孔周期性排列,使得荧光光子散射加强,利于形成反馈放大和出射随机激光。另外,图4(a)中,随机激光峰位显示了一定的周期性,峰的间距约为0.5 nm或1 nm。那么是否有回音壁模式(whispering gallery mode,WGM)激光输出呢?即荧光光子在PCF包层的圆周面上来回反射并得到干涉加强。由公式
$$D_{cal} \approx \frac{{{\lambda ^2}}}{{\pi n\Delta \lambda }}$$ (3) 取激光辐射峰的中心位置波长
$\lambda = 602$ nm,间距$\Delta \lambda {\rm{ = 1}}$ nm,折射率$n_e = 1.692$ ,计算得出$D_{cal} = 68.21$ μm,大于实际光子晶体光纤包层的直径$D_0{\rm{ = 62}}{\rm{.44}}$ μm。不符合WGM模式激光输出规律[16]。设置泵浦能量为210 μJ,探测角度为0°(光纤探头垂直光纤轴)、30°、45°、60°,测得激光辐射谱如图4(c)所示。可以看出激光辐射方向较广。WGM模式激光的特点为垂直光纤轴向方向出射,偏离光纤轴向方向强度急剧减弱,因此可以排除回音壁模式。当加热样品至液晶的各向同性温度时,发现随机激光峰消失,只能观察到荧光峰。在早期实验报道,全反射型PCF(中心为实芯)样品中也得到相同的结果[11]。这说明,填充液晶混合物的PCF载体中的随机激光辐射行为仍源于掺杂染料的液晶混合物,微孔周期性排列的PCF起到加强散射载体的作用。 -
为进一步分析PCF载体的影响,由PCF样品端面入射,在另一端面测量激光辐射谱,并改变光纤样品的温度使液晶折射率发生变化。采用光纤耦合系统,将泵浦光精确对准光纤端面。泵浦能量为30 μJ,测量结果如图5所示,激光辐射谱580~605 nm,FWHM约为0.3 nm。加热PCF样品,当温度达到70 ℃时,液晶进入各向同性态(
$\Delta n = 0$ ),随机激光峰消失,只能看到较宽的荧光辐射峰。在PCF中,手性向列相液晶形成许多不同取向的微畴,导致取向顺序
$[\delta S{\rm{ = }}S({{r}} + \delta {{r}}) - S({{r}}) \ne 0]$ 的空间不均匀性,从而使介电张量$[\delta \varepsilon = \varepsilon ({{r}} + \delta {{r}}) - \varepsilon ({{r}}) \ne 0]$ 。随着温度的升高,染料掺杂手性向列相液晶的螺距(P)将缩短,由公式(1)和(2)得,$l_{tr}$ 和D将会减小,散射强度增强,随机激光强度增加。但同时,升温过程中液晶趋向于无序状态,此时所有不同取向的液晶微畴都重新取向,$\delta S$ 和$\delta \varepsilon $ 会减小,$l_{tr}$ 和D将会增加,导致散射强度减弱,随机激光强度降低。两种效应同时存在,使得样品在加热过程中随机激光峰强度不能单调增大或减小,呈现了先增大后减弱的趋势。这进一步说明,在PCF载体中,随机激光的产生源于液晶的强散射作用。
Random laser radiation behavior of liquid crystal in photonic crystal fiber carrier
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摘要: 将向列相液晶TEB30A、手性剂S-811、激光染料PM597的混合物填充空芯光子晶体光纤中,以Nd: YAG倍频532 nm激光作为泵浦光源,测量激光辐射谱,研究了光子晶体光纤载体中的随机激光辐射行为。泵浦激光侧面入射,侧面出射随机激光波长范围为590~605 nm,半高全宽约为0.3 nm;辐射方向较广。泵浦光端面入射,端面出射随机激光波长范围为580~605 nm,半高全宽约为0.3 nm。加热样品至各向同性温度时,端面和侧面激光辐射被关断。由实验结果得出,光子晶体中随机激光辐射源于微孔中填充的染料掺杂液晶混合物。手性向列相液晶中光子传输平均自由程和液晶分子介电张量的涨落随温度的变化,是影响出射激光强度的主要因素。Abstract: A hollow-core photonic-crystal fiber filled with a mixture of nematic liquid crystal TEB30A, chiral agent S-811 and laser dye PM597 was pumped by a frequency-doubled Nd:YAG laser with a wavelength of 532 nm. The laser emission spectra was measured and the random laser radiation behavior in the photonic-crystal fiber carrier was investigated. When side-pumping was applied to the fiber, the emitted random laser with a wider radiation direction from the side face had a wavelength range of 590−605 nm and an FWMH of 0.3 nm. When end-pumping was employed to the fiber, the emitted random laser from the end face had a wavelength range of 580−605 nm and an FWMH of 0.3 nm. After the sample was heated to the isotropic temperature, the laser emission with both pumping methods was shut down. The experimental results demonstrate that the dye doped liquid crystal mixture in the micropore induce the random laser emission in the photonic-crystal. The change in the mean free path of photon transport and the fluctuation of the dielectric tensor of chiral nematic liquid crystals with temperature are the main factors affecting the laser intensity.
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图 3 PCF白光透射谱。(a) 探测角度
$\theta $ 不同时无填充的PCF样品;(b) 无填充与填充液晶混合物后的PCF样品,$\theta {\rm{ = 0}}^\circ $ ;(c) PCF样品中光场分布Figure 3. White light transmission spectrum of PCF. (a) Unfilled PCF with different detection angles θ; (b) Unfilled PCF and liquid crystal mixture filled PCF,
$\theta {\rm{ = 0}}^\circ $ ; (c) Light field distribution in PCF -
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