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CN113867063B - 一种铁电螺旋液晶材料及其实现二次谐波增强的方法 - Google Patents

一种铁电螺旋液晶材料及其实现二次谐波增强的方法 Download PDF

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CN113867063B
CN113867063B CN202111267015.0A CN202111267015A CN113867063B CN 113867063 B CN113867063 B CN 113867063B CN 202111267015 A CN202111267015 A CN 202111267015A CN 113867063 B CN113867063 B CN 113867063B
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谢晓晨
黄明俊
赵秀虎
李金星
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Abstract

本发明公开一种铁电螺旋液晶材料及其实现二次谐波增强的方法。该方法通过手性分子掺杂铁电向列相液晶得到具有超高极性螺旋结构的铁电螺旋液晶，其介电常数大于104。此类液晶材料具备超强的非线性光学效应，可以激发高强度的二次谐波(比如，与LiNbO3非线性(NLO)结晶材料的非线性系数相当)。本发明基于铁电螺旋液晶的周期性螺旋结构可调性，通过手性分子浓度调谐铁电螺旋液晶的分子螺距(即调控极化周期)，首次实现在高流动性液体材料中的准相位匹配技术，从而实现不亚于商用NLO结晶材料的二次谐波增强。

Description

一种铁电螺旋液晶材料及其实现二次谐波增强的方法
技术领域
本发明属于非线性光学材料制备及应用领域，公开了一类新颖的具有周期性螺旋结构的超高极性流体材料可满足准相位匹配条件，达到二次谐波增强的制作方法及其在激光倍频调制方面的应用。
背景技术
1961年，P.A.Franken等人首次报道了激光的非线性变频现象，开启了非线性光学研究的新领域。对于非线性光学过程，一般采用光在介质总的极化响应来描述：
Figure BDA0003327115230000011
式中，P和E分别为极化强度和电场矢量，χ为极化率，ε0为真空介电常数，P(1)＝ε0χ(1)·E为线性极化强度，描述了各种线性光学现象，其余各项为非线性项，用来描述光与物质的非线性作用。在诸多非线性光学现象中研究最为广泛的是二阶非线性光学效应，特别是三波混频，它是目前激光频率变换与扩展研究的最主要方法。在三波混频过程中，当两束入射光的频率相等，即ω1＝ω2＝ω时，出射倍频光ω3＝ω1+ω2＝2ω，即光学倍频效应，又称为二次谐波现象，通常入射光称为基频光，出射光称为倍频光。
对于二次谐波产生的强度，若不考虑走离效应，可简化为以下的关系式：
Figure BDA0003327115230000012
式中I2ω和Iω分别表示倍频光和基频光的强度，L为光束穿过样品的厚度，deff为有效二阶非线性系数，
Figure BDA0003327115230000013
为相位失配因子，其中λ为入射光的波长。由于介质的色散效应，一般地n2ω≠nω，即Δk≠0，称为相位不匹配条件。倍频光强度随着晶体长度L的增加成周期性的变化，这里称Lc＝π/Δk为相干长度。在L从0增加Lc的过程中，倍频光强度随着L的增加单调增加，能量从基频光向倍频光转换；而当L从Lc增加2Lc的过程中，倍频光随之L的增加而单调减小，能量反过来又从倍频光向基频光转换，此后一直这样往复。因此在相位不匹配的条件下，非线性转化效率极低。只有满足相位匹配条件Δk＝0，即基频光和倍频光在介质中的传播速度相等，或折射率相等，倍频光的强度才能不断地增长1962年，J.A.Armstrong与N.BLogembergen等人提出了准相位匹配(Quasi-PhaseMatching)的技术。假设泵浦激光振幅恒定，信号波长可以表示为非线性光学介质中存在的畴数量的总和，通常，信号幅度的变化率为：
Figure BDA0003327115230000021
对于周期性极化的非线性光学晶体而言，在每一个相干长度的畴内，晶体的晶轴反转180°，导致非线性极化率χ改变符号。对于有n个畴的介质，χ可表示为：
χ＝χ0(-1)n
则当泵浦激光走过n个畴时产生信号的总幅度为：
Figure BDA0003327115230000022
产生的二次谐波强度可表示为：
Figure BDA0003327115230000023
Figure BDA0003327115230000024
与双折射相位匹配相比，准相位匹配是利用非线性介质光学性质的周期性分布补偿相位失配的。每当光在非线性介质中传播经历了一个相干长度Lc时，通过人工调制引入一个π的相位补偿，使得能量持续地从基频光向倍频光转换。准相位匹配技术对非线性介质的耦合光波波矢方向和偏振方向没有严格的限制，只需要人工引入一个恰当的极化周期性结构。非线性光学晶体的极化周期可根据Sellmeier方程和波矢的关系来计算，即Λ＝2mLc＝mλ/2(nω-n2ω)(m为奇数)。
实现准相位匹配过程的关键在于调谐适当的周期性结构的非线性介质，具有反向平行畴结构的非线性光学晶体是应用于准相位匹配技术的主要非线性介质。非线性光学晶体具有自发的极化，且自发极化可以被外电场改变。目前，已有多种加工方法用于制备准相位匹配的铁电畴结构，其中最常用的是利用图案化电极和高压电场极化法，还有早期的生长条纹法、近年来发展的光辅助极化、全光极化等方法。这些方法制备过程繁琐，对设备要求高，制造具有高质量和可靠的周期性极化晶体具有挑战性，并且只有使用某些晶体材料才有可能。制备的细节和成功率在很大程度上取决于材料——不仅是材料类型，还有缺陷密度、化学计量、表面处理等。周期性极化只能应用于厚度相当有限的晶体，对于不同的过程，需要许多不同的极化周期。另请注意，精确预测所需极化周期需要精确的折射率(Sellmeier)数据。伴生高阶过程可以产生额外波长的光，这可能会以各种方式产生干扰。
2017年，英国约克大学的Mandle博士和Goodby教授合成了一种具有大电偶极子的楔形分子。研究发现，该分子在高温表现为普通向列相，但在低温下(小于133℃)呈现出一种新型的、具有铁电特征的向列相结构，即分子排列产生自发极化，向列相分子偶极矩在空间分布上变得有序，形成具有特定取向的畴。同年，日本九州大学的Kikuchi Hirotsugu教授也发现了一种具有极高介电常数的极性向列相液晶，该材料还表现出极强的二次谐波响应等特性。目前，这种新型向列相的基础研究尚处于起步阶段，但其极强的介电和非线性光学特征使其具有很高的应用价值。
通过向铁电向列相液晶中加入手性分子可以得到具有周期性螺旋结构的铁电螺旋液晶，与传统的非线性光学晶体相比，铁电螺旋液晶在一个完整周期内(即一个螺距内)具有类似于非线性光学晶体的周期性畴结构，且铁电向列相液晶的自发极化沿着分子长轴方向排列，当液晶分子沿着螺旋轴螺旋排列时，每扭转180°(即半个螺距)时保留了极化特征，这样当液晶分子沿着螺旋轴扭转360°(一个完整的螺距)时相当于非线性光学晶体的一个极化周期，具有两个相反方向的周期性极化畴结构。铁电螺旋液晶的极化率可以用类似于非线性光学晶体的极化率来表示：
Figure BDA0003327115230000031
在铁电螺旋液晶体系中，极化周期也可通过Sellmeier方程和波矢的关系来计算，同时和铁电螺旋液晶的周期性螺旋结构(pitch)也有关系，即pitch＝Λ＝2mLc＝mλ/2(n2ω-nω)(m为奇数)，故可通过调谐铁电螺旋液晶的周期性螺旋结构来实现准相位匹配技术，与传统的非线性光学介质的二次谐波强度与相干长度Lc模拟结果见附图3。
铁电螺旋液晶对电场有快速的响应，可以通过向铁电螺旋液晶施加面内电场来调控液晶的周期性极化结构，且这种电场调控是可逆的。当向铁电螺旋液晶施加一个正的电场(E＞0)时，与正电场方向一致的极化方向不发生改变，与正电场方向相反的极化会向施加电场的方向旋转，破坏了螺旋结构的对称性，从而改变周期性极化结构；撤去电场时，被改变的极化结构又会恢复到初始状态(E＝0)；同样地，当改变电场的方向时，即向铁电螺旋液晶施加一个负的电场(E＜0)时，与负电场方向一致的极化方向不发生改变，与负电场方向相反的极化会向施加电场的方向旋转，从而可以对周期性极化结构进行调控，撤去电场时，被改变的极化结构又会恢复到初始状态(E＝0)(详见附图1)。同时，与传统的非线性光学晶体相比，液晶具有良好的流动性性，容易制备成器件，而且铁电螺旋液晶的周期性极化结构十分容易调谐。铁电向列相液晶的发现，使得铁电螺旋液晶液晶成为一种潜在的具有周期性极化结构的可满足准相位匹配条件的非线性介质，从而实现二次谐波增强，在激光倍频领域具有广泛的应用前景。
发明内容
目前，利用准相位匹配技术实现二次谐波增强还集中在调谐具有周期性结构的非线性光学晶体领域，这些方法大多制备过程繁琐，对设备要求高。本发明利用了铁电向列相液晶和手性分子耦合制备出具有周期性螺旋结构的铁电螺旋液晶材料。所述这种铁电螺旋液晶材料具有极强的二次谐波响应特征，自身即可激发光子，同时这种铁电螺旋液晶的周期性螺旋结构方便调谐，可调谐至准相位匹配所需的极化周期从而实现二次谐波增强，这在准相位匹配技术的应用领域尚处首例。而且相比较传统的准相位匹配技术，这种铁电螺旋液晶材料无需复杂的设备、周期方便可调、制作简单等特点，在光学倍频领域有更加广泛的应用前景。
本发明的目的是通过如下措施来达到：
一种铁电螺旋液晶材料，铁电螺旋液晶材料通过将手性分子和铁电向列相液晶以一定的质量比均匀混合，所述手性分子占混合物中的质量分数为0.4％～1.5％，所述铁电螺旋液晶材料在具有传统胆甾相周期性螺旋结构的基础上，具有宏观的螺旋极性，所述的宏观极性是由铁电向列相液晶提供；所述的周期性螺旋结构是由手性分子和铁电向列相液晶耦合形成胆甾相液晶提供的。
进一步地，所述铁电向列相液晶在倍频光和基频光处的折射率n2ω和nω，上述铁电螺旋液晶材料的极化周期根据公式pitch＝Λ＝2mLc＝mλ/2(n2ω-nω)(m为奇数)来计算。
进一步地，所述铁电螺旋液晶材料的螺距(pitch)，通过改变手性分子和铁电向列相液晶的质量配比(0.4～1.5％质量分数的手性分子)来实现。
进一步地，所述的铁电螺旋液晶材料在128～65℃内，具有高介电常数和极强的二次谐波响应特征。
进一步地，所述高介电常数为ε～104。
进一步地，所述铁电螺旋液晶材料的二次谐波响应特征为石英晶体的3-10倍。
进一步地，所述铁电螺旋液晶材料的螺距(pitch)调至准相位匹配所需的极化周期时可实现二次谐波增强。
进一步地，所述的铁电螺旋液晶材料随着不同的手性掺杂剂浓度具有可调的螺旋周期，可调谐至满足准相位匹配条件的极化周期。
进一步地，所述的铁电螺旋液晶材料在不同厚度的液晶盒中能够均匀分布一定数量(0～100(但不限于此))的周期性螺旋结构，详见附图2铁电螺旋液晶波长转换示意图中液晶分子的排布。
进一步地，所述的铁电螺旋液晶材料达到准相位匹配技术的极化周期时，其二次谐波强度随着厚度的增大而增大，详见附图5和7，分别是1.1％R811/RM734样品不同厚度下焦点处和平行光处的SHG信号值随温度的变化。
一种利用准相位匹配技术实现二次谐波增强的制作方法：
两片由聚酰亚胺平行摩擦配向的玻璃基板，制成不同厚度的液晶盒，利用毛细作用将铁电螺旋液晶液晶灌入液晶盒中。在370～440K下退火处理半小时，使胆甾相形成稳定的平面织构。
进一步地，所述铁电螺旋液晶螺距可由手性分子浓度配比调节，当手性分子浓度在一定范围之间时，铁电螺旋液晶的螺距也会发生变化，通过调节螺距至准相位匹配的极化周期可实现SHG增强。
与现有技术相比，本发明具有如下优点和有益效果：
本发明所述的铁电螺旋液晶材料具有极强的二次谐波响应，其非线性光学特性能够与晶体中的石英相媲美，这在流体软材料中是十分罕见的。通过改变手性分子掺杂浓度，可以调节分子螺距，达到准相位匹配条件从而实现二次谐波增强，相较于没有掺杂手性分子的极性向列相，其二次谐波强度提高了4倍以上，该技术与现有的调谐合适周期性结构的非线性光学晶体相比，制备工艺简单，螺距方便可调，对设备要求不高，同时对温度敏感性不高，能在较宽的温度范围内工作。与一般的非线性光学晶体相比，铁电螺旋液晶能够通过手性分子掺杂浓度便利的调节分子螺距，而且具有柔软、易加工和成膜特性，能够实现很多晶体无法应用的工作场景，也更具有成本优势，能够较好的应用于激光倍频调制等领域。
附图说明
图1是二次谐波增强的原理图；
图2是铁电螺旋液晶波长转换示意图，铁电螺旋液晶的非线性光学效应，使波长为2λ的入射光转化为波长为λ的光波；
图3是传统非线性光学介质和铁电螺旋液晶的二次谐波与周期长度(表示为相干长度Lc的n倍)之间的关系图；
图4是不同浓度手性分子和铁电向列相液晶混合物的SHG信号值(y轴为与石英激发二次谐波强度之比)；
图5是1.1％R811/RM734样品不同厚度下焦点处的SHG信号值随温度的变化(y轴为与石英激发二次谐波强度之比)；
图6是1.1％R811/RM734样品不同厚度下焦点处的SHG信号最大值；
图7是1.1％R811/RM734样品不同厚度下在平行光路处的SHG信号值随温度的变化(平行光路处石英信号几乎为0，样品依然有很强的信号)；
图8是1.1％R811/RM734样品不同厚度下在平行光路处的SHG信号最大值。
具体实施方式
下面结合实施例，对本发明作进一步地详细说明，但不用来限制本发明的范围。
实施例1
手性分子掺杂浓度为1.1％的铁电螺旋液晶的制备方法如下：
使用三氯甲烷为溶剂，分别配制0.05％质量分数的手性分子和5％质量分数的铁电向列相液晶溶液，再按手性分子：铁电向列相液晶的质量比为1.1/98.9的配制混合物溶液，真空干燥后得到均匀混合物，标记为1.1％R811/RM734。
Figure BDA0003327115230000061
为所述铁电向列相液晶，R1、R2为甲基。
Figure BDA0003327115230000062
为所述手性分子，R1、R2为-C6H13。
实施例2
手性分子掺杂浓度为1.0％的极性胆甾相液晶的制备方法如下：
使用三氯甲烷为溶剂，分别配制0.05％质量分数的手性分子和5％质量分数的铁电向列相液晶溶液，再按手性分子：极性向列相液晶的质量比为1.0/99的配制混合物溶液，真空干燥后得到均匀混合物，标记为1.0％R811/RM734。
实施例3
手性分子掺杂浓度为0.9％的极性胆甾相液晶的制备方法如下：
使用三氯甲烷为溶剂，分别配制0.05％质量分数的手性分子和5％质量分数的铁电向列相液晶溶液，再按手性分子：极性向列相液晶的质量比为0.9/99.1的配制混合物溶液，真空干燥后得到均匀混合物，标记为0.9％R811/RM734。
实施例4
二次谐波增强方法的实现如下：
准备两片涂有聚酰亚胺膜的玻璃基板(1cm2)，用天鹅绒布摩擦取向后制备成液晶盒，液晶盒的厚度方便可调。将配制好的铁电螺旋液晶加热至液相，液晶会在毛细作用下进入液晶盒，其结构如附图2所示。在400K下退火处理半小时，使胆甾相形成稳定的平面织构。若使用1064nm的脉冲激光作为光源，由于极性胆甾相非线性光学特征，会产生532nm的二次谐波，对应的，改变手性分子掺杂浓度至1.1％，调节螺距使其满足准相位匹配技术的极化周期，以达到二次谐波增强的效果。使用光电倍增探测器对出射二次谐波进行探测，与同等条件下石英的二次谐波响应光强进行对比(见附图4、5、6、7、8)。图4是不同浓度手性分子和铁电向列相液晶混合物的SHG信号值，可以看出当手性分子浓度为1.1％时的SHG信号值最大；图5是1.1％R811/RM734样品不同厚度下焦点处的SHG信号值随温度的变化；图6是1.1％R811/RM734样品不同厚度下焦点处的SHG信号最大值，厚度越厚，极化周期越多，从而SHG信号值也越大；图7是1.1％R811/RM734样品不同厚度下在平行光路处的SHG信号值随温度的变化；图8是1.1％R811/RM734样品不同厚度下在平行光路处的SHG信号最大值厚度越厚，极化周期越多，从而SHG信号值也越大。
上述实施例为本发明较佳的实施方式，但本发明的实施方式并不受上述实施例的限制。对于本发明所属技术领域的技术人员来说，在不脱离本发明构思的前提下，还可以做出若干简单推演或者替换，都应当是为属于本发明的保护范围。

Claims (7)

1.一种铁电螺旋液晶材料，其特征在于，铁电螺旋液晶材料通过将手性分子和铁电向列相液晶以一定的质量比均匀混合，所述手性分子占混合物中的质量分数为0.4%~1.5%，所述铁电螺旋液晶材料在具有传统胆甾相周期性螺旋结构的基础上，具有宏观的螺旋极性，所述的宏观的螺旋极性是由铁电向列相液晶提供；所述的周期性螺旋结构是由手性分子和铁电向列相液晶耦合形成胆甾相液晶提供的；
所述铁电向列相液晶在倍频光和基频光处的折射率n2ω和nω，上述铁电螺旋液晶材料的极化周期根据公式pitch=Λ=2mL c=mλ/2(n2ω-nω)来计算，m为奇数；
所述铁电螺旋液晶材料的螺距pitch，通过改变手性分子和铁电向列相液晶的质量配比，0.4~1.5%质量分数的手性分子实现；
所述的铁电螺旋液晶材料在128~65℃内，具有高介电常数和极强的二次谐波响应特征。
2.根据权利要求1所述铁电螺旋液晶材料，其特征在于，所述高介电常数为ε~104。
3.根据权利要求1所述铁电螺旋液晶材料，其特征在于，所述铁电螺旋液晶材料的二次谐波响应特征为石英晶体的3-10倍。
4.权利要求1~3任一项所述铁电螺旋液晶材料实现二次谐波增强的方法，其特征在于，所述铁电螺旋液晶材料的螺距（pitch）调至准相位匹配所需的极化周期时可实现二次谐波增强。
5.根据权利要求4所述铁电螺旋液晶材料实现二次谐波增强的方法，其特征在于，所述的铁电螺旋液晶材料随着不同的手性掺杂剂浓度具有可调的螺旋周期，可调谐至满足准相位匹配条件的极化周期。
6.根据权利要求4所述铁电螺旋液晶材料实现二次谐波增强的方法，其特征在于，所述的铁电螺旋液晶材料在不同厚度的液晶盒中能够均匀分布的周期性螺旋结构。
7.根据权利要求4所述铁电螺旋液晶材料实现二次谐波增强的方法，其特征在于，所述的铁电螺旋液晶材料达到准相位匹配技术的极化周期时，其二次谐波强度随着厚度的增大而增大，分别是1.1%R811/RM734样品不同厚度下焦点处和平行光处的SHG信号值随温度的变化。
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