利用动态光散射技术测量Cu:KTN晶体中三维超晶格结构的动态弛豫过程

自上世纪被发现以来,铁电功能材料一直是新型材料中的研究热点。铁电材料特性众多,包括铁电性、介电性、压电性、热电性等。在光学领域,由于其出众的电光效应、声光效应、光折变效应和非线性光学效应,铁电功能材料在光波导、光开关、显示设备、声光调制设备、光学信息存储用以形成二次谐波产生、光学参量振荡等方面均有重要应用。此外,其也被广泛应用于薄膜存储器、高容量电容、可调谐微波器件、正温度系数热敏电阻、压电传感器、换能器、电机、热释电传感器、红外传感器、成像仪和显示设备中。

在过去的半个世纪中,钽铌酸钾(KTN)晶体因其出色的电光特性和较宽的透明光谱范围在研究和应用领域受到了广泛关注。因此,其已被应用于电光调制、光束偏转、电控镜头等等。最近的研究表明,当处在居里温度(顺电-铁电相变温度)附近时,KTN晶体会表现出各种奇特的临界现象,例如高二次电光系数、无尺度光学效应和光束反衍射传播、空间孤子和三维超晶体等。这些现象都与顺电-铁电弛豫相变及伴随相变出现的极性纳米微区有关。以三维超晶体为例,这种现象出现在具有内置一维组份振荡种子的KTN样品中。在这种样品中,伴随顺电-铁电相变过程,内置的一维有序性将以相同的空间尺寸扩展到整个三维空间中。由于其尺寸和可见光波长在数值上十分接近,这种三维结构会引起类似于X射线衍射的可见光衍射现象。

顺电-铁电弛豫相变的机理至今尚无定论,目前对其进行探究的一个重要方法就是测量极性纳米微区/极化团簇的弛豫时间。通过弛豫时间的测定,就可以用几个弛豫函数的和来描述弛豫过程。三维超晶体本质上是一个动态弛豫过程,这使得其可以被几个包含主要特征弛豫时间的弛豫函数所表征。

南开大学武鹏飞教授领导的课题组研究了KTN晶体中的三维超晶体现象,阐释了这种结构的物理机理并测量了其特征弛豫时间。相关研究结果发表在Chinese Optics Letters 2020年第18卷第2期上(Quanxin Yang, Xin Zhang, Hongliang Liu, Xuping Wang, Yingying Ren, Shan He, Xiaojin Li and Pengfei Wu. Dynamic relaxation process of 3D super crystal structure in Cu:KTN crystal [J]. Chinese Optics Letters, 2020, 18(2): 021901),并被主编选为Editors’ Pick。

实验上采用了称为动态光散射的技术完成特征弛豫时间的测量。该技术在悬浮液或溶液中的粒径测量以及聚合物的尺寸分布领域中应用广泛,而应用于铁电微结构的弛豫时间测量的动态光散射技术有两个关键点:一个是需要使用垂直、水平偏振片来提高信噪比,另一个是采用自相关方法来提取信号。

由三维超晶体引起的强烈可见光衍射现象。

研究结果表明,由于极性纳米微区和极化团簇之间的强烈耦合和相互作用,与晶体固有弛豫过程相对应的弛豫时间比实际构成这种结构的极性纳米微区和极化团簇的固有弛豫时间更长。此外,顺电-铁电弛豫相变过程中形成的极化玻璃态对应的无序背景会进一步限制嵌入的微结构的响应速度和频率,使整个系统的弛豫过程变得相当迟滞。三维超晶体在衍射功能器件领域显示出巨大的潜力。

具有不同弛豫时间自相关函数的拟合过程。

这项工作为三维超晶体的应用提供了理论准备。此外,报道的弛豫过程为研究人员对铁电弛豫机理的探究提供了全新的视角。

Dynamic relaxation process of 3D super crystal structure in Cu:KTN crystal

Due to the multitudinous properties such as instance, ferroelectricity, dielectricity, piezoelectricity, and pyroelectricity, ferroelectric functional material has become a research hotspot among all the innovative materials since it has been discovered in the last century. And in optics field, as owning outstanding electrooptic effect, acousto-optic effect, photorefractive effect and non-linear optics, ferroelectric functional material has been applied as the key component in optical waveguide, optical switch, acousto-optic modulation device, optical information storage device for second harmonic generation, optical parametric oscillation etc. Moreover, it has been utilized in thin film memory, high capacity capacitors, tunable microwave devices, positive temperature coefficient thermistors, piezoelectric transducers, piezoelectric and pyroelectric sensors, infrared sensors and imagers, display devices, etc.

In the past half century, potassium tantalate-niobate crystal (KTN) has attracted a lot of attention in the research and application field because of its exceptional electro-optic (EO) properties and wide transparent wavelength range. Thus, it has been used in EO modulation, beam deflection, electric-controlled lens, and so on. Recent researches show that in proximity of the Curie temperature (paraelectric-ferroelectric phase transition temperature), KTN crystal will present a few kinds of fascinating critical phenomena, i.e. giant EO effect, scale-free optics and anti-diffraction beams propagating, solitons, and 3D super crystal. All the mentioned phenomena are related to paraelectric-ferroelectric relaxation phase transition and the accompanied changes of PNRs. Specifically, 3D super crystal appears in KTN sample with build-in 1D compositional oscillating seed. In this kind of KTN, as paraelectric-ferroelectric phase transition takes place, the build-in 1D order will transfer to the whole volume with the same spatial scale simultaneously. Formed 3D structure can cause an optical diffraction phenomenon similar to typical X-ray diffraction since the scale of this structure and the wavelength of the laser are numerically comparable just as the relationship between crystal lattice and X-ray.

Theory of paraelectric-ferroelectric relaxation phase transition is not certainly defined, and an important method to clarify it is to measure the relaxation time of PNRs/polarized clusters. With measured relaxation time, relaxation process can be described by a sum of relaxation functions. 3D super crystal is essentially dynamic relaxation, which can be illustrated by some kinds of relaxation functions with a main characteristic relaxation time.

The research group led by Prof. Pengfei Wu from Nankai University researched the 3D super crystal phenomenon in customized KTN crystal. Physical mechanism and characteristic relaxation time were studied and demonstrated. The relevant research results are published in Chinese Optics Letters, Vol. 18, Issue 2, 2020 (Quanxin Yang, Xin Zhang, Hongliang Liu, Xuping Wang, Yingying Ren, Shan He, Xiaojin Li and Pengfei Wu. Dynamic relaxation process of 3D super crystal structure in Cu:KTN crystal [J]. Chinese Optics Letters, 2020, 18(2): 021901), and is selected as Editors’ Pick.

For the characteristic relaxation time measurement, technique named dynamic light scattering (DLS) which has been applied in the field of particle diameter measurement or size distribution profile in suspension and polymers is employed. There are two pivotal points for DLS used in the relaxation time measurement of ferroelectric microstructure: one is the utilization of vertical and horizontal polarizers for SNR increment, and the other is the auto-correlation method for signal extraction.

Strong optical diffraction phenomenon caused by the 3D super crystal.

Research results indicate that with strong coupling and interaction of PNRs and polarized clusters, relaxation time corresponding to intrinsic relaxation process of 3D super crystal is rather longer than that of polarization clusters and PNRs, though the 3D super crystal structure is essentially formed by them. Also, glassy disorder background formed during the paraelectric-ferroelectric relaxation phase transition can further restrict the response speed and the frequency of embedded microstructures, making the relaxation process of the whole system become rather tardy.

Fitting processes of auto-correlation functions with different relaxation times.

This work provides theoretical basis for the application of 3D super crystal, which shows huge potential in functional diffraction devices. Moreover, the reported relaxation process gives us a brand-new vision on the ferroelectric relaxation mechanism.