光子器件新星:二维钙钛矿光腔

主编点评:

自从2010年的诺贝尔物理奖颁发给物理学家Andre Geim和Konstantin Novoselov表彰他们对石墨烯(graphene)的贡献,科学家们对二维材料的兴趣与日俱增。

和三维材料相比,二维材料在一个维度只有单层原子,这一独特的几何结构让它拥有三维材料所没有的特殊性能。比方说通过引入疏水的有机阳离子破坏钙钛矿材料的三维结构的对称性,形成二维的钙钛矿结构以后,因为只有一层原子,电子密度和其传输特性以及电声耦合(electron-phonon coupling)效应出现极大变化。通过调节起间隔作用的阳离子以及非对称的晶格结构,和三维材料相比,有更多的自由度来调节材料的能带结构、非线性光学和光电性能。

近年来,二维钙钛矿因为其优越的光电性能在太阳能电池、光电探测器以及发光二极管方面的应用备受瞩目,堪称明星级别的材料。另一方面,在光学器件里,能增强光—物质相互作用的光学腔(cavity)是很多功能元件的基础,比如激光、调制器、频率转换器等。

如何利用二维钙钛矿材料的光电特点并将其与应用广泛的光学腔结合,在功能器件上实现更多的突破是一个值得探索的课题。针对这个需求,国家纳米科学中心的刘新风教授课题组对二维钙钛矿材料的光腔应用及其内在的光与物质相互作用进行了讨论和展望。相关综述作为封面文章发表在Photonics Research 2020年第11期上。

杨兰主编 华盛顿大学

光子器件的小型化、集成化及快速响应迫切需要推进光在微纳尺度上耦合与传播的控制。发光介质与光学微腔的耦合不仅实现了光的定向传播还抑制了光的损耗,更重要的是微腔中限域的光子还可以和介质产生相互作用。

光学微腔作为耦合过程中重要的组成部分,其品质的高低直接影响到耦合状态的好坏。而增益介质的选择在光腔构建中具有重要意义。一方面它需要尽可能高的量子效率,降低无辐射过程的损耗;另一方面在工艺上能参与构建高质量光腔,保持高的光腔品质因子。因此什么样的材料能同时满足这两个方面的需求,这一问题引起了人们的思考。

近年来,人们发现卤铅钙钛矿材料在光子学器件应用上具有先天性优势。因为其具有在室温下高效率的激子(注:受库仑势束缚的电子—空穴对复合体)发光、在高载流子浓度下较低的俄歇速率(注:电子与空穴复合时,把能量或者动量,通过碰撞转移给另一个电子或者另一个空穴的速率,影响激子寿命)以及高质量钙钛矿单晶易于制备等优点。

虽然目前人们在基于三维钙钛矿的发光二极管、光泵浦激光以及激子极化激元等领域都有了很多研究。然而许多三维钙钛矿材料具有激子束缚能较低以及缺陷态密度较高的缺点,从而在发光性能及器件应用方面存在局限,因此引起了人们对低维度钙钛矿材料的研究热情。

二维钙钛矿材料具有介电屏蔽效应和单一维度内的量子限域效应,这增加了激子复合速率;而且二维钙钛矿材料还具有二维层状特征和相对更高的量子效率的优点,因而在微纳尺度光腔构建上具有独特优势。

目前基于二维钙钛矿材料的光腔应用正引起人们的关注,当然其中也有一系列有待解决的问题,例如二维钙钛矿材料的独特性质对光腔的构建提出了什么要求,以及如何选择合适的光腔和钙钛矿的形态等。

针对该研究现状,国家纳米科学中心的刘新风教授课题组对二维钙钛矿材料的光腔应用及其内在的光与物质相互作用进行了讨论和展望。相关综述作为封面文章发表在Photonics Research 2020年第11期上(Shuai Zhang, Yangguang Zhong, Fan Yang, et al. Cavity engineering of two-dimensional perovskites and inherent light-matter interaction[J]. Photonics Research, 2020, 8(11): 11000A72)。

01 二维钙钛矿材料独特的光学性质

相对于三维钙钛矿具有更高的量子产率、更低的缺陷密度以及更单纯的激子发光;材料中同时存在平面内和垂直平面的光学跃迁途径,这会引起独特的光学各向异性;以及更容易出现自陷态的激子发光。

02 制备方法和微腔的构建途径

样品制备方面可分为:旋涂薄膜法、溶液结晶法、化学气相沉积法;

光腔的组建方式可分为:利用单晶微纳结构的自组装光腔、垂直腔面发射的法布里—珀罗腔、周期性的光腔阵列及光子晶体结构。

构建二维钙钛矿的光学腔,包括基于单晶的自组装光腔,垂直平面的法布里-珀罗腔和光子晶体结构。

03 基于二维钙钛矿光学微腔的应用

包括光子—激子强耦合、光子激光和光电器件:

在构建强耦合体系方面,回顾了在不同光腔体系中实现的激子与光腔模式的强耦合。特别是在基于单晶片的分布式布拉格反射镜(DBR)光腔中观察到了自旋依赖的极化激元相互作用和玻色—爱因斯坦凝聚现象;

在光子激光方面,指出单层无机骨架的二维钙钛矿材料存在光增益受到抑制的现象,而在准二维(多层)钙钛矿材料中此效应得到了改善。同时在混合相的二维钙钛矿材料中,容易存在高维度激子态上的光增益现象。

在光电器件方面,指出光腔对特定能量处吸收和发射的增强有利于提高器件性能和构建新型功能器件。例如提高非线性的双光子发射、构建基于二维钙钛矿纳米线的光电探测器等。

04 在光腔应用方面的机遇和挑战

刘新风教授认为,在样品制备方面,可控生长大面积单晶薄膜将会是下一步的目标;从二维到准二维钙钛矿材料的转变将如何影响光增益和激子极化激元的形成仍需要进一步的研究;二维钙钛矿光腔有望在非线性光学器件集成方面实现突破,但环境稳定性及潜在的重金属污染等问题也需要解决。

Cavity engineering of two-dimensional perovskites and inherent light-matter interaction

The pursuit of optical-electrics with small size, integration and fast operation requires the efficient control of photon coupling and propagation at nanoscale. The coupling of luminescent medium to optical cavity promotes the directional transmission and reduce the loss of light. Moreover, the confined light can interact with medium.

The choices of gain materials have significant effects to optical cavities. On the one hand, the high quantum efficiency of gain materials can reduce the nonradiative processes; on the other hand, the ease of integration with high -quality optical cavity improves the quality factor of optical modes.

In recent years, people have found room-temperature, highly efficient excitonic emission of lead -halide perovskites. Meanwhile, Auger recombination at high carrier density is also relatively low. In addition, high-quality single crystal perovskites can be easily produced by solution methods or chemical vapor deposition (CVD). All of these make perovskite advantageous materials for photonic devices.

Room-temperature LEDs, optical pumped lasing and exciton-polariton have been obtained in three-dimensional (3D) perovskites ABX3 (A=CH3NH3, CH(NH2)2, Cs; B= Cl, Br, I). However, the relative low value of exciton binding energy together with high trap density gives the way to investigate the emissive properties and applications of low dimensional perovskites.

In two dimensional perovskites, the steric effects of large organic cations induce the separation of inorganic frameworks by organic layers and lead to dielectric screen and quantum confinement effect. In spectroscopy, this can be viewed as the excitonic absorption and emission varied with different inorganic layer number. Due to their two-dimensional (2D) layered property and relative high quantum efficiency, they have unique advantages in optical cavity engineering at nano/micro size.

So far cavity applications of 2D perovskites have gained broad interests, yet with a series of problems. Questions need to be answered such as the demand of cavity construction considering the unique properties of 2D perovskites, the arrangement of suitable cavity structures and morphology of 2D perovskites.

In view of the current research status, Prof. Xinfeng Liu's group from National Center for Nanoscience and Technology made constructive discussions on the cavity application of 2D perovskites and inherent light-matter interaction. The review is selected to be on the Cover of Photonics Research, vol. 11, 2020 (Shuai Zhang, Yangguang Zhong, Fan Yang, et al. Cavity engineering of two-dimensional perovskites and inherent light-matter interaction[J]. Photonics Research, 2020, 8(11): 11000A72).

The review initially discusses the unique optical properties of 2D perovskites, and makes comparison with 3D perovskites. Firstly, higher quantum yield, lower trap density and more excitonic emission can be expected in 2D perovskites. In addition, optical anisotropy of 2D perovskite can be generated by the coexistence of in-plane and out of plane optical transition routes, which is different from many 2D materials such as transition metal cluster chalcogenides. The scattering between exciton and lattice of 2D perovskites results in the exciton-phonon coupling. Moreover, localized excitons induce the deformation of surrounding lattice, and self-trapped excitonic emission can be easily occurred in 2D perovskites.

The next section of this review introduces the synthesis of 2D perovskites and fabrication of their microcavities. Spin-coating thin film, solution crystallization and chemical vapor deposition (CVD) are used for material preparation. Based on the morphology of material source, different cavity structures can be fabricated, such as self-assembled cavity using single crystal micro structure of 2D perovskite, vertical Fabry–Pérot cavity and periodical cavity arrays like photonic crystal structure.

The construction of two-dimensional perovskite based optical cavities, including self-assembled single crystal cavity, vertical Fabry–Pérot cavity and photonic crystal structure.

The main content discusses the cavity application of 2D perovskites, including strong exciton-photon coupling, photonic laser and optoelectronic devices. In the field of strong coupling, the review summarizes the observation of strong coupling in different cavity structures. The quality factor of optical cavity plays an important role in the nonlinear performance of exciton polariton. Especially in the single crystal embedded distributed Bragg reflector cavity, spin-dependent polariton-polariton interaction and BEC have been observed. In the field of photonic laser, the review points that optical gain of single inorganic layer 2D perovskite is suppressed due to strong exciton-phonon coupling and Auger recombination processes. The obstacle is released in quasi 2D perovskite (multi inorganic layer). Hybrid quasi 2D perovskite could form fast excitonic transmission between different energy states and finally contribute to the optical gain on the lowest energy state. Finally, in the aspect of optoelectronic applications based on 2D perovskite cavities, the review indicates that optical cavity can enhanced the absorption and emission at certain energy region. This can improve the optoelectronic performance and promote the design of new-type of functional devices such as enhanced nonlinear two-photon absorption and 2D perovskite nanowire-based photon detector.

At last, the review discusses the opportunities and challenges of 2D perovskites-based cavity applications. In the field of material preparation, the controlled growth of large scale and high quality single crystal film is the next target. For the understanding of cavity mechanism, the influence of different dimensionality (from 2D to quasi-2D) to optical gain and the generation of exciton polariton need further exploration. 2D perovskite cavities have great potential in many functional devices such as nonlinear, integrated photonic devices. However the environmental stability and potential heavy metal pollution should also be considered.