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在F-P腔中,光在与其传播方向垂直的两个端面间来回振荡,腔模Δλ满足以下条件:
$$ {\rm{\Delta \lambda }} = \dfrac{{{{\rm{\lambda }}^2}}}{{2{{L}}\left( {{{n}} - {\rm{\lambda }}\dfrac{{{{{\rm d}n}}}}{{{\rm{d\lambda }}}}} \right)}} $$ 式中:n为折射率;L为腔长;
$\left( {n - \lambda \dfrac{{{\rm d}n}}{{{\rm d}\lambda }}} \right)$ 为群折射率。自构型腔由钙钛矿材料本身相对的端面作为反射镜形成,目前已经有纳米线、三角锥、四边形、六边形、八边形、微球等形状的钙钛矿微腔的研究报道。然而,WGM模式的钙钛矿微腔还未实现激子与光子的强耦合,目前在自构型钙钛矿微腔中实现强耦合主要发生在自构型F-P腔中。纳米线(Nanowire, NW)在两个维度上空间受限,具有许多与体材料截然不同的新奇特性。沿着钙钛矿纳米线长度方向的端面可以自然形成F-P腔,这样的腔往往具有较小的模式体积,并且具有波导的性质,更容易观察到光子与激子的强耦合。Zhu等人[28]于2015年首次通过低温溶液法生长了高质量的CH3NH3PbI3(MAPbI3)纳米线,室温下的MAPbI3纳米线单晶在功率密度为220 nJ/cm2的脉冲激光泵浦下产生激射,出射的激光表现出显著的线偏振。随后Zhou等人[29]通过CVD生长了三角形截面的CsPbX3纳米棒。他们发现CVD生长温度对晶型有着显著影响:立方相纳米片在高生长温度下形成,而单斜相纳米棒在低生长温度下形成。通过调整卤素阴离子可得到几乎覆盖整个可见光谱区域的室温下光泵浦激光。Park等人[30]发现无法用F-P腔模式计算得到的腔长来解释所制备的CsPbBr3纳米线激光不均匀的模式间距,通过改变探测角度排除了横模叠加的因素后,他们将其归因于在脉冲激光泵浦的过程中存在光子与激子的强耦合。
为了探索钙钛矿纳米线中可能影响激子极化激元的产生和激射的条件,需要对纳米线本身(材料、尺寸和形貌)和激光泵浦的条件进行进一步研究。Zhang等人[31]于2018年报道了甲胺铅溴(MAPbBr3)钙钛矿纳米线的激射与纳米线尺寸和泵浦功率的相关性,图4(a)给出了实验的示意图。在较小直径的纳米线中,由于光子模式体积较小,振子强度和拉比劈裂能较大(见图4(b),尺寸为0.27 µm× 5.8 µm的MAPbBr3纳米线的激子极化激元色散曲线)。他们研究了不同尺寸纳米线的振子强度和拉比劈裂能,当光子模式体积减小到3.6 μm3以下时,观察到光子与激子耦合强度急剧增加(见图4(c),拉比劈裂能与不同长度纳米线的有效模式体积的关系,有效模式体积更小的纳米线中的腔极化激元表现出更大的拉比劈裂能),群速度显著降低。而随着泵浦功率的增加,载流子屏蔽效应导致振子强度减弱,使得激子与光子的耦合强度减弱。随后Du等人[32]在CVD合成的CsPbBr3纳米线中实现了沿长度方向的F-P腔模与激子的强耦合,图4(d)给出了具有高度平整表面的CsPbBr3纳米线的SEM照片。通过对空间分辨光致发光光谱的分析(见图4(e),从CsPbBr3纳米线的波导端收集的发射光谱),得到了高达656 meV的真空拉比劈裂能(见图4(f),沿纳米线长度方向的激子极化激元色散曲线),并且通过比较激射光谱和波导输出光谱,证实了室温下CsPbBr3纳米线中低阈值(8 μJ/cm2)激子极化激元的激射行为。
图 4 自构型F-P微腔中的激子-光子耦合强度的调控
Figure 4. Regulation of exciton-photon coupling intensity in self-organized F-P microcavities
开展激子极化激元在低维钙钛矿中传输和介电特性方面的研究对激子极化激元器件的实现有着重要意义。Shang等人[33]借助空间分辨光致发光的手段研究了CVD生长的CsPbBr3纳米线中激子极化激元对光吸收和慢光效应的增幅作用(见图5(a),沿着CsPbBr3纳米线光传输的示意图)。他们的工作证明了室温下钙钛矿纳米线中粒子以激子极化激元的形式传播,并表现出显著的空间色散。随着光子能量的增加或者纳米线维度的降低,光子-激子耦合强度增加,激子极化激元中激子的成分显著提升(见图5(b),对于不同尺寸的纳米线,LPB中激子的成分与ΔE的关系),从而提高了激子极化激元的有效质量(见图5(c),对于不同尺寸的纳米线,LPB有效质量与ΔE的关系)。有效质量更大的激子极化激元显著提高了钙钛矿纳米线对光的吸收,更适用于调控光子和增强光与物质相互作用。
图 5 自构型F-P微腔中激子极化激元的传输和激射行为研究
Figure 5. Propagation and lasing behavior of exciton-polaritons in self-organized F-P microcavities
连续光(continuous-wave, CW)泵浦钙钛矿激光是迈向电泵浦钙钛矿激光的必经之路。Zhu等人[34]通过溶液法制备了单晶CsPbBr3纳米线,并实现了77 K下连续光泵浦极化激元激射,其阈值为6 kW/cm2。Shang等人[35]于2020年研究了反溶剂法生长的CsPbBr3纳米带在连续光泵浦下的极化激元激射。如图5(d)所示,他们选用蓝宝石衬底作为导热材料,在78 K下实现了阈值为2.6 kW/cm2的连续光泵浦激光(见图5(e),随着泵浦功率增加,从自发辐射到激光的转变),并指出利用高热导率、低反射率的衬底和薄的增益材料是减小光热效应、实现连续光泵浦激光的关键。将温度降至7.8 K时,他们实现了130 W/cm2的低阈值连续光泵浦(见图5(f),在不同的拟合因子β下,对数-对数坐标下积分强度与功率密度在7.8 K时的关系,β为辐射到特定光学模式中自发辐射的占比)。表面平整的带状纳米线激光器将更适合与其他光电器件集成,他们的工作为实现片上高性能低阈值激光器集成的大规模应用奠定了基础。
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非自构型F-P腔指作为增益介质的钙钛矿与外部的反射镜构成的组合结构。与自构型F-P腔相比,外加DBR或者金属反射镜的非自构型F-P腔能实现更高的品质因子Q,光损耗较低,更容易实现腔模与钙钛矿激子的强耦合。并且由于外部反射镜的封装,使得腔内的钙钛矿具备更高的稳定性,更有利于钙钛矿激子极化激元的相关研究。
将钙钛矿与F-P腔进行耦合时,钙钛矿的激子能级与微腔的腔模并不总是完全重叠的,通常用失谐度Δ来描述腔模能量与激子能量的差值,即Δ=Ecav−Eex。Zhang等人[9]通过改变DBR中间CsPbBr3的厚度来控制高品质F-P腔的模式,从而调控失谐度Δ。如图6(a)、(b)所示,腔内的极化激元凝聚的稳态/亚稳态可通过改变失谐度或泵浦功率来调制,最终极化激元的状态由二维空间限制下极化激元-激子池的散射和量子化极化激元模式之间的散射决定。较大负失谐(Δ=−118 meV)条件下,激子极化激元中的光子权重更高,此时激子极化激元通过热弛豫达到凝聚的效率很低,更倾向于发生激子极化激元之间的散射,并最终在LPB有限动量位置形成亚稳态凝聚。而当激子极化激元中激子的权重更大(Δ=−36 meV)时,LPB底部的凝聚可通过热弛豫实现。该结果揭示了可以通过腔结构或泵浦功率密度的调制来实现控制定向传播的极化激元凝聚,从而实现与非线性极化激元集成器件相关的应用。
图 6 非自构型F-P微腔中激子极化激元凝聚和调控
Figure 6. Condensation and regulation of excition-polaritons in non-self-organized F-P microcavities
借助非自构型微腔带来的特性,Su等人[36]观察到室温下钙钛矿微米线中长程相干的激子极化激元凝聚。他们将CVD生长的CsPbBr3微米线转移到沉积了65 nm的SiO2作为隔离层的DBR衬底上,并在钙钛矿微米线上旋涂一层50 nm的聚甲基丙烯酸甲酯(PMMA)以保护钙钛矿,所制备的F-P腔Q值高达1150。经实验发现,在脉冲光泵浦下,当泵浦功率密度达到阈值0.8 μJ/cm2时,观察到发射强度急剧增大,而发射线宽迅速减小。当泵浦功率密度超过阈值时,CsPbBr3微米线实空间的发射呈现出清晰的干涉条纹(见图6(c),微米线微腔中微米线的泵浦位置(红色虚线圈)和探测位置(蓝色虚线圈)示意图;图6(d)、(e)分别为极化激元凝聚阈值以上的微米线微腔实空间图像的测量和模拟结果),此时在CsPbBr3微米线中出现了长程相干的激子极化激元凝聚以及凝聚体的传播行为,其传播群速度为10 μm/ps,大于GaAs微腔中极化激元传播群速度。
随着微纳加工技术的进步,人们可以通过构建光子晶格的周期性势场俘获激子极化激元凝聚态,用于实现超冷原子的可控模拟。Su等人[37]在DBR与CsPbBr3集成的微腔基础上,用PMMA涂覆在CsPbBr3上并用电子束光刻的手段制备了一维微柱阵列晶格(图6(f)),当泵浦光垂直入射时,观察到激子极化激元很好地被限制在晶格内,并且由于限域效应表现出离散的能级,通过测量单个微柱基态极化激元的发光,得出s轨道对应的基态为2.232 eV,p轨道对应的第一激发态为2.273 eV,并且观察到了一维钙钛矿人工晶格中极化激元凝聚的长程空间相干性(见图6(g),室温下一维钙钛矿晶格凝聚区长程空间相干性的构建)。这项工作展示了晶格设计在激子极化激元调控方面的巨大潜力。他们在后续的工作中还对电子束光刻的PMMA环形势阱[38]以及基于Su-Schrieffer-Heeger(SSH)模型的激子极化激元拓扑绝缘体[39]进行了报道。前者利用各向异性有效质量引起的对称性破缺,实现了两个相反的自旋轨道模式的耦合,从而形成了实空间中锁相的花瓣状模式,在动量空间中形成多个离散的激子极化激元凝聚态;而后者利用卤化物钙钛矿微腔的各向异性和强光子自旋轨道耦合,演示了室温下微腔中锯齿链状(见图6(h),通过将PMMA间隔层图案化形成的钙钛矿之字形晶格的示意图)的相位通过对线偏振的改变实现从非平凡拓扑相到平凡拓扑相的切换,并且证明了室温下脉冲光泵浦使激子极化激元凝聚成拓扑极化激元边缘态的可行性。
与纯光子相比,激子极化激元具有高出几个数量级的非线性系数,这种非线性最早在GaAs/InGaAs量子阱的极化激元参量振荡中观测到[40-41]。参量振荡是一种非线性过程,能够相干地产生信号光和闲频光,从而构成宽带可调谐源和混频器的基础。大部分基于极化激元的器件受限于传统半导体较弱的激子束缚能,需要低温条件,而F-P腔与钙钛矿的耦合恰好能解决这个难题。最近,Wu等人[42]首次观察到室温下F-P腔中的CsPbBr3多个极化激元分支的参量散射。他们分析了在高于激子极化激元凝聚阈值泵浦下相关的角分辨发射光谱,由于CsPbBr3单晶的各向异性,极化激元下能支在面内波矢k=0处出现沿晶轴方向的线性极化分裂,产生LPB 1和LPB 2两个能支。在阈值Pth为0.35 μW的脉冲光泵浦下,极化激元在LPB 1的最低能量处发生凝聚,并开始散射到LPB 2上。当泵浦功率增加,散射源的极化激元密度增大,其非线性极化激元相互作用也变得更强。在1.6Pth的脉冲泵浦下,闲频光与信号光的强度达到很好的平衡,这也意味着从瑞利散射区过渡到了非线性参量散射区。
Exciton-polaritons in Fabry-Pérot microcavity based on halide perovskites (Invited)
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摘要: 当激子与腔光子间的相互作用强于激子和腔光子的衰减时,激子能级与腔模之间产生强耦合,形成的准粒子被称为激子极化激元。激子极化激元有效质量小,同时具有较强的非线性,在慢光和低功耗发光器件等方面具有巨大的应用前景。传统Ⅲ-Ⅴ族无机半导体材料激子束缚能较弱,而有机半导体材料非线性系数较小等问题限制着室温条件下激子极化激元的应用。卤化物钙钛矿材料具有高吸收系数、长扩散长度、高缺陷容忍度以及低非辐射复合率等一系列优异的光电性质,并且具有高的激子束缚能和振子强度,成为研究光与物质强相互作用的理想材料。文中从卤化物钙钛矿结构和法布里-珀罗(Fabry-Pérot, F-P)微腔类型两方面介绍了近年来卤化物钙钛矿与F-P微腔强耦合在激子极化激元方面的研究进展。首先回顾了极化激元的研究背景和卤化物钙钛矿的基本光电特性,其次介绍了三维钙钛矿和二维层状钙钛矿各自的特点以及与F-P微腔强耦合的相关研究,随后对钙钛矿的自构型和非自构型F-P微腔激子极化激元的调控与相关应用进行了讨论,最后总结和展望了卤化物钙钛矿激子极化激元面临的挑战以及未来研究方向。Abstract: When the interaction between excitons and cavity photons is stronger than the decay of excitons and cavity photons, a strong coupling occurs between exciton energy level and cavity mode, thereby generating the quasi-particles called exciton-polaritons. The small effective mass and strong nonlinearity of exciton-polariton make it great potential in the applications of slow light and low-power-consumption light emission devices. However, weak exciton binding energy of traditional III-V inorganic semiconductor materials and weak nonlinearity of organic semiconductor materials limit their application of exciton-polaritons at room temperature. In contrast, halide perovskites have a series of excellent photoelectric properties such as high absorption coefficient, long diffusion length, high defect tolerance, and low rates of nonradiative recombination. Furthermore, with large exciton binding energy and oscillator strength, halide perovskites become an ideal material for studying strong interaction between light and matter. The research progress on exciton-polaritons based on the strong coupling between halide perovskite and Fabry-Pérot (F-P) microcavities was introduced from two aspects: the structure kinds of halide perovskites and the type of F-P microcavities. Firstly, the research background of polaritons and the basic photoelectric properties of halide perovskites were reviewed. Secondly, the respective characteristics of three-dimensional perovskites and two-dimensional layered perovskites and related research on strong coupling with F-P microcavities were introduced. Afterwards, the regulation and application of self-organized and non-self-organized F-P microcavities to perovskite exciton-polaritons were discussed. Finally, the challenges and future research directions of halide perovskite exciton-polaritons were summarized and prospected.
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Key words:
- exciton-polariton /
- perovskite /
- microcavity /
- strong coupling
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