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TMD是一种丰富的天然的材料,其相邻的两层由范德华力连接。其化学式为MX2,其中M是第六主族的过渡金属元素(M=Mo、W),X是硫族元素(X=S、Se、Te)。TMD自发现其在单层极限带隙从间接带隙转变为直接带隙以来,一直是一组极具吸引力的半导体。相比于3维材料,直接带隙2维TMD有着更高的激子结合能(0.5 eV~1.0 eV)[31-34],并且对于部分TMD,比如2维MoS2,其单层光致发光(photoluminescence,PL)较其3维形态有所提高。它们为研究具有新的自旋谷自由度的半导体材料中的2维激子提供了一个几乎理想的系统。
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要实现极化激元激光首先要实现激子与微腔模式的强耦合,而实现强耦合的最常见方法是将工作介质嵌入两个DBR形成的法布里-珀罗(Fabry-Perot, F-P)光学微腔中[35-39]。当激子与电磁场之间的能量交换率高于系统的其它能量耗散率(即光学损耗和激子非辐射损耗)时,就会发生强激子-光子耦合。能量交换率通常与激子振荡强度有关,这也是TMD和钙钛矿成为在室温下实现强耦合的良好材料的一大原因。当满足这些条件时,系统可以用两种新的本征态来描述,分别称为上激子极化激元(upper polariton branch,UPB)和下激子极化激元(lower polariton branch,LPB)。在耦合振荡模型中,UPB、LPB的能量[9]为:
$ \begin{gathered} E_{\mathrm{UPB}, {\rm{L P B}}}=\left(E_{\mathrm{ex}}+E_{\mathrm{cav}}\right) / 2+\mathrm{i}\left(\hbar \varGamma_{\mathrm{ex}}+\hbar \varGamma_{\mathrm{cav}}\right) / 2 \pm \\ \sqrt{V^2-\frac{\left(E_{\mathrm{ex}}-E_{\mathrm{cav}}+\mathrm{i} \hbar \varGamma_{\mathrm{ex}}-\mathrm{i} \hbar \varGamma_{\mathrm{cav}}\right)^2}{4}} \end{gathered} $
(1) 式中:$ \hbar $是约化普朗克常数;Eex是激子能量;Ecav是腔模式能量;Γcav和Γex分别为腔模式和激子的半峰全宽;V为光与物质相互作用势能,而当Eex=Ecav时,拉比分裂能量[9]为:
$ \hbar \varOmega_{\text {Rabi }}=2 \sqrt{V^2-\frac{\left(\hbar \varGamma_{\mathrm{ex}}-\hbar \varGamma_{\mathrm{cav}}\right)^2}{4}} $
(2) 早在2014年,LIU团队在2维TMD中观测到了激子-极化激元[9]。该实验在MoS2嵌入SiO2/Si3N4的DBR组成的光学微腔中实现了激子与光子的强耦合,拉比分裂为$ \hbar $ΩRabi=46 meV±3 meV。随后,其它的TMD也陆续实现了激子极化激元强耦合,比如,2016年,FLATTEN等人首次在室温下实现了激子极化激元的强耦合[40]。该团队构造了一种基于WS2材料的开放式结构(见图 1a),微腔的一侧是DBR,另一侧是银镜,中间夹着WS2;图 1b是当保持模式数q=3时,拉比分裂的反交叉图像;由于是开放式结构,其拉比分裂将随腔的模式数而改变(见图 1c),通过改变腔长度以改变模的数量,可以得到模的数量变化对拉比分裂能量的影响,图 1c中纵坐标相同的3个点为同一模式数下的重复测量,从左至右模式数依次为q=3,4,…, 12,当模式数增加时,拉比分裂能量减小;图 1d中的红线为图 1b中激子能量与腔能量相交处的垂直切面图,此时腔长L=0.185 μm,绿线和蓝线为拟合的洛伦兹峰,可以得到拉比分裂为$ \hbar $ΩRabi= (70±2) meV。
同年,HU团队利用MoS2材料实现了塔姆极化激元与激子的强耦合,拉比分裂为54 meV[31]。2017年,WURDACK团队则利用GaAs中的万尼尔(Wannier)型激子、单层MoSe2中强束缚谷激子和塔姆-等离子体-极化激元器件中的微腔光子间的强耦合机制,形成了混合激子-极化激元强耦合[41]。
2020年,XIE等人从理论上研究了具有单个金属纳米棒的镜面纳米颗粒(nanoparticle on mirror,NPoM)系统支持的磁偶极子模式与单层WS2中的激子之间的强耦合[29],其结构如图 2a所示;该团队证明了仅有少数激子(激子数N<10)参与时,就可以产生高达220 meV的拉比分裂(见图 2b),比以往实验中实现的强耦合高了一个数量级;该团队还提供了一种量子理论模型,用于解释这种强耦合机制,其与模拟的结果符合良好(见图 2c),并且指出:随着该系统的非相干程度的增加,UPB布居数将增加(见图 2d),这为未来实现极化激元激光提供了全新的思路。
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相干性是满足激光标准的基础。然而,迄今发现的这些单层半导体中的极化激元由于极化激元动力学的不可控和相干耦合减弱,很难支持强非线性相互作用和量子相干性,这为2维激子-极化激元的研究带来了不小的阻碍[42]。相干性减弱表现为分裂线宽比(splitting-to-linewidth,SLR)较小。大的SLR可以在高抽运密度下保持不受干扰的相干耦合,支持强非线性极化激元相互作用,并减缓相对于非线性相互作用率的极化激元泄漏率,这对极化激元凝聚至关重要[43]。
激子-极化激元组成是控制极化激元凝聚态非线性相互作用的重要物理量[44-45],可以通过腔-激子的失谐直接管理。由于难以控制激子部分,这种失谐通常通过对光子部分进行繁琐的修改来实现[44, 46-49]。2017年,LIU等人利用TMD中特有的Wannier-mott激子,以其对温度的敏感性来控制激子与极化激元的耦合强度与失谐,进而达到调节SLR的目的[50],该团队设计了一种紧凑的结构,通过将脆弱的2维WS2夹于含氢硅酸盐(hydrogen silsesquioxane,HSQ)和Al2O3保护层中,再在上下各放上7.5对、12.5对SiO2/Si3N4分布式DBR以实现强耦合激子极化激元,其强耦合的特性可从图 3中的双支极化激元色散特征中看出[50]。图 3a中纵轴表示光子能量,灰度表示反射率。在TM偏振下,激子与腔模式耦合,形成UPB与LPB,UPB的色散在小入射角时变平,而LPB的色散在大角度时变平。空穴光子色散以蓝色虚线表示,而激子能量(2.078 eV)则以红色虚线表示,极化激元分支用品红色曲线标识。在入射角θ的正弦值sin θ=0.25时,拉比分裂直接读数约为40 meV,因此SLR大于3.3。通过改变温度,调节Wannier-mott激子,进而控制对应的样品失谐,从而表现出不同的拉比分裂(见图 3b)。130 K、210 K、230 K时的拉比分裂为39 meV、37 meV、36 meV。色散曲线与PL分布(见图 3c)非常一致,证实了在这种非共振抽运过程中,经过散射和热弛豫后的极化激元态是稳定的。极化激元中激子与光子的权重系数可由Hopfield系数给出。随着腔体失谐由负值(130 K)到接近零值(210 K)再到正值(230 K),Hopfield系数的变化表明,LPB可以灵活地从更像光子的状态调谐到光子-激子混合状态,并在小入射角下调谐到更像激子的状态(见图 3d)。这种可调谐性提供了随意控制极化激元组成的自由,从而优化了极化激元动力学。这项工作为极化激元凝聚提供了基础,还为维持单层TMD中谷极化激元相干性带来了希望。
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BEC的实现对于极化激元激光的相干性有着重要的意义。在高温下实现BEC通常需要外界抽运能量,当能量超过一定值时,则会出现明显的阈值现象:PL的强度有明显的非线性增长,而峰宽则由于凝聚效应迅速下降到某个值附近,表现出相干性。近年来有不少团队致力于实现单层TMD中的BEC,比如,2018年,WALDHERR等人使用GaAs量子阱与单层MoSe2实现了超低温(4.2 K)下的BEC[51];2021年,ANTON-SOLANAS团队利用GaAs和MoSe2实现了BEC以及对极化激元的谷调控[52]。这些工作为日后实现2维TMD极化激元激光铺平了道路。
2021年,ZHAO团队首次实现了室温下2维TMD材料的超低阈值极化激元激光[53]。如图 4a所示,实验微腔为全介质λ/2微腔,由单层WS2、DBR和两个SiO2间隔层组成;在强耦合以及抽运光源作用下,激子-极化激元通过受激散射发生BEC,LPB色散表明大量粒子凝聚在基态(见图 4b);在高于阈值的抽运能量下,极化激元发出陷阱态发射以及LPB发出非凝聚背景发射,在PL谱中表现为非常细的洛伦兹峰以及背景发射宽峰(见图 4c);随着抽运能量的增加,洛伦兹峰变得更加的尖细,并且发生蓝移,充分体现了激子-极化激元的BEC现象,在迈克耳孙干涉仪下,在时间延迟Δt=0的情况下有着不错的可见度,进一步说明了发出的激光的相干性(见图 4d);如图 4e所示,通过减小微腔模式的失谐,激光阈值可以降低,最低可达到约0.06 W/cm2,这些工作为日后低阈值极化激元激光提供了良好的平台;图 4f中清楚地展示了在阈值附近出现的PL随抽运能量的非线性增长,这与图 4c相对应。
不同于前面提到的LPB的BEC,2023年,CHEN团队报告了在可转移WS2单层微腔中观察到上极化激元分支的BEC(见图 5a)[54]。该团队设计了一种将WS2夹在两块由SiO2/SiNx层交替构成的DBR组成的光学微腔中。实验中,UPB的PL比LPB的更强,在增大功率时,出现了明显的阈值现象(见图 5b),PL出现强烈的非线性增长,峰宽降低为原本的1/4倍,能量则出现1 meV的蓝移,极化激元时间相干性增加;通过调节迈克耳孙干涉仪的时间延迟可以测得当抽运功率从0.6Pth增加到2Pth时(Pth为阈值能量),相干时间(δt)从55 fs增加到138 fs(见图 5c、图 5d);该实验中,强耦合机制下拉比分裂为30 meV(见图 5e)。模型模拟与实验结果表明,只有当UPB和下极化激元之间的转换时间长于或相当于极化激元的寿命时,才会发生UPB凝聚,在长寿命极化激元的情况下,UPB衰变到LPB的速度要比在腔外衰变的速度快得多。正因为如此,几乎所有的UPB在逃逸出空腔之前都会首先转化为LPB,从而使得UPB分支难以被探测到。随着受激散射导致的极化激元密度增加,这一转换时间会变得更快,最终快到足以与极化激元的寿命相媲美。这种效应也是上极化激元凝聚阈值低于下极化激元的重要因素。该工作对极化激元激光器的设计和开发具有实际意义,因为在极化激元激光器中,LPB和UPB凝聚态之间的竞争起着至关重要的作用。
2维材料中极化激元激光的研究进展
Research progress on polariton lasing in 2-D materials
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摘要: 极化激元激光是一种新型激光,以半导体中激子-极化激元的玻色-爱因斯坦凝聚(BEC)的相干性实现超低阈值激光。不同于传统3维有机无机材料,2维过渡金属硫族化合物与2维钙钛矿以其高激子结合能、高振子强度、直接带隙、范德华特性、谷极化特性,有利于实现激子与腔模式的强耦合以及激子-极化激元的BEC, 在极化激元激光的进一步发展中表现出巨大的潜力。介绍了2维材料中的极化激元激光的原理和进展,分别从2维过渡金属硫族化合物和2维钙钛矿的特性出发,重点综述了其中激子与腔模式的强耦合,极化激元的相干性的调控以及BEC的实现,并对2维材料中的极化激元激光的未来发展进行了展望。Abstract: Polariton lasing is a new type of lasing that realizes ultra-low threshold lasing with the coherence of Bose-Einstein condensation(BEC) of exciton-polariton in semiconductors. Unlike conventional 3-D organic and inorganic materials, 2-D transition metal dichalcogenides and 2-D perovskite show great potential in the further development of polariton lasing due to their high exciton binding energies, high oscillator strengths, direct band gaps, van der Waals properties, and valley polarization properties, which are conducive to the realization of strong coupling of exciton and cavity modes and BEC of exciton-polariton. In this review, the principle and progress of polariton lasing in 2-D transition metal chalcogenides and perovskite were mainly focused on, starting with development of strong coupling between exciton and cavity modes. Then, the modulation of the coherence of polariton, the realization of the BEC and polariton lasing was introduced. Finally, an outlook will be given on the future development of polariton lasing in 2-D materials.
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