Abstract:With the advent of the post-Moore era, the demand for large-capacity and high-speed information processing has caused the application of semiconductor devices to shift from electronic integration to photonic integration. High-performance micro-nano lasers are an important part of achieving photonic integration. Varieties of semiconductor materials have promoted the rapid development of semiconductor micro-nano lasers. In recent years, with the advent of a large number of new semiconductor materials (such as two-dimensional semiconductors, lead halide perovskites, etc.), it is expected that the performances of semiconductor micro-nano lasers will be further improved. Perovskite materials have excellent optical properties such as high light absorptions, high defect tolerances, and large exciton binding energy, which make them excellent candidate materials for high-gain, low-threshold semiconductor micro-nano lasers. The Fabry-Perot (F-P) resonator laser is a type of perovskite laser with extensive research, simple structure and high application value. In this paper, we take lead halide perovskite F-P resonator laser for example, and summarize its working mechanism and recent research results, by starting from two aspects of photon laser with exciton and photon weak coupling and strong coupling polariton laser. And we introduce the lasing principle and influencing factors of F-P structure lasers with perovskite materials as both gain medium and resonant cavity and F-P cavity lasers with perovskite as only gain medium in detail. Finally, the current challenges of perovskite F-P resonant lasers are summarized, and the possible prospects of its further development are also presented. Keywords:perovskite/ Fabry-Pérot cavity/ lasing/ polariton
钙钛矿材料的发光性质也易受到温度的影响, 随着温度的升高, 载流子热运动剧烈, 造成激子解离, 发光量子产率降低, 光学增益下降, 激光阈值升高[32]. 同时, 温度升高会产生晶格热膨胀和晶格振动频率的增加[6], 晶格膨胀使得钙钛矿离子间距拉大, 激子受到晶格的散射作用减小, 导致出射激光强度提高, 而晶格振动频率增加使得钙钛矿内电子与声子的耦合作用增加, 这种非弹性碰撞导致能量损失, 使得出射激光强度降低, 因此, 温度对钙钛矿的激光行为的影响较为复杂. 北京大学张青教授课题组[33]研究了CsPbBr3纳米线的自发辐射(SE)行为与激光行为随温度的变化, 结果如图3所示. 研究发现, 温度影响在低温(78—178 K)时以晶格热膨胀为主, 而在高温(195—295 K)下以电子-声子相互作用为主. 图 3 CsPbBr3纳米线的温度依赖性激光行为 (a) 78 ?295 K的不同温度下, 纳米线的2D伪色彩图的激光光谱; (b) 提取的SE (红点)和激光(蓝点)能量随温度变化; (c) SE与激光峰的能量差在温度从78 K增加到295 K的变化图. 红色实线为拟合结果[33] Figure3. Temperature-dependent lasing behavior of a CsPbBr3 NW: (a) The lasing spectra of a 2D pseudo-color plot of the NW at different temperatures from 78 K to 295 K; (b) the extracted SE (red dots) and lasing (blue dots) energies as a function of temperature; (c) plot of the change in the energy difference with temperature increasing from 78 K to 295 K. The red solid line is the fitting result[33].
除尺寸与温度影响因素外, 钙钛矿F-P腔在不同激发光强下激光行为也有所不同. 如前所述, 钙钛矿激光行为的过程经历SE至ASE再到STE, 因此钙钛矿激光的激发强度与发射强度关系呈现S形曲线[34]. 而在更高激发下激子吸收达到饱和, 载流子浓度增加, 超过Mott密度后, 强烈的库仑屏蔽作用使得激子解离为电子-空穴等离子体(EHP)[35], 同时由于高载流子浓度下的带填充效应, 导致激光模式峰蓝移[13]. 美国哥伦比亚大学研究人员[36]研究了CsPbBr3纳米线(图4(a)和图4(b))中的这一激光行为, 并通过图4(c)光路测量瞬态反射光谱(TR)证明了激光饱和现象与饱和处激子振荡向EHP振荡的转变, 并证明了载流子冷却过程和载流子/激子向非简并电子-空穴等离子体(n-EHP)及简并电子-空穴等离子体(d-EHP)的相变过程, 最终将纳米线激光发射归于n-EHP与等离子体基元耦合的受激发射(载流子动力学模型如图4(d)). 图 4 (a)在蓝宝石衬底上生长的三棱柱纳米线的扫描电子显微镜图像; (b) 纳米线中最低阶波导模式的有限元模拟图(电场极化方向由青色箭头表示); (c) 用于时间分辨的Kerr门控实验的光学装置的图示. 线性偏振 (I)在脉冲光穿过Kerr介质时变为椭圆偏振光 (II), 经过偏振器变为垂直于原始入射偏振的偏振光 (III); (d) 光激发的载流子动力学过程示意图, 激光来源于冷EHP与等离子基元的耦合的受激发射[36] Figure4. (a) Scanning electron microscopy images of triangular nanowires grown on sapphire substrate; (b) FEM simulation of the lowest order waveguiding mode in a nanowire (The electric field polarization is depicted by the cyan arrows); (c) illustration of the optical setup for time-resolved Kerr gating experiment. The linear polarization (I) becomes elliptical (II) as it passes through the Kerr medium with the pump pulse. A final polarizer (III) filters polarization perpendicular to the original incident polarization; (d) cartoon describing the carrier dynamics from photoexcitation which results in a hot electron hole plasma (hot EHP) through carrier cooling to a cold electron hole plasma (cold EHP) finishing with stimulated emission coupled with plasmon emission[36].
F-P微腔钙钛矿激光中, 钙钛矿材料仅作为增益介质, 以有源层的形式置于多层基底结构之上或DBR之间, 在垂直方向实现振荡的平面型激光器, 与自身作为微腔的F-P结构钙钛矿材料相比, 光束约束能力较大, 光学损耗较小, 有利于激光应用. F-P型微腔的结构为不同折射率介质的叠层薄膜[23,38], 依据薄膜干涉原理, 对特定波长的光具有高反射率, 目前的反射率R可达99% 以上, 如图5所示, 因此微腔结构对腔内光子有更好的局域作用, $ \varGamma $值较大, 激光阈值较低. 同时DBR的封装大幅提高了钙钛矿激光的环境稳定性, 光激发下稳定激光输出可达数十小时[39]. 图 5 (a) 钙钛矿DBR激光器的示意图结构, DBR叠层由10对HfO2·SiO2组成; (b) HfO2·SiO2 DBR的实测反射率光谱与设计模拟值匹配较好; (c) 高反射率部分的放大图, 显示峰值反射率为99.5%, 光谱范围覆盖在2.145?2.398 eV (517?578 nm)[38] Figure5. (a) Schematic structure of mixed-cation perovskite DBR laser. The sputtered dielectric DBR stack consists of 10 pairs of HfO2/SiO2; (b) measuredreflectivity spectrum of a standalone HfO2/SiO2 DBR showing a reasonably good match with pre-deposition design simulation; (c) zoom-in view of the high-reflection band showing peak reflectivity of 99.5%, and a broad spectral window covering from 2.145 eV to 2.398 eV (517?578 nm) wherein reflectivity exceeds 99% [38].
由于钙钛矿薄膜的热导率较低, 温度对钙钛矿F-P腔激光的影响较大, 复旦大学陈张海教授课题组[19]制备了CH3NH3PbI3钙钛矿DBR结构, 研究了温度对F-P微腔钙钛矿激光的影响, 结果发现升温会抑制激子辐射复合, 阈值提高, 半高宽增加, 激光相干性下降. 因此研究人员常采用蓝宝石等热导率较高的材料作为基底, 降低材料热损耗, 提高Q值. 对于钙钛矿有源层, 可利用钙钛矿薄膜本身具有的多相共存, 不同相之间的能量和激子传输能够促进粒子数在最终相的积累, 快速实现粒子数反转, 提高钙钛矿薄膜的增益. 西安交通大学吴朝新教授课题组[39]利用CsPbBr3薄膜中立方相与斜方相间的能量传递提高了薄膜中的激子传输速率, 增大了薄膜的增益系数, 制备的DBR结构在400 nm飞秒激光泵浦下获得最低阈值低达1.7 μJ/cm2的绿色激光. 北京大学冉广照教授课题组[40]利用Ruddlesden-Popper (RP) 钙钛矿(PEA)2Csn–1PbnBr3n+1微晶薄膜不同n相间的能量传输提高增益, 制备的DBR结构在接近连续泵浦的8 ns脉冲光激发下产生室温阈值为500 μJ/cm2的激光输出, 如图6所示. 同时由于多相混合, 制备的钙钛矿薄膜具有光学各向异性, 产生的激光具有较高的偏振度(图6(d)). 图 6 (a) Ruddlesden-Popper钙钛矿DBR腔体结构示意图; (b)器件发射光谱随泵浦强度的变化; (c)腔模式的积分强度强度(蓝色球)和半高宽(粉红色菱形)与泵浦强度的关系图; (d)旋转分析仪测得的强度极坐标图(红球), 黑线为马吕斯定律拟合曲线[36] Figure6. (a) Schematic diagram of cavity architecture; (b) pump fluence dependence of the emission spectra for the device; (c) the plots of integrated intensity (blue ball) and FWHM (pink diamond) of the cavity mode and the resulting lasing peak as a function of the pump fluence; (d) intensity polar plot through a rotational analyzer (red ball). The black line is the fitting curve from Malus’s law [36].
高激发下钙钛矿材料中的激子会发生解离, 导致实际产生激射的激子数量减少, Q值降低, 激光阈值升高. 美国布朗大学研究人员[41]采用混合阳离子钙钛矿Cs0.17FA0.83PbBr3薄膜降低上述影响, 该钙钛矿材料的激子结合能较大, 能有效抑制高激发下的激子解离, 产生激子增益的激光发射(图7), 激光阈值较相同条件下的FAPbBr3降低了30%, 同时光稳定性和环境稳定性有所提升(图7(g)). 另外, 也可采用具有高发光量子产率的量子点、纳米线等, 提高材料增益, 实现低阈值激光[42]. 不过, 虽然F-P微腔有效降低了钙钛矿激光阈值, 但其尺寸较大, 集成应用仍具有挑战. 图 7 (a) 激光泵浦钙钛矿激光器的示意图(脉冲激光波长为3.493 eV, ${\tau }_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=0.34~\mathrm{n}\mathrm{s}$, 重复频率1 kHz); (b) 以对数值表示的器件光输出, 阈值能量密度为13.5$ \pm $1.4 μJ/cm2, 空圆圈表示发射光谱的半高宽; (c) 不同激发强度下的发射光谱, 观察到2.244 eV (λ = 552.5 nm)的单模激光, 半高宽为0.996 meV (Δλ = 0.245 nm); (d) 在对数值下在不同激发强度下器件发射的伪彩色图, 显示了单模操作和大背景抑制比(> 20 dB); (e) 泵能量密度接近或高于阈值时的器件的近场图像; (f) 器件发射的远场方向图, 在横向发散约5°; (g) 相同泵浦下(1.5倍阈值强度)测得的器件寿命. 绿色正方形为激光输出强度, 橙色圆圈为激光峰半高宽, 显示了持续的激光操作[41] Figure7. (a) Schematic structure of a vertically pumped perovskite laser with a long-pass filter to block the pump residue. The pulsed pump source is acompact diode-pumped solid state laser (3.493 eV, τpulse= 0.34 ns, 1 kHz repetition rate). (b) Device light output with increasing pump fluence expressed in a log–log plot with threshold energy density of 13.5$ \pm $1.4 μJ/cm2. The empty circles record the FWHM linewidth of the emission spectrum. (c) Emission spectra under different excitation levels. Spectrally coherent single-mode lasing at 2.244 eV (λ = 552.5 nm) with FWHM linewidth of 0.996 meV (Δλ = 0.245 nm) is observed. (d) Pseudo-color plot of laser emission under different levels of excitation in a logarithmic scale demonstrating the single mode operation and large background suppression ratio (>20 dB). (e) Near-field images of a device with pump energy densities near and above the threshold. (f) Far-field pattern of laser emission, with ~5° divergence in the transverse plane. (g) Device (longevity) lifetime measured under the same subnanosecond pumping source (1.5 times the threshold intensity). Green squares monitor the lasing output intensity, while the orange circles track the FWHM linewidth of the emission, indicating persistent lasing operation [41].
极化子激光最早在Ⅲ-Ⅴ 族和 Ⅱ-Ⅵ 族半导体微腔中实现, 但传统无机发光半导体激子结合能较小, 难以在室温下产生极化子. 而钙钛矿材料激子结合能较大, 激子俄歇寿命较短, 有利于在室温下产生稳定的极化子. 目前已在具有F-P结构的钙钛矿纳米线、纳米片等结构中实现了极化子激光, 北京大学张青教授课题组[46]使用CsPbBr3纳米线在400 nm飞秒激光泵浦下, 实现阈值低达8 μJ/cm2的极化子激光. 美国哥伦比亚大学研究人员[47]同样使用CsPbBr3纳米线, 在450 nm连续光泵浦下实现阈值为6 kW/cm2的极化子激光, 如图9所示. 图 9 (a) 20 μm的纳米线PL光谱图, 泵浦强度范围为0.25?7.80 kW/cm2; (b) 积分能量强度与泵浦强度的关系, 阈值为6 kW/cm2; (c)?(e) 20, 14和5 μm的纳米线阈值处的极化子激光荧光图; (f) 14 μm 纳米线腔光子(蓝线虚线)与激子(绿线)在77 K下的能量色散图, 红点表示LPB瓶颈处实验测得的极化子模式[46] Figure9. (a) PL spectra of a 20 μm long NW obtained with increasing excitation light power densities in the range of 0.25?7.80 kW/cm2. Inset (b) shows the integrated power density plotted against the power density, showing threshold near 6 kW/cm2. The right side shows fluorescence images above the polariton lasing threshold for NWs with length of (c) 20 μm, (d) 14 μm and (e) 5 μm. Thescale bad in (c) is 5 μm. (f) Dispersions of the cavity photon (blue dashed) and exciton(green) at 77 K of the 14 μm long NW without coupling, the red dots show the experimental CW polaritonmodes near the bottleneck region of the LPB [46].
为证明极化子的产生, 研究人员常通过数值模拟的E-K曲线与实验光谱所测得的模式能量与π/L值关系相比较, 说明激光的产生来源于极化子[48], 并通过角分辨光谱(ARPL)观察阈值处极化子的BEC过程[49]. 在F-P结构钙钛矿(钙钛矿纳米线)中产生极化子并实现极化子激光的关键在于光子与激子合适的耦合强度, 由于F-P结构钙钛矿对光子的束缚能力较弱, 产生的激光常包含光子激光成分(来自未耦合的激子复合)与极化子激光成分, 因此激子与光子耦合强度(后文写为耦合强度)越高, 激光中的极化子激光成分越高, 激光阈值越低. 耦合强度受到成分、温度、尺寸和激发强度的影响, 合适的影响因素将有助于耦合强度的不断提高. 首先耦合强度对卤素成分敏感, 卤素成分由I到Cl, 钙钛矿激子结合能不断增加, 钙钛矿激子浓度增加, 耦合强度增大[50]. 其次, 如前所述, F-P腔尺寸也会影响耦合强度, 北京大学张青教授课题组[46]从实验上验证了CsPbBr3中Rabi劈裂能与F-P腔尺寸的关系, 随着纳米线尺寸的减小, 振荡强度提高, 耦合强度增大, 如图10所示, 最终得到了高达656 meV的劈裂能. 同时耦合强度增加引起钙钛矿材料光吸收系数的增加, 有助于激光阈值的进一步减小[51]. 图 10 CsPbBr3纳米线尺寸依赖的耦合强度 (a)?(d)宽度为3.31, 1.99, 1.19和0.78 μm, 长度为11.01, 14.02, 16.41和20.77 μm的纳米线从一端激发在另一端收集的发光光谱. 光谱在洛伦兹拟合下展现出多个F-P模式峰; (e)?(f)上方对应的纳米线Z方向的能量-波矢曲线, 内部图片为对应纳米线横向电场分布图[50] Figure10. Size-dependent light-matter coupling strength in CsPbBr3micro/nanowires: (a)?(d) Emission spectra acquired from the waveguided end and excited from the other end of CsPbBr3 nanowires with width of 3.31, 1.99, 1.19 and 0.78 μm and lengths of 11.01, 14.02, 16.41 and 20.77 μm, respectively. The spectra show multiple Fabry-Pérot interference peaks which have been fitted by Lorentzian line shapes to determine the resonance energies. (e)?(f) Corresponding energy-wavevector dispersion curves in the Z-direction (along nanowire length) of above four CsPbBr3 nanowires, respectively. The insets are normalized electric field distribution at the cross-section of these four CsPbBr3 nanowires [50].
利用DBR微腔, 可有效提高光束约束因子, 因此有利于极化子的产生和高极化子激光成分的实现[49]. 对于微腔结构的极化子研究由来已久, 在传统无机半导体微腔及有机半导体微腔中均已实现极化子激光, 但传统无机半导体微腔需要复杂的外延生长技术, 而有机半导体中的Frenkel型激子使得极化子与极化子间的散射作用较小, 导致受激散射阈值较高. 钙钛矿材料能够在DBR微腔中实现较高成分的极化子, 以及量子阱结构下有效的BEC过程. 新加坡南洋理工大学熊启华教授课题组[58]合成了较大尺寸CsPbBr3纳米片并放入DBR结构中, 如图11所示, 实现了低Q值下的阈值为12 μJ/cm2的极化子激光, 与Q值相当的DBR结构光子激光相比, 阈值有所下降. 作者通过非平衡凝聚模型研究了阈值附近的PL峰的蓝移变化, 揭示了极化子实现激光输出的过程中, 经历了阈值以下极化子-激子相互作用到阈值以上极化子-极化子散射作用的转变(图11(c)). 图 11 CsPbCl3微腔极化子激光 (a) 不同泵浦强度下的基态发光光谱; (b) 基态发光强度与半高宽与泵浦强度的关系图, 半高宽的变化拐点位于阈值Pth= 12 μJ/cm2附近; (c) 极化子发光能量蓝移与泵浦强度关系图, 阈值处蓝移趋势发生变化, 从极化子-激子相互作用转变为极化子-极化子相互作用[58] Figure11. Characterizations of CsPbCl3 microcavity polariton lasing: (a) Ground-state emission spectra under different pumping fluences; (b) ground-state emission intensity and full width at half maximum as a function of pump fluence. A line width narrowing occurs near the threshold of Pth= 12 μJ/cm2, along with a sharp increase of emission intensity; (c) energy blueshift with respect to the polariton emission energy at the lowest pump fluence as a function of pump fluence. The blueshift trend below the threshold is attributed to polariton–reservoir interaction while the trend above the threshold corresponds to polariton–polariton interaction [58].
DBR结构中同时存在腔模式和布拉格模式[59], 在RP钙钛矿作为有源层的DBR结构中, 发现了钙钛矿激子与腔模式光子以及DBR中的布拉格模式光子的耦合杂化态, 耦合杂化态的出现使得极化子占据状态分布发生变化, 因此具有更小的有效质量和更长寿命, 有利于实现低阈值极化子激光. 其次, 与F-P结构钙钛矿极化子激光类似, 微腔结构极化子的状态会受到有源层厚度的影响. 北京大学张青教授课题组[60]研究发现, 当DBR之间的CsPbBr3薄片厚度较小时, 腔内振荡的光子能量增大, 激子与光子能量的负失谐值减小(失谐值$ {\varDelta } $负向减小), 极化子的LPB曲线更靠近激子色散曲线, 形成类激子的极化子, 极化子-极化子散射作用增加, BEC过程容易实现(图12(a)), 激光阈值随之减小(图12(b)), 阈值均在1.0 μJ/cm2以下, 低于当下钙钛矿DBR 光子激光, 阈值达到最低. 不过, 由于光子与激子在波矢空间的重叠减小, 耦合强度有所减小(图12(b)), 阻碍了阈值的进一步降低. 同时由于DBR结构中的极化子成分极高, 因此耦合强度与激光阈值的关系与F-P结构钙钛矿极化子激光不同. 当钙钛矿薄片厚度较大时, 光子能量减小, 激子与光子能量负失谐值增加, LPB曲线倾向于光子色散曲线, 形成类光子的极化子, 极化子-极化子散射作用减小, 激光阈值增加(图12(b)), 激子与光子的耦合强度有所上升, 极化子倾向于留在由横向约束确定的局部模式中. 当薄片厚度继续增加, 负失谐能进一步增大, F-P腔极化子模式逐渐消失, 本征PL峰占据优势, 如图12(c)所示, 即发光来源于钙钛矿薄片材料本身, 激光模式变为WGM模式. 因此, 应设计合适的有源层厚度, 获得阈值较低、较为稳定的极化子激光. 图 12 不同失谐值下极化子在不同能量和动量的分布 (a) 失谐值为–36 meV(蓝色方块)、–85 meV(绿色圆圈)、–118 meV(红色三角)的极化子数量(取对数)在低于阈值和阈值处不同能量分布, 其中–36 meV失谐值下的分布可在阈值下和阈值处拟合为玻尔兹曼分布和玻色分布; (b) 极化子凝聚的阈值与耦合强度与失谐值的关系, 不同颜色区域表示在亚稳态(零动量)和稳态(有限动量)产生的极化子凝聚; (c) 钙钛矿DBR结构的发光光谱(低于阈值, 黑色线)和反射光谱(红色线), 从下到上钙钛矿厚度不断增加(负失谐能增加). 区域Ⅰ 为–28和–56 meV, 区域Ⅱ 为–64, –80, –103和–114 meV, 区域Ⅲ 为–129和–182 meV[60] Figure12. Polariton distributions at different energy and momentum with different detuning energy: (a) Polariton population (log scale) versus energy at threshold and below threshold for detuning energies of –36 (blue squares), –85 (green circles), and –118 meV (red triangles), respectively. The values are normalized to the population of the emission peak at the threshold. The polariton population at a detuning energy of –36 meV can be fitted (blue solid line) using the Maxwell–Boltzmann distribution below threshold or Bose–Einstein distribution at threshold; (b) threshold (blue dots and circles) of the polariton condensation and corresponding coupling strength (pink squares) as a function of detuning energy. Two different regions indicated by different colors of the background and green dash line represent detuning for generating polariton condensation at stationary (at finite momentum) and metastable states (zero momentum); (c) normalized emission (below threshold, black solid line) and reflection spectral (red solid line) of perovskite/DBR cavity with increasing perovskite thickness (also the more negative detuned energy) from bottom to up. Based on different relaxation processes of polaritons at different detuning energies, three different regions (I–III) are introduced. Region I: Δ = –28 and –56 meV. Region II: Δ = –64, –80, –103, and –114 meV. Region III: Δ = –129 and –182 meV [60].