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Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures.
Scientific Reports ( IF 3.8 ) Pub Date : 2020-02-13 , DOI: 10.1038/s41598-020-59457-7 Mohammad Adnan 1 , Jeremy J Baumberg 2 , G Vijaya Prakash 1
Scientific Reports ( IF 3.8 ) Pub Date : 2020-02-13 , DOI: 10.1038/s41598-020-59457-7 Mohammad Adnan 1 , Jeremy J Baumberg 2 , G Vijaya Prakash 1
Affiliation
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Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200-250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η2σ2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.
中文翻译：
二维无机-有机混合结构中各种激子的线性和非线性光学探测。
非线性光学特性（例如两（或多）光子吸收（2PA））对于快速光学切换，体内成像等技术上的重要应用特别感兴趣。高强度的红外超短脉冲深入样品，揭示了一些潜在的结构扰动（层间变形，层内起皱），还提供了有关新的激发态及其弛豫的信息。自然自组装的无机有机多量子阱（IO-MQW）在室温激子发射特征（结合能〜200-250 meV）下显示出效用。这些Mott型激子对这些软二维（2D）和弱范德华层状混合体的自组装过程，无机网络畸变，厚度和层间畸变非常敏感。我们展示了由红外飞秒激光脉冲激发的来自这些IO-MQW的强室温非线性激发强度相关的双光子吸收诱导的激子光致发光（2PA-PL）。严格限制的激子显示出明显不同的单光子和双光子激发光致发光能量：来自自由激子（2.41 eV）耦合到完全对准的MQW，以及来自能量下移的激子（2.33 eV），其源于局部皱缩的分层体系结构。由单光子吸收（1PA-PL）引起的高强度飞秒诱导的PL表示吸收饱和和激子-激子ni灭，随着激发强度的增加，PL的辐射弛豫时间通常从270 ps减少到190 ps。通过广泛的红外激发调谐，2PA-PL激发的起源被认为是由在带隙以下延伸的激子暗态引起的。观察到的双光子吸收系数（β〜75 cm / GW）和双光子激发截面（η2σ2〜110GM），进一步支持了2PA激发源的证据。大面积单晶血小板上的1PA-和2PA-PL空间映射均显示了自由和深层皱缩激子并存，并伴有一些缺陷诱发的陷阱态发射的痕迹。我们得出的结论是，双光子吸收诱导的PL对很少到很多单层的自组装过程，晶体堆积和深能级缺陷高度敏感。这项研究为定制许多2D材料类的非线性特性铺平了道路。
更新日期：2020-02-13
中文翻译：
二维无机-有机混合结构中各种激子的线性和非线性光学探测。
非线性光学特性（例如两（或多）光子吸收（2PA））对于快速光学切换，体内成像等技术上的重要应用特别感兴趣。高强度的红外超短脉冲深入样品，揭示了一些潜在的结构扰动（层间变形，层内起皱），还提供了有关新的激发态及其弛豫的信息。自然自组装的无机有机多量子阱（IO-MQW）在室温激子发射特征（结合能〜200-250 meV）下显示出效用。这些Mott型激子对这些软二维（2D）和弱范德华层状混合体的自组装过程，无机网络畸变，厚度和层间畸变非常敏感。我们展示了由红外飞秒激光脉冲激发的来自这些IO-MQW的强室温非线性激发强度相关的双光子吸收诱导的激子光致发光（2PA-PL）。严格限制的激子显示出明显不同的单光子和双光子激发光致发光能量：来自自由激子（2.41 eV）耦合到完全对准的MQW，以及来自能量下移的激子（2.33 eV），其源于局部皱缩的分层体系结构。由单光子吸收（1PA-PL）引起的高强度飞秒诱导的PL表示吸收饱和和激子-激子ni灭，随着激发强度的增加，PL的辐射弛豫时间通常从270 ps减少到190 ps。通过广泛的红外激发调谐，2PA-PL激发的起源被认为是由在带隙以下延伸的激子暗态引起的。观察到的双光子吸收系数（β〜75 cm / GW）和双光子激发截面（η2σ2〜110GM），进一步支持了2PA激发源的证据。大面积单晶血小板上的1PA-和2PA-PL空间映射均显示了自由和深层皱缩激子并存，并伴有一些缺陷诱发的陷阱态发射的痕迹。我们得出的结论是，双光子吸收诱导的PL对很少到很多单层的自组装过程，晶体堆积和深能级缺陷高度敏感。这项研究为定制许多2D材料类的非线性特性铺平了道路。
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