Adnan Mohammad, Baumberg Jeremy J, Vijaya Prakash G
Nanophotonics Lab, Department of Physics, Indian Institute of Technology Delhi, New Delhi, 110016, India.
Nanophotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom.
Sci Rep. 2020 Feb 13;10(1):2615. doi: 10.1038/s41598-020-59457-7.
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 (ησ ~ 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.
非线性光学性质，如双（或多）光子吸收（2PA），在快速光开关、体内成像等技术上重要的应用中具有特殊意义。高强度红外超短脉冲可深入探测样品，揭示几种潜在的结构扰动（层间畸变、层内褶皱），还能提供有关新激发态及其弛豫的信息。天然自组装无机 - 有机多量子阱（IO - MQWs）因其室温激子发射特性（束缚能约为200 - 250 meV）而具有实用性。这些莫特型激子对这些柔软的二维（2D）和弱范德华层状杂化物的自组装过程、无机网络畸变、厚度和层间畸变高度敏感。我们展示了由红外飞秒激光脉冲激发的这些IO - MQWs在室温下具有强烈的非线性激发强度依赖性双光子吸收诱导激子光致发光（2PA - PL）。强受限激子表现出明显不同的单光子和双光子激发光致发光能量：来自与完美排列的MQWs耦合的自由激子（2.41 eV）以及源于局部褶皱层状结构的能量下移激子（2.33 eV）。高强度飞秒诱导的单光子吸收光致发光（1PA - PL）表明吸收饱和以及激子 - 激子湮灭，随着激发强度增加，PL辐射弛豫时间典型地从270 ps减少到190 ps。通过广泛的红外激发调谐，2PA - PL激发的起源被认为来自延伸到带隙以下的激子暗态。观察到的双光子吸收系数（β ~75 cm/GW）和双光子激发截面（ησ ~ 110GM），进一步支持了2PA激发起源的证据。在大面积单晶片上的1PA和2PA - PL空间映射都表明自由和深层褶皱激子与一些缺陷诱导的陷阱态发射痕迹共存。我们得出结论，双光子吸收诱导的PL对少数到多层的自组装过程、晶体堆积和深层缺陷高度敏感。这项研究为定制许多二维材料类别的非线性性质铺平了道路。因此，我们的结果为利用层状无机 - 有机和其他金属有机框架探索基本现象和新型光电子应用开辟了新途径。
