Izadshenas Jahromi Saeid, Słowik Karolina
Institute of Physics, Faculty of Physics, Astronomy, and Informatics, Nicolaus Copernicus University in Toruń, ul. Grudzikadzka 5, 87-100, Toruń, Poland.
Sci Rep. 2025 Jan 31;15(1):3956. doi: 10.1038/s41598-025-87198-y.
Two-photon absorption in molecules, of significance for high-resolution imaging applications, is typically characterised with low cross sections. To enhance the TPA signal, one effective approach exploits plasmonic enhancement. For this method to be efficient, it must meet several criteria, including broadband operational capability and a high fluorescence rate to ensure effective signal detection. In this context, we introduce a plus-shaped silver nanostructure designed to exploit the coupling of bright and dark plasmonic modes. This configuration considerably improves both the absorption and fluorescence of molecules across near-infrared and visible spectra. By fine-tuning the geometrical parameters of the nanostructure, we align the plasmonic resonances with the optical properties of specific TPA-active dyes, i.e., ATTO 700, Rhodamine 6G, and ATTO 610. The expected TPA signal enhancement is evaluated using classical estimations based on the assumption of independent enhancement of absorption and fluorescence. These results are then compared with outcomes obtained in a quantum-mechanical approach to evaluate the stationary photon emission rate. Our findings reveal the important role of molecular saturation determining the regimes where either absorption or fluorescence enhancement leads to an improved TPA signal intensity, considerably below the classical predictions. The proposed nanostructure design not only addresses these findings, but also might serve for their experimental verification, allowing for active polarization tuning of the plasmonic response targeting the absorption, fluorescence, or both. The insight into quantum-mechanical mechanisms of plasmonic signal enhancement provided in our work is a step forward in the more effective control of light-matter interactions at the nanoscale.
分子中的双光子吸收对高分辨率成像应用具有重要意义，其典型特征是截面较低。为了增强双光子吸收（TPA）信号，一种有效的方法是利用等离子体增强。要使这种方法有效，必须满足几个标准，包括宽带操作能力和高荧光速率，以确保有效信号检测。在此背景下，我们引入了一种十字形银纳米结构，旨在利用亮暗等离子体模式的耦合。这种结构显著改善了分子在近红外和可见光谱范围内的吸收和荧光。通过微调纳米结构的几何参数，我们使等离子体共振与特定TPA活性染料（即ATTO 700、罗丹明6G和ATTO 610）的光学性质相匹配。基于吸收和荧光独立增强的假设，使用经典估计来评估预期的TPA信号增强。然后将这些结果与量子力学方法获得的结果进行比较，以评估稳态光子发射率。我们的研究结果揭示了分子饱和度在决定吸收或荧光增强导致TPA信号强度改善的区域方面的重要作用，这大大低于经典预测。所提出的纳米结构设计不仅解决了这些发现，还可能用于其实验验证，允许针对吸收、荧光或两者对等离子体响应进行主动偏振调谐。我们工作中对等离子体信号增强的量子力学机制的深入了解是在纳米尺度上更有效控制光与物质相互作用方面向前迈出的一步。
