Nonlinear spectroscopy of a single nanoobject via a plasmonic waveguide

自然界是如何优化光合作用的光吸收和能量传输的？在生物温暖潮湿的环境中，量子力学相干性起作用了吗？所有的蛋白质复合体都是相同的，还是相同功能存在不同的溶液？这些问题的答案需要单一蛋白质复合体的超快光谱。单分子(或蛋白质)荧光成像和光谱分析是一项成熟的技术。它绕过了光谱特性的集合平均，并提供了访问它们的时间演变的途径，而不需要同步集合。然而，可获得的时间尺度受到荧光寿命的限制，更快的过程，如相干衰减和激发传输，对观察者是隐藏的。非线性光学光谱学使我们能够访问这些超短时间尺度。因此，单个发射体必须在很短的延迟内与两个光子相互作用。在这些条件下，荧光团的光漂白将可探测到的光子总数限制在约100万个。到目前为止，只有很少且相当有限的单分子非线性实验取得了成功。在这里，我们建议结合最近的两项创新来克服这一限制。我们将利用等离子体光波导通过珀塞尔效应来提高荧光团的光稳定性。这将增加探测到的光子总数，从而提高信噪比，使微小的非线性效应可见。我们将把这种样品设计与相位调制荧光检测二维光谱学结合起来。这种荧光激发和数据分析的方法将使我们能够从有限的光子中提取最大的信息。为了验证我们的新设置，我们将研究单个染料分子中的振动相干性，这是线性光谱学无法获得的，但被认为对光收集具有决定性影响。然后我们切换到紫色细菌的单一光合蛋白复合体。2D光谱将阐明吸收带之间的相干程度，它的衰变，以及它从复杂到复杂的变化。

How does Nature optimize the absorption of light and the transport of energy for photosynthesis? Does quantum mechanical coherence play a role in the warm and wet environment of biology? Are all protein complexes identical or do different solutions exist for the same function? The answer to these questions requires ultrafast spectroscopy of a single protein complex. Single molecule (or protein) fluorescence imaging and spectroscopy is a well-established technique. It circumvents ensemble averaging of spectroscopic properties and gives access to their temporal evolution without the need to synchronize the ensemble. However, the accessible timescales are limited by the fluorescence lifetime and faster processes such as decay of coherence and transport of excitation are hidden from the observer. Nonlinear optical spectroscopy allows us to access these ultrashort timescales. A single emitter has thus to interact with two photons within a short delay. Under these conditions, photobleaching of the fluorophore limits the total number of detectable photons to about one million. Only very few and rather limited nonlinear experiments on single molecules have been successful so far. Here we propose to combine two recent innovations to overcome this limitation. We will use plasmonic waveguides to increase the photostability of fluorophores by the Purcell effect. This will boost the total number of detected photons and thus the signal-to-noise ratio, making tiny nonlinear effects visible. We will combine this sample design with phase-modulated fluorescence-detected two-dimensional spectroscopy. This method of fluorescence excitation and data analysis will allow us to extract the maximum information from the limited number of photons. To validate our new setup, we will investigate the vibrational coherence in a single dye molecule, which is not accessible by linear spectroscopy but is thought to have a decisive influence on light harvesting. We then switch to single photosynthetic protein complexes of purple bacteria. 2d spectra will elucidate the degree of coherence between the absorption bands, its decay, and its variation from complex to complex.