Coherent Control of Plasmonic Hotspots in Nanoantennas

金属等离子体纳米天线可用于在纳米尺度上限制光，并增强其附近样品物体或纳米天线本身的光信号。这种创造光学热点的能力，以及它们在尺寸、形状和材料组成方面几乎无限的多样性，开辟了广泛的应用领域，并继续激发等离子体领域的巨大研究努力。特别是非线性光学和光谱学，由于其高阶场依赖性，有可能从表面等离子体共振提供的局部场增强中获益。此外，局部表面等离子体的快速响应时间，通常低于10秒，使它们有可能适用于超快应用，例如光信息处理、开关和光谱学。本课题旨在利用超快激光脉冲整形显微镜控制单纳米天线内部的非线性热点分布。我们的方法是基于这样一个假设：相位整形可以用来操纵空间上不同的等离子体模式的叠加，以及它们的光谱干扰，从而产生纳米天线的二次谐波响应。首先，我们将证明仅通过改变宽带激光脉冲的光谱相位，二次谐波产生（SHG）的空间分布可以达到纳米精度。这种空间控制是通过超分辨率方法来检测的，在这种方法中，我们通过远场和尖端增强近场实验来跟踪SHG信号的峰值位置。数值模拟将支持观测到的热点分布偏移，并指导优化纳米天线的设计，以提供最大的可控性。除了确定性相位变化，我们将在模拟和实验中使用自学习算法。其次，我们的目标是控制纳米天线发射的二次谐波光的角分布，这将证实我们的空间光谱干扰假设。第三，我们将研究所获得的热点控制是否可以用于纳米材料的时间和空间分辨光谱。为此，我们将在纳米天线上沉积一组选定的二维材料，包括石墨烯，MoSe2和MoS2，这些材料使用成熟的聚合物转移提供最大的可控性。使用两个具有不同时间延迟的相位控制脉冲，我们将创建两个不同的热点分布作为泵浦脉冲和探测脉冲。然后，我们将在空间上跟踪二维材料的不同光学响应以及纳米天线的SHG，以探测可能的时间相关关系。我们的研究结果将大大提高我们对非线性等离子体和相干控制的理解，并有望在纳米尺度上为二维材料的非局部光学特性提供新的见解。

Metal plasmonic nanoantennas can be used to confine light on the nanometer scale, and to enhance the optical signals of sample objects in their proximity or the nanoantenna itself. This capability of creating optical hotspots, together with their virtually unlimited diversity in size, shape, and material composition opened up a wide range of applications and continues to stimulate enormous research efforts in the field of plasmonics. Non-linear optics and spectroscopy, in particular, have the potential to strongly benefit from local field enhancement provided by surface plasmon resonances due to their higher order field dependence. Furthermore, the fast response time of localized surface plasmons, typically below 10 fs, makes them potentially suitable for ultrafast applications, for example in optical information processing, switching and spectroscopy. In this project we aim at controlling the non-linear hotspot distribution inside single nanoantennas using ultrafast laser pulse shaping microscopy. Our approach is based on the hypothesis that phase shaping can be used to manipulate the superposition of spatially distinct plasmon modes and of their spectral interference that generates the second harmonic response of the nanoantenna. First, we will show that by varying only the spectral phase of a broadband laser pulse, the spatial distribution of the second harmonic generation (SHG) can be manipulated with nanometer accuracy. This spatial control is detected using a super-resolution approach, in which we track the peak position of the SHG signal in the far-field and by tip-enhanced near-field experiments. Numerical simulations will be carried out to support the observed shifts in the hotspot distributions and to guide the design of optimized nanoantennas providing maximum controllability. In addition to deterministic phase variations we will use self-learning algorithms in both simulations and experiments. Second, we aim at controlling the angular distribution of the second harmonic light emitted by the nanoantenna, which would confirm our hypothesis of spatial-spectral interference. Third, we will investigate if the achieved hotspot-control can be utilized for the time and spatially resolved spectroscopy of nanomaterials. To this end we will deposit a selected set of 2D materials including graphene, MoSe2 and MoS2 on the nanoantennas that provide maximum controllability using well-established polymer transfer. Using two phase-controlled pulses with varying temporal delay, we will create two different hotspot distributions acting as pump and probe pulses. We will then spatially track the distinct optical responses of the 2D materials together with the SHG of the nanoantenna to probe possible time dependent correlations. Our results will substantially improve our understanding of non-linear plasmonics and coherent control and are expected to provide new insight into the non-local optical properties of 2D materials on the nanoscale.