Coherent Control of Plasmonic Hotspots in Nanoantennas

金属等离子体纳米天线可以用来将光限制在纳米尺度上，并增强样品物体附近或纳米天线本身的光信号。这种创造光学热点的能力，加上它们在大小、形状和材料组成上几乎无限的多样性，开辟了广泛的应用领域，并继续刺激等离子体领域的巨大研究努力。尤其是非线性光学和光谱学，由于它们的高阶场依赖性，表面等离子体共振提供的局域场增强具有很大的潜力。此外，局域表面等离激元的快速响应时间通常低于10fs，这使得它们有可能适合于超快应用，例如在光学信息处理、开关和光谱学中。在这个项目中，我们的目标是利用超快激光脉冲整形显微镜来控制单个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，即相位整形可以用来操纵空间上不同的等离子体激元模式及其光谱干涉的叠加，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，就可以以纳米精度操纵二次谐波的空间分布。这种空间控制是使用超分辨率方法检测的，在该方法中，我们跟踪倍频信号在远场中的峰值位置，并通过尖端增强的近场实验来检测。将进行数值模拟，以支持观察到的热点分布的偏移，并指导提供最大可控性的优化纳米天线的设计。除了确定性的相位变化，我们还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光的角分布，这将证实我们的空间光谱干涉假说。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精选的2D材料，包括石墨烯、MoSe2和MoS2，利用成熟的聚合物转移提供最大的可控性。利用两个具有不同时间延迟的相控脉冲，我们将产生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2D材料的不同光学响应以及纳米天线的倍频，以探索可能的时间相关关系。我们的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，并有望为在纳米尺度上理解2D材料的非局域光学性质提供新的见解。

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.