Coherent Control of Plasmonic Hotspots in Nanoantennas

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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.