Kohärente Kontrolle Plasmonischer Brennpunkte in Nanoantennen
Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
Zusammenfassung der Projektergebnisse
This project focused on the control of the non-linear flux in plasmonic nanoantennas using phase shaping of ultrafast laser pulses. The proposed control scheme aims to exploit the phase sensitive spatial-spectral interference of different plasmon modes within single nanoantennas. Laser pulse shaping is achieved using the spatial light modulator (SLM) purchased through this grant, which we integrated into a newly developed 4f-setup and combined with a home-built confocal / near-field optical microscope. In the beginning of the project we realized that the purchased SLM model exhibits unexpectedly strong spatial heterogeneities in its optical performance. Whereas the characteristics of the SLM is within its specifications according to the supplier, the observed heterogeneities precluded the deterministic laser pulse shaping based on the provided calibration data and procedure. We solved this problem by establishing and implementing a comprehensive look-up table instead, that resulted in the needed accuracy. In careful investigations of the fundamental laser spectrum during phase shaping we found a significant amplitude modulation due to phase-wrapping at the SLM. We developed and implemented an efficient correction procedure to deal with this problem. Using our optimized platform we achieved the following important results in the field of ultrafast nanooptics: As the starting point we addressed the possibility of phase-control of the hotspot position in plasmonic nanostructures by phase shaping using numerical simulations. We first implemented a numerical tool box for electromagnetic field calculations and extended it to perform non-linear optics simulations that allowed us to calculate the SH flux at different positions within the nanostructure. To achieve maximimum controlability, we developed a new optimization scheme that combines an evolutionary algorithm with a neural network (NN). This combination was found to significantly speed up optimization and to flexibly adapt to variations of the initial problem. Our results showed that it is possible to control the intensity of second harmonic generation (SHG) hotspots in L-shaped gold nanoantennas on nanometer length scales by phase shaping. Initially, we aimed at experimentally controlling the SHG response. However, SHG in single nanoantennas turned out to not provide sufficiently strong signal intensities at moderate excitation powers that would allow for extended observation times without photo-degradation. Instead, we switched to the detection of near-degenerate four-wave mixing (ND-FWM). As for SHG, the resulting signal intensity of ND-FWM depends on the relative phase between the spectral components of the laser pulse and thus should also be applicable to the proposed coherent control scheme. We then demonstrated the experimental control of the near-degenerate four-wave mixing intensity in single L-shaped gold nanoantennas by phase shaping. While the intensity of a particular nanoantenna could be increased by nearly 50 % by applying an optimized spectral phase profile, other nanoantennas with different lengths showed a simultaneous signal decrease by about 30 %. This intensity control was found to be completely reversible and reproducible. In a further step, we demonstrated that this intensity control is connected to a variation of the spatial distribution of the ND-FWM hotspots within the sub-wavelength sized nanoantenna. In this experiment we detected a small but reproducible shift of the peak position of the detected ND-FWM signal depending on the applied spectral phase. This can be considered as a proof-of-concept of our proposed scheme for nanoscale hotspot control by laser phase shaping. We investigated the ultrafast response of single layers of MoSe2 and graphene without nanoantennas using pump-probe microscopy to establish the basis for our control experiments. We also studied the possible non-linear response of MoSe2 coupled to bow-tie nanoantennas and demonstrated that Raman scattering in graphene becomes non-linear in case of pulsed excitation due to a complex interplay of interfering quantum pathways. We are now applying the developed scheme to control the optical response of graphene and MoSe2 coupled to nanoantennas. We believe that our results are important for the field of ultrafast nanooptics and that they constitute important steps towards our vision to control optical fields on nanometer length scales in terms of ’scanning near-field optical microscopy without moving parts’.
