This publication presents an innovative approach to study the plasmonic enhancement effect in nonlinear signal generation (NSG) from gold gratings. Plasmon-enhanced NSG requires specially designed plasmonic nanostructures to optimally mediate the optical near and far fields such that maximum NSG efficiency can be achieved. In this context, plasmonic gratings are one effective type of nanostructures because they provide well-defined lattice momentum for photon-to-plasmon coupling. How to design a suitable plasmonic grating for nonlinear signal generation is, however, challenging because NSG processes can involve multiple frequencies coving a wide spectral range. To address this issue and find the optimal grating design, an azimuthally chirped grating (ACG) platform is particularly suited as it provides azimuthal angle-dependent grating periods and offers a spatially resolved plasmonic enhancement effect for NSG. This work studies the enhancement effect of plasmonic ACGs for surface-enhanced two-photon excited photoluminescence (TPPL) and second harmonic generation (SHG).

The capability of ACGs to serve as a spatially and spectrally resolved antenna that mediates the far and near optical fields of multiple input and output beams at different frequencies is demonstrated. The results show distinct spatial distributions of SHG and TPPL signals, revealing the difference in the underlying mechanisms. This information is valuable for the targeted design of effective plasmonic nanostructures for other nonlinear optical processes like, e. g., the molecule-sensitive coherent anti-Stokes Raman scattering (CARS). By understanding the complicated nonlinear enhancement effect of plasmonic nanostructures, the team aims further to design nanostructures for surface-enhanced CARS with sensitivity down to the single-molecule level.

This study is the result of a successful collaboration between the two Research Departments Spectroscopy and Imaging as well as Nanooptics. Here, the know-how of the Spectroscopy and Imaging Research Department in nonlinear spectroscopy and imaging with the competencies of the Research Department of Nanooptics in plasmonics complement each other in an ideal way. Furthermore, this study is also of major importance for the Collaborative Research Center CRC 1375 NOA where Leibniz IPHT is a central partner of. 

The work was funded by the CRC 1375 NOA (Subprojects C1 and C5).