Proposal summaryBiophotonics is an emerging interdisciplinary field. Modern laser spectroscopic methods in combination withmicroscopy open up exciting new ways to study biological objects. For biological applications, multi-photonor non-linear spectroscopy can offer several advantages over one-photon excitation. The aim of this projectis to explore novel spectroscopic strategies based on multi-photon excitation. In order to overcome lowsignalproblems related to multi-photon spectroscopy, we will exploit plasmonics and perform multi-photonspectroscopy in the enhanced local optical fields of gold- and silver nanostructures. I propose a combinationof three different plasmonics-enhanced two-photon spectroscopic methods for multimodal two-photonsensing and imaging. This combination can provide information on morphological structures and function ofbiological systems along with chemical information about molecular composition, structure, and interactions.A key aspect of this new concept will be the implementation of spectroscopically multifunctionalnanosensors with plasmonic nanoparticles as basic building blocks. Following multi-photon excitation, thesesensors deliver information on their environment inferred from a set of multiple surface-enhancedspectroscopic signatures.The proposed research will generate fundamental knowledge about multi-photon driven processes inenhanced local optical fields and about multi-photon interaction of light and biological matter. This mightopen up entirely new directions to advance nanobiophotonics. In particular, the outcome of the research willstimulate the new field of two-photon sensing and imaging, which has the capability to advance ourunderstanding of biological systems and processes. The project is expected to have broad impact andcontribute to the development of multi-photon optical sensing and multi-photon excited photophysics andphotochemistry with applications in physics, chemistry, energy technology, biotechnology and medicine.