Fluctuation-dominated materials for advanced photonics

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The main aim of this project is to develop nanostructured materials for novel photonic applications in which a well-designed disorder induces huge fluctuations of local electromagnetic fields. In these structures, the nonlinear response of the ‘hot spots’ with largest fields will dominate and, on average, strongly increase the nonlinear response of the sample. This approach may lead to a whole new class of ‘fluctuation-dominated materials’ -- for photonics, but mutatis mutandis for other wave-based technologies as well. For two model systems, metallic nanoporous gold nanoparticles (‘nanosponges’) and dielectric nanostructured silicon nano-needles (‘nanograss’), we established field-enhancement factors and electromagnetic mode confinements that come close to the best values achievable with state-of-the-art top-down nanotechnology, e.g., in nano antenna produced via focus-ion-beam lithography. Experimental near-field scanning spectroscopy and ultrafast photoemission microscopy of a single gold nanosponge prepared within this project resolved long-lived localized plasmonic hot spots with sub-10 nm resolution in excellent agreement with theoretical calculations. The present successful close cooperation of material science and nanofabrication with, on the one hand most advanced optical characterization methods, and on the other hand a detailed theoretical analysis and modelling of the relevant near-field optics and solid-state physics will be continued with the following goals: (i) nanofabrication of materials with optimized disorder for further enhanced field-fluctuations based on a deeper understanding of hot-spot formation; (ii) new experiments for space- and time-resolved studies of the locally enhanced non-linear response and further increase of the latter; (iii) design, fabrication and investigation of hybrid materials where small, typically atom- or molecule-sized emitters with suitable quantum-optical properties are, e.g., infiltrated into nanosponge pores. The coupling of quantum-emitters to hot-spot modes allows optical functionalities far surpassing those of the constituents. Finally, we plan (iv) to enter completely unchartered territory by the study of the interaction of our fluctuation-dominated materials with strong Terahertz fields which will modulate the interaction of the long-lived, ultra-localized hot spots. It should be emphasized, that the paradigm of ‘fluctuation-dominated materials’ has been confirmed for systems as different as metallic nanosponges and dielectric silicon nanograss. This indicates that the underlying physical mechanisms will be present in many systems with tailored disorder, making the SPP 1839 the ideal context and fruitful environment for the planned research and cooperation.