Metal nanostructures show pronounced electromagnetic resonances that arise from localized surface plasmons. These collective oscillations of free electrons in the metal give rise to confined electromagnetic near fields. Surface-enhanced spectroscopy exploits the near-field intensity to enhance the optical response of nanomaterials by many orders of magnitude.
Plasmons are classified as bright and dark depending on their interaction with far-field radiation. Bright modes are dipole-allowed excitations that absorb and scatter light. Dark modes are resonances of the electromagnetic near field only that do not couple to propagating modes. The suppressed photon emission of dark plasmons makes their resonances spectrally narrow and intense, which is highly desirable for enhanced spectroscopy as well as storing and transporting electromagnetic energy in nanostructures. The suppressed absorption, however, prevents us from routinely exploiting dark modes in nanoplasmonic systems.
The original premise of this project was to use spatially patterned light beams and exploit retardation to excite dark plasmons with far-field radiation. This approach was suggested to activate the excitation of dark modes, while their radiative decay remain suppressed. We wanted to harvest dark modes for surface-enhanced Raman scattering providing an enhancement that may be tailored to specific vibrations. While this idea worked, it proved less fruitful than we had hoped originally. The main obstacles was the extremely strong dependence of the dark mode excitation on the focusing conditions.
We then developed a second concept for the plasmon polaritons with long lifetimes that proved to be a real game changer in light-matter interaction, polaritonics, and nanostructured materials. We synthesized and studied plasmonic nanoparticle supercrystals as novel artificial materials with record-strong light-matter coupling. These are three-dimensional lattices made of spherical or otherwise regularly shaped metallic nanoparticles. The initial supercrystals were face-centered cubic lattices of gold nanoparticles, but over the project we also worked on supercrystals made of silver mecon nanoparticles for body-centered cubic lattices, and octahedron-tetrahedron combinations for fluorite structures and many more. The interaction between the plasmons on the nanoparticles in the crystals leads to collective plasmons with transverse polarization and outstanding light-matter coupling. We showed how to use such materials for superior enhanced spectroscopy and photocatalysis.
Our project unlocked novel technologies based on nanoplasmonic properties. This class of plasmon-polaritons is useful for enhanced spectroscopy, catalysis and future implementations of quantum technology.
