Periodic Reporting for period 4 - DarkSERS (Harvesting dark plasmons for surface-enhanced Raman scattering)
Reporting period: 2022-10-01 to 2024-09-30
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.
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.
The initial year of the project focused on setting up the team and the labs to produce. We developed labs for modulation-type spectroscopy, combined transmission-reflection studies, and near-field optical micrscopy. The initial success in our experiments was met by the shut down of the labs during the covid-19 pandemic. While we were able to resume some of the experiments over summer 2020, the project became behind schedule, also due to the loss of key personel.
On the theory side, we developed tools for the symmetry analysis of dark modes and collective plasmons in plasmonic oligomers and plasmonic supercrystals. Based on the analysis we derived selection rules for linear and non-linear optics using linearly polarized light and many implementations of structured light. The group theory analysis was combined with numerical simulations to demonstrate the feasibility of non-linear optics in such structures.
A novel idea for the excitation of dark modes was to exploit field retardation in addition to structured light. This idea proved particularly fruit- and successful. We developed a concept to describe propagating plasmonics modes in supercrystals made of gold nanoparticles. We showed that such material can be described as a system with deep strong light-matter coupling and demonstrated first consequences of this regime of extreme interaction strength between light and materials, which lead to a break-through publication of the project. This research line became the main focus of the project. We demonstrated:
- surface-enhanced Raman scattering and surface-enhanced infrared absorption
- surface-enhanced photocatalysis
- the tightly structured electromagnetic fields inside supercrystals
- more complex lattice structures and their effect on plasmon-polaritons in supercrystals
- surface and volume plasmon-polaritons and their lifetime
On the theory side, we developed tools for the symmetry analysis of dark modes and collective plasmons in plasmonic oligomers and plasmonic supercrystals. Based on the analysis we derived selection rules for linear and non-linear optics using linearly polarized light and many implementations of structured light. The group theory analysis was combined with numerical simulations to demonstrate the feasibility of non-linear optics in such structures.
A novel idea for the excitation of dark modes was to exploit field retardation in addition to structured light. This idea proved particularly fruit- and successful. We developed a concept to describe propagating plasmonics modes in supercrystals made of gold nanoparticles. We showed that such material can be described as a system with deep strong light-matter coupling and demonstrated first consequences of this regime of extreme interaction strength between light and materials, which lead to a break-through publication of the project. This research line became the main focus of the project. We demonstrated:
- surface-enhanced Raman scattering and surface-enhanced infrared absorption
- surface-enhanced photocatalysis
- the tightly structured electromagnetic fields inside supercrystals
- more complex lattice structures and their effect on plasmon-polaritons in supercrystals
- surface and volume plasmon-polaritons and their lifetime
This project successfully synthesized and characterized plasmonic supercrystals exhibiting record-breaking light-matter interactions. These supercrystals were subsequently demonstrated to be highly effective substrates for enhanced spectroscopy, providing significant improvements in sensitivity and uniformity of signal enhancement. Furthermore, their unique optical properties were successfully leveraged to create highly efficient bimetallic photocatalysts for sunlight-driven hydrogen generation, showcasing the potential of plasmonic nanostructures for addressing critical challenges in sustainable energy production. The findings demonstrate a significant advancement in the field of plasmonics, opening new avenues for research in advanced spectroscopy, catalysis, and sustainable energy technologies.