Periodic Reporting for period 2 - DarkSERS (Harvesting dark plasmons for surface-enhanced Raman scattering)
Reporting period: 2019-10-01 to 2021-03-31
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.
In this project we use spatially patterned light beams and exploit retardation to excite dark plasmons with far-field radiation. By this the distribution of the electric field component matches the dark electromagnetic eigenmode. This approach activates the excitation of dark modes, while their radiative decay remains suppressed. We harvest dark modes for surface-enhanced Raman scattering providing an enhancement that may be tailored to specific vibrations. Another feature of dark modes is their strong coupling to the vibrations of nanostructures, which will be used to amplify vibrational modes and, ultimately, induce phonon lasing.
Our project aims at enabling technologies that unlock a novel range of nanoplasmonic properties. We want to produce a novel class of plasmon-polaritons to be used for enhanced spectroscopy as well as in 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.
In this project we use spatially patterned light beams and exploit retardation to excite dark plasmons with far-field radiation. By this the distribution of the electric field component matches the dark electromagnetic eigenmode. This approach activates the excitation of dark modes, while their radiative decay remains suppressed. We harvest dark modes for surface-enhanced Raman scattering providing an enhancement that may be tailored to specific vibrations. Another feature of dark modes is their strong coupling to the vibrations of nanostructures, which will be used to amplify vibrational modes and, ultimately, induce phonon lasing.
Our project aims at enabling technologies that unlock a novel range of nanoplasmonic properties. We want to produce a novel class of plasmon-polaritons to be used for enhanced spectroscopy as well as in future implementations of quantum technology.
The initial months of the project focused on hiring team members and setting up the labs to produce and exploit structured light. We were able to hire personal with special expertise in the production and exploitation of structured light that was key for the implementation of the project. We set up a lab for modulation-type spectroscopy and combined existing experiments with setups to produce and characterize structured light. We also learnt how to produce the necessary gold nanostructures using electron-beam lithography and characterized their far-field properties. 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 direct observation of absorption and scattering by dark modes via cylindrical vectors beams and/or beams with angular momentum is currently behind schedule.
We developed the symmetry analysis for dark modes in plasmonic oligomers and derived selection rules for linear and non-linear optics using, e.g. radially polarized light or other forms of structured light. The group theory analysis was combined with numerical simulations to demonstrate the feasibility of non-linear optics in such structures. This part of the work has been completed and a set of novel selection rules has been put down.
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 two breakthrough publications in 2020. We currently develop this research line further to combine it with enhanced spectroscopy and the read-out of extreme light-matter coupling via embedded molecular probes in the nanoparticle crystal.
We developed the symmetry analysis for dark modes in plasmonic oligomers and derived selection rules for linear and non-linear optics using, e.g. radially polarized light or other forms of structured light. The group theory analysis was combined with numerical simulations to demonstrate the feasibility of non-linear optics in such structures. This part of the work has been completed and a set of novel selection rules has been put down.
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 two breakthrough publications in 2020. We currently develop this research line further to combine it with enhanced spectroscopy and the read-out of extreme light-matter coupling via embedded molecular probes in the nanoparticle crystal.
The progress beyond the state of the art can be summarized as follows:
1) We developed the selection rules for the excitation of nanosystems by structured light. They allow to tailor light fields for the excitation of a specific electromagnetic mode.
2) We demonstrated the excitation of dark plasmon modes using field retardation in nanoparticle multilayers. We showed that such modes are well suited to harvest hot electrons, because they have reduced radiative damping. A set of design rules allows optimizing for the desired excitation frequency.
3) We worked out a description of nanoparticle supercrystals as materials with deep strong light-matter coupling. For this we envisioned an artificial crystal that is entirely composed of plasmonic nanoparticles. We showed that this material will extremely strongly interact with light.
4) In collaboration with the Lange group (Hamburg) we synthesis nanoparticle supercrystals and studied theiry optical properties. We verified that our supercrystals showed the strongest light-matter coupling of any material reported so far. In first experiments we confirmed one of the predictions for the novel physics expected for deep strong coupling. The strongly structured electromagnetic modes inside the nanocrystals are extremely promising for enhanced spectroscopy and vibrational pumping.
1) We developed the selection rules for the excitation of nanosystems by structured light. They allow to tailor light fields for the excitation of a specific electromagnetic mode.
2) We demonstrated the excitation of dark plasmon modes using field retardation in nanoparticle multilayers. We showed that such modes are well suited to harvest hot electrons, because they have reduced radiative damping. A set of design rules allows optimizing for the desired excitation frequency.
3) We worked out a description of nanoparticle supercrystals as materials with deep strong light-matter coupling. For this we envisioned an artificial crystal that is entirely composed of plasmonic nanoparticles. We showed that this material will extremely strongly interact with light.
4) In collaboration with the Lange group (Hamburg) we synthesis nanoparticle supercrystals and studied theiry optical properties. We verified that our supercrystals showed the strongest light-matter coupling of any material reported so far. In first experiments we confirmed one of the predictions for the novel physics expected for deep strong coupling. The strongly structured electromagnetic modes inside the nanocrystals are extremely promising for enhanced spectroscopy and vibrational pumping.