A combination of helium- and gallium-ion beam milling together with a fast and reliable sketch-and-peel technique is used to fabricate gold nanorod dimer antennas with an excellent quality factor and with gap distances of less than 6 nm. The high fabrication quality of the sketch-and-peel technique compared to a conventional ion beam milling technique is proven by polarisation-resolved linear dark-field spectromicroscopy of isolated dimer antennas. We demonstrate a strong coupling of the two antenna arms for both fabrication techniques, with a quality factor of more than 14, close to the theoretical limit, for the sketch-and-peel–produced antennas compared to only 6 for the conventional fabrication process. The obtained results on the strong coupling of the plasmonic dimer antennas are supported by finite-difference time-domain simulations of the light-dimer antenna interaction. The presented fabrication technique enables the rapid fabrication of large-scale plasmonic or dielectric nanostructures arrays and metasurfaces with single-digit nanometer scale milling accuracy.
Squaraines are prototypical quadrupolar charge-transfer
chromophores
that have recently attracted much attention as building blocks for
solution-processed photovoltaics, fluorescent probes with large two-photon
absorption cross sections, and aggregates with large circular dichroism.
Their optical properties are often rationalized in terms of phenomenological
essential state models, considering the coupling of two zwitterionic
excited states to a neutral ground state. As a result, optical transitions
to the lowest S1 excited state are one-photon allowed, whereas the
next higher S2 state can only be accessed by two-photon transitions.
A further implication of these models is a substantial reduction of
vibronic coupling to the ubiquitous high-frequency vinyl-stretching
modes of organic materials. Here, we combine time-resolved vibrational
spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical
simulations to test and rationalize these predictions for nonaggregated
molecules. We find small Huang–Rhys factors below 0.01 for
the high-frequency, 1500 cm–1 modes in particular,
as well as a noticeable reduction for those of lower frequency modes
in general for the electronic S0 → S1 transition. The two-photon
allowed state S2 is well separated energetically from S1 and has weak
vibronic signatures as well. Thus, the resulting pronounced concentration
of the oscillator strength in a narrow region relevant to the lowest
electronic transition makes squaraines and their aggregates exceptionally
interesting for strong and ultrastrong coupling of excitons to localized
light modes in external resonators with chiral properties that can
largely be controlled by the molecular architecture.
Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Twodimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton−plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule−gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.
We explore dynamics of strong exciton-plasmon couplings using two-dimensional electronic spectra displaying pronounced Rabi oscillations of their cross-peaks. Radiative couplings govern the polariton nonlinearity and provide first access to the two-quantum-excitations of the coupled system.
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