The strong interaction between light and matter states presents a fascinating avenue toward tailoring material properties on the nanoscale, such as energy and charge transport, and toward steering of chemical reactions. Plasmonic nanostructures are particularly powerful objects for manipulating the microscopic interactions between spatially highly localized fields and quantum emitters. For sufficiently strong interactions, the resulting hybridization of the light-like plasmons with excitations of the electronic material, usually excitons, leads to the formation of new eigenstates called polaritons. The new optical properties that arise from this strong coupling are often investigated by probing the linear optical response of these polaritons. In the resulting spectra, an energetic splitting between the two polariton states is usually taken as evidence for strong coupling. There is, however, a complementary, much less investigated characteristic of such strong couplings: They give rise to a coherent, i.e., oscillatory, transfer of energy  between the coupled excitonic and plasmonic subsystems. These “Rabi oscillations” are an important property of every strongly coupled system. These characteristic signatures of the energetic polariton splitting in the quantum dynamics are, however, rarely investigated since they often occur on exceedingly short femtosecond timescales.

Time-resolved experiments can, in principle, probe these coherent energy transfer processes directly and thus get access to the underlying couplings. For this a pulse duration that is shorter than the Rabi oscillation period, often only few tens of femtoseconds, is required. An early study, performed in our group some years ago, used femtosecond pump-probe spectroscopy to resolve exciton-plasmon Rabi oscillations [1]. Such a pump-probe experiment gives, however, limited insight into the coherent couplings. The impulsive excitation by the broadband pump pulse does not allow for tracing out the excitation pathways. By now, an improved approach called two-dimensional electronic spectroscopy (2DES), originally developed for nuclear magnetic resonance spectroscopy, has been established in the optical domain as the method of choice to investigate coherent couplings and energy transport. This technique produces a series of energy-energy maps that allow to correlate the excitation and detection of the system while maintaining a high temporal resolution, in our case ~10 fs. 2DES is particularly powerful for investigating both the energetics and the temporal evolution of strongly coupled systems. An important prediction for strong coupling is that the 2DES maps should show not only diagonal polariton peaks but also cross-peaks between them, indicating their coherent coupling. In the time domain, these peaks should then oscillate with the Rabi period. Such signatures in a 2DES experiment have not been reported for plasmonic systems so far.

Since we are interested in probing the quantum dynamics of strong exciton-plasmon couplings, we designed and fabricated a plasmonic nanostructure to investigate its ultrafast optical response using 2DES. We deposited a 10-nm molecular aggregate on a plasmonic nanoslit array as depicted in Fig. 1a. This type of periodic nanostructure creates a spatially modulated plasmon field that is strongly enhanced in the nanoslit region. The plasmon resonance energy can conveniently be tuned by changing the angle of incidence of the laser light. The molecular aggregate [2] is based on squaraine molecules and forms superradiant exciton states that can effectively couple to the plasmon. The linear dispersion, i.e. the angle-dependent energy positions of the optical resonances (Fig. 1b), indeed shows a clear anticrossing between the upper and lower polariton resonances, the spectral signature for strong coupling.

To investigate the coherent dynamics we performed a series of angle-dependent 2DES measurement, Fig. 2 shows the results for an angle with finite detuning between exciton and plasmon. When we first saw these results, we were excited that the data showed clearly the anticipated cross-peaks and that they indeed were oscillating in time with a period that matches their energy splitting. However, a closer inspection of the data and in particular their angle-dependence revealed strong deviations from the expected Rabi oscillations in the simple exciton-plasmon coupling picture. Instead, the coherent dynamics and cross-peaks did not only represent the coherent energy transfer between the exciton and plasmons, but an additional energy transfer between the excitons. We learned that this surprising observation is caused by the spatially strongly structured plasmon field of the periodic nanostructure, which creates two distinct types of excitons in our system. Those excitons in the slit region, where the field enhancement is large, are coupled strongly to the plasmon, while those excitons in-between the slits are only weakly coupled to the plasmon. This spatial variation in the coupling strength results in a coherent and temporally oscillating transport of energy from regions inside the slits to outside the slits and back. It is this coherent energy transport that causes the Rabi oscillations in our experimental data. The plasmon therefore induces a long-range energy transport between regions with different local field strength. We were able to confirm this conclusion by simulating our 2DES data. We could only reproduce the experimental data when accounting for these two types of excitons in our simulations.

Such insight into the intriguing quantum dynamics of strongly coupled nanostructures cannot be obtained from linear optical spectra. Our study shows that 2DES is an exceptionally powerful tool for probing strong coupling phenomena in the time domain that gives much new insight into the underlying microscopic couplings. Ideally, we would of course like to probe these ultrafast quantum dynamics in time and space. New experiments that combine 2DES with optical far-field or near-field superresolution techniques or photoelectron microscopy can, in principle, provide this information. Experimental work towards realizing such experiments is currently underway in several research laboratories in the world.

1. Vasa, P., et al., Real-time observation of ultrafast Rabi oscillations between excitons and plasmons in metal nanostructures with J-aggregates. Nature Photonics, 2013. 7: p. 128-132.
2. Quenzel, T., et al., Plasmon-Enhanced Exciton Delocalization in Squaraine-Type Molecular Aggregates. Acs Nano, 2022. 16(3): p. 4693-4704.