Ultrafast Nano-Optics

Ultrafast Rabi oscillations in a molecular/metal-hybrid nanostructure

Guiding and manipulating light on a nanoscale is of high interest to increase the speed and reduce the size of optoelectronic devices. Thereby surface plasmon polaritons (SPPs), optical exitations at the interface between a metal and a dielectric, can play an important role in their realization. However, the weak nonlinearity of the SPPs needs to be compensated which can be achieved by coupling the SPPs to strong nonlinear optical resonances such as excitons (Xs) in molecular or semiconducting nanostructures.

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The aim of the project is to measure the coherent energy transfer of a coupled hybrid nanostructure consisting of a J-aggregate coated onto a gold array (figure 1). The coupling leads to a splitting of the resonance-energies of the two systems (figure 3). The predicted Rabi oscillations between the excitonic quantum emitters and the arising SPP field occur on a timescale of several tenths of femtoseconds which makes it essential to have even shorter laser pulses.

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These pulses were generated by using a NOPA-system with 20fs-pulses centered around 700nm. In a pump-probe set-up, as seen in figure 2, reflectivity spectra were recorded in the presence and absence of the pump pulse and the differential reflectivity DeltaR/R(omega, tau) were calculated.
Clear oscillations at the polariton resonance could be found indicating the energy transfer between the two systems on a timescale of roughly 50 fs (figure 4).

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By coupling excitons with plasmons the advantage of the nanoscale localization of the plasmons can be combined with the strong nonlinearity of the excitons. This can open new ways to build coherent, all-optical ultrafast plasmonic devices.
In a next step the pump-probe setup will be extended such that 2-d-spectroscopy2 can be made on the same sample. This will give further insight into the energy-dynamics of the sample.

1    Vasa, P. et al. Real-time observation of ultrafast Rabi oscillations between excitons and plasmons in metal nanostructures with J-aggregates. Nat. Photonics 7, 128-132, doi:10.1038/nphoton.2012.340 (2013).  + Supporting online material

2    Brixner, T., Mancal, T., Stiopkin, I. V. & Fleming, G. R. Phase-stabilized two-dimensional electronic spectroscopy. J. Chem. Phys. 121, 4221-4236, doi:10.1063/1.1776112 (2004). 


Project members:

Ephraim Sommer, Dr. Antonietta DeSio, Dr. Wei Wang, Dr. Ralf Vogelgesang