Ultrafast nanooptics

Exciton-plasmon interaction in metal-semiconductor hybrid nanostructures (Lienau, Runge)

Ultrafast nanooptics

Exciton-plasmon interaction in metal-semiconductor hybrid nanostructures (Lienau, Runge)


A major problem in plasmonics are the short lifetimes and the corresponding small propagation lengths of surface plasmon polaritons (SPPs). These are due to the unavoidable ohmic losses in the metal structures. An idea to obtain long propagation lengths which are essential in plasmonic circuitry is to resonantly and coherently couple the SPPs to gain media. For this purpose, we fabricate and characterize metal-semiconductor hybrid nanostructures where we bring structures with optical gain, namely quantum wells and dye-molecules, very close to the metal surfaces on which we excite SPPs. We investigate their coupled behavior with ultrafast non-linear pump-probe experiments. Furthermore, we develop FEM and FDTD calculations as a tool for the interpretation of our experimental findings and structure optimization.

Setup for ultrafast pump-probe experiments

The experimental setup for angle-resolved pump probe measurements is shown in Fig.1. The output pulse of an OPA-System is first precompressed for material dispersion with chirped mirrors and than split into pump and probe pulses. The sample is mounted inside a cryostat and cooled with liquid nitrogen. The Setup is computer controlled and the desired excitation angle can be easily adjusted. With this setup a time resolution in the order of 10fs is achieved.

Basics of SPP-Exciton coupling

To study the radiative coupling between excitons and surface plasmon polaritons (SPP) the hybrid nanostructure shown in Fig.2 is used. It consists of a gold nano-slit grating, coated with a thin polymer film of J-aggregated dye molecules.
Due to the gold grating, far-field light can be coupled to specific plasmon modes. The cyanine dye has a large excitonic dipole moment and forms photostable J-aggregate layers. A J-aggregate exciton will couple to the vacuum SPP field. Excitons located outside of the slits have a strongly reduced radiative coupling. Theoretical calculations are performed to optimize the geometrical sample parameters. Fig.3. shows the theoretical field-distribution of the grating-sample, calculated by FEM and FDTD methods.

Selected results in a nutshell

1. Formation of new states

The coupling of excitons and surface plasmon polaritons (SPP) leads to formation of new eigenstates (see Fig.4). They are called exciton-surface plasmon polaritons or excimons. Their dispersion relations repel each other. Due to the coupling between the two states, energy is transferred between them. The probability to find the system in the second state after the first one is excited, oscillates with the Rabi frequency.

2. Incoherent exciton excitations allows to manipulate the Rabi splitting

By performing pump-probe experiments with fully incoherent optical excitation, it was shown, that the Rabi splitting of the coupled system could be changed. Fig. 5 shows angle-resolved differential spectra of the metal-J-aggregate hybrid structure for three different pump fluences. Two prominent features are observed:

1. the anti-crossing between J-aggregate exciton and SPP, resulting in the formation of coupled X-SPP modes (black and white dashed lines).

2. Due to the bleaching of exciton resonance with increasing pump fluence, an essentially complete disappearance of the anti-crossing occurs.

3. Future application: ultrafast switches and photonic transistors

The change of the Rabi splitting has remarkable influence of the optical properties of the hybrid structures. The pump laser induces a transient shift of the coupled polariton resonances, without affecting their dephasing properties. That shift results in pronounced polaritonic nonlinearities. The shaded area in Fig. 6 shows, that the pump-induced exciton creation reduces the sample reflectivity around 710nm near the LP resonance, from 95% down to about 40%. This reflectivity change occurs on a fast, sup-ps time scale. Therefore such layered hybrid structures are interesting for future applications like ultrafast switches or photonic transistors.


Ultrafast manipulation of the Rabi splitting in metal-molecular aggregate hybrid nanostructures
P. Vasa, R. Pomraenke, G. Cirmi, E. De Re, W.Wang, S. Schwieger, D. Leipold, E. Runge, G. Cerullo, and C. Lienau
Phys. Status Solidi C 8 (2011) 1113
Surface plasmon polariton - exciton interaction in metal-semiconductor and metal-dye nanostructures
S Schwieger, P Vasa, R Pomraenke, C Lienau and E Runge
Journal of Physics: Conference Series 210 (2010) 012001
An Unusual Marriage: Coupling Molecular Excitons to Surface Plasmon Polaritons in Metal Nanostructures
P. Vasa, C. Lienau
Angewandte Chemie-International Edition 49 (2010) 2476
Ultrafast Manipulation of Strong Coupling in Metal-Molecular Aggregate Hybrid Nanostructures
Vasa, P. Pomraenke, R. Cirmi, G. De Re, E. Wang, W. Schwieger, S. Leipold, D. Runge, E. Cerullo, C. Lienau
ACS Nano 4 (2010) 7559
Ultra-fast nano-optics
P. Vasa, C. Ropers, R. Pomraenke, C. Lienau
Laser & Photonics Reviews 3 (2009) 483
Coherent exciton - surface plasmon polariton interaction in hybrid metal semiconductor nanostructures
P. Vasa, R. Pomraenke, S. Schwieger, Y. I. Mazur, Vas. Kunets, P. Srinivasan, E. Johnson, G. J. Salamo, E. Runge and C. Lienau
Physica Status Solidi C 6 (2009) 466
Calculation and interpretation of surface plasmon polariton features in the reflectivity of metallic nanowires arrays
P. Scholz, S. Schwieger, P. Vasa, and E. Runge
International Journal of Modern Physics 22 (2008) 4442
Theory of the surface plasmon polariton-exciton interaction in multi-layer systems
S. Schwieger, P. Vasa, E. Runge
Physica Status Solidi B-Basic Solid State Physics 245 (2008) 1071
Coherent Exciton-Surface-Plasmon-Polariton Interaction in Hybrid Metal-Semiconductor Nanostructures
P. Vasa, R. Pomraenke, S. Schwieger, Yu I Mazur, Vas Kunets, P. Srinivasan, E. Johnson, J. E. Kihm, D.S. Kim, E. Runge, G. Salamo, C. Lienau
Physical Review Letters 101 (2008) 116801
Optical spectroscopy of single-walledcarbon nanotubes: From excitonic effects towards control of the radiative lifetime
R. Pomraenke, P. Vasa, C. Lienau
Physica Status Solidi B-Basic Solid State Physics 245 (2008) 1033
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