We investigated the interactions between localized plasmons in gold nanorods and excitons in J-aggregates and were able to track an anticrossing behavior of the hybridized modes both in the extinction and in the photoluminescence spectra of this hybrid system. We identified the nonlinear optical behavior of this system by transient absorption spectroscopy. Finally using magnetic circular dichroism spectroscopy we showed that nonmagnetic organic molecules exhibit magnetooptical response due to binding to a plasmonic nanoparticles. In our experiments we also studied the effect of detuning as well as the effect of off- and on resonance excitation on the hybrid states
We focus on fabricating organic/inorganic halide perovskites with controlled dimensionality, size and composition and studying the optical and electrical properties of the resulting nanocrystals. By partially exchanging the most commonly used organic cation methylammonium for a cation with a larger chain we are able to fabricate two-dimensional nanoplatelets down to a single unit cell thickness.1 Through absorption and photoluminescence measurements we find that this leads to a strong-quantum size effect in the perovskites while additionally increasing the exciton bind energy to several hundreds of meV. We employ several fabrication techniques to increase the fluorescence quantum yield to be able to investigate single particles, and to study energy transport between individual nanocrystals by time-resolved spectroscopic methods. Our findings can lead to improvements in not only photovoltaic devices, but also for light-harvesting and light-emitting devices, such as LEDs and lasers.
(1) Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Nano Letters 2015, 15, 6521.
Strong coupling of plasmons and excitons can form hybrid states, the so called “plexcitons”. Especially with molecular J-aggregated dye molecules strong interaction becomes observable even under ambient conditions. In our work we investigate the nature of plexcitonic states formed in gold nanorods coated with a cyanine dye by transient absorption spectroscopy. We demonstrate that plexcitons show tunable plasmonic and excitonic non-linear optical behaviour. Our experimental data can be explained on the basis of a simple Lorentz oscillator model. We find that both the quality factor and the coupling strength between plasmons and excitons can be optically manipulated on an ultrashort time scale.
T. Simon, D. Melnikau, A. Sánchez-Iglesias, M. Grzelczak, L. Liz-Marzán, Y. Rakovich, J. Feldmann and A. Urban, Exploring the optical non-linearities of plasmon-exciton hybrid resonances in coupled colloidal nanostructures, submitted (2016)
This paper reports on transient switch-off of the emission of an electrically pumped quantum dot laser with perturbation by a short optical pulse. This laser response is explained in terms of hot carrier absorption on intraband optical transitions leading to the transient suppression of the laser mode and, hence, to the switch-off. The switching time constant is determined to be as fast as 2 ps.
We have demonstrated a low power, optically-controllable, THz attenuator capable of high contrast ratios using a mixed type-I/type-II quantum well sample. When high free-carrier densities are optically excited in the quantum wells by a cw-laser, the transmitted THz intensity can be controllably reduced. Normally in quantum well samples high carrier densities cannot be achieved using low power excitation densities, because the carrier lifetime is so short. This is not the case for out sample which consists of 20 periods of a narrow and a wide GaAs well. After electron-hole pairs are created via optical excitation in the narrow well, they are separated in space, because the electronics are rapidly transferred into the wide well via an x-valley in the barrier material. The carrier lifetime at low sample temperatures i therefore extremely long, 0.48 ms, leading to high carrier densities. Using an optical excitation power of 2.1 mW from a cw-HeNe laser, the transmitted THz intensity can be reduced by 60 percent.
We present results of the optically excited dynamics in semiconductor quantum wells on short length and time scales. Nonlinear optical experiments are performed with high temporal and high spatial resolution. To interpret the experimental findings calculations are performed on different approximation levels. Two different time regimes are investigated: in the incoherent time regime we study the dynamics of heating, cooling, and the formation of excitons by measuring the temporal behavior of the lateral expansion rate of locally created electron-hole pairs or excitons. A monomolecular exciton formation process is found. The experimental results in this regime are well reproduced by the Boltzmann equation for incoherent exciton densities with phenomenological scattering rates. In addition we have performed a microscopic density matrix analysis for the heating scenario where we have modeled explicitly the initial transformation of coherent excitations into incoherent exciton densities. It is found that the heating due to scattering with acoustic phonons gives reasonable agreement with the observed rates. In the coherent time regime a spatio-temporal beating is observed. This unexpected non-monotonic modulation of the spatial width arises from excitonic wave-packets which modulate the detected lateral profile of the optical nonlinearity in a characteristic way. It is explained by the superposition of various signal components which are detected simultaneously due to the collinearity of our experiment. This effect is illustrated by calculations using a simplified model on the Hartree-Fock level.
The mesoscopic structure of water has long been a subject of discussion. We postulate that, on the mesoscopic scale, liquid water forms nm-size ice-like crystals and that his structure is responsible for absorption in the THz-frequency range. However, until the recent development of Thz-time domain spectroscopy (THz-TDS), it was difficult to determine the optical constants in this frequency range with a good signal-to-noise ratio and hence to study the absorption properties of water. Here we report on the optical properties of water in the frequency range 0.05-1.4 THz and discuss the mesoscopic structure of water. We use THz-TDS based on photoconductive dipole antennas gated by a 150 femtosecond laser pulses to generate and detect the THz- frequency pulses. A new theoretical approach is also presented which were use to explain the absorption behavior in the measured THz frequency range. In this theory, molecular plasma oscillations of H3O2 complexes, that are distinctly separate from the H5O2+ complexes which form an underlying crystalline lattice, are assumed to be responsible for absorption in the THz- frequency range. This model provides good agreement to our data.
We investigate the photophysics in composite systems consisting of fullerene molecules and a conjugated polymer. Photoluminescence (PL) quenching experiments are used to study the photoinduced electron transfer that occurs after photoexcitation of the conjugated polymer In blend systems with various fullerene concentrations we find a strong concentration dependant PL-quenching. By doping a ladder type poly(p-phenylene) (LPPP) with 5 weight percent of a C60-fullerene the polymer PL is quenched by more than one order of magnitude. Time-resolved measurements show that the photoinduced electron transfer can not be described by a single rate. The nonexponential PL-decay is due to complex processes in more detail we have prepared will define heterostructures comprising a self-assembled fullerene monolayer and a thin spin-coated polymer layer. From PL- quenching experiments on these samples we infer a value of 14 nm for the diffusion length of neutral excitations in LPPP.
The nature of the optical excitations at the dipole forbidden optical gap of crystalline C60 is investigated by linear and non-linear spectroscopic techniques. The resonances observed both in the absorption and emission spectra are identified with Herzberg-Teller vibronic coupling of the electronic states to intramolecular vibrational modes. The highly structured fluorescence spectra of both C60 single crystals and crystalline C60 films are interpreted in terms of Frenkel exciton emission from X-traps.
We present an electrical pump optical probe experiment to investigate the temporal and spectral gain dynamics of an actively modelocked diode laser. The temporal behavior of the gain is studied with picosecond time resolution by measuring the transmission of a modelocked Ti:sapphire laser synchronized to the semiconductor laser through the active region of the diode. Our results show that the gain transients exhibit a strong temporal asymmetry which leads to incomplete modelocking.