Jasmina Hranisavljevic

Photo-initiated energy transfer in nanostructured complexes observed by near-field optical microscopy

We report an apertureless near‐field optical study on nanostructured objects formed by J‐aggregates adsorbed on silver (Ag) nanoparticles. Near‐field images reveal that the enhanced near‐field from the dressed particles (DP) resonantly excited plasmon oscillation is efficiently absorbed by the J‐aggregates. The sensitivity of the near‐field images recorded at the harmonics of the probe vibration frequency suggests that the DP is releasing part of the absorbed energy radiatively upon interaction with the probe. The role of the probe in providing this new radiative relaxation channel is further confirmed as fluorescence from the J‐aggregates on the particle is detected on the particle location only. We based the interpretation of our results on the near‐field optical response from a bare Ag particle excited at the plasmon resonance as well as on far‐field emission and transient absorption experiments.

Visualizing charge movement near organic heterojunctions with ultrafast time resolution via an induced Stark shift

We introduce a method to monitor photoinduced charge separation processes in organic donor-acceptor heterostructures. This approach utilizes a transient Stark shift of the exciton band of a molecular J-aggregate, deposited as a thin probe layer adjacent to the organic heterojunction. The high temporal dynamic range of this approach, from 100 femtoseconds to nanoseconds and longer, enables the entire charge separation process to be followed in both space and time. More broadly, this method can be applied to characterize photoinduced charge injection and separation processes in different materials and architectures, where sub-picosecond time resolution is needed at high spatial resolution.

Modulation of the exciton lifetime of j-aggregates on metal nanoparticles and nanoparticle arrays explored through ultrafast spectroscopy and near-field optical microscopy

Illumination of metal nanoparticles at the plasmon resonance produces enhanced evanescent fields on the nanoparticles’ surfaces. The unusual strength of the field make it a target for exploring photoinduced phenomena at the nanoscale, if efficient functionalization or coating of the nanoparticle surface with appropriate chromophores is possible. One direction is to use cyanine dyes that form monolayers of J-aggregates on the surface of noble metal nanoparticle colloids. The unique, collective electronic properties of J-aggregates produce excitons with enormous extinction coefficients that are of interest for their efficient energy transfer, electron transfer, and nonlinear optical properties. In that vein, we report our results on time-resolved spectroscopy and near-field scanning optical microscopy (NSOM) of J-aggregate exciton dynamics on Ag and Au nanoparticle colloids. Ultrafast transient absorption studies show that J-aggregate exciton lifetimes on Ag nanoparticles are much longer than on Au nanoparticles, with a 300 ps lifetime that is two orders of magnitude longer than the electronic processes in the nanoparticles themselves. Complementary NSOM studies of the colloids show that fluorescence from the J-aggregates on the Ag nanoparticles is induced by the scanning probe. These results may be significant for improving the nanophotonic performance of hybrid materials for nanoscale applications.

Observation of coherent optical coupling within inorganic-organic hybrid nanoparticles

We report on the study of the optical properties of hybrid systems made from molecular aggregates (J-aggregates) and metallic nanoparticles (Au and Ag). Both entities have outstanding optical properties that have been extensively addressed both through experimental and theoretical efforts. The J-aggregates were chosen for their linear and non-linear excitonic response that shows among other intriguing properties, superradiant emission, ultrafast optical switching, and electroluminescence. These J-aggregates form spontaneously on the surface of Au and Ag nanoparticles, used primarily to excite the organic shell through optical near-field interaction. Steady state as well as ultrafast spectroscopic measurements was used to characterize the nature of the interaction between the two constituents as well as to follow the dynamics of the exciton in the aggregate upon near-field excitation from the particle. For both Au and Ag, Mie scattering theory calculations of the hybrid ground state absorption spectra account for the experimental observations and reflect the coherent coupling between the excitonic states in the aggregate and the nanoparticles electronic transition dipoles. Furthermore we show that the dipole-dipole coupling strength between the individual molecules in the aggregate is increased by ~30% on the surface of the particle compared to its value in solution. The different dynamics of these nanosystems have been probed using femtosecond spectroscopy and reveals contrasted relaxation pathways for the exciton when interacting with Ag with respect to Au as well as a delay dependent excitonic coherent length.

Temporal and Spatial Imaging of Energy Flow at the Nanoscale via Molecular Plasmonics

Efforts to spatially and temporally resolve photoinduced energy and charge transfer in hybrid plasmonic nanostructures are discussed. The ability to use these nanostructures to characterize photoinduced energy and charge transfer processes important for solar energy conversion is described.

Ultrafast energy flow in hybrid plasmonic materials

Nanoscale materials absorb, propagate, and dissipate energy very differently than their bulk counterparts. Furthermore, hybrid nanostructures consisting of molecular and plasmonic materials with strongly coupled electronic states can produce new optical states and decay pathways that provide additional handles with which to externally control energy flow in complex nanostructured systems. In this talk, we discuss our recent studies of electromagnetic coupling and associated temporal dynamics of molecular excitations with plasmonic resonances supported by either localized or extended planar geometries. Recent experimental results and theoretical analysis for observing and controlling coherences between molecular excitations and plasmonic polarizations are shown. Advances will explore new directions in ultrafast manipulation of energy dissipation processes in hybrid plasmonic structures, as well as ultrafast addressing and switching in plasmonics-based circuit architectures. Also discussed are recent synthetic advances in the creation of hybrid materials. Ultimately, these studies may impact a range of next-generation optical materials and devices, of relevance to new energy conversion materials, nanoscale photocatalysis, or plasmon-enhanced sensors.

Plasmonic heterostructures for addressable nanophotonics

Plasmonics applications will benefit if reliable means to alter plasmon absorption and damping properties via external inputs are found. We are working towards this goal by functionalizing noble metal films with polarizable, excitonic molecular films. Examples include molecular j-aggregates, whose excitonic absorptions can be photobleached to modify plasmon absorption properties. We report two developments in this area. The first is the observation of coherent polarization coupling between the exciton of a molecular J-aggregate and the electronic polarization of noble metal nanoparticles. The second is a new far-field method to directly observe surface plasmon propagation, demonstrating that the lateral intensity decay length is affected by a change of the interface property. The method relies on the detection of the intrinsic lossy modes associated with plasmon propagation in thin films. We also uniquely introduce a method to excite a broad spectral distribution of surface plasmon simultaneously throughout the visible spectrum allowing surface plasmon based spectroscopy to be performed.