P. Vasa

Ultrafast Manipulation of Strong Coupling in Metal−Molecular Aggregate Hybrid Nanostructures

We demonstrate an ultrafast manipulation of the Rabi splitting energy Ω(R) in a metal-molecular aggregate hybrid nanostructure. Femtosecond excitation drastically alters the optical properties of a model system formed by coating a gold nanoslit array with a thin J-aggregated dye layer. Controlled and reversible transient switching from strong (Ω(R) ≃ 55 meV) to weak (Ω(R) ≈ 0) coupling on a sub-ps time scale is directly evidenced by mapping the nonequilibrium dispersion relations of the coupled excitations. Such a strong, externally controllable coupling of excitons and surface plasmon polaritons is of considerable interest for ultrafast all-optical switching applications in nanoscale plasmonic circuits.

Ultrasmall bullets of light--focusing few-cycle light pulses to the diffraction limit.

We demonstrate an essentially dispersion-free and diffraction-limited focusing of few-cycle laser pulses through all-reflective microscope objectives. By transmitting 6-fs-pulses from a Ti:sapphire oscillator through an all-reflective 0.5 NA objective, we reach a focus with a beam diameter of 1.0 µm, preserving the time structure of the pulses. The temporal and spatial pulse profile is recorded simultaneously using a novel tip-enhanced electron emission autocorrelator, indicating a focal volume of these pulses of only 1.8 µm3. We anticipate that the demonstrated technique is of considerable interest for inducing and probing optical nonlinearities of individual nanostructures.

Tunneling-barrier controlled excitation transfer in hybrid quantum dot-quantum well nanostructures

A systematic spectroscopic study of the carrier transfer between quantum dot (QD) and quantum well (QW) layers is carried out in a hybrid dot-well system based on InAs QDs and InGaAs QWs. We observe a strong dependence of the QD and QW photoluminescence (PL) both on the dot-well barrier thickness and height. For thick (or high) barriers QD and QW systems accumulate independently sufficient photogenerated carrier densities to be seen in PL even at low nonresonant excitation power. For thin (or low) barriers it is impossible to detect the PL signal from QW at low excitation densities due to effective carrier transfer from QW to QDs. Strong state-filling effects of the excited QD states influence the carrier transfer efficiencies. By investigating the carrier dynamics using time-resolved spectroscopy and the state-filling effects in the continuous wave excitation regime the basic characteristics of interlevel, intersublevel, and dot-well relaxation are determined. The mechanisms of the dot-well coupling are ...

Strictly monolayer large continuous MoS2 films on diverse substrates and their luminescence properties

Despite a tremendous interest on molybdenum disulfide as a thinnest direct band gap semiconductor, single step synthesis of a large area purely monolayer MoS2 film has not yet been reported. Here, we report a CVD route to synthesize a continuous film of strictly monolayer MoS2 covering an area as large as a few cm2 on a variety of different substrates without using any seeding material or any elaborate pretreatment of the substrate. This is achieved by allowing the growth to take place in the naturally formed gap between a piece of SiO2 coated Si wafer and the substrate, when the latter is placed on top of the former inside a CVD reactor. We propose a qualitative model to explain why the MoS2 films are always strictly monolayer in this method. The photoluminescence study of these monolayers shows the characteristic excitonic and trionic features associated with monolayer MoS2. In addition, a broad defect related luminescence band appears at ∼1.7 eV. As temperature decreases, the intensity of this broad fe...

Generation of stable colloidal gold nanoparticles by ultrashort laser-induced melting and fragmentation

We report on generation of stable colloidal gold nanoparticles by ultrashort laser-induced melting and fragmentation. Irradiation of colloidal gold nanoparticles (of initial size larger than 25 nm) by 56 fs long, near-IR pulses of moderate fluence (1.3–5.3 J cm−2) generates very small (2.5 nm) nanoparticles with a narrow size distribution (±0.5 nm). Systematic measurements show the final size of fragmented nanoparticles to be (i) very weakly dependent on the original size and particle shape as well as of pump laser wavelength (800 nm, 1200 nm and 1350 nm), but (ii) strongly dependent on laser parameters; moreover, fragmentation is effectively controllable by pulse fluence and irradiation time. The fragmented particles appear to be contaminant free and have high crystalline quality. We find that the fragmented particles are stable over a time period of more than three months. Stable, contaminant-free, crystalline colloidal gold nanoparticles of sizes around 3 nm, with very narrow size distribution, have potential utility in diverse nanotechnological applications, ranging from biologically relevant imaging to nanoscopic generators of high-frequency mechanical vibrations in the GHz range.

Observation of Lorentzian lineshapes in the room temperature optical spectra of strongly coupled Jaggregate/metal hybrid nanostructures by linear two-dimensional optical spectroscopy

We analyze the linear optical reflectivity spectra of a prototypical, strongly coupled metal/molecular hybrid nanostructure by means of a new experimental approach, linear two-dimensional optical spectroscopy. White-light, broadband spectral interferometry is used to measure amplitude and spectral phase of the sample reflectivity or transmission with high precision and to reconstruct the time structure of the electric field emitted by the sample upon impulsive excitation. A numerical analysis of this time-domain signal provides a two-dimensional representation of the coherent optical response of the sample as a function of excitation and detection frequency. The approach is used to study a nanostructure formed by depositing a thin J-aggregated dye layer on a gold grating. In this structure, strong coupling between excitons and surface plasmon polaritons results in the formation of hybrid polariton modes. In the strong coupling regime, Lorentzian lineshape profiles of different polariton modes are observed at room temperature. This is taken as an indication that the investigated strongly coupled polariton excitations are predominantly homogeneously broadened at room temperature. This new approach presents a versatile, simple and highly precise addition to nonlinear optical spectroscopic techniques for the analysis of line broadening phenomena.

Coherent exciton - surface plasmon polariton interactions in hybrid metal semiconductor nanostructures

We report an experimental study of the coherent coupling between Surface Plasmon Polaritons (SPPs) and Quantum Well (QW) excitons in a hybrid metal- semiconductor nanostructure, consisting of a gallium arsenide quantum well placed in a close proximity of a metal nanoslit array. Exciton-SPP coupling is probed by low-temperature angle resolved spectroscopy. Our results give evidence of a distinct modification of the exciton dispersion relation due to interaction of the exciton with SPP fields at both interfaces of the metal film. An analysis of the experimental data within a coupled oscillator model indicates coupling strengths of several tens of meV.

Field-level characterization of the optical response in J-aggregate/metal hybrid nanostructures by chirp-compensated spectral interferometry

We use chirp-compensated spectral interferometry to fully probe the optical response of strongly coupled excitons (Xs) and surface plasmon polaritons (SPPs) in J-aggregate/metal hybrid nanostructures at a field level. The effect of higher order chirp in the interferometric arms on the spectral phase retrieval is theoretically and experimentally analyzed. A quantitative description of the X-SPP polariton response is precisely obtained by simultaneously fitting the measured spectra and phases to a Fano lineshape model. We further deduced the time structure of the reemitted polariton field by Fourier transform to the time domain, which shows clear damped polarization oscillations within an ultrashort time scale of 100 fs.

Electrostatic engineering of charge clouds around DNA inhibits strand breakages

Femtosecond laser-induced damage to plasmid DNA in aqueous medium manifests itself as strand breaks arising from interactions with slow OH-radicals (and electrons) in the laser-induced plasma generated in water. Such strand breakages are found to be significantly inhibited upon addition of salts (in physiologically significant concentrations) to the aqueous medium. A simple model rationalizes our observations and suggests the feasibility of electrostatically “engineering” DNAs ion atmosphere so as to prevent damage-inducing free radicals accessing DNA strands. These findings may aid development of new strategies for cancer treatment.

Spectroscopy and dynamics of charge transfer excitons in type-II band aligned quantum confined heterostructures

We illustrate effect of charge transfer (CT) in type-II quantum confined heterostructure by comparing CdSe quantum dots (QDs), CdSe/CdTe heterostructure quantum dots (HQDs) and CdSe/CdTe/CdSe quantum well-quantum dots (QWQDs) heterostructures. CdSe core QDs were synthesized using a kinetic growth method where QD size depends on reaction time. For shell coating we used modified version of successive ionic layer adsorption and reaction (SILAR). Size of different QDs ∼5 to 7 nm were measured by transmission electron microscopy (TEM). Strong red shift from ∼597 to ∼746 nm in photoluminescence (PL) spectra from QDs to QWQDs shows high tunability which is not possible with single constituent semiconductor QDs. PL spectra have been recorded at different temperatures (10K-300K). Room temperature time correlated single photon counting (TCSPC) measurements for QDs to QWQDs show three exponential radiative decay. The slowest component decay constant in QWQDs comes around eight fold to ∼51 ns as compared to ∼6.5 ns in HQD suggesting new opportunities to tailor the radiative carrier recombination rate of CT excitons.