Lev Chuntonov

Trimeric Plasmonic Molecules: The Role of Symmetry

Artificial plasmonic molecules possess excitation modes that are defined by their symmetry and obey group theory rules, just like conventional molecules. We follow the evolution of surface-plasmon spectra of plasmonic trimers, assembled from equal-sized silver nanoparticles, as gradual geometric changes break their symmetry. The spectral modes of an equilateral triangle, the most symmetric structure of a trimer, are degenerate. This degeneracy is lifted as the symmetry is lowered when one of the vertex angles in opened, which also leads to a subtle transition between bright and dark modes. Our experimental results are quantitatively explained using numerical simulations and plasmon hybridization theory.

Vacuum Rabi splitting in a plasmonic cavity at the single quantum emitter limit

The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light–matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations.

Correlating electron tomography and plasmon spectroscopy of single noble metal core-shell nanoparticles.

The 3D structure reconstruction of gold core-silver shell nanoparticles by electron tomography is combined with optical dark-field spectroscopy. Electron tomography allows segmentation of the particles into core and shell subvolumes and facilitates avoiding Bragg diffraction artifacts inherent in 2D images. This advantage proves essential for accurate correlation of plasmon spectra and structure. We find that for the nanoparticles of near-spherical shape studied here the plasmon resonances depend on the relative size of the core and shell, rather than on their exact shapes and concentricity. A remarkable dependence of the spectral shape on the permittivity of the surrounding medium is also demonstrated, suggesting that core-shell nanoparticles can be used as ratiometric sensors with a very high dynamic range.

Maximal Raman Optical Activity in Hybrid Single Molecule-Plasmonic Nanostructures with Multiple Dipolar Resonances

We show that a hybrid system built of a plasmonic nanoparticle cluster and a single molecule can attain maximal Raman optical activity (ROA), converting linearly polarized light into purely circularly polarized light at the Raman-scattered frequency. In contrast to standard molecular ROA, the effect described here does not involve magnetic modes and is attributed to off-resonance excitation of electric-dipole plasmon modes of the nanoparticle cluster. A model based on a combination of harmonic oscillators excited at the frequency of the Raman-scattered light is shown to successfully capture the physics of the effect.

Kinetics of exchange between zero-, one-, and two-hydrogen-bonded states of methyl and ethyl acetate in methanol.

It has recently been shown that the ester carbonyl stretching vibration can be used as a sensitive probe of local electrostatic field in molecular systems. To further characterize this vibrational probe and extend its potential applications, we studied the kinetics of chemical exchange between differently hydrogen-bonded (H-bonded) ester carbonyl groups of methyl acetate (MA) and ethyl acetate (EA) in methanol. We found that, while both MA and EA can form zero, one, or two H-bonds with the solvent, the population of the 2hb state in MA is significantly smaller than that in EA. Using a combination of linear and nonlinear infrared measurements and numerical simulations, we further determined the rate constants for the exchange between these differently H-bonded states. We found that for MA the chemical exchange reaction between the two dominant states (i.e., 0hb and 1hb states) has a relaxation rate constant of 0.14 ps(-1), whereas for EA the three-state chemical exchange reaction occurs in a predominantly sequential manner with the following relaxation rate constants: 0.11 ps(-1) for exchange between 0hb and 1hb states and 0.12 ps(-1) for exchange between 1hb and 2hb states.

Enhancement of intermediate-field two-photon absorption by rationally shaped femtosecond pulses

We extend the powerful frequency-domain analysis of femtosecond two-photon absorption to the intermediate-field regime of considerable absorption yields, where additionally to the weak-field nonresonant two-photon transitions also four-photon transitions play a role. Consequently, we rationally find that the absorption is enhanced over the transform-limited pulse by any shaped pulse having a spectral phase that is antisymmetric around one-half of the transition frequency and a spectrum that is asymmetric around it (red or blue detuned according to the system). The enhancement increases as the field strength increases. The theoretical results for Na are verified experimentally.

Pulse-bandwidth dependence of coherent phase control of resonance-mediated ( 2 + 1 ) three-photon absorption

We study in detail coherent phase control of femtosecond resonance-mediated

Frequency-domain coherent control of femtosecond two-photon absorption: intermediate-field versus weak-field regime

(2+1)

Vibrational dynamics of a non-degenerate ultrafast rotor: The (C12,C13)-oxalate ion

three-photon absorption and its dependence on the spectral bandwidth of the excitation pulse. The regime is the weak-field regime of third perturbative order. The corresponding interference mechanism involves a group of three-photon excitation pathways that are on resonance with the intermediate state and a group of three-photon excitation pathways that are near resonant with it. The model system of the study is atomic sodium (Na), for which experimental and numerical-theoretical results are obtained. Prominent among the results is our finding that with simple proper pulse shaping an increase in the excitation bandwidth leads to a corresponding increase in the enhancement of the three-photon absorption over the absorption induced by the (unshaped) transform-limited pulse. For example, here, a 40 nm bandwidth leads to an order-of-magnitude enhancement over the transform-limited absorption.

Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays

Coherent control of femtosecond two-photon absorption in the intermediate-field regime is analysed in detail in the powerful frequency domain using an extended fourth-order perturbative description. The corresponding absorption is coherently induced by the weak-field non-resonant two-photon transitions as well as by four-photon transitions involving three absorbed photons and one emitted photon. The interferences between these two groups of transitions lead to a difference between the intermediate-field and weak-field absorption dynamics. The corresponding interference nature (constructive or destructive) strongly depends on the detuning direction of the pulse spectrum from one-half of the two-photon transition frequency. The model system of the study is atomic sodium, for which both experimental and theoretical results are obtained. The detailed understanding obtained here serves as a basis for coherent control with rationally-shaped femtosecond pulses in a regime of sizeable absorption yields.