Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes-Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.

Authors:

Satyendra Nath Gupta, Ora Bitton, Tomas Neuman, Ruben Esteban, Lev Chuntonov, Javier Aizpurua & Gilad Haran

Glutathione Self-Assembles into a Shell of Hydrogen-Bonded Intermolecular Aggregates on “Naked” Silver Nanoparticles

A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the β-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using ¹³C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native β-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons’ anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.

Authors: Arghyadeep Basu, Alexander Vaskevich, and Lev Chuntonov*

Surface-enhanced ultrafast two-dimensional vibrational spectroscopy with engineered plasmonic nano-antennas

Development of noble metal nanostructure substrates that provide strong near-field enhancements enables applications of linear and nonlinear infrared (IR) spectroscopies to study minute sample quantities, such as nanometer thick films and molecular monolayers. Large near-field enhancements of the electric fields used for spectroscopic interrogation of molecules at the nanostructure surface result in enhancement of the spectroscopic signatures. This enhancement scales with the nonlinear order of the method, providing particularly large signal gains for third- and fifth-order IR methods, reaching 10⁶ and 10⁸ raw enhancement factors, not adjusted to the amount of interrogated sample. In this perspective, we overview the advances in the development of nano-arrays of antenna-like nanostructures for mid-IR measurements and illustrate their use in linear and especially nonlinear two-dimensional IR approaches. We discuss how studies of the interaction mechanisms between light, plasmonic antennas, and molecular excitations benefit from the nonlinear two-dimensional time-resolved methods, which involve high-order scaling of the signal with the excitation field, high sensitivity to signal localization, and coherence of the excitation over a broad bandwidth. On the other hand, we demonstrate how studies of molecular structure and ultrafast dynamics by these advanced spectroscopic methods benefit from surface enhancement of signals by plasmonic antennas.

Authors: Lev Chuntonov and Igor V. Rubtsov

Dual-frequency surface-enhanced two-dimensional vibrational spectroscopy of thin layers at an interface

Ultrafast spectroscopy of molecular systems involving hydrogen- (H−) bonding has been at the forefront of fundamental chemical and physical research for several decades. Among the spectroscopic observables of the ultrafast dynamics is the pure dephasing of vibrationally excited molecules. Using third-order nonlinear vibrational spectroscopy, including polarization-selective transient grating measurements of vibrational lifetime and orientational diffusion as well as two-dimensional infrared spectroscopy, we determined different individual line shape components of hydroxyl stretching (νOH) excitations in a homologous series of chlorophenols and obtained the corresponding pure dephasing rates. The pure dephasing rates are correlated with vibrational anharmonicity of the νOH mode, which is tuned remotely from the hydroxyl site by changing the position of the chlorine substituents on the phenol ring. We found that in molecules where the hydroxyl group is in its free form, the pure dephasing rates scale linearly with the mode’s anharmonicity such that assuming it is dominated by the third-order diagonal term, the ultrafast dynamics follow the prediction of the Kubo–Oxtoby theory. However, in the intramolecularly H-bonded ortho-chlorophenols, this trend is reversed, and the pure dephasing slows down by ∼50% for an increase in anharmonicity of only a few wavenumbers. Because the νOH mode’s anharmonicity is known to reflect the H-bonding strength, our results suggest that intramolecular H-bonding can serve as a mechanism of protection from fluctuating forces exerted by the solvent. Such an effect can be relevant for ultrafast dynamics in biomolecules, where H-bonding plays a central role.

Authors: Amit Akiva, Lev Chuntonov

Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas. Excited population assessment relies on a novel reference-sample-free evaluation of the field enhancement with 5th- and 3rd-order nonlinear infrared spectroscopies and on quantitative modeling of coherent excitation dynamics. This study opens opportunities for precise population control utilizing population inversion for vibrational transitions using weak fields.

Authors: Robert T. Mackin, Bar Cohn, Lev Chuntonov, and Igor V. Rubtsov

Plasmonic Trimers for Dual-Frequency Surface-Enhanced Two-Dimensional Infrared Spectroscopy

Extension of surface-enhanced two-dimensional infrared spectroscopy (SE-2DIR) to dual-frequency experiments allows studying dynamics and energy transport in thin molecular films by tagging and probing vibrational modes on different sites of the molecule. Measurements of cross-peaks involving transitions largely separated in frequency by SE-2DIR require plasmonic nanostructures with resonant excitations at the corresponding frequencies, where the associated enhanced near-fields spatially overlap and different molecular transitions are simultaneously enhanced in the same molecule. Gold trimer infrared antennas localize enhanced fields within the gap formed by their arms. We exploit the symmetry of trimer antennas to individually tune frequencies of the in-plane plasmonic excitations to match molecular transitions of interest. Dual-frequency SE-2DIR measurements are demonstrated on 4-azidobutyrate-N-hydroxysuccinimide ester with the cross-peaks between the carbonyl and azido stretching vibrational modes, separated by 370 cm^–1, and the carbonyl and C–N–C stretching modes, separated by 550 cm^–1. Excitation with cross-polarized laser pulses allows appropriate plasmon excitations in resonance with the relevant molecular transitions to be selectively accessed. Our approach, based on the rational plasmon mode engineering, achieves significant enhancement of the cross-peak signals involving largely separated transition frequencies, which is not possible with single broadband plasmon modes.

Authors: Robert T. Mackin, Bar Cohn, Ben Engelman, Adi Goldner, Igor V. Rubtsov and Lev Chuntonov

Two-Dimensional Infrared Spectroscopy Reveals Molecular Self-Assembly on the Surface of Silver Nanoparticles

The conformation of molecules, peptides, and proteins, self-assembled into structured monolayers on the surface of metal nanoparticles (NPs), can strongly affect their properties and use in chemical or nanobiomedical applications. Elucidating molecular conformations on the NP surface is highly challenging, and the microscopic details mostly remain elusive. Using polarization-selective third-order two-dimensional ultrafast infrared spectroscopy, we revealed the highly ordered intermolecular structure of γ-tripeptide glutathione on the surface of silver NPs in aqueous solution. Glutathione is an antioxidant thiol abundant in living cells; it is extensively used in NP chemistry and related research. We identified conditions where the interaction of glutathione with the NP surface facilitates formation of a β-sheet-like structure enclosing the NPs. A spectroscopic signature associated with the assembly of β-sheets into an amyloid fibril-like structure was also observed. Remarkably, the interaction with the metal surface promotes formation of a fibril-like structure by a small peptide involving only two amino acids.

Authors: Anup Ghosh, Amit Kumar Prasad, Lev Chuntonov

Quantifying conformations of ester vibrational probes with hydrogen-bond-induced Fermi resonances

Solvatochromic shifts of local vibrational probes report on the strength of the surrounding electric fields and the probe’s hydrogen bonding status. Stretching vibrational mode of the ester carbonyl group is a popular solvatochromic reporter used in the studies of peptides and proteins. Small molecules, used to calibrate the response of the vibrational probes, sometimes involve Fermi resonances (FRs) induced by inter-molecular interactions. In the present work, we focus on the scenario where FR does not appear in the infrared spectrum of the ester carbonyl stretching mode in aprotic solvents; however, it is intensified when a hydrogen bond with the reporter is established. When two molecules form hydrogen bonds to the same carbonyl oxygen atom, FR leads to strong hybridization of the involved modes and splitting of the absorption peak. Spectral overlap between the Fermi doublets associated with singly and doubly hydrogen-bonded carbonyl groups significantly complicates quantifying different hydrogen-bonded conformations. We employed a combination of linear and third-order (2DIR) infrared spectroscopy with chemometrics analysis to reveal the individual line shapes and to estimate the occupations of the hydrogen-bonded conformations in methyl acetate, a model small molecule. We identified a hydrogen-bond-induced FR in complexes of methyl acetate with alcohols and water and found that FR is lifted in larger molecules used for control experiments—cholesteryl stearate and methyl cyanoacetate. Applying this methodology to analyze acetonitrile-water solutions revealed that when dissolved in neat water, methyl acetate occupies a single hydrogen-bonding conformation, which is in contrast to the conclusions of previous studies. Our approach can be generally used when FRs prevent direct quantification of the hydrogen bonding status of the vibrational probe.

Authors: Anup Ghosh, Bar Cohn, Amit K Prasad, Lev Chuntonov

Artificial plasmonic molecules and their interaction with real molecules

Plasmonic molecules are small assemblies of nanosized metal particles. Interactions between the particles modify their optical properties and make them attractive for multiple applications in spectroscopy and sensing. In this review, we focus on basic properties rather than on applications. Plasmonic molecules can be created using either nanofabrication methods or self-assembly techniques in solution. The interaction of plasmonic molecules with light leads to excitations that are classified using the concept of normal modes. The simplest plasmonic molecule is a dimer of particles, and its lowest energy excitation takes the form of a symmetric dipolar mode. More complex excitations take place when a larger number of particles is involved. The gaps between particles in a plasmonic molecule form hotspots in which the electromagnetic field is concentrated. Introducing molecules into these hotspots is the basis of a vast spectrum of enhanced spectroscopies, from surface-enhanced Raman scattering to surface-enhanced fluorescence and others. We show in this review how these spectroscopic methods can be used to characterize the fields around plasmonic molecules. Furthermore, the strong fields can be used to drive new phenomena, from plasmon-induced chemical reactions to strong coupling of quantum emitters with the plasmonic fields. We systematically discuss these phenomena, introducing in each case the theoretical basis as well as recent experimental realizations.

Authors: Gilad Haran, Lev Chuntonov

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas

Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule–plasmon interaction. The mutual radiative damping of plasmonic and molecular transitions leads to the spectroscopic signal of a molecular vibrational excitation to be enhanced by up to a factor of 50 in the case of linear spectroscopy and over 2000 in the case of third-order spectroscopy. A qualitative explanation for the observed effect is given by the extended coupled oscillators model, which takes into account both near-field and radiative interactions between the plasmonic and molecular transitions.

Authors: Andrey Gandman, Robert T Mackin, Bar Cohn, Igor V Rubtsov, Lev Chuntonov

Optics lab	Wet lab	Email
Solid State Institute, 324	Schulich Faculty of Chemistry, 312	chunt @ technion.ac.il
T: +972-4-829-3933	T: +972-4-829-3946