The goal of our research is to advance fundamental understanding of light-matter interactions at subwavelength scale. We utilize both classical and quantum theories to scrutinize general physical aspects of optical properties of various materials ranging from dispersive dielectrics to nonlinear crystals. Such systems exhibit interesting phenomena, some of which we use to investigate how individual atoms and molecules behave in the vicinity of these materials. For example, silver diffraction gratings may support strong electromagnetic field gradients as shown in the figure on the left. These gradients may be used to trap quantum objects.

RESEARCH HIGHLIGHTS

OPTICS AT THE NANOSCALE

COLLABORATIONS

EXPERIMENTS

Adi Salomon [Bar-Ilan University, Ramat Gan, Israel]
Renaud Vallee [CNRS, Bordeaux, France]
THEORY

Abraham Nitzan [University of Pennsylvania]
Eric Charron [University of Paris South XI, Orsay, France]
Ruth Pachter [Air Force Research Lab, WPAFB]
Tamar Seideman [Northwestern University]

FUNDING

First observation of collective exciton resonances in hybrid systems

In collaboration with Renaud Vallee we recently submitted manuscript to Scientific Reports, in which we discuss the first observation of the collective exciton resonance in plasmonic opals. We observe experimentally the transition from a conventional strong coupling regime exhibiting the usual upper and lower polaritonic branches to a more complex regime, where a third nondispersive mode is seen, as the concentration of J-aggregates is increased. The numerical simulations confirm the presence of the third resonance. We attribute its physical nature to collective molecule-molecule interactions leading to the collective electromagnetic response. A simple analytical model is proposed to explain the physics of the third mode. The nonlinear dependence on molecular parameters followed from the model are confirmed in a set of rigorous numerical studies. It is shown that at the energy of the collective mode molecules oscillate completely out of phase with the incident radiation acting as an effective thin metal layer.

Figure above shows the Rabi splitting vs. square root of the molecular concentration. The conventional dependence is shown as a dashed line. The actual results significantly deviate from the known model at high molecular concentrations. Rigorous numerical simulations and a simple analytical model shed light onto the physical nature of the new mode. It is shown that such a mode corresponds to the collective molecular exciton resulting from strong molecule-molecule interaction. The dependence of the energy of this mode on various material parameters is confirmed by numerical simulations and further explained using a simple analytical model. It is demonstrated that the molecules oscillate out-of-phase with the incident radiation at the energy of the collective mode acting as an effective metallic layer. The physical nature of the observed mode, closely related to the collective molecule-molecule coupling, has all the attributes of a superradiant mode.

Photon echo in exciton-plasmon materials

Recently published manuscript explores the dynamics of photon echo experiments in hybrid materials. We investigate the dynamics of photon echo exhibited by exciton-plasmon systems under strong coupling conditions. Using a self-consistent model based on coupled Maxwell-Bloch equations we investigate femtosecond time dynamics of ensembles of interacting molecules optically coupled to surface plasmon supporting materials. It is shown that observed photon echoes under two pulse pump-probe sequence are highly dependent on various material parameters such as molecular concentration and periodicity. Simulations of photon echoes in exciton-plasmon materials reveal a unique signature of the strong exciton-plasmon coupling, namely a double-peak structure in spectra of recorded echo signals. This phenomenon is shown to be related to hybrid states (upper and lower polaritons) in exciton-plasmon systems under strong coupling conditions. It is also demonstrated that the double-peak echo is highly sensitive to mild deviations of the coupling from resonant conditions making it a great tool for ultrafast probes.

The figure shows the photon echo signal simulated in a hybrid system comprising interacting molecules and periodic array of slits. What is seen is very different from the conventional photon echo generated by a 1-D ensemble: the observed echo has a clear double-peaked structure. The frequency-time analysis of the echo signal reveals two peaks reveals the fact that the double-peaked structure has its origins in the interaction between surface plasmons and molecules.

The role of ro-vibrational molecular states in exciton-plasmon materials

In collaboration with Eric Charron we recently published new studies, in which we extended the model of exciton-plasmon materials to include a ro-vibrational structure of molecules using wave-packet propagations on electronic potential energy surfaces. Our model replaces conventional two-level emitters with more complex molecules, allowing us to examine the influence of alignment and vibrational dynamics on strong coupling with surface plasmon-polaritons. We apply the model to a hybrid system comprising a thin layer of molecules placed on top of a periodic array of slits. Rigorous simulations are performed for two types of molecular systems described by vibrational bound-bound and bound-continuum electronic transitions. Calculations reveal new features in transmission, reflection, and absorption spectra, including the observation of significantly higher values of the Rabi splitting and vibrational patterns clearly seen in the corresponding spectra. We also examine the influence of anisotropic initial conditions on optical properties of hybrid materials, demonstrating that the optical response of the system is significantly affected by an initial pre-alignment of the molecules. Our work demonstrates that pre-aligned molecules could serve as an efficient probe for the subdiffraction characterization of the near-field near metal interfaces.

Figure above shows the transmission spectra obtained for the period array of slits with no molecules (black), slits covered by a thin layer of interacting two-level molecules (red), interacting diatomic molecules described by bound-bound electronic transitions (green), and bound-continuum transitions (blue).

Topical review: optics of exciton-plasmon nanomaterials

In collaboration with Abraham Nitzan we recently finished a comprehensive topical review of research in optics of exciton-plasmon systems. This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued. This review will appear soon in Journal of Physics: Condensed Matter.