Collective Polariton Research Articles

Sub-picosecond collapse of molecular polaritons to pure molecular transition in plasmonic photoswitch-nanoantennas

Molecular polaritons are hybrid light-matter states that emerge when a molecular transition strongly interacts with photons in a resonator. At optical frequencies, this interaction unlocks a way to explore and control new chemical phenomena at the nanoscale. Achieving such control at ultrafast timescales, however, is an outstanding challenge, as it requires a deep understanding of the dynamics of the collectively coupled molecular excitation and the light modes. Here, we investigate the dynamics of collective polariton states, realized by coupling molecular photoswitches to optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an ultrafast collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation at room temperature. Through a synergistic combination of experiments and quantum mechanical modelling, we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state.

Cavity induced collective behavior in the polaritonic ground state

Cavity quantum electrodynamics provides an ideal platform to engineer and control light-matter interactions with polariton quasiparticles. In this work, we investigate collective phenomena in a system of many particles in a harmonic trap coupled to a homogeneous cavity vacuum field. The system couples collectively to the cavity field, through its center of mass, and collective polariton states emerge. The cavity field mediates pairwise long-range interactions and enhances the effective mass of the particles. This leads to an enhancement of localization in the matter ground state density, which features a maximum when light and matter are on resonance, and demonstrates a Dicke-like, collective behavior with the particle number. The light-matter interaction also modifies the photonic properties of the polariton system, as the ground state is populated with bunched photons. In addition, it is shown that the diamagnetic A^2A2 term is necessary for the stability of the system, as otherwise the superradiant ground state instability occurs. We demonstrate that coherent transfer of polaritonic population is possible with an external magnetic field and by monitoring the Landau-Zener transition probability.

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s−1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons

Charge-transfer chemical reactions in nanofluidic Fabry-Pérot cavities

We investigate the chemical reactivity of molecular populations confined inside a nanofluidic Fabry-P\'erot cavity. Due to strong light-matter interactions developing between a resonant electromagnetic cavity mode and the electric dipole moment of the confined molecules, a collective polariton excitation is formed. The former gets dressed by environmental vibrational and rotational degrees of freedom of the solvent. We call the resulting polariton dressed by its cloud of environmental excitation a ``reacton'', since it further undergoes chemical reactions. We characterize how the reacton formation modifies the kinetics of a photoisomerization chemical reaction involving an elementary charge-transfer process. We show that the reaction driving force and reorganization energy are both modulated optically by the reactant concentration, the vacuum Rabi splitting, and the detuning between the Fabry-P\'erot cavity frequency and targeted electronic transition. Finally, we compute the ultrafast picosecond dynamics of the whole photochemical reaction. We predict that despite optical cavity losses and solvent-mediated nonradiative relaxation, measurable signatures of the reacton formation can be found in state-of-the-art pump-probe experiments.

Observation and Active Control of a Collective Polariton Mode and Polaritonic Band Gap in Few-Layer WS2 Strongly Coupled with Plasmonic Lattices.

Two-dimensional semiconductors host excitons with very large oscillator strengths and binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS2 coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of a complete polariton band gap with energy scale comparable to the exciton-plasmon coupling strength. A coupled oscillator model reveals that the collective mode arises from the cooperative coupling of the excitons to the plasmonic lattice diffraction orders via exciton-exciton interactions, leading to ultrastrong coupling. The emergence of the collective mode is accompanied by a superlinear increase of the polariton mode splitting as a function of the square root of the exciton oscillator strength. The presence of these many body effects, which are enhanced in systems which lack bulk polarization, not only allows the formation of a collective mode with periodically varying field profiles, but also further enhances the exciton-plasmon coupling. By integrating the hybrid WS2-plasmonic lattice device with a field-effect transistor, we demonstrate active tuning of the collective mode and the polariton band gap. We also report electrically tunable waveguiding in the polariton band gap region through a line defect, which can be turned off with gate bias that can extinguish the collective mode and the polariton band gap. These systems provide new opportunities for obtaining a deeper and systematic understanding of many body cooperative phenomena in two-dimensional materials coupled with periodic photonic systems and for designing more complex and actively controllable polaritonic devices including switchable polariton lasers, waveguides, and optical logical elements.

Scaling the Artificial Polariton Bandgap at Infrared Frequencies Using Indium Tin Oxide Nanorod Arrays

Artificial polariton bandgaps at infrared frequencies are investigated by exploiting the strong coupling of electromagnetic waves with induced electric dipoles in two‐dimensional (2D) indium tin oxide nanorod arrays (ITO‐NRAs). The electric dipoles originate from the collective oscillations of free electrons within the individual ITO nanorods undergoing plasmonic resonance. Controlling the near‐field interactions among the neighboring electric dipoles allows for manipulation of the collective polariton modes that are manifested as a polariton bandgap. A theoretical model is developed to understand the coupled phenomena underlying the unique characteristics of plasmon–polariton bandgaps. With high‐degree geometric control of the ITO‐NRAs, it is experimentally demonstrated that reducing the spacing between ITO nanorods in a square array strengthens the near‐field interactions and thus results in a redshift as well as broadening of the polariton bandgap. Furthermore, arranging ITO‐NRAs in a rectangular lattice breaks the symmetry with respect to the principle axis, which leads to a splitting of the collective polariton modes owing to the competition between the quasi‐longitudinally and quasi‐transversely coupled plasmon–polariton modes. The work highlights the use of a classical dipole coupling method for scaling polariton bandgaps to the infrared in artificial plasmonic lattices, thereby offering a new design dimension for infrared sensing, absorbers, and optical communications.

Quantum nondemolition measurement of small photon numbers using stored light

We propose a scheme for realizing a quantum nondemolition (QND) measurement of a small number of optical photons. Our scheme has two stages: First, we couple a propagating light pulse with fixed photon number to a trapped cold atomic gas within a cavity, such that the pulse is stored within a collective polariton mode. Second, a small-photon-number measurement is engineered by monitoring the cavity-transmission spectrum. Since the polariton mode profile is preserved during the process of detecting the spectrum, photon-number QND measurements could be achieved by retrieving the light pulse from the polariton mode. We also discuss a method which uses QND measurements to generate small-photon Fock states.

Dark-state polaritons in a degenerate two-level system

We investigate the formation of dark-state polaritons in an ensemble of degenerate two-level atoms admitting electromagnetically induced transparency. Using a generalization of microscopic equation-of-motion technique, multiple collective polariton modes are identified depending on the polarizations of two coupling fields. For each mode, the polariton dispersion relation and composition are obtained in a closed form out of a matrix eigenvalue problem for arbitrary control field strengths. We illustrate the algorithm by considering the Fg = 2 → Fe = 1 transition of the D1 line in 87Rb atomic vapor. In addition, an application of dark-state polaritons to the frequency and/or polarization conversion, using D1 and D2 transitions in cold Rb atoms, is given.

Strong coupling regime in semiconductor microcavities

In a Quantum Well embedded in a planar optical microcavity, excitons can be strongly coupled to the electromagnetic standing wave and form 2D polaritons. If N Quantum Wells are placed in the microcavity, they cooperate to a collective polariton state. The luminescence dynamics under resonant excitation is modified by the cavity and varies continuously with the detuning between the exciton and the cavity mode. However, since only 10 % of the exciton radiant states are in the strong coupling regime, as soon as scattering by acoustic phonons has occurred, mainly states in the weak coupling regime are populated and govern the luminescence dynamics, As a consequence, cavity effects are significant only under resonant excitation and for short time delays. To improve the modifications of spontaneous emission, it seems promising to study pillar microcavities where the light is confined in all directions and could be strongly coupled to a larger fraction of exciton states.

Tensor-product structure of a new electromagnetic propagator for nonlocal surface optics of metals

With emphasis on the physical interpretation, the structure of a recently constructed electromagnetic propagator describing, within the framework of the semiclassical infinite-barrier model, the nonlocal optical properties of adjacent vacuum-metal half-spaces, is analyzed. The tensor-product structure of the rotational-free and divergence-free parts of the propagator is determined, and the direct, the indirect, and the self-field contributions are identified. On the basis of a plane-wave expansion, the physical role of the different propagator terms is studied. The contributions to the propagator from the collective polariton and plasmon excitations are separated from those stemming from the electron-hole pair excitations. Via a damped-wave picture of the propagator, contact with a new local propagator formalism of Sipe is established. The flexibility of the present formalism is demonstrated by investigating (i) the excitation of nonlocal surface waves by an oscillating dipole and (ii) the concept of surface-dressed dipole polarizability in the nonlocal regime.