6 research outputs found
Photochemical Initiation of Polariton Propagation
Placing a material inside an optical cavity can enhance transport of
excitation energy by hybridizing excitons with confined light modes into
polaritons, which have a dispersion that provides these light-matter
quasi-particles with low effective masses and very high group velocities. While
in experiments polariton propagation is typically initiated with laser pulses,
tuned to be resonant either with the polaritonic branches that are delocalized
over many molecules, or with an uncoupled higher-energy electronic excited
state that is localized on a single molecule, practical implementations of
polariton-mediated exciton transport into devices would require operation under
low-intensity incoherent light conditions. Here, we propose to initiate
polaritonic exciton transport with a photo-acid, which upon absorption of a
photon in a spectral range not strongly reflected by the cavity mirrors,
undergoes ultra-fast excited-state proton transfer into a red-shifted
excited-state photo-product that can couple collectively with a large number of
suitable dye molecules to the modes of the cavity. By means of atomistic
molecular dynamics simulations we demonstrate that cascading energy from a
photo-excited donor into the strongly coupled acceptor-cavity states can indeed
induce long-range polariton-mediated exciton transport
Tuning the Coherent Propagation of Organic Exciton-Polaritons through the Cavity Q-factor
Transport of excitons in organic materials can be enhanced through polariton
formation when the interaction strength between these excitons and the confined
light modes of an optical resonator exceeds their decay rates. While the
polariton lifetime is determined by the Q(uality)-factor of the optical
resonator, the polariton group velocity is not. Instead, the latter is solely
determined by the polariton dispersion. Yet, experiments suggest that the
Q-factor also controls the polariton propagation velocity. To understand this
observation, we performed molecular dynamics simulations of Rhodamine
chromophores strongly coupled to Fabry-P\'erot cavities with various Q-factors.
Our results suggest that propagation in the aforementioned experiments is
initially dominated by ballistic motion of upper polariton states at their
group velocities, which leads to a rapid expansion of the wavepacket. Cavity
decay in combination with non-adiabatic population transfer into dark states,
rapidly depletes these bright states, causing the wavepacket to contract.
However, because population transfer is reversible, propagation continues, but
as a diffusion process, at lower velocity. By controlling the lifetime of
bright states, the Q-factor determines the duration of the ballistic phase and
the diffusion coefficient in the diffusive regime. Thus, polariton propagation
in organic microcavities can be effectively tuned through the Q-factor.Comment: arXiv admin note: text overlap with arXiv:2209.0730
Enhanced Excitation Energy Transfer under Strong Light-Matter Coupling: Insights from Multi-Scale Molecular Dynamics Simulations
Transfer of excitation energy is a key step in light harvesting and hence of
technological relevance for solar energy conversion. In bare organic materials
energy transfer proceeds via incoherent hops, which restrict propagation
lengths to nanometers. In contrast, energy transport over several micrometers
has been observed in the strong coupling regime where excitations hybridise
with confined light modes to form polaritons. Because polaritons have group
velocity, their propagation should be ballistic and long-ranged. However,
experiments indicate that organic polaritons propagate in a diffusive manner
and more slowly than their group velocity. Here, we resolve this controversy by
means of molecular dynamics simulations of Rhodamine molecules in a
Fabry-P\'erot cavity. Our results suggest that polariton propagation is limited
by the cavity lifetime and appears diffusive due to reversible population
transfers between bright polaritonic states that propagate ballistically at
their group velocity, and dark states that are stationary. Furthermore, because
long-lived dark states transiently trap the excitation, propagation is observed
on timescales beyond the intrinsic polariton lifetime. These atomistic insights
not only help to better understand and interpret experimental observations, but
also pave the way towards rational design of molecule-cavity systems for
achieving coherent long-range energy transport
Tuning the Coherent Propagation of Organic ExcitonâPolaritons through the Cavity Qâfactor
Transport of excitons in organic materials can be enhanced through polariton formation when the interaction strength between these excitons and the confined light modes of an optical resonator exceeds their decay rates. While the polariton lifetime is determined by the Q(uality)-factor of the optical resonator, the polariton group velocity is not. Instead, the latter is solely determined by the polariton dispersion. Yet, experiments suggest that the Q-factor also controls the polariton propagation velocity. To understand this observation, the authors perform molecular dynamics simulations of Rhodamine chromophores strongly coupled to FabryâPĂ©rot cavities with various Q-factors. The results suggest that propagation in the aforementioned experiments is initially dominated by ballistic motion of upper polariton states at their group velocities, which leads to a rapid expansion of the wavepacket. Cavity decay in combination with non-adiabatic population transfer into dark states, rapidly depletes these bright states, causing the wavepacket to contract. However, because population transfer is reversible, propagation continues, but as a diffusion process, at lower velocity. By controlling the lifetime of bright states, the Q-factor determines the duration of the ballistic phase and the diffusion coefficient in the diffusive regime. Thus, polariton propagation in organic microcavities can be effectively tuned through the Q-factor.peerReviewe
Identifying Vibrations that Control Non-adiabatic Relaxation of Polaritons in Strongly Coupled Molecule-Cavity Systems
The strong lightâmatter coupling regime, in which excitations of materials hybridize with excitations of confined light modes into polaritons, holds great promise in various areas of science and technology. A key aspect for all applications of polaritonic chemistry is the relaxation into the lower polaritonic states. Polariton relaxation is speculated to involve two separate processes: vibrationally assisted scattering (VAS) and radiative pumping (RP), but the driving forces underlying these two mechanisms are not fully understood. To provide mechanistic insights, we performed multiscale molecular dynamics simulations of tetracene molecules strongly coupled to the confined light modes of an optical cavity. The results suggest that both mechanisms are driven by the same molecular vibrations that induce relaxation through nonadiabatic coupling between dark states and polaritonic states. Identifying these vibrational modes provides a rationale for enhanced relaxation into the lower polariton when the cavity detuning is resonant with specific vibrational transitions.peerReviewe
Controlling Exciton Propagation in Organic Crystals through Strong Coupling to Plasmonic Nanoparticle Arrays
Exciton transport in most organic materials is based on an incoherent hopping process between neighboring molecules. This process is very slow, setting a limit to the performance of organic optoelectronic devices. In this Article, we overcome the incoherent exciton transport by strongly coupling localized singlet excitations in a tetracene crystal to confined light modes in an array of plasmonic nanoparticles. We image the transport of the resulting excitonâpolaritons in Fourier space at various distances from the excitation to directly probe their propagation length as a function of the exciton to photon fraction. Excitonâpolaritons with an exciton fraction of 50% show a propagation length of 4.4 ÎŒm, which is an increase by 2 orders of magnitude compared to the singlet exciton diffusion length. This remarkable increase has been qualitatively confirmed with both finite-difference time-domain simulations and atomistic multiscale molecular dynamics simulations. Furthermore, we observe that the propagation length is modified when the dipole moment of the exciton transition is either parallel or perpendicular to the cavity field, which opens a new avenue for controlling the anisotropy of the exciton flow in organic crystals. The enhanced excitonâpolariton transport reported here may contribute to the development of organic devices with lower recombination losses and improved performance.peerReviewe