Entangled photon pair excitation and time-frequency filtered multidimensional photon correlation spectroscopy as a probe for dissipative exciton kinetics

TL;DR

The paper addresses the challenge of resolving dissipative kinetics in densely packed $2$-exciton manifolds by coupling state-selective excitation with entangled photons to time-frequency filtered photon coincidence spectroscopy. Using a Frenkel exciton model for LHCII, it shows that entangled photon pairs can prepare narrowband $2$-exciton distributions and that multidimensional coincidence measurements with spectral-temporal filtering can classify and monitor the ensuing transport dynamics. The results demonstrate selective excitation of targeted $2$-exciton states, followed by transport-based redistribution and cascaded emission, with filtering parameters enabling tuning of pathway contributions. This approach offers a principled route to enhanced resolution in molecular spectroscopy and sensing, potentially enabling refined insights into exciton dynamics in light-harvesting complexes and related systems.

Abstract

In molecular aggregates, multiple delocalized exciton states interact with phonons, making the state-resolved spectroscopic monitoring of dynamics challenging. We propose a protocol that combines photon-entanglement-enhanced narrowband excitation of two-exciton states with time-frequency-filtered two-photon coincidence counting. It can alleviate bottlenecks associated with probing exciton dynamics spread across multiple spectral and temporal windows. We demonstrate that non-classical correlations of entangled photon pairs can be used to prepare narrowband two-exciton population distributions, circumventing transport in mediating states. The distributions thus created can be monitored using time-frequency-filtered photon coincidence counting, and the pathways contributing to photon emission events can be classified by tuning filtering parameters. Numerical simulations for a light-harvesting aggregate highlight the ability of this protocol to achieve selectivity by suppressing or amplifying specific pathways. Combining entangled photonic sources and multidimensional photon counting allow promising applications to spectroscopy and sensing.

Figures (8)

• Figure 1: (A) Illustration of the proposed protocol, consisting of the driving, evolution, and detection stages. The process is assisted by entangled photon pairs and temporal-spectral filtering of emitted photons. In (B), three plausible snapshots of the dynamics (I)-(III), featuring increasingly complex pathways, that may be monitored using the protocol is displayed. In the simulation, the exciton transport within both manifolds is accounted for. Since these pathways may be simultaneously present in the dynamics, the protocol allows for selective probing and aids in constructing a mechanistic narrative.
• Figure 2: (A) Diagrammatic illustration (using the Schwinger-Keldysh loop representation) of the pathways involved in the proposed protocol, consisting of the driving, evolution, and emission stages. (B)-(C) Diagrammatic illustration (using the Liouville superoperator representation, suitable for dissipative systems) of the pathways involved in filtered emission (B) and excitation (C). The combinatorial pairing of five possible excitation diagrams and four possible emission diagrams gives rise to all possible pathways that contribute to the final signal. The corresponding key equations are presented in Section \ref{['sec:results']}.
• Figure 3: The two-exciton population distribution map, constructed by sequentially targeting each of the $105$ states in that manifold, using degenerate entangled photon pairs i.e., $\omega_i=\omega_p/2=E_{\mathrm{target}}^{(2)}/2$. The left panel serves as a reference simulation while the temporal width of the SPDC pump (entanglement time) is varied in the middle panel (right panel). Dispersal of the plot, in the horizontal cut, away from diagonal signifies a reduction in the narrowband excitation. For discussion, see section \ref{['subsec:entexcite']}
• Figure 4: The population distribution for the target state $f_{07}^{}$ which is dominantly excited via the intermediate states $e_{07}^{}$ and $e_{09}^{}$ is displayed for variations in the entangled photon parameters (rows) and for two distinct transport times (columns): $t=0\, \text{fs}$ (immediately after the pulse) and $t=50 \, \text{fs}$. In the upper panel, the entanglement time is $\tilde{T}_{\text{ent}} =10 \, \text{fs}$ and the SPDC pump width is $\tau_0= 150 \, \text{fs}$. In the middle panel (lower panel), the width is reduced (entanglement time is increased) to $\tau_0= 50 \, \text{fs}$ ($\tilde{T}_{\text{ent}} =30 \, \text{fs}$). For detailed discussion, refer to Section \ref{['subsec:entexcite']}.
• Figure 5: The population distribution for the target state $f_{83}^{}$ which is dominantly excited via the intermediate states $e_{07}^{}$ and $e_{09}^{}$ is displayed for variations in the entangled photon parameters (rows) and for two distinct transport times (columns): $t=0\, \text{fs}$ (immediately after the pulse) and $t=250 \, \text{fs}$. In the upper panel, the entanglement time is $\tilde{T}_{\text{ent}} =10 \, \text{fs}$ and the SPDC pump width is $\tau_0= 150 \, \text{fs}$. In the middle panel (lower panel), the width is reduced (entanglement time is increased) to $\tau_0= 50 \, \text{fs}$ ($\tilde{T}_{\text{ent}} =30 \, \text{fs}$). For detailed discussion, refer to Section \ref{['subsec:entexcite']}.