Electromagnetically-Induced Transparency Bridges Disconnected Light-Harvesting Networks

TL;DR

This work addresses enhancing energy-transfer efficiency in artificial light-harvesting networks by inserting a three-site bridge between distant outer antennae and the reaction center. It employs electromagnetically-induced transparency (EIT) under two-photon resonance $E_1=E_3$ with tunable single-photon detuning $\Delta = E_2 - E_1$ to create a dark state that suppresses population of the lossy intermediate site. In natural PSI, modulating bridge energies to satisfy EIT conditions yields higher efficiency than the unmodified network, achieving up to $\eta \approx 0.9729$ at $E \approx 13940\ \text{cm}^{-1}$ within a specific energy window; detuning generally reduces performance. In an artificial LH setup, the EIT bridge similarly boosts efficiency from about $0.53$ (large detuning) to approximately $0.97$ at resonance, illustrating a general design principle for reconnecting disconnected energy-transfer networks and guiding artificial light-harvesting implementations.

Abstract

The energy-transfer efficiency of the natural photosynthesis system seems to be perfectly optimized during the evolution for millions of years. However, how to enhance the efficiency in the artificial light-harvesting systems is still unclear. In this paper, we investigate the energy-transfer process in the photosystem I (PSI). When there is no effective coupling between the outer antenna (OA) and the reaction center (RC), the two light-harvesting networks are disconnected and thus the energy transfer is inefficient. In order to repair these disconnected networks, we introduce a bridge with three sites between them. We find that by modulating the level structure of the 3-site bridge to be resonant, the energy transfer via the dark state will be enhanced and even outperform the original PSI. Our discoveries may shed light on the designing mechanism of artificial light-harvesting systems.

Figures (6)

• Figure 1: (a) Schematic of disconnected light-harvesting networks repaired by an EIT bridge. Each dot represents a chlorophyll with a ground state and an excited state. A bridge with three sites is inserted between the OA and the RC to connect them. Under the single-excitation condition, $|m\rangle$ represents the excitation on site $m$, while the other sites are in the ground state. $\kappa$ is the spontaneous-emission rate, and $\Gamma$ represents the charge-separation rate in the RC. For the simplified 3-site bridge model, $\Gamma'$ is the effective rate of site 3 in the bridge to the RC. (b) The position of the Mg atoms of the chlorophyll molecules in PSI. Blue, green and red dots respectively represent the OA, the bridge and the RC sites. The gray dots are the other sites in the PSI.
• Figure 2: The population dynamics on the intermediate site $|2\rangle$ and the target site $|3\rangle$ with different detunings $\Delta$s, where $J_{12}=J_{23}=J/\sqrt{2}$. The initial state is $|1\rangle$ in (a) and $|2\rangle$ in (b) and (c). Solid red, dash-dotted green, dashed blue, and dotted black lines denote $\Delta/J=0, 1, 2, 5$, respectively.
• Figure 3: The effect of the detuning of the intermediate site $\Delta$ on the energy transfer efficiency $\eta$, with $J_{12}=55.57~\textrm{cm}^{-1}$, $J_{23}=-37.04~\textrm{cm}^{-1}$, and $J_{13}=-0.36~\textrm{cm}^{-1}\ll J_{12}, ~J_{23}$ for the cluster A4-A28-A40 in PSI, $\kappa^{-1}=1$ ns, $\Gamma'^{-1}=10$ ps.
• Figure 4: The population dynamics of the OA, the RC and the bridge with $\kappa^{-1}=1$ ns and $\Gamma^{-1}=10$ ps. The level structure is (a) "extremely bad" with $\Delta=1000~\textrm{cm}^{-1}$, (b) the same as the natural PSI, (c) modified to $E_1=E_2=E_3=14006~\textrm{cm}^{-1}$, (d) modified to $E_{1}=E_{2}=E_{3}=13940~\textrm{cm}^{-1}$, whose efficiency $\eta=0.9729$ is the highest in the parameter regime.
• Figure 5: The energy-transfer efficiency of the light-harvesting network where the energy levels of the bridge are modified to be $E_1=E_2=E_3=E$. Dashed line represents the efficiency where the energy levels of the bridge are the same as the natural PSI. In the green area, i.e. $13752 ~\textrm{cm}^{-1}< E < 14199 ~\textrm{cm}^{-1}$, the bridge with modified energy levels works better than the natural PSI.