[论文解读] Polaritons in Living Systems: Modifying Energy Landscapes in Photosynthetic Organisms Using a Photonic Structure

David M. Coles, Lucas C. Flatten+9 more|arXiv (Cornell University)|Feb 6, 2017

Strong Light-Matter Interactions参考文献 18被引用 27

一句话总结

本研究通过将光合细菌（Chlorobaculum tepidum）嵌入光学微腔中，在活体光合细菌中实现了强激子-光子耦合，形成混合的‘活体极化子’——一种光子与激子的叠加态准粒子。通过调节微腔长度，可原位调控极化子能量，从而实现实时、非侵入式控制活体系统中的能量转移路径。

ABSTRACT

Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum (Cba. tepidum), a member of the green sulphur bacteria (GSB) family, light is absorbed by large antenna complexes called chlorosomes. The exciton generated is transferred to a protein baseplate attached to the chlorosome, before traveling through the Fenna-Matthews-Olson (FMO) complex to the reaction center. The energy levels of these systems are generally defined by their chemical structure. Here we show that by placing bacteria within a photonic microcavity, we can access the strong exciton-photon coupling regime between a confined cavity mode and exciton states of the chlorosome, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have an energy distinct from that of the exciton and photon, and can be tuned in situ via the microcavity length. This results in real-time, non-invasive control over the relative energy levels within the bacteria. This demonstrates the ability to strongly influence living biological systems with photonic structures such as microcavities. We believe that by creating polariton states, that are in this case a superposition of a photon and excitons within a living bacteria, we can modify energy transfer pathways and therefore study the importance of energy level alignment on the efficiency of photosynthetic systems.

研究动机与目标

• 探索活体光合生物中的激子与微腔中受限光子模式之间的强耦合行为。
• 研究光子结构是否能非侵入式地调节活体生物系统中的能级。
• 展示混合准粒子——‘活体极化子’的形成，其中光子组分可实现相干能量交换。
• 提供一个实时研究能级对齐如何影响光合效率的平台。
• 为光学杂化生物系统与其他光电子材料以实现能量收集开辟新途径。

提出的方法

• 使用两个半透明金属镜面制造平面光学微腔，并通过压电执行器调节腔长。
• 将活体Chlorobaculum tepidum细菌悬浮液置于镜面之间，以实现氯小体激子与腔模之间的相互作用。
• 通过光谱透射测量观察到当腔模扫过氯小体激子能量时，极化子分支出现反交叉现象。
• 应用耦合振子模型（方程 S4）拟合观测到的极化子分支能量，并提取Rabi劈裂能量（ħΩ）和混合系数。
• 通过Fabry-Perot模式间距和折射率匹配校准腔长，折射率n ≈ 1.35–1.38，具体取决于腔模和细菌负载量。
• 基于测得的劈裂能量和模场体积，使用Rabi劈裂公式（方程 S6）估算参与强耦合的偶极子数量。

实验结果

研究问题

• RQ1 活体光合细菌是否能在光学微腔中支持强激子-光子耦合？
• RQ2 能否通过调节腔长，原位动态调节生物激子的能级？
• RQ3 所得极化子态中光子与激子成分的占比如何？
• RQ4 参与耦合的偶极子数量如何影响极化子耦合强度？
• RQ5 活体极化子的形成是否可用于探测或调控光合系统中的能量转移路径？

主要发现

• 观察到氯小体激子与腔光子之间的强耦合，表现为透射光谱中极化子分支的清晰反交叉现象。
• 测得Rabi劈裂能量（ħΩ）约为10 meV，表明强耦合，拟合结果得出不同腔模的等效折射率为1.35–1.38。
• 下极化子分支和上极化子分支表现出不同的能量色散特性，其中上分支表现出更强的光子特征，下分支表现出更强的激子特征。
• 基于耦合振子模型计算出的混合系数（αc 和 αx）证实，极化子是光子态与激子态的相干叠加。
• 基于Rabi劈裂和模场体积（q=2模时V ≈ 15(λ/n)³），估算参与强耦合的偶极子数量约为10^4–10^5。
• 透射强度在趋近激子能量时单调减小，与下极化子分支中激子特征增强的现象一致。