In physics and other science disciplines, it has long been a fundamental issue to make a bridge connecting effectively and efficiently an object in the microscopic world and the observer in the macroscopic world. This is by no means an easy task and it needs great cautions, skills and wisdoms to accomplish.

The vacuum Rabi splitting, which stems from a single photon interaction with a quantum emitter (a single atom, molecule, or quantum dot), is a fundamental quantum phenomenon of long interest. Intrinsically this effect is reflected in the internal energy splitting of quantum emitter state, while extrinsically it is reflected by the spectral splitting in either photoluminescence, or fluorescence, or scattering, or absorption spectrum. Naturally there arises a fundamental and interesting question regarding whether the extrinsic spectral splitting observed by an experimenter can be exactly identical to and thus faithfully reflect the intrinsic energy level splitting of the quantum emitter state.

The quantity of vacuum Rabi splitting, ?E∝Q/√V, where Q is the quality factor while V is the modal volume of the optical resonant cavity. This energy level splitting is usually very small, at the micro-electro volt (μeV) level. But two prominent ways can be used to increase Rabi splitting, one is to reduce the modal volume V, the other is to increase the quality factor Q.

In 2007, this splitting was increased to a few meV level by embedding a semiconductor quantum dot into a high-Q photonic crystal microcavity. Recently, many groups used plasmonic nanocavities with an extremely small modal volume V of hot spot to enhance Rabi splitting. In particular, many reports have claimed that using J-aggregates coupling to highly localized plasmon, for instance, in the nanogap formed between two closely-packed gold nanoparticles or between a sharp silver tip and a silver substrate, can produce an extremely large spectral splitting at the level of several hundred meV or even close to 1eV, the researchers called it giant Rabi splitting. Besides, this Rabi splitting is proportional to √N, where N is the number of excitons (modeled as a two-level quantum states) in J-aggregates.

More importantly, it is generally believed that this splitting originates purely from quantum interaction between excitons and plasmons, and thus should faithfully reflect the intrinsic energy level splitting of the J-aggregate quantum emitter. If this so-called giant Rabi slitting is truly the energy level splitting of microscopic particles (excitons here), it really can be called a miracle that reflects the unprecedented power of human being in manipulating, shaping and changing the microscopic world.

However, do we human beings really have so great a power? For a responsible scientist, the answer to this fundamental problem of course should be subject to a very solid scientific double-check. Prof. Zhi-Yuan Li, an expert in nanophotonics, optical physics and quantum physics from School of Physics and Optoelectronic Technology, South China University of Technology, together with his PhD student Bo Wang and Xian-Zhe Zeng, a visiting undergraduate student from School of Physics, Peking University, took this issue into careful and deliberate examination, trying to figure out the truth. They have carried out a deep theoretical research of plasmon-molecule interaction and set up a theoretical model which can handle complicated quantum and optical interactions and distinguish their individual contribution to the observed spectral splitting that are hard to elucidate via pure experimental studies. Related research results are published in Photonics Research, Vol. 8, Issue 3, 2020 (Bo Wang, Xian-Zhe Zeng, Zhi-Yuan Li. Quantum versus optical interaction contribution to giant spectral splitting in a strongly coupled plasmon–molecules system[J]. Photonics Research, 2020, 8(3): 03000343).

The team first noticed that in these experiments claiming giant Rabi splitting, the observed quantity, strictly speaking, is rather a giant spectral splitting. So, a problem naturally arises whether this extrinsic spectral splitting is equivalent to the intrinsic energy level splitting (the original meaning of Rabi splitting).

To explicitly answer this question, the team also noticed that the plasmon-molecule interaction takes place not only in the quantum mechanical level in terms of single-molecule Rabi splitting but also in the classical optical level in terms of multiple scattering between molecules and plasmons. When the plasmon nanogap involves more molecules, as is the case in most experiments, there also exists multiple scattering among these molecules and their multiple scattering interaction with the nanogap plasmon. In short, the observed extrinsic spectral splitting in principle should stem both from the quantum interaction of single-molecule with plasmons (Rabi splitting) and from the classical optical interaction of multiple molecules with plasmons.

More precisely, the quantum interaction of highly localized plasmon hot spot field upon each individual molecule will cause an energy level splitting, the true Rabi splitting, with the quantity depending on the molecule exciton dipole moment and the field amplitude. This quantum interaction will change a lot the response of the molecule (compared with usual weak-field situation) against the incident light, including the dipole moment strength and its spectral lineshape. This molecule, with a greatly modified optical response, will in turn affect the interaction of external light with the nanogap plasmons. Obviously, this is a classical optical interaction. When N molecules are involved, they will together significantly modify the optical property (more specifically the effective refractive index) of the background medium sensed by this nanogap plasmon and thus greatly change the scattering spectrum of the molecule-plasmon system against the incident light signal, leading to the observed spectra of optical signal with a feature of giant splitting.

This expectation has been confirmed by the team’s numerical simulations, which show that the scattering spectrum is very sensitive to the surrounding medium filling the plasmonic nanogap, where an apparent spectral shift is clearly visible upon changing the refractive index of the medium. This high sensitivity is of course well known in the community of nanophotonics, and also well recognized as a purely optical effect originating from the complicate optical interaction between the highly localized nanogap plasmon hot spot with the medium surrounding it.

To be more quantitative, the team developed a Lorentzian model to approximately describe molecules and plasmon, describe the optical response of all the molecules, and calculate the effective refractive index of the molecule system as a filling medium against the nanogap plasmon. Then they found that the collective optical interaction is dominant to generate the giant splitting (in scattering spectra), which is also proportional to √N, over the quantum interaction of single-molecule Rabi splitting.

Therefore, the observed giant spectral splitting is not a pure quantum Rabi splitting effect, but rather a mixture contribution from the large spectral modulation by the collective optical interaction of all molecules with plasmons and the modest quantum Rabi splitting of single-molecule strongly coupled with plasmons. The physical insights and theoretical model developed by this team can offer a more reasonable angle to look into the complicated molecule-plasmon strong interacting system. This theory can help to figure out the true quantity of the intrinsic energy level splitting of microscopic molecule, answer the question of how large the true Rabi splitting can be, and more importantly, find out the true power of human being and the limitation to change the microscopic world.

This work also suggests that nature behaves more complicated than it hints at a first glance, and therefore, one should develop a broad vision when looking into light-matter interaction at nanoscale in order to get deep physical insight and find the most important key to a seemingly simple optical phenomenon.

Prof. Huakang Yu, an optics expert from the same optics group led by Zhi-Yuan Li, says: “Investigating interactions between surface plasmon polaritons and metallic nanostructures is very popular recently. The strong localized electric fields of nanogap plasmons could lead to significant spectral changes, i.e., Rabi splitting, which is from the full quantum mechanical description with various parametric inputs. This research successfully explained the observed spectral splitting from the classical optical interaction of multiple molecules with plasmons, thus questioned the validity of the pure quantum mechanial descriptions presented previously. Indeed, the research results reflects the fact that experimental imperfections such as surface-ligand interactions or inhomogeneity could possibly contribute to the measured plasmonic resonance spectrum, making the experiments of quantum plasmonics not easy to handle.”

The next thing the team will do is to extend the model from a multiple-molecule system to a single molecule involving multiple microscopic excitons, clarify the true energy splitting of each exciton, and find out the contribution of quantum and optical interaction in this single molecule level (but still not the single exciton level). Besides, it is also interesting to construct a more accurate model that can describe well the quantum and optical interaction in the strong interacting molecule-plasmon system and disclose possible mutual competing and cooperating action of these two effects.

Strong molecule-plasmon quantum interaction together with the molecule-molecule and molecule-plasmon optical interaction in a plasmon nanogap. Generally, the extrinsically visible spectral splitting observed experimentally is not equivalent to the invisible intrinsic energy level splitting of molecule.