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Acta Physica Sinica

Superradiance study of molecular clusters in different cavities formed by scanning tunneling microscope tip and substrate

Zhihua LIU, Siyuan LYU, Meng Ran Zhou, Luxia Wang

Published Jan 1, 2025• 0 citations
Publisher

Abstract

The plasmon cavity system composed of a scanning tunneling microscope tip and a substrate has attracted much attention due to its ability to break through the diffraction limit, enhance the electromagnetic field by hundreds of times, and localize it on a nanometer or even sub-nanometer scale. This plasmon cavity system can serve as an advanced platform for studying superradiance phenomena on an ultrafast scale. Methylene blue molecules have many applications in the field of optics due to their significant light absorption and fluorescence emission characteristics. In this work, macroscopic quantum electrodynamics and open quantum system theory are used to explore the radiation dynamics of methylene blue molecular clusters with three different configurations: cyclic, two-dimensional planar, and one-dimensional chain, in specific scanning tunneling microscope nanocavity and picocavity. Taking the cyclic molecular clusters for example, the radiation effects of different external field excitations on the molecular clusters in the cavity are studied. The research results indicate that for the same molecular cluster configuration, the scanning tunneling microscope picocavity has a more significant superradiance intensity, while the scanning tunneling microscope nanocavity has a longer duration of superradiance. From the perspective of symmetry, the one-dimensional chain molecular clusters only have axial symmetry, while the two-dimensional planar and cyclic molecular clusters have both axial symmetry and central symmetry. The cyclic molecular clusters also have multiple rotational symmetries. Therefore, within the same scanning tunneling microscope cavity, the higher the symmetry of the arrangement of molecular clusters, the easier it is to generate significant superradiance pulses. In addition, because of its higher spatial resolution and stronger local field enhancement effect, the picocavity of scanning tunneling microscope is more sensitive to changes of external conditions such as excitation wavelength. These results indicate that the occurrence and enhancement of superradiance can be effectively controlled by reasonably designing the cavity structure and geometric configuration of molecular clusters, and the time scale of superradiance pulses can be extended to the picosecond order, which provides new ideas and methods for practical applications in the fields of optics and nanotechnology in the future.

DOI
10.7498/aps.74.20241720

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The plasmon cavity system composed of a scanning tunneling microscope tip and a substrate has attracted much attention due to its ability to break through the diffraction limit, enhance the electromagnetic field by hundreds of times, and localize it on a nanometer or even sub-nanometer scale. This plasmon cavity system can serve as an advanced platform for studying superradiance phenomena on an ultrafast scale. Methylene blue molecules have many applications in the field of optics due to their significant light absorption and fluorescence emission characteristics. In this work, macroscopic quantum electrodynamics and open quantum system theory are used to explore the radiation dynamics of methylene blue molecular clusters with three different configurations: cyclic, two-dimensional planar, and one-dimensional chain, in specific scanning tunneling microscope nanocavity and picocavity. Taking the cyclic molecular clusters for example, the radiation effects of different external field excitations on the molecular clusters in the cavity are studied. The research results indicate that for the same molecular cluster configuration, the scanning tunneling microscope picocavity has a more significant superradiance intensity, while the scanning tunneling microscope nanocavity has a longer duration of superradiance. From the perspective of symmetry, the one-dimensional chain molecular clusters only have axial symmetry, while the two-dimensional planar and cyclic molecular clusters have both axial symmetry and central symmetry. The cyclic molecular clusters also have multiple rotational symmetries. Therefore, within the same scanning tunneling microscope cavity, the higher the symmetry of the arrangement of molecular clusters, the easier it is to generate significant superradiance pulses. In addition, because of its higher spatial resolution and stronger local field enhancement effect, the picocavity of scanning tunneling microscope is more sensitive to changes of external conditions such as excitation wavelength. These results indicate that the occurrence and enhancement of superradiance can be effectively controlled by reasonably designing the cavity structure and geometric configuration of molecular clusters, and the time scale of superradiance pulses can be extended to the picosecond order, which provides new ideas and methods for practical applications in the fields of optics and nanotechnology in the future.
In this article, we address fluorescence of single molecule inside a plasmonic picocavity by proposing a semi-classical theory via combining the macroscopic quantum electrodynamics theory and the open quantum system theory. To gain insights into the experimental results [Nat. Photonics, 14, 693 (2020)], we have further equipped this theory with the classical electromagnetic simulation of the pico-cavity, formed by single atom decorated silver STM tip and a silver substrate, and the time-dependent density functional theory calculation of zinc phthalocyanine molecule. Our simulations not only reproduce the fluorescence spectrum as measured in the experiment, confirming the influence of extreme field confinement afforded by the picocavity, but also reveal Rabi oscillation dynamics and Mollow triplets spectrum for moderate laser excitation. Thus, our study highlights the possibility of coherently manipulating the molecular state and exploring non-linear optical phenomena with the plasmonic picocavity.
In this study, we present the superradiance effects of N identical molecular emitters inside Fabry–Pérot (FP) cavity systems in the framework of macroscopic quantum electrodynamics. The proposed theory can estimate the enhancement of spontaneous emission rates, including the effects of cavity geometry and material dielectric properties. Moreover, our study connects the multichromophoric excitation energy transfer theory and the Tavis–Cummings model via identical cooperative decay rates and dipole–dipole shifts. For molecules resonant with the cavity photon mode, we find that the enhancement is not exactly proportional to N even in a perfect FP cavity due to the effects of electrostatic dipole–dipole interactions. Moreover, in a silver FP cavity, our simulation shows that the enhancement saturates for a large N, indicating that the cavity photon mode can lead to only the collective emission of a limited number of molecules. For molecules in the proximity of the silver mirror, we demonstrate that under specific conditions, the subradiance of molecular emitters can occur through surface plasmon polaritons. Our study not only provides new insights into the mechanism of superradiance effects but also demonstrates that collective effects such as superradiance and subradiance can be controlled by manipulating the dielectric environments.
We demonstrate single-photon superradiance from artificially constructed nonbonded zinc-phthalocyanine molecular chains of up to 12 molecules. We excite the system via electron tunneling in a plasmonic nanocavity and quantitatively investigate the interaction of the localized plasmon with single-exciton superradiant states resulting from dipole-dipole coupling. Dumbbell-like patterns obtained by subnanometer resolved spectroscopic imaging disclose the coherent nature of the coupling associated with superradiant states while second-order photon correlation measurements demonstrate single-photon emission. The combination of spatially resolved spectral measurements with theoretical considerations reveals that nanocavity plasmons dramatically modify the linewidth and intensity of emission from the molecular chains, but they do not dictate the intrinsic coherence of the superradiant states. Our studies shed light on the optical properties of molecular collective states and their interaction with nanoscopically localized plasmons.
Coupling between quantum emitters and modes of light is foundational to studies of quantum systems. Coherent interactions between matter and light have greatly advanced the development of quantum technologies. When an ensemble of atoms interacts with light that is confined within a cavity---a resonator for photons---light also mediates atom-atom interactions, leading to collective phenomena that cannot be extrapolated from microscopic behaviors. Understanding how interactions among quantum particles lead to macroscopic phenomena, such as phase transitions and breaking of symmetries, remains a challenge in modern physics. In this dissertation, I present results addressing this challenge in the context of mesoscopic arrays of optical-tweezer-trapped atoms interfaced with a high-finesse optical cavity. In particular, the physics we investigate arises from the collective scattering of light. Utilizing the ability to place atoms with nanometer-scale precision, we engineer superradiant and subradiant states, where control over interference of the emission from multiple atoms leads to collectively enhanced or suppressed amounts of scattered light. Building on this work, we investigate the emergence of optomechanical self-organization, mediated by superradiance in the cavity electromagnetic field. By coupling atoms to the same cavity mode, the atoms exert forces on one another through the exchange of photons, leading them to autonomously order themselves spatially. We characterize mesoscopic fluctuations and their effect on symmetry-breaking dynamics associated with self-organization. Finally, we extend this framework to theoretically investigate how the phase of the atom-cavity coupling can be altered to generate higher symmetries. Our analysis reveals first-order phase transitions and instabilities induced by non-reciprocal forces. Altogether, this work establishes a path towards realizing mesoscopic systems of complex phases of matter in cavity quantum electrodynamics with unprecedented programmability.
Strong coupling with single molecules in plasmonic picocavities has emerged as a resource for room-temperature quantum control with nanoscale light. Tip-based nanoprobes can measure the local dynamics of individual molecular picocavities, but the overhead associated with sampling an inhomogeneous picocavity distribution can be challenging for scalability. We propose a macroscopic approach in which an ensemble of molecular picocavities acts as a nonlinear plasmonic metamaterial. Using a quantum optics perspective, we study theoretical performance limits for optical cross-phase modulation in the system, taking into account realistic distributions of picocavity volumes and molecular transition frequencies. The medium nonlinearity is mediated by local strong coupling with the lowest vibronic emission sideband of individual organic chomophores. The local vacua change the refractive index of the medium at the frequency of a weak probe field ωp\omega_pωp​, set to drive the bare zero-phonon absorption band. Refractive index variations Δn/n\Delta n/nΔn/n of a few percent, relative to a molecule-free scenario, are feasible with dilute ensembles. The probe phase evolution can be switched off in the presence of a signal field at a higher frequency ωs\omega_sωs​, for intensities as low as 10 kW/cm2^22. The mechanism for optical switching involves a novel (ωp+ωs\omega_p+\omega_sωp​+ωs​) two-photon absorption channel, assisted by local vacuum fields. Our work paves the way for future studies of plasmonic metamaterials that exploit strong light-molecule interactions, for applications in optical state preparation and control.