On-chip Plasmonic Slit Cavities Achieve Room-Temperature Strong Coupling with Deterministically Positioned Quantum Dots

Strong coupling between light-emitting materials and nanoscale cavities represents a crucial step towards advanced technologies including quantum information processing and highly efficient photon sources. Jin Qin, Benedikt Schurr, and Patrick Pertsch, alongside colleagues at Universität Würzburg and Trinity College Dublin, now demonstrate a significant advance in achieving this strong coupling at room temperature within a compact, on-chip device. The team successfully couples colloidal quantum dots to plasmonic slit cavities, positioning the dots with remarkable precision using a technique that monitors light emission in real time. This achievement overcomes a major hurdle in the field, enabling the creation of scalable and potentially electrically tunable plasmonic platforms for integrated photonic circuits and active light-matter interactions, opening new avenues for room-temperature quantum technologies.

Plasmonic Cavities Enhance Quantum Emitter Coupling

Strong coupling between quantum emitters and optical cavities is essential for advancements in quantum information processing, high-purity single-photon sources, and nonlinear quantum devices. This research demonstrates strong coupling by integrating quantum emitters into plasmonic cavities, structures that confine light to extremely small volumes. The team precisely positions individual quantum emitters within these cavities using dielectrophoresis, a technique employing non-uniform electric fields, enhancing light-matter interaction and allowing control over the quantum emitter’s properties. The investigation reveals a significant Stark effect, a shifting of energy levels due to an applied electric field, on the quantum emitter when positioned within the plasmonic cavity, indicating a strong modification of its quantum properties.

Dielectrophoresis and Photoluminescence Characterisation

Atomic Force Microscopy confirms the physical proximity of quantum dots to the plasmonic resonators after observing spectral splitting in photoluminescence measurements, validating the coupling. Photoluminescence spectroscopy serves as the primary method for observing and characterizing the coupling, with voltage sweeps applied during measurements to investigate the Stark effect and the influence of external fields. Dielectrophoresis is used to fabricate the coupled structures, with feedback control implemented during the fabrication process to ensure precision. Analysis of the photoluminescence spectra extracts quantum dot resonances and determines coupling strengths.

Significant fluctuations in photoluminescence intensities and quantum dot resonances are observed, even without an applied voltage, attributed to internal electric fields within the quantum dots and their surrounding environment. The asymmetric response of the photoluminescence intensity to positive and negative applied electric fields is explained by the interplay between the applied field and the internal electric field, which can increase or reduce electron-hole wavefunction overlap, thus decreasing photoluminescence intensity. The fluctuating internal electric field contributes to the observed spectral diffusion, making accurate quantification of the Stark effect challenging. Coupling strengths vary between different coupled structures, likely due to variations in quantum dot dipole orientation and the number of quantum dots coupled to the plasmonic mode. The dielectrophoresis fabrication process with feedback control proves reliable, consistently fabricating coupled structures.

Strong Coupling Between Quantum Dots and Plasmonic Cavities

Scientists have achieved strong coupling between quantum dots and plasmonic slit cavities on a single chip at room temperature, demonstrating a significant advance in nanophotonic device development. The team fabricated nanoscale slit cavities using focused helium ion milling, creating robust and reproducible structures with precisely defined dimensions. Measurements reveal clear photoluminescence resolved Rabi splitting, a hallmark of strong light-matter interaction, in each fabricated cavity, indicating the formation of a hybrid light-matter state known as a polariton, confirming coherent energy exchange between the quantum dots and the cavity. The experiments demonstrate that the strength of the Rabi splitting varies between devices, directly correlating with the number of quantum dots effectively coupled to the cavity mode.

Researchers utilized dielectrophoresis to position individual or multiple quantum dots near the slit tip, where the plasmonic field is maximized, enabling on-demand strong coupling. Analysis of the cavity modes confirms the resonant behavior essential for achieving strong coupling, with well-defined Fabry-Pérot modes observed within the slit cavity. While electrical tuning of the quantum dot resonance via the quantum-confined Stark effect was explored, significant spectral diffusion at room temperature limited observable tunability. This breakthrough paves the way for integrating quantum photonic circuits with established nanofabrication techniques, offering a clear pathway toward scalable and functional quantum technologies. The results confirm the potential of this platform for developing advanced devices based on cavity quantum electrodynamics, particularly for applications in integrated photonics and quantum information processing.

Quantum Dot Coupling via Nanophotonic Cavities

This work demonstrates a functioning on-chip platform achieving strong interaction between light and matter, specifically coupling quantum dots to plasmonic slit cavities. Researchers successfully positioned quantum dots within the cavity’s strongest electromagnetic field using dielectrophoresis, a technique refined with real-time photoluminescence feedback, enabling the fabrication of numerous coupled structures. Measurements reveal clear spectral splitting in the emitted light, confirming strong coupling between the quantum dots and the cavity, consistent with theoretical predictions. Analysis of multiple fabricated devices indicates that variations in coupling strength correlate with the number of quantum dots interacting with the cavity mode, providing a means to control light-matter interaction. The team also explored electrical control of the quantum dot’s properties, demonstrating modulation of light emission intensity; however, the observation of spectral shifts caused by applied electric fields was limited by fluctuations in the quantum dot’s emission spectrum at room temperature. These results establish a foundation for scalable, electrically tunable quantum plasmonic systems with potential applications in quantum information processing, tunable photonics, and integrated quantum light sources.