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Advanced Quantum Technologies

Vacuum‐Rabi and Autler‐Townes Splitting in Quantum Dot Molecules Embedded in Coupled Cavities

Xiaolei Liu, Chunchao Yu, Wenxing Yang, Boyun Wang

Published May 1, 2026• 0 citations
Publisher

Abstract

ABSTRACT Optical properties in quantum dot molecules (QDMs), which are implanted in coupled cavities, are researched theoretically. The QDMs are composed of three QDs, and a multi‐level system, including direct and indirect excitons, which can be formed due to tunneling effects therein. Under the strong coupling condition, double Autler‐Townes splitting (ATS) occurs. Then, we implant the QDMs in the coupled cavities, and double Vacuum‐Rabi splitting (VBS) is created together. Four transparency windows are demonstrated. Moreover, the effects of the coupling strengths and frequency detunings on the system absorption spectrum are discussed. Especially, the group index in QDMs controlled by the coupling strengths are explored and group velocity with a reduced factor of 10 6 is revealed. Such a system may be used in on‐chip QD‐based optical devices.

DOI
10.1002/qute.70318

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Journal of the Optical Society of America B•Oct 24, 2025•1 citation
ABSTRACT Optical properties in quantum dot molecules (QDMs), which are implanted in coupled cavities, are researched theoretically. The QDMs are composed of three QDs, and a multi‐level system, including direct and indirect excitons, which can be formed due to tunneling effects therein. Under the strong coupling condition, double Autler‐Townes splitting (ATS) occurs. Then, we implant the QDMs in the coupled cavities, and double Vacuum‐Rabi splitting (VBS) is created together. Four transparency windows are demonstrated. Moreover, the effects of the coupling strengths and frequency detunings on the system absorption spectrum are discussed. Especially, the group index in QDMs controlled by the coupling strengths are explored and group velocity with a reduced factor of 10 6 is revealed. Such a system may be used in on‐chip QD‐based optical devices.
In the present study, the effect of the geometrical shape of quantum dots on the nonlinear absorption coefficient of semiconductor quantum dot molecules (QDMs) is theoretically investigated. The nonlinear optical response of QDMs is analyzed using the density matrix formalism, because this theoretical framework provides a rigorous and unified method to describe the interaction between electromagnetic fields and confined carriers, enabling the derivation of both linear and higher-order nonlinear susceptibilities. Accordingly, the nonlinear optical absorption of double quantum dot molecules, triple quantum dot molecules, and quadruple quantum dot molecules coupled through tunneling is studied and compared. We show that the optical spectral response of QDMs is significantly affected by their shape and the strength of tunneling between coupled quantum dots of a QDM. The results indicate that the shape of quantum dots can significantly affect nonlinear optical absorption. In addition, the tunneling intensity between the closely quantum dots strongly affects the nonlinear absorption, and the width and number of transparency windows.
The interaction of excitons with localized surface plasmons in hybrid nanostructures containing semiconductor quantum dots and metal nanoparticles, under specific conditions, may generate products with collective optical properties that have an abundance of potential applications in the area of nanotechnology. In the present study, we explore the behavior of the linear absorption and dispersion properties of the double-semiconductor quantum dot molecule in the presence of a spherical metal nanoparticle. We find that a transparency window arises in the absorption spectrum, the width of which decreases with the decrease in the electron tunnelling rate. In the low-electron-tunnelling regime, slow light is generated, an effect closely associated with tunneling-induced transparency. The enhancement of the tunneling rate induces a broadening in the transparency window, occurring due to the Autler–Townes splitting. The investigation of the impact of the distance between the quantum dot and the metal nanoparticle on the slowdown factor and the width of the transparency window shows that, by transposing the metal nanoparticle closer to the double-semiconductor quantum dot molecule, the transparency window widens.
We theoretically study cross-polarized pump-probe spectra of a quantum dot (QD) embedded in a microcavity in the cavity quantum electrodynamics treatment. Recently, observations of a vacuum Rabi splitting for a QD in microcavity have been reported. The vacuum Rabi splitting is the level separation of one-excitation dressed states (1e states). However, two-excitation dressed states (2e states) have not been observed in this system. We discuss the possibility to observe pump-probe signal coming from the cross-polarized 2e states, in which the energy levels of a QD are modeled by a four-level system consisting of the ground state, right- and left-polarized excitons, and a biexciton. The signal peaks due to the cross-polarized 2e states are found when the quality factor of the cavity is extremely large. The spectral intensity is dramatically enhanced due to the cavity effect. The enhancement rate is independent of the coupling constants between the exciton (biexciton) and a cavity photon when a well-defined vacuum Rabi splitting is formed. However, a strong suppression of the signal occurs at a condition where the energy of the 1e state coincides with the energy difference between 1e and cross-polarized 2e states.
Based on a calculation model, we study the interference phenomena of serially coupled V-type and Λ-type triple quantum dots (CTQDs) driven simultaneously by a strong driving field and a weak probe field. Strongly depending on the configuration of the three-level CTQD, the probe absorption spectra, which are shown in the tunneling current, exhibit various quantum coherence properties. In the case where the two pairs of transitions of the CTQD have a small eigenfrequency difference Δω, the double-coupling effect of the driving field results in two Autler—Townes doublets and one weak Mollow triplet in one spectrum. With the value of Δω increasing, only one Autler—Townes splitting remains due to the single-coupling of the field. We also find that the effect of spontaneous emission of phonons may lead to an obvious background current, which can be used to distinguish which transition is driven by the driving field in experiment. The interesting quantum property of a CTQD revealed in our results suggests its potential applications in quantum modulators and quantum logic devices.
The system optical response in quantum dot molecule (QDM)-embedded coupled cavities with photothermal effect is investigated theoretically. Couple resonator-induced transparency can be found first when only considering the photon tunneling between the double cavities. After implanting QDMs with tunneling effects into the right cavity, three transparency windows are observed due to QDMs and tunneling-induced transparency (TIT) therein. The fourth transparency window is formed due to photothermally induced transparency (PTIT). The results indicate the system absorption can be tuned by changing the system coupling strengths and frequency detunings. Furthermore, the transmission and group delay of the probe field are examined. Especially, the group delay in different transparency windows can be adjusted by the tunneling effect in the QDMs, and the group delay in PTIT can also be controlled by the system coupling strengths, etc. The research provides a new way of using the TIT to control the PTIT, which may be applied to design novel nanophotonic devices.