Electrically tunable orbital coupling and quantum light emission from O-band quantum dot molecules

Abstract: We present the observation of electrically tunable quantum coupling of orbital states in individual InAs/InGaAs quantum dot molecules emitting in the telecom O-band (~1300 nm). By tuning the static electric field along the growth axis of the QD-molecule, we observe pronounced anticrossings between excitonic transitions and determine the dependence of the interdot electron tunnel coupling on the interdot separation. As the electric field applied along the growth axis of the QD-molecules increases, positively charged exciton complexes sequentially emerge in the time-integrated emission spectra due to electron escape from the system while holes remain trapped. Moreover, for strong pumping, biexciton emission from the O-band molecules is identified. We demonstrate single-photon emission from the InAs/InGaAs QD-molecule emitting around 1300 nm with a g(2)(0) = 0.017(2) and explore the impact of tuning orbital coupling on the second-order correlation function.

• The paper demonstrates electrically tunable orbital coupling using a p-i-n diode and varied GaAs barriers to precisely control excitonic states.
• The methodology employs molecular beam epitaxy and hyperspectral imaging to quantify the strong quantum-confined Stark effect and tunnel-coupling energies.
• The work confirms high-purity single-photon emission and scalable integration potential for telecom quantum networks through controlled multi-exciton dynamics.

Electrically Tunable Orbital Coupling and Quantum Light Emission in O-band InAs/InGaAs Quantum Dot Molecules

Introduction

This work investigates the quantum coupling between orbital states in vertically stacked InAs/InGaAs quantum dot molecules (QDMs) designed for emission in the telecom O-band near 1.3 μm. Alignment with the fiber-optic low-loss window enables integration with existing telecommunication infrastructure. The core innovation is the demonstration of electrically tunable tunnel-coupling between quantum dots, enabling detailed control over excitonic transitions, charge configurations, and quantum light emission. The study utilizes advanced molecular beam epitaxy (MBE) growth, p-i-n diode architectures, and hyperspectral imaging (HSI) to systematically probe and tune QDM characteristics.

Experimental Realization and Characterization

Four distinct MBE-grown samples were engineered: a single quantum dot (QD) reference and three QDMs with varying interdot GaAs barriers (3, 5, and 10 nm). The QDMs are embedded in the intrinsic region of a vertical p-i-n diode with AlGaAs barriers, supporting high tunability of excitonic energies via electric field (Stark effect) while inhibiting thermal carrier escape.

Micro-photoluminescence (μPL) and voltage-dependent HSI facilitate high-resolution analysis of excitonic transitions as a function of field. Stark shift measurements reveal pronounced quantum-confined Stark effect (QCSE), with permanent excitonic dipole moments up to 0.52 nm and polarizabilities of 0.27 μeV·kV⁻²·cm²—values that surpass those typical for In(Ga)As/GaAs QDs, emphasizing the strong spatial electron-hole separation achievable in this system.

Quantum Coupling and Tunnel Barrier Dependence

Quantum coupling is evidenced by pronounced anticrossings (ACs) in the μPL spectra as electric field tunes spatially indirect excitons (electron and hole in different QDs) into resonance with direct excitons (confined in the same QD). The energy splitting at these anticrossings quantitatively determines the tunnel-coupling energy ($\Delta_{tc}$). Statistical analysis using HSI demonstrates that as the GaAs tunnel barrier is increased from 3 nm to 5 nm, the mean $\Delta_{tc}$ decreases sharply from 1.72 meV to 0.65 meV. For a 10 nm barrier separation, anticrossings are completely suppressed, signifying loss of coherent interdot coupling. This exponential sensitivity to interdot spacing reflects enhanced carrier confinement in O-band InAs QDs as compared to 930 nm systems.

Probabilistically, 74% of QDMs with a 3 nm barrier and 32% with a 5 nm barrier exhibit observable anticrossings. The sharp suppression of tunnel coupling with thicker barriers delineates the regime for coherent QDM operation and confirms the stringent requirements for O-band QDM integration.

Charge Complex Dynamics and Multi-Particle States

Appreciable electric fields induce sequential charging of the QDMs, as verified by abrupt steps and blueshifts in emission energies corresponding to increasing numbers of spectator holes (X⁺, X²⁺, ..., X⁵⁺ complexes). The observed energy offsets between charge states (e.g., X⁺−X ~ +3.65 meV, X²⁺−X⁺ ~ +1.5 meV) are consistent with Coulomb interaction scaling in large, In-rich dots. The field-driven transition dynamics are supported by the presence of a carrier reservoir effect, wherein the lower dot acts as a temporary electron or hole reservoir for the upper dot.

Multi-exciton features, including biexcitons (XX), are resolved under elevated excitation powers. The field and power-dependent PLV spectra exhibit multiple anticrossings (AC2–AC4) assigned to hybridized charged and neutral excitonic states, with coupling energies comparable to those of the neutral exciton. The nature of these anticrossings, including identification of charged (trions, e.g., X⁺, X⁻) and neutral biexcitonic transitions, is substantiated by the power-law dependence of integrated μPL intensity, which scales with power exponents indicative of the underlying recombination process (1 for exciton, ~2 for biexciton).

Quantum Light Emission

Second-order photon correlation measurements confirm the single-photon character of excitonic emission from O-band QDMs. Antibunching with $g^{(2)}(0) = 0.017 \pm 0.002$ (substantially below the threshold for Poissonian light) is achieved, with multimodal behavior emerging at higher excitation powers due to spectral line broadening and multi-exciton formation. Biexciton emission is associated with characteristic bunching at short delays, with the timescales of decay tracking the lifetimes of the underlying charge complexes.

The integration of electrically tunable QDMs with strong on-demand biexciton emission and robust single-photon purity at a telecom-relevant wavelength establishes the platform's suitability for quantum photonic applications.

Implications and Prospects

This work demonstrates deterministic, field-tunable engineering of orbital coupling and charge configurations in O-band QDMs. The platform offers multiple pathways for coherent manipulation of spin-charge states, deterministic emission of structured photonic entanglement, and scalable integration with silicon photonics. The strong modulation of tunnel coupling with atomic-scale precision in barrier thickness and the direct electrical control over excitonic configuration lay the groundwork for advanced protocols in quantum communication and entanglement distribution.

On a practical level, the integration pathway is compatible with on-chip nanophotonic resonators and quantum network nodes, given the emission in the fiber telecom window. The observed high purity and efficiency of single-photon emission, and the ability to electrostatically cycle through multi-exciton complexes, support deterministic entangled-photon sources and quantum logic primitives.

Theoretically, the results contribute to a deeper understanding of correlated carrier dynamics, Coulomb-mediated hybridization, and the interplay between quantum confinement and field-induced tunneling in strongly coupled nanostructures. The observed suppression of tunnel coupling with enhanced carrier confinement further delineates the limitations and design space for O-band QDMs.

Anticipated future directions include the integration of such QDMs into cavity-QED architectures, the study of ultrafast coherent control of molecular states, in situ manipulation of orbital and spin coherence for quantum error correction, and the realization of multi-photon entangled states with deterministic charge control.

Conclusion

Electrically tunable InAs/InGaAs quantum dot molecules emitting in the telecom O-band demonstrate coherent control of orbital coupling, multi-exciton charging, and high-purity quantum light emission. The platform supports deterministic initialization and manipulation of complex excitonic states, addresses key integrative challenges for fiber-based quantum networks, and constitutes a flexible testbed for the study and deployment of semiconductor quantum photonic technologies.