Researchers have identified biexciton emission at 1.3 microns from InAs/InGaAs quantum dot molecules, a wavelength important for applications using the “O-band” of telecommunications. The work demonstrates single-photon emission with a measured g(2)(0) = 0.017 ± 0.002, indicating strong quantum coherence suitable for advanced communication protocols. By electrically tuning a static electric field along the growth axis, the team observed positively charged exciton complexes sequentially emerging, revealing how electrons escape the system while holes remain trapped within these unique structures.

Electrically Tunable Coupling in InAs/InGaAs Quantum Dot Molecules

The fabrication of InAs/InGaAs quantum dot molecules with surface densities of less than one per square micrometer has been achieved. This scalable synthesis, reported by P. S. Huber-Loyola and colleagues, directly correlates structural and optical properties of the quantum dot molecules, offering a mapped parameter space for precise control. Analysis of these vertically stacked quantum dots revealed pronounced anticrossings between excitonic transitions when a static electric field was applied along the growth axis.

The study identified biexciton emission from the O-band molecules, a critical step toward applications requiring correlated photon pairs. The impact of tuning the orbital coupling on the second-order correlation function was also explored. “The development of single-photon sources (SPSs) based on semiconductor quantum dot (QD) and QD-molecule (QDM) nanostructures emitting in the telecommunications bands is essential for integration with fiber optics and scalable silicon nanophotonic platforms,” the researchers write, highlighting the practical implications of their findings.

O-Band Quantum Dot Molecule Synthesis and Scalability

Electrical tuning of quantum dot molecule orbital coupling reveals a progressive transfer of spectral weight toward increasingly positively charged excitonic complexes, a phenomenon observed as static electric field strength increases. The team’s analysis correlated these optical properties with structural data obtained through transmission electron microscopy. Scalability of the synthesis process was confirmed through voltage-dependent scanning hyperspectral imaging, providing statistical information across numerous individual O-band quantum dot molecules.

The probability of observing anticrossings reached 74% for samples with a 3-nanometer gallium arsenide tunnel barrier, decreasing to 32% for 5-nanometer barriers. No anticrossings were observed in samples with a 10-nanometer barrier, suggesting a loss of vertical correlation between the paired quantum dots forming the molecules. While not every emitter exhibited emission within the telecom O-band, analysis of data from approximately 60 quantum emitters per sample revealed consistent trends. The team demonstrated single-photon emission with a g(2)(0) = 0.017 under continuous-wave excitation.

Biexciton Emission and Single-Photon Characteristics at 1.3 Microns

Increasing the electric field applied to InAs/InGaAs quantum dot molecules induces a sequential emergence of positively charged exciton complexes, a phenomenon observed through changes in the time-integrated emission spectra. This occurs as electrons leave the system while holes become trapped within the structure. The observed progression of spectral weight toward higher positive charge states confirms the accumulation of holes, providing insight into the charge dynamics within these nanoscale structures.

Vertical Alignment and Tunnel Coupling in QD Molecules

This electrical control over orbital coupling is critical for manipulating quantum states within the nanostructures. The team used a low-temperature technique, extending the high spatial resolution (HSI) method with a bias-voltage sweep at each scan step, to identify quantum dot molecules based on anticrossings (AC) in the spectra. Identifying ACs with excitation power (𝑃exc) lower than 1μ⁢W ensured that observed signals originated from the exciton, providing direct statistics of Δt, the orbital tunnel coupling, as a function of 𝑡GaAs.

The absence of anticrossings when the total lower wetting layer to upper wetting layer separation reached 17nm (𝑡GaAs+𝑡SRL=17nm) confirms that tunnel coupling diminishes rapidly as barrier thickness increases. Broad-range photoluminescence voltage data from sample (ii) recorded with weak (300 nW) and strong (5μ⁢W) excitation further illustrates these dynamics.

Second-Order Correlation Function and Orbital Coupling Tuning

The observed emission wavelength falls within the telecom O-band, making these structures promising candidates for integration with existing fiber optic networks. This phenomenon arises because the applied field facilitates the escape of electrons while effectively trapping holes within the quantum dot structure, revealing a subtle control over charge carrier dynamics.

Further investigation revealed pronounced anticrossings between neutral and charged excitonic states as the electric field was adjusted, allowing for determination of the dependence of the interdot electron tunnel coupling on the interdot separation. Coupling of orbital states within the quantum dot molecule is evidenced by these anticrossings in the optical spectra, which originate from electron tunneling between the quantum dots.

To achieve this wide tunability of exciton energy and orbital coupling, the O-band quantum dot molecules were surrounded by AlGaAs barriers and embedded within the intrinsic region of a vertical p-i-n diode. InAs coverage of the lower and upper quantum dot layers proved to be a critical parameter for achieving vertical alignment of the quantum dot molecules grown in the gradient regime.

Telecom-Band Potential for Quantum Communication and Computation

This structural control directly impacts the potential for scalable quantum technologies, as the observed anticrossings signify effective electron interaction between the paired quantum dots. The study systematically mapped the parameter space for this interaction, revealing how variations in the barrier thickness influence the quantum properties of the resulting molecules. The team’s analysis of around 60 quantum emitters per sample allowed for a statistical assessment of the growth process and the identification of optimal conditions for achieving strong vertical correlation between the quantum dots.

👉 More information
🗞 Electrically Tunable Orbital Coupling and Quantum Light Emission from O-Band Quantum Dot Molecules
✍️ P. S. Avdienko et al.
🧠 DOI: http://link.aps.org/doi/10.1103/xx5b-rlgx

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