🔬 optics

Integrated Whispering-Gallery Microlaser-Waveguide Platform for On-Chip Electrical Excitation of InGaAs Quantum Dots

This paper reports the fabrication and characterization of an integrated quantum photonic platform featuring an electrically driven whispering-gallery-mode microlaser coupled to a ridge waveguide, which successfully enables on-chip optical excitation of InGaAs quantum dots to generate spectrally tunable single-photon emission with a second-order correlation value of $g^{(2)}(0) \approx 3.5\%$ .

Original authors: Léo J. Roche, Peter Gschwandtner, Chirag C. Palekar, Setthanat Wijitpatima, Yuhui Yang, Monika Emmerling, Sven Höfling, Tobias Huber-Loyola, Stephan Reitzenstein

Published 2026-06-23

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Léo J. Roche, Peter Gschwandtner, Chirag C. Palekar, Setthanat Wijitpatima, Yuhui Yang, Monika Emmerling, Sven Höfling, Tobias Huber-Loyola, Stephan Reitzenstein

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a tiny, self-contained factory on a single computer chip. The goal of this factory is to produce "perfect" single particles of light (photons) one by one, which are essential for future quantum computers and ultra-secure communication.

Usually, to make these particles, scientists have to use huge, bulky lasers sitting outside the chip and shine light onto it through a window. This paper describes a clever new way to build the laser inside the chip itself, right next to the factory, so everything fits in a tiny space.

Here is how they did it, explained with simple analogies:

1. The Setup: A Whispering Gallery and a Slide

The researchers built a device with three main parts:

The Whispering Gallery (The Laser): Think of this as a tiny, circular marble track (a micropillar) about 7 micrometers wide (thinner than a human hair). Inside this track, light bounces around the walls like a whisper echoing in a cathedral. Because the light is trapped and bouncing so efficiently, it builds up enough energy to become a laser.
The Slide (The Waveguide): Next to the circular track, they built a straight "slide" (a ridge waveguide).
The Factory (The Quantum Dots): At the very end of the slide, they placed a tiny "factory" containing special atoms called Quantum Dots.

The Magic Trick: The researchers placed the circular track and the slide so close together that they almost touch. In the world of light, when two things get this close, the light doesn't just stay in the circle; it "leaks" out and slides directly into the straight slide. This is called evanescent coupling. It's like if you had a spinning top next to a ramp; if they are close enough, the spinning motion transfers to the ramp without them actually touching.

2. Tuning the Gap: The Goldilocks Zone

The team tested many devices where the distance between the circular track and the slide was different (ranging from very close to a bit further apart).

Too Close: If the gap was too small (under 200 nanometers), the light leaked out of the laser too fast. It was like trying to fill a bucket with a hole in the bottom; you had to pour water in much faster (higher electrical current) just to get the laser to start working.
Just Right: They found a "Goldilocks" distance (around 500 nanometers) where the laser worked efficiently. It started lasing with a very low amount of electricity, and the light flowed smoothly into the slide.

3. The Process: Lighting the Factory

Once the laser was running, the light traveled down the slide and hit the "factory" (the Quantum Dots) at the end.

The Spark: The light from the laser hit the Quantum Dots, exciting them.
The Result: The excited Quantum Dots responded by firing off their own single particles of light.
The Tuning: The researchers could also apply a small electrical voltage to the factory at the end of the slide. This acted like a fine-tuning knob, slightly shifting the color (wavelength) of the light the factory produced. This is known as the Quantum-Confined Stark Effect.

4. The Proof: One by One

The most important test was to see if the factory was actually producing single photons one by one, rather than a messy stream of many.

They used a special detector to count the light particles.
The results showed that the light came out in a very clean, single-file line. The measurement confirmed that the device was successfully generating single photons with high purity (about 96.5% pure, with a tiny bit of background noise).

Summary

In short, the researchers successfully built a self-contained, electrically powered light factory on a chip.

They created a tiny laser that runs on electricity.
They guided that laser's light into a channel without using big external mirrors or lenses.
That light excited a tiny quantum dot at the end of the channel.
The quantum dot responded by emitting high-quality, single photons.

This proves that we can build complex, quantum-ready circuits entirely on a single chip, making them smaller, more efficient, and easier to scale up for future technologies.

Technical Summary: Integrated Whispering-Gallery Microlaser–Waveguide Platform for On-Chip Electrical Excitation of InGaAs Quantum Dots

Problem Statement
The realization of scalable integrated quantum photonic circuits (IQPCs) requires deterministic, on-demand single-photon sources with high brightness and indistinguishability. While solid-state quantum dots (QDs) are promising candidates, their practical integration faces challenges. Electrical excitation via current injection is non-resonant and often suffers from charge noise and timing jitter, limiting photon indistinguishability. Conversely, strictly resonant optical excitation offers superior coherence but typically relies on bulky external lasers, hindering scalability. The authors identify a need for a compact, electrically driven on-chip excitation scheme that can provide coherent optical pumping to QDs without the footprint and complexity of external laser systems.

Methodology
The authors fabricated a monolithic quantum photonic device integrating an electrically driven whispering-gallery-mode (WGM) micropillar laser with a ridge waveguide. The device architecture relies on a p–i–n diode structure grown via molecular beam epitaxy (MBE). The active region consists of InGaAs QDs embedded within a GaAs cavity, sandwiched between AlGaAs/GaAs distributed Bragg reflectors (DBRs).

Key fabrication and experimental steps included:

Device Design: Two electrically contacted WGM micropillars (7.2 µm diameter) were positioned adjacent to a ridge waveguide. The gap distance ( $d_{gap}$ ) between the pillars and the waveguide was varied (100 nm to 900 nm) to optimize evanescent coupling.
Coupling Mechanism: One end of the waveguide features a Bragg outcoupler, while the opposite end incorporates a waveguide-integrated micropillar (WIM) containing QDs and an aperture contact. This WIM allows for spectral tuning of the QDs via the quantum-confined Stark effect (QCSE).
Characterization: The devices were tested in a cryostat at 4 K and 20 K. Electroluminescence (EL) was analyzed to determine lasing thresholds, quality factors (Q-factors), and coupling efficiencies as a function of $d_{gap}$ .
Excitation Scheme: The WGM microlaser was electrically driven to emit coherent light, which was evanescently coupled into the ridge waveguide. This light propagated to the WIM, where it optically excited the embedded QDs. The emission from the QDs was collected and analyzed for quantum optical properties, specifically the second-order autocorrelation function $g^{(2)}(\tau)$ .

Key Contributions and Results

Gap-Dependent Lasing Characteristics: The study systematically investigated how the pillar-to-waveguide gap distance affects microlaser performance. Devices with smaller gaps ( $<200$ nm) exhibited significantly higher lasing threshold currents (up to 3.5 times higher than reference devices) due to enhanced out-coupling losses. The quality factor ( $Q$ ) was found to increase with gap distance, following a theoretical model where coupling losses dominate at small separations.
Efficient On-Chip Coupling: For a reference device with a 500 nm gap, the authors demonstrated efficient evanescent coupling. The input-output characteristics of the waveguide-transmitted signal matched the direct emission from the microlaser, with a lasing threshold of approximately 24 µA. The emission linewidth narrowed from 0.1 nm to 0.03 nm at threshold, confirming the onset of temporal coherence.
Wetting Layer vs. WGM Contribution: The authors quantified the power contributions of the wetting layer and the WGM mode to the total excitation power. Above the lasing threshold, the WGM mode became the dominant power source, contributing over 60% of the total intensity at 1 mA injection current.
On-Chip QD Excitation and Tuning: The coherent emission from the WGM laser successfully excited QDs in the WIM. The resulting photoluminescence showed a power dependence consistent with a double exciton state. Furthermore, the emission wavelength of the QDs was spectrally tuned by up to 0.35 nm by applying a bias voltage to the WIM, demonstrating control via the QCSE.
Single-Photon Emission: Under continuous-wave (CW) on-chip excitation, the selected QD emission line exhibited strong photon antibunching. The measured second-order autocorrelation value was $g^{(2)}(0) = (3.49 \pm 0.01)\%$ , confirming the generation of high-purity single photons.

Significance
The paper claims that these results constitute an important step toward the development of low-footprint, deterministic, and scalable single-photon sources for QD-based integrated quantum photonic circuits. By demonstrating that an electrically driven on-chip microlaser can effectively provide coherent optical excitation to a separate QD emitter, the work addresses the challenge of integrating high-performance excitation sources directly into quantum circuits. The successful observation of single-photon emission with high purity under these conditions validates the feasibility of this architecture for future IQPCs, offering a pathway to replace bulky external lasers with compact, electrically controlled on-chip alternatives.