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arXiv:2608.12081

Design of monolithic microcavities for enhancing organic quantum emitters

Tim Hebenstreit, Jaime Gimeno Balaguer, Fridtjof Betz, Felix Binkowski, Maja Colautti, Jan Renger, Costanza Toninelli, Sven Burger, Stephan Götzinger

quant-phSubmitted Aug 12, 2026• Announced Aug 13, 2026

Abstract

Single organic molecules are a well-established platform for high-quality single-photon generation: they can emit lifetime-limited photons with high purity and indistinguishability. However, their emission is accompanied by a pronounced red-shifted phonon sideband and higher-order vibrational peaks, which reduce the fraction of photons emitted into the desired narrowband zero-phonon line. The standard approach to suppressing this unwanted emission is to integrate the emitter into a monolithic photonic nanostructure that provides Purcell enhancement. However, incorporating organic materials using clean-room techniques has proven challenging, and has in fact so far prevented their integration into monolithic microcavities altogether. As a result, efficient, narrowband organic single-photon sources for applications in quantum information processing have remained elusive despite their considerable promise. Here, we propose three monolithic microcavity designs tailored to provide sufficient Purcell enhancement for organic quantum emitters. The Purcell effect induced by these cavities preferentially enhances emission into the 0-0 zero-phonon line, increasing spectral purity, photon extraction, and shortening the excited-state lifetime, which in turn relaxes the requirements for generating indistinguishable photons. The cavities have been optimized using Bayesian optimization and the adaptive Antoulas-Anderson (AAA) algorithm for rational approximation, which offer global optimization and the efficient reconstruction of spectra from scattering simulations, respectively. These structures are compatible with both standard clean-room processing and single-molecule preparation techniques. Therefore, they offer a clear route to realize high-quality, practically monochromatic organic single-photon light sources.

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Single organic molecules are a well-established platform for high-quality single-photon generation: they can emit lifetime-limited photons with high purity and indistinguishability. However, their emission is accompanied by a pronounced red-shifted phonon sideband and higher-order vibrational peaks, which reduce the fraction of photons emitted into the desired narrowband zero-phonon line. The standard approach to suppressing this unwanted emission is to integrate the emitter into a monolithic photonic nanostructure that provides Purcell enhancement. However, incorporating organic materials using clean-room techniques has proven challenging, and has in fact so far prevented their integration into monolithic microcavities altogether. As a result, efficient, narrowband organic single-photon sources for applications in quantum information processing have remained elusive despite their considerable promise. Here, we propose three monolithic microcavity designs tailored to provide sufficient Purcell enhancement for organic quantum emitters. The Purcell effect induced by these cavities preferentially enhances emission into the 0-0 zero-phonon line, increasing spectral purity, photon extraction, and shortening the excited-state lifetime, which in turn relaxes the requirements for generating indistinguishable photons. The cavities have been optimized using Bayesian optimization and the adaptive Antoulas-Anderson (AAA) algorithm for rational approximation, which offer global optimization and the efficient reconstruction of spectra from scattering simulations, respectively. These structures are compatible with both standard clean-room processing and single-molecule preparation techniques. Therefore, they offer a clear route to realize high-quality, practically monochromatic organic single-photon light sources.

Semiconductor quantum dots (QDs) coupled with optical microcavities provide an excellent platform for generating high-performance single-photon sources. However, the current microcavity structures are difficult to solve the tradeoff between single-photon purity and indistinguishability without additional filtering, due to challenges in suppressing broad phonon sideband spectrum and two-photon emission in the practical experiments. To address these issues, we have developed a novel asymmetric microcavity that supports two nondegenerate polarized modes, both resonant with a single quantum dot. One cavity mode is broadband and is used for excitation to reduce two-photon emission intensity, while the other is narrowband and is used for collection to enhance the indistinguishability of single-photon sources. This asymmetric microcavity also has the ability to overcome at least half of the photon efficiency loss under polarization filtering. Moreover, our model has employed an excitation pulse that resonates with cavity modes eliminating spectral distortion of intracavity fields, and efficiently achieving deterministic population inversion of QDs compared to an elliptical cavity. Our scheme paves the way to develop the excellent performance single-photon sources with higher brightness, near-unity purity, and indistinguishability without any additional filtering in practical experiments.

Sources of pure and indistinguishable single-photons are critical for near-future optical quantum technologies. Recently, color centers hosted by two-dimensional hexagonal boron nitride (hBN) have emerged as a promising platform for high luminosity room temperature single-photon sources. Despite the brightness of the emitters, the spectrum is rather broad and the single-photon purity is not sufficient for practical quantum information processing. Here, we report integration of such a quantum emitter hosted by hBN into a tunable optical microcavity. A small mode volume of the order of λ 3 allows us to Purcell enhance the fluorescence, with the observed excited state lifetime shortening. The cavity significantly narrows the spectrum and improves the single-photon purity by suppression of off-resonant noise. The complete device, including all optics, driving electronics, and control units, is compact and integrated in a small volume of 10 × 10 × 10 cm 3, allowing for portable usage in mobile applications.

Generating single photons on demand in silicon is a challenge to the scalability of silicon-on-insulator integrated quantum photonic chips. While several defects acting as artificial atoms have recently demonstrated an ability to generate antibunched single photons, practical applications require tailoring of their emission through quantum cavity effects. In this work, we perform cavity quantum electrodynamics experiments with ensembles of artificial atoms embedded in silicon-on-insulator microresonators. The emitters under study, known as W color centers, are silicon tri-interstitial defects created upon self-ion implantation and thermal annealing. The resonators consist of circular Bragg grating cavities, designed for moderate Purcell enhancement ( F p = 12.5) and efficient luminescence extraction (η coll = 40% for a numerical aperture of 0.26) for W centers located at the mode antinode. When the resonant frequency mode of the cavity is tuned with the zero-phonon transition of the emitters at 1218 nm, we observe a 20-fold enhancement of the zero-phonon line intensity, together with a 2-fold decrease of the total relaxation time in time-resolved photoluminescence experiments. Based on finite-difference time-domain simulations, we propose a detailed theoretical analysis of Purcell enhancement for an ensemble of W centers, considering the overlap between the emitters and the resonant cavity mode. We obtain a good agreement with our experimental results assuming a quantum efficiency of 65 ± 10% for the emitters in bulk silicon. Therefore, W centers open promising perspectives for the development of on-demand sources of single photons by harnessing cavity quantum electrodynamics in silicon photonic chips.

Combining resonant excitation with Purcell-enhanced single quantum dots (QDs) stands out as a prominent strategy for realizing high-performance solid-state single-photon sources. However, optimizing photon extraction efficiency requires addressing the challenge of effectively separating the excitation laser from the QDs’ emission. Traditionally, this involves polarization filtering, limiting the achievable polarization directions and the scalability of polarized photonic states. In this study, we have successfully tackled this challenge by employing spatially orthogonal resonant excitation of QDs, deterministically coupled to monolithic Fabry–Perot microcavities. Leveraging the planar microcavity structure, we have achieved spectral filter-free single-photon resonant fluorescence. The resulting source produces single photons with a high extraction efficiency of 0.87 and an indistinguishability of 0.963(4).

Monolithic integration of single-photon sources into silicon photonics offers a compelling solution for realizing scalable quantum telecommunication architectures. Among the candidate silicon color centers, the C-center features a zero-phonon emission in the telecom L-band, corresponding to the spectral window of minimum transmission loss in optical fibers. Despite this advantage, isolating single C-centers has remained an unrealized challenge in solid-state quantum optics. In this work, we report the isolation of a single silicon C-center. We enhance the spontaneous emission rate of the zero-phonon line by coupling the single emitter to a nanophotonic crystal cavity, observing a fivefold reduction in the excited-state lifetime compared to bulk conditions, which corresponds to a Purcell factor lower bound of 33. Second-order autocorrelation measurements confirm single-photon emission from the investigated defect, obtaining an optical purity of

g^{(2)}(0) = 0.23

without background correction. Our results establish a native telecom L-band quantum light source in silicon, laying the foundation for ultra-low-loss and long-distance quantum applications.

Solid-state emitters are excellent candidates for developing integrated sources of single photons. Yet, phonons degrade the photon indistinguishability both through pure dephasing of the zero-phonon line and through phonon-assisted emission. Here, we study theoretically and experimentally the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity as a function of temperature. We show that a large coupling to a high quality factor cavity can simultaneously reduce the effect of both phonon-induced sources of decoherence. It first limits the effect of pure dephasing on the zero-phonon line with indistinguishabilities above 97% up to 18 K. Moreover, it efficiently redirects the phonon sidebands into the zero-phonon line and brings the indistinguishability of the full emission spectrum from 87% (24%) without cavity effect to more than 99% (76%) at 0K (20K). We provide guidelines for optimal cavity designs that further minimize the phonon-induced decoherence.