April 13, 2024

A breakthrough in monolithic Fabry-Perot microcavities

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1. a, Conceptual illustration of the strain-tunable single-photon source. b, Structure of integrated FP microcavity and fundamental mode electric field distribution. c, 3D-FDTD simulation design. Credit: Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

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1. a, Conceptual illustration of the strain-tunable single-photon source. b, Structure of integrated FP microcavity and fundamental mode electric field distribution. c, 3D-FDTD simulation design. Credit: Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

Self-assembled semiconductor quantum dots (QDs) represent a confined three-dimensional nanostructure with discrete energy levels, which are similar to atoms. They are capable of producing highly efficient and indistinguishable single photons on demand and are important for exploring fundamental quantum physics and various applications in quantum information technologies. Leveraging traditional semiconductor processes, this materials system also offers a scalable and compatible platform for natural integration.

For an ideal QD single-photon source, a widely adopted approach to obtain photons with high extraction efficiency and indistinguishability is to embed QDs in Purcell-enhanced photonic cavities. However, the spatially random distribution of QDs makes it difficult to deterministically couple them to photonic structures.

Currently, accurate alignment of their spatial positions depends on precise optical fluorescence positioning techniques, and one of the ideal strategies for wavelength alignment involves the introduction of voltage tuning.

Current state-of-the-art QD single-photon sources are based on open Fabry-Perot (FP) cavity structures or elliptical micropillars. The former achieves position and wavelength alignment by precisely adjusting the top and bottom mirrors, but discrete structures are sensitive to environmental vibrations. The latter’s insulated structure hinders voltage transfer, making effective wavelength tuning a challenge.

Currently, this structure still relies on temperature adjustment within a small range, significantly reducing the device’s performance. Achieving effective integration of tension tuning into a microcavity structure while ensuring precise alignment of spatial position and wavelength remains a formidable challenge.

In a recent study published in Light: Science and Applications, the collaborative efforts of Jiawei Yang, Ying Yu, Siyuan Yu of Sun Yat-sen University, and Yan Chen of the National University of Defense Technology addressed these challenges by innovatively combining FP microcavities with a piezoelectric actuator, developing a tunable monolithic microcavity structure per wavelength. This innovative approach eliminates the need for etching semiconductor materials, avoiding surface defects and facilitating effective voltage conduction.

As shown in Fig. 1a, the FP microcavity designed in this work is integrated into a piezoelectric substrate. Since the QDs are located in the thin film, stress can be transmitted effectively. This structure does not require etching of semiconductor materials, effectively avoiding the influence of sidewall defects on QD emission.

In the FP microcavity structure depicted in Fig. 1b, the vertical confinement of the optical field is formed by upper and lower Bragg reflectors, while the lateral confinement of the optical field is created by a parabolic SiOtwo defect. The simulated efficiency of the single photon source can reach 0.95, with a Purcell factor of 40 (Fig. 1c). Furthermore, the fundamental mode has a Gaussian-like far-field distribution, facilitating coupling into optical fibers.


2. a, Monolithic FP thin film microcavity integrated into a piezoelectric substrate under an optical microscope. b, Fluorescence image of the QD coupled to the microcavity. c, QD emission scan in microcavity mode. d, Increase in brightness when the QD is coupled to the fundamental mode of the cavity. Credit: Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

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2. a, Monolithic FP thin film microcavity integrated into a piezoelectric substrate under an optical microscope. b, Fluorescence image of the QD coupled to the microcavity. c, QD emission scan in microcavity mode. d, Increase in brightness when the QD is coupled to the fundamental mode of the cavity. Credit: Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

In the experimental implementation, high-precision wide-field optical positioning technology was used to place QDs in the center of the FP microcavities (Fig. 2b). Subsequently, the thin film microcavity containing a single QD was integrated onto a PMN-PT (100) substrate using microtransfer printing technology (Fig. 2a).

A tuning range of 1.3 nm was achieved through voltage sweep (Fig. 2c), which is the largest wavelength tuning range reported for all microcavity structures to date. A remarkable 50-fold increase in brightness is obtained when the QD is brought with the microcavity fundamental mode, a 50-fold increase in brightness is achieved (Fig. 2d).


Figure 3. a, Coupling of the QD with the H-polarized mode. b, Rabi oscillations under resonant pulsed resonance excitation. c, Service life measurement. d, Single photon purity. and, single-photon indistinguishability. Credit: Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

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Figure 3. a, Coupling of the QD with the H-polarized mode. b, Rabi oscillations under resonant pulsed resonance excitation. c, Service life measurement. d, Single photon purity. and, single-photon indistinguishability. Credit: Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

Furthermore, when the QD coupled to H-polarized mode (Fig. 3a), a peak APD count rate of 2.88 Mcps is recorded under pulse resonance fluorescence (Fig. 3b), with an extraction efficiency of single photon polarized extraction of 0.58 and a fast lifetime of 100 ps.

Compared to QDs in planar structures, this represents a tenfold reduction in lifetime (Fig. 3c). The Hanbury Brown and Twiss correlation measurement extracts a single photon purity of 0.956 (Fig. 3d), meaning a low probability of multiple photons. Two-photon interference experiments highlight an impressive 0.922 photon indistinguishability (Fig. 3e).

In summary, researchers have developed a monolithic FP microcavity structure with the advantages of optimal exploitation of the Purcell effect, a compact footprint, and integration capabilities. By deterministic incorporation of a single QD into the microcavity, high-performance single photon sources with high simultaneous extraction efficiency, high purity and high indistinguishability are achieved.

Regarding future developments, charge stabilization or spin injection using closed electrical devices can be implemented directly in the structure to realize low-noise single-photon emission or spin-photon entanglement/a linear cluster state.

Furthermore, strain tuning can also be employed to erase the spectral inhomogeneity between different QDs and address the FSS. These aspects are fundamental in realizing high-performance sources of entangled photon pairs.

Most intriguingly, the simplicity and versatility of the cavity scheme pave the way for establishing a new manufacturing paradigm for quantum light sources, in which various types of solid quantum light sources (including semiconductor QDs, defects, etc.) ) with different emitting materials and operating wavelengths could be co-fabricated on the same PMN-PT platform. This potential breakthrough could significantly advance scalable quantum photonic technologies in the future.

More information:
Jiawei Yang et al, Tunable quantum dots in monolithic Fabry-Perot microcavities for high-performance single-photon sources, Light: Science and Applications (2024). DOI: 10.1038/s41377-024-01384-7

Diary information:
Light: Science and Applications

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