Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride

Abstract: Hexagonal boron nitride (h-BN) is a tantalizing material for solid-state quantum engineering. Analogously to three-dimensional wide-bandgap semiconductors like diamond, h-BN hosts isolated defects exhibiting visible fluorescence, and the ability to position such quantum emitters within a two-dimensional material promises breakthrough advances in quantum sensing, photonics, and other quantum technologies. Critical to such applications, however, is an understanding of the physics underlying h-BN's quantum emission. We report the creation and characterization of visible single-photon sources in suspended, single-crystal, h-BN films. The emitters are bright and stable over timescales of several months in ambient conditions. With substrate interactions eliminated, we study the spectral, temporal, and spatial characteristics of the defects' optical emission, which offer several clues about their electronic and chemical structure. Analysis of the defects' spectra reveals similarities in vibronic coupling despite widely-varying fluorescence wavelengths, and a statistical analysis of their polarized emission patterns indicates a correlation between the optical dipole orientations of some defects and the primitive crystallographic axes of the single-crystal h-BN film. These measurements constrain possible defect models, and, moreover, suggest that several classes of emitters can exist simultaneously in free-standing h-BN, whether they be different defects, different charge states of the same defect, or the result of strong local perturbations.

Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride

The research paper titled "Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride" presents an in-depth exploration of quantum emitters embedded in two-dimensional materials, focusing on the photophysical characteristics of hexagonal boron nitride (h-BN). This work highlights the growing interest in two-dimensional materials for their potential application in quantum technologies.

Study Overview

The study elucidates the photonic properties of individual quantum emitters in suspended, single-crystal h-BN films. By analyzing the spectral, temporal, and spatial characteristics of these emitters, it aims to unravel the underlying electronic and chemical structures responsible for visible fluorescence. The paper reports on the creation of stable single-photon sources within h-BN that maintain their emission characteristics over several months in ambient conditions, emphasizing their suitability for applications in quantum sensing and photonics.

Methodology and Results

The researchers adopted a systematic approach to isolate and characterize quantum emitters in suspended h-BN. By employing optical and scanning electron microscopy, they identified regions with isolated defect emission. The study utilized techniques such as photoluminescence (PL) imaging, spectroscopy, and photon correlation measurements to analyze the properties of these emitters. Key findings include:

• Spectral Diversity: The emission spectra of single emitters exhibited significant variety, with features extending between 550-700 nm. Some spectra presented clearly defined zero-phonon lines (ZPLs) and phonon sidebands (PSBs), while others displayed broad spectral distributions suggesting a complex defect structure.

• Spectral Analysis: The study employed models of electron-phonon coupling to fit emission spectra, extracting parameters like the Huang-Rhys factor and highlighting similarities in the vibronic coupling of defects, despite differences in fluorescence wavelengths.

• Photon Emission Dynamics: Photon autocorrelation measurements indicated the presence of optical cycles involving three or more electronic levels for some defects. The tested three-level models revealed the participation of metastable states with lifetimes approaching microseconds, suggesting pathways for spin physics involvement.

• Polarization Dependence: Emission spectra indicated variations in absorptive and emissive dipole orientations, with weak statistical alignment to the crystallographic axes of h-BN, pointing to complex internal structures or strain conditions across the membrane.

Implications and Future Work

This study provides a foundation for understanding the interaction of optical dipoles with the host material's crystallography and electronic structure, which is crucial for the design of two-dimensional quantum photonic devices. The identification of substrate-dependent effects highlights the importance of substrate choice in experimental setups. The presence of film-induced strain effects also suggests potential directions for engineering the electronic properties of quantum emitters through mechanical manipulation.

Future work could investigate the effects of lowering h-BN thickness to strictly monolayer films, which would magnify electromechanical influences. Additionally, the combination of theoretical models with experimental results could enhance the understanding of defect emissions and offer strategies to tailor emitter properties for specific quantum technological applications.

In sum, the paper presents a thorough analysis of quantum emitters in h-BN, showcasing the material's potential as a platform for developing advanced quantum devices. This research is pivotal for advancing defect engineering in low-dimensional materials, promising to open new avenues in quantum information science, nanophotonics, and nanoscale sensing.