X-ray Studies on Single-Photon Emission in hBN Stimulated by Ultrafast Laser Pulses

Yifeng Cao
Brookhaven National Laboratory – Upton, New York, USA
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Single-photon emission (SPE) is a phenomenon where individual photons are generated one at a time, serving as fundamental components for various strategic applications, including quantum cryptography, quantum sensing, and secure communications. In hexagonal boron nitride (hBN), defect-based single-photon emitters were discovered only recently, sparking considerable interest due to their high brightness, frequency tunability, spin coherence properties, and operability at room temperature. SPE in hBN originates from localized defects in the crystal structure, making it a promising platform for quantum technologies.

Recently, using a combined approach of photoluminescence (PL) and Resonant Inelastic X-ray Scattering (RIXS) at the N-K edge, we revealed that the large tunability of SPE in hBN, particularly in plasma-treated samples, arises from harmonics of an elementary excitation at 285 meV, extending up to 2.5 eV into the ultraviolet region. This mechanism highlights the influence of defect interactions with lattice vibrations. As a working hypothesis, the RIXS/PL process could vary depending on whether the excitation laser selectively excites specific energy states or the cross-section of the RIXS/PL process varies for the same SPE depending on the laser.

To further explore this behavior, we combined RIXS measurements with ultrafast laser stimulation (1030 nm and 200 fs) to investigate the response of SPE in pristine and defective hBN. In pristine hBN, no modes were initially detected in the absence of laser excitation, as expected. However, following the first exposure to laser light, multiple modes emerge, matching those observed in plasma treated defective samples. Remarkably, these modes (harmonics of 285 meV) intensified during laser exposure and continued to grow even after the laser was turned off. In defective hBN, the elementary excitation at 285 meV was present without laser stimulation, and upon laser exposure, a substantial increase in the mode intensity was observed.Unexpectedly, the intensity of the harmonics continued to rise even after the laser was switched off, eventually doubling the initial emission intensity.

This study provides compelling evidence that laser stimulation can be used to write and activate defects in pristine hBN, a phenomenon of significant technological relevance for on-demand, remote-controlled quantum emitters. The ability to induce SPE without direct physical contact, using laser beams that can be delivered through waveguides or optical fibers, opens new pathways for integrating hBN-based quantum sources into scalable photonic circuits. Furthermore, the delayed enhancement of SPE intensity suggests the presence of a long-lived phase in hBN, revealing previously unexplored defect dynamics that could be critical for future applications in quantum memory, quantum sensing, and optically-controlled qubits. This work highlights the potential for non-invasive, laser-driven quantum emission control, offering a robust method for manipulating quantum states in solid-state materials.