Nanoscale resolved mapping of the dipole emission of hBN color centers with a scattering-type scanning near-field optical microscope

Color centers in hexagonal boron nitride (hBN) are promising candidates as quantum light sources for future technologies. In this work, we utilize a scattering-type near-field optical microscope (s-SNOM) to study the photoluminescence (PL) emission characteristics of such quantum emitters in metalorganic vapor phase epitaxy grown hBN. On the one hand, we demonstrate direct near-field optical excitation and emission through interaction with the nanofocus of the tip resulting in a sub-diffraction limited tip-enhanced PL hotspot. On the other hand, we show that indirect excitation and emission via scattering from the tip significantly increases the recorded PL intensity. This demonstrates that the tip-assisted PL (TAPL) process efficiently guides the generated light to the detector. We apply the TAPL method to map the in-plane dipole orientations of the hBN color centers on the nanoscale. This work promotes the widely available s-SNOM approach to applications in the quantum domain including characterization and optical control.

💡 Research Summary

In this work the authors combine scattering‑type scanning near‑field optical microscopy (s‑SNOM) with tip‑assisted photoluminescence (TAPL) to investigate color centers in hexagonal boron nitride (hBN) that were grown by metal‑organic vapor‑phase epitaxy (MOVPE). The study demonstrates two distinct near‑field contributions to the recorded photoluminescence (PL): a tightly confined “dot” originating from direct excitation of a color center by the nanofocus at the apex of a metallic atomic‑force‑microscopy (AFM) tip (tip‑enhanced PL, TEPL), and a broader “arc” that results from interference between the direct far‑field excitation beam and light scattered by the tip (tip‑assisted PL, T‑APL).

The experimental configuration uses a standard gold‑coated AFM tip (Arrow‑NCPt) illuminated with a 532 nm continuous‑wave laser. The tip acts as an optical antenna, converting the incident p‑polarized beam into a highly confined near‑field spot of roughly 30 nm. While scanning the sample, a full PL spectrum is recorded at each pixel. Without the tip, the PL map of a single emitter shows a diffraction‑limited spot of about 1 µm × 4 µm. With the tip present, two sub‑diffraction features appear: a circular hotspot with a full width at half‑maximum (FWHM) of ~110 nm (the dot) and an arc‑shaped feature of ~1 µm diameter and 209 nm FWHM (the arc).

The dot is identified as TEPL, i.e., direct near‑field excitation of the emitter. Its intensity is relatively weak because the tip oscillates in tapping mode (20 nm amplitude) and the near‑field decays exponentially with tip‑sample distance; therefore the TEPL contribution is significant only during the brief portion of the oscillation when the tip is closest to the emitter. In contrast, the arc originates from interference between the direct excitation beam and the light scattered by the tip. The authors develop an analytical interference model that incorporates the tip position, the collection numerical aperture (NA = 0.72), and Gaussian angular distributions of both beams. The model reproduces the observed arc intensity profiles and predicts higher‑order interference fringes (n = 2, 3) that become visible at higher excitation powers (500 µW). Importantly, the arc intensity depends only weakly on tip height, so it contributes throughout the entire tapping cycle, leading to a much stronger overall PL signal.

Quantitatively, the presence of the tip increases the total PL intensity by a factor of six, while the peak intensity of the arc is about twice that of the dot. The authors attribute this enhancement to two mechanisms: (A) the tip concentrates the excitation field, increasing the absorption probability of the color center, and (B) the tip redirects the emitted photons into the acceptance cone of the off‑axis parabolic mirror, improving collection efficiency.

A key application demonstrated is the nanoscale mapping of the in‑plane dipole orientation of individual hBN color centers. By analyzing the angular dependence of the arc intensity across the scanned area and fitting it with the interference model, the authors extract the dipole axis with a spatial resolution limited by the tip apex (~30 nm). The results show that most emitters possess an in‑plane dipole, information that is crucial for designing plasmonic antennas or waveguides that efficiently couple to these quantum emitters.

The study also explores the bleaching behavior of the emitters under strong near‑field excitation. When the excitation power is increased to 500 µW, the TEPL dot bleaches rapidly at the tip position, confirming that the nanofocus delivers a highly localized, intense field capable of modifying the emitter’s photostability. This observation underscores the need to balance excitation intensity and emitter lifetime in practical quantum‑photonic devices.

Finally, the authors verify that the observed phenomena are not substrate‑specific: similar dot‑and‑arc patterns are obtained on hBN transferred onto Si/SiO₂, indicating that the interference‑driven T‑APL does not rely on the metallic gold substrate. The MOVPE‑grown hBN exhibits low background PL and a relatively uniform distribution of emitters, making it a promising platform for scalable quantum‑light sources.

In summary, the paper establishes s‑SNOM‑based TAPL as a powerful tool for (i) achieving sub‑diffraction PL imaging of hBN color centers, (ii) quantitatively separating direct near‑field and tip‑mediated interference contributions, and (iii) extracting dipole orientation with nanometer precision. These capabilities open new avenues for deterministic integration of hBN quantum emitters with nanophotonic structures, advancing the development of on‑chip quantum communication, computation, and sensing technologies.