A scalable method for cavity-enhanced solid-state quantum sensors

Photoluminescent color centers in diamond and hexagonal boron nitride (hBN) are powerful nanoscale solid-state quantum sensors that are explored in a plethora of quantum technologies. Methods for integrating them into macroscopic structures that improve their sensitivity and enable their large-scale deployment are highly sought after. Here, we demonstrate cavity-enhanced photoluminescence (PL) of fluorescent nanodiamonds (FNDs) and hBN nanoparticles (NPs) embedded in polymer-based thin-film optical cavities on the centimeter scale. The cavity resonances efficiently modulate the spectral PL peak position of nitrogen-vacancy (NV) centers in FNDs across the NV PL spectrum and lead to an up to 2.9-fold Purcell-enhancement of the NV PL decay rate. The brightness of hBN NPs increases by up to a factor of three and the PL decay rate is enhanced by up to 13-fold inside the cavities. Finally, we find a 4.8 times improved magnetic field sensitivity of 20 nm FNDs in thin-film cavities due to cavity-enhanced optically detected magnetic resonance contrast and PL brightness. Our study demonstrates a low-cost and scalable method for the fabrication of quantum sensor-doped thin-film cavities, which is an important step toward the development of advanced quantum sensing technologies.

💡 Research Summary

The authors present a scalable, low‑cost approach to integrate solid‑state quantum sensors—fluorescent nanodiamonds (FNDs) containing nitrogen‑vacancy (NV) centers and hexagonal boron‑nitride (hBN) nanoparticles—into polymer‑based Fabry‑Pérot thin‑film microcavities. The devices are fabricated on centimeter‑scale substrates by sequential electron‑beam evaporation of a 100 nm silver mirror, spin‑coating of a polymer layer (PVP for FNDs, PMMA for hBN) doped with the quantum emitters, and deposition of a semi‑transparent top silver mirror. By varying the polymer thickness (≈107–210 nm) the cavity resonance wavelength (λ_res) can be tuned continuously from 500 nm to 800 nm, covering the zero‑phonon lines of NV⁰ (575 nm) and NV⁻ (637 nm) as well as the broad emission of hBN defects.

Using a 520 nm picosecond pulsed laser for excitation, the authors compare photoluminescence (PL) spectra, time‑resolved decay, and optically detected magnetic resonance (ODMR) contrast for emitters inside versus outside the cavity on the same chip. For FNDs, when λ_res aligns with both the excitation and emission bands (λ_res≈500–570 nm) the cavity provides up to a ten‑fold increase in PL intensity (enhancement factor η≈10). When only the emission couples to the cavity (λ_res≈600–700 nm) the enhancement remains positive but reduced (η≈1–3). For λ_res>700 nm the excitation is largely reflected, leading to PL suppression. Lifetime measurements reveal a Purcell‑enhanced decay: the average lifetime drops from 16.3 ns (no cavity) to 5.6 ns (cavity λ_res=650 nm), corresponding to a 2.9‑fold increase in the radiative decay rate. This demonstrates that the cavity mode density directly speeds up NV emission.

hBN nanoparticles exhibit a much broader and particle‑dependent emission (550–800 nm). Despite this variability, more than 90 % of the particles show a 2–10× brightness increase inside the cavity, and the decay rate can be accelerated up to 13‑fold. The authors argue that, beyond the Purcell effect, the proximity of hBN aggregates to the silver mirrors induces additional surface‑plasmon‑related enhancements, which are largely independent of the cavity resonance.

Finally, magnetic‑field sensing performance is evaluated using ODMR on 20 nm and 100 nm FNDs. The cavity‑enhanced devices display a 4.8‑times larger ODMR contrast compared with control regions, translating into a proportional improvement in magnetic‑field sensitivity. This improvement stems from the combined effects of higher PL brightness and faster spin‑dependent fluorescence dynamics.

Overall, the work establishes a practical route to fabricate large‑area quantum‑sensor‑doped optical cavities using only spin‑coating and thin‑film deposition. The method is compatible with a wide range of solid‑state emitters, allows deterministic spectral tuning of the cavity mode, and yields substantial gains in both optical and spin‑based sensing metrics. The authors envision applications in magnetic imaging of micro‑electronic circuits, high‑throughput biomedical sensing, and integration of quantum sensors into flexible or planar platforms, marking a significant step toward commercial quantum‑technology devices.