Polymer thin-film optical cavities boost the magnetic sensing sensitivity of nanodiamond quantum sensors nearly fivefold while enabling centimeter-scale fabrication with standard manufacturing techniques.
(Nanowerk Spotlight) Somewhere inside many diamonds lies a flaw. Not the inclusions that diminish a gemstone’s value, but something far stranger: individual atoms out of place, frozen in the crystal lattice. When that misplaced atom is nitrogen sitting next to an empty site where a carbon atom should be, the result is a nitrogen-vacancy center, a quantum system so sensitive it can detect the magnetic field from a single electron spinning in a nearby molecule.
These atomic defects glow red when struck by green laser light, and that glow shifts in response to magnetic fields, temperature changes, and electric fields at the nanoscale. Physicists recognized early that this sensitivity could transform fields from medical diagnostics to semiconductor manufacturing.
A thin layer of nitrogen-vacancy sensors placed over a microprocessor, for instance, could map current flow through individual transistors, catching defects invisible to conventional inspection. Positioned near neurons, the same sensors could track the magnetic signatures of brain activity without electrodes penetrating tissue.
Yet diamond itself has blocked these ambitions. The material is expensive, resists machining, and bonds poorly with other substances. High-quality diamond chips currently max out at 4 mm, too small for wafer-scale manufacturing. Nanodiamonds sidestep some limitations by dispersing easily across surfaces or mixing into polymers. But shrinking the crystals exacts a toll: the particles emit less light and lose quantum coherence faster, eroding the sensitivity that makes nitrogen-vacancy centers useful.
Optical cavities offer one way forward. Trapping light between mirrors amplifies interactions with quantum emitters through a phenomenon called the Purcell effect, which accelerates the rate at which emitters radiate. Scientists have demonstrated this enhancement for individual nanoparticles positioned inside photonic crystal structures or between precision-machined micro-mirrors. None of these approaches, however, scales beyond one particle at a time.
A team at RMIT University in Melbourne, Australia, has now found a scalable alternative. In a paper published in Advanced Science (“A Scalable Method for Cavity‐Enhanced Solid‐State Quantum Sensors”), the researchers describe embedding fluorescent nanodiamonds into polymer thin films sandwiched between silver mirrors, creating centimeter-scale optical cavities using standard semiconductor fabrication equipment. The technique achieved a 4.8-fold improvement in magnetic field sensitivity for 20 nm nanodiamonds, potentially clearing a path toward practical quantum-sensing devices.
Application and design of quantum sensor-doped microcavity thin-films. a) Schematic illustration of the application of the team’s thin-films to the magnetic imaging of currents in a microcircuit via cavity–enhanced quantum sensors. b) Schematic of the cavity and control (’no cavity’) devices with embedded quantum sensor particles. The cavity resonance energy is controlled by the polymer layer thickness L, allowing tuning of the spectral position of the cavity resonance. c) Typical reflectivity spectrum of the cavity resonance (top) and photoluminescence (PL) spectra of FNDs inside (red trace) and outside the cavity (black trace). d) A photograph of a typical thin-film microcavity-coated substrate, with clearly distinguishable cavity (green) and no cavity (clear) regions. e) Confocal PL image of the microcavity in panel (c), showing PL from FNDs uniformly dispersed throughout the thin-film. (Image: Reproduced from DOI:10.1002/advs.202517593, CC BY) (click on image to enlarge)
The cavity follows a Fabry-Pérot design, named after the French physicists who invented it in 1899. The principle is simple: two parallel mirrors face each other, and light bounces back and forth between them. Only wavelengths that fit a whole number of times into the gap resonate and build up intensity; other wavelengths destructively interfere and fade. The result is a device that selectively amplifies specific colors of light while suppressing others.
In the RMIT implementation, a 100 nm silver mirror forms the fully reflective base. Above it sits a polymer layer containing the quantum sensors. A semi-transparent 25 nm silver mirror caps the structure, allowing some light to escape for detection. The resonance wavelength depends on polymer thickness, which the team varied by adjusting spin-coating speeds. This produced devices with resonances spanning 500 to 800 nm, covering the nitrogen-vacancy emission spectrum.
The researchers tested two types of quantum sensors. Fluorescent nanodiamonds approximately 120 nm in diameter contained nitrogen-vacancy centers at a concentration of 1 part per million. A parallel set of devices used 70 nm particles of hexagonal boron nitride, a layered crystal similar in structure to graphite that hosts its own light-emitting defects suitable for quantum sensing.
Cavity resonances strongly modulated the nanodiamond emission. By tuning polymer thickness, the researchers shifted the peak emission wavelength from 600 to 730 nm. Emission profiles also narrowed compared to nanodiamonds outside the cavity, selectively enhancing the negatively charged nitrogen-vacancy state most useful for sensing.
Photoluminescence decay measurements revealed that nanodiamonds inside cavities resonant near 650 nm exhibited lifetimes of 5.63 ns, compared to 16.3 ns outside the cavity. This nearly threefold acceleration matches expectations for Purcell enhancement. Total brightness stayed largely unchanged, indicating the cavity boosted the radiative rate rather than introducing non-radiative losses.
The hexagonal boron nitride nanoparticles responded even more dramatically. Decay rates increased by up to 13-fold inside cavities, with brightness rising up to threefold. Yet the enhancement did not track cavity resonance wavelength, and the theoretical Purcell factor could not fully account for the magnitude. Surface plasmon interactions with the silver mirrors, or electron transfer between particles and metal, may play a role.
The decisive test used optically detected magnetic resonance, a technique that converts magnetic fields into optical signals. Microwave radiation drives transitions between spin states, producing a dip in photoluminescence at the resonance frequency of 2.87 GHz. The depth of this dip, called the contrast, governs sensitivity.
For 20 nm nanodiamonds, contrast increased 2.8-fold inside the cavity while spectral linewidth held steady. Combined with threefold brighter emission, this yielded a 4.8-fold sensitivity improvement. The cavity-enhanced particles could detect fields as small as 1 μT/√Hz. Larger 100 nm nanodiamonds showed a more modest 1.4-fold gain, likely because they already emit more brightly and had less to gain from the enhancement.
A tradeoff accompanies the enhancement. The polymer layer and bottom mirror form a physical barrier that holds sensing particles at least 104 nm away from any measurement target. Magnetic fields weaken with distance, so this standoff reduces signal strength. The 4.8-fold sensitivity gain compensates for standoffs up to about 495 nm, beyond which thinner non-cavity films would perform better.
The hexagonal boron nitride cavities yielded no usable magnetic resonance signals. The silver mirrors may have attenuated microwaves needed to drive spin transitions, or shortened excited-state lifetimes may have suppressed transitions to the metastable state required for readout.
What distinguishes this approach is manufacturability. Spin-coating polymer suspensions onto mirrored substrates requires no nanoscale alignment or single-particle manipulation. The researchers demonstrated devices on both silicon wafers and quartz, the latter permitting microwave delivery for sensing experiments. The standoff geometry permits a spatial resolution of approximately 156 nm, finer than the roughly 590 nm diffraction limit of current optical systems. In practice, the microscope optics, not the sensor geometry, limit resolution.
The work establishes that thin-film cavities can enhance quantum sensors at scales relevant to commercial fabrication. The paper suggests that replacing silver with dielectric mirrors in future devices could reduce optical losses and improve microwave transmission, potentially yielding even greater sensitivity gains. With centimeter-scale quantum-sensing films now achievable, applications from microelectronics inspection to biomedical imaging move closer to practical implementation.
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