Rotating fluorescent nanodiamond assemblies with focused Laguerre-Gaussian beams

Optical tweezers which utilize structured light fields enable the rotation of trapped nanoparticles through the transfer of orbital angular momentum (OAM) from holographically generated Laguerre-Gaussian (LG) modes. In this research we use OAM transfer to demonstrate controlled rotation of bright fluorescent nanodiamond clusters assembled in a focused higher-order LG beam. We find that the assemblies can be effectively rotated in a two-dimensional optical trap with orbital frequencies of up to 5 Hz. We use video tracking to explore the Brownian dynamics of such a trapping arrangement and look at the impact of orientation stability on measurements of optically detected magnetic resonance (ODMR) with an applied weak external magnetic field. By collecting ODMR spectra at multiple points along the orbit, we show that the constrained two-dimensional motion can provide additional insights for vector magnetic field reconstruction.

💡 Research Summary

In this work the authors demonstrate controlled two‑dimensional rotation of bright fluorescent nanodiamond (ND) clusters using a high‑order Laguerre‑Gaussian (LG) beam that carries orbital angular momentum (OAM). A 1064 nm infrared laser is phase‑modulated by a spatial light modulator (SLM) to generate an LG mode with azimuthal index l = 7 (TEM₀₇) and radial index m = 0. The beam is tightly focused through a high‑NA objective into a thin (≈150 µm) microfluidic chamber. Freely dispersed NDs are driven to the top glass‑water interface by the combination of axial radiation pressure and lateral gradient forces; because the NDs are essentially transparent at 1064 nm, the dominant forces are purely optical.

Within the high‑intensity ring of the LG beam the particles self‑assemble into elongated, ellipsoidal clusters. The OAM of the LG mode exerts a continuous torque on these clusters, causing them to orbit the beam axis along an elliptical trajectory. By varying the laser power the orbital frequency can be tuned from ≈0.5 Hz up to ≈5 Hz, with lower‑order LG modes and higher powers giving faster tangential velocities but reduced angular stability. The authors record the motion at 40 fps for 11 s, extracting the centre‑of‑mass position and the orientation of the major axis of each cluster from each frame.

Analysis of the trajectories shows that the orbit is not perfectly circular; intensity hot‑spots in the beam profile produce non‑uniform tangential speeds. Angular stability is quantified as the deviation between the cluster’s major axis and the local tangent of the fitted ellipse. The mean deviation is |µ| = 1.4° with a standard deviation σ = 6.3°, while the overall angular spread over a full orbit is about ±13°. Using the NV‑centre Hamiltonian, the authors estimate that such an angular spread would broaden the ODMR resonance by ΔE/|B| ≈ ±4.6 MHz mT⁻¹ for a magnetic field of ≈1.3 mT. In practice, the measured ODMR linewidths (11–14 MHz) are comparable to those obtained from static bright NDs, indicating that the observed angular diffusion does not dominate the spectral width under the experimental conditions.

For magnetic resonance sensing, a co‑aligned 532 nm green laser is focused onto a fixed point on the orbital path to excite the NV‑centres. The fluorescence is collected with an avalanche photodiode (APD) while a microwave (MW) field (2–4 GHz) is swept to drive the spin transitions. The fluorescence time trace exhibits periodic peaks each time the rotating cluster passes through the excitation spot, confirming that the cluster’s orientation remains essentially constant during each transit. ODMR spectra acquired at a single orbital position show the characteristic zero‑field doublet centred at 2.87 GHz with a linewidth of ≈14 MHz. When a weak external magnetic field (≈1.3 mT) is applied, the doublet splits by ≈11 MHz, and the linewidth remains unchanged, demonstrating that the rotation does not introduce additional dephasing.

By accumulating ODMR spectra over many orbital cycles, the authors achieve rapid convergence of the averaged spectrum (≈20 s for a clear doublet). They calculate a shot‑noise‑limited DC magnetic‑field sensitivity of 40 ± 8 µT Hz⁻¹ᐟ² based on a photon count rate of ≈450 kcts s⁻¹, contrast ≈1.1 % and linewidth ≈11 MHz. Further experiments varying the external field strength (0.2–1.4 mT) and the orbital phase (0°, 90°, 180°) reveal that the ODMR response is symmetric under half‑orbit rotations, indicating minimal rotation about the long axis of the cluster. Simulations of the expected ODMR spectra for different orbital positions agree with the measurements, confirming that the 2‑D rotational motion provides a well‑defined geometric reference for vector magnetic‑field reconstruction.

In summary, the study shows that high‑order LG beams can be used to trap and rotate self‑assembled nanodiamond clusters with good angular stability, enabling simultaneous optical manipulation and NV‑centre magnetic sensing. The constrained 2‑D rotation yields repeatable orientation relative to an external field, allowing the extraction of vector magnetic‑field information from ODMR spectra collected at multiple orbital positions. This approach opens a pathway toward fast, orientation‑controlled nanoscale magnetometry and could be extended to more complex optical potentials for advanced quantum sensing applications.