New Colloid Enhances Near-Infrared Fluorescence in Nanoparticles

Researchers have developed a novel colloidal suspension of neodymium-doped sodium gadolinium fluoride nanoparticles that demonstrates significantly improved near-infrared fluorescence properties. The study, published in Results in Optics, focuses on the preparation of these nanoparticles and their optical performance under pulsed pumping conditions. By engineering the surface chemistry and selecting specific solvents, the team achieved a formulation that outperforms traditional dimethyl sulfoxide-based dispersions in key optical metrics. This work contributes to the broader effort to create liquid gain media that combine the thermal management advantages of fluids with the spectral precision of solid-state lasers.

The core of the research involves the synthesis of core-shell nanoparticles with an 8 nanometer doped core and a 1 nanometer inert shell, resulting in an overall diameter of approximately 10 nanometers. The doped core contains 5 mol% neodymium ions substituted into the sodium gadolinium fluoride lattice. Transmission electron microscopy confirmed that the particles are quasi-spherical with distinct boundaries and good monodispersity. X-ray diffraction analysis verified that the crystal structure is pure hexagonal beta-NaGdF4, with no detectable impurities or heterophases. This structural purity is critical for minimizing non-radiative decay pathways that can quench luminescence in rare-earth-doped materials.

The synthesis process utilized a modified high-temperature co-precipitation method to create the core nanoparticles, followed by a shell growth step to encapsulate the doped core. Surface ligand modification was performed using oleic acid and tri-n-octylphosphine oxide to stabilize the particles in solution. The choice of ligands is significant because the phosphoryl group of the phosphine oxide can coordinate with undercoordinated rare-earth surface sites, potentially reducing non-radiative decay. The nanoparticles were then dispersed in a co-solvent mixture of carbon tetrachloride and deuterated chloroform at a 5:1 volume ratio. This solvent system was selected for its refractive index, which closely matches that of the nanoparticles to minimize scattering losses.

Optical characterization revealed that the optimized colloid exhibits a fluorescence lifetime of 223.84 microseconds, which is longer than the 201.73 microseconds measured for the unoptimized dimethyl sulfoxide-based sample. The instrument-reported photoluminescence quantum yield increased from approximately 19.8% to 30.0% in the optimized formulation. Additionally, the transmittance at 1058 nanometers improved from 94.6% to 98.9%, indicating lower optical attenuation. These improvements suggest that the combined surface-ligand and solvent optimization effectively reduces non-radiative decay and scattering, leading to a more efficient fluorescent medium.

Judd-Ofelt analysis was performed to quantify the radiative properties of the neodymium ions in the colloid. The effective neodymium number density in the suspension was estimated to be 5.73 x 10^18 per cubic centimeter. The analysis yielded radiative parameters that correspond to a radiative lifetime of approximately 576 microseconds. The calculated branching ratio for the 1058 nanometer transition, which is the primary near-infrared emission line, was 0.524. The peak stimulated-emission cross-section at this wavelength was determined to be 3.93 x 10^-20 square centimeters. These spectroscopic parameters are essential for evaluating the potential of the colloid as a laser gain medium, although they do not alone prove the existence of net optical gain.

To test the laser potential, the colloid was subjected to pulsed pumping experiments using both nanosecond and femtosecond laser pulses. In the nanosecond pumping test, an optical parametric oscillator operated at 796 nanometers with a 10 nanosecond pulse duration. Pulse energies were varied from 10 to 35 millijoules. While visible intracavity luminescence and scattering were observed, the signal was insufficient for reliable energy-resolved spectral analysis. No laser oscillation or amplified spontaneous emission narrowing was detected over the tested pump-energy range. The lack of a clear threshold suggests that the current formulation does not yet support stable laser action under nanosecond pumping conditions.

The femtosecond pumping test used a 1 kilohertz repetition rate with 200 femtosecond pulse durations at 796 nanometers. Pulse energies were varied from 10 to 27 microjoules. Broad near-infrared emission bands centered near 1058 nanometers were recorded at incident pulse energies of 15, 20, and 25 microjoules. The intensity of the emission increased with pulse energy, but the spectra retained a broad fluorescence profile without any narrow laser-like features. No abrupt narrowing or other threshold-like spectral changes were resolved. These results are assigned to pump-dependent fluorescence rather than laser oscillation, indicating that further optimization is required to achieve net optical gain.

The study highlights the challenges in developing liquid laser media based on nanoparticle dispersions. While the optimized colloid shows improved fluorescence performance compared to previous formulations, it has not yet demonstrated the necessary conditions for laser operation. The authors note that direct gain measurements, quantitative cavity-loss characterization, and more detailed pump-dependent spectral measurements are required to determine whether this colloid can support laser action. The current results provide a solid foundation for future work, but they do not constitute a demonstration of a working liquid laser.

Future research should focus on increasing the neodymium concentration, optimizing the cavity design, and reducing residual scattering losses. The authors also recommend direct concentration determination and replicate-level uncertainty reporting to improve the precision of the optical measurements. Separation of scattering and absorption contributions is another area that requires further investigation. By addressing these issues, researchers may be able to bridge the gap between enhanced fluorescence and practical laser operation in colloidal media. This work represents a step forward in the development of liquid gain media with improved thermal and optical properties.

Subject of Research: Preparation and near-infrared fluorescence of Nd:NaGdF4 nanoparticle colloids under pulsed pumping

Article Title: Preparation and near-infrared fluorescence of Nd:NaGdF4 nanoparticle colloids under pulsed pumping

Article References: Chen, Q., Guo, G., Yang, K., Chen, S., Wang, Y., & Wang, K. (2026). Preparation and near-infrared fluorescence of Nd:NaGdF4 nanoparticle colloids under pulsed pumping. Results in Optics, Article 101188. https://doi.org/10.1016/j.rio.2026.101188

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101188

Keywords: Preparation, near-infrared, fluorescence, NaGdF4, nanoparticle, colloids, under, pulsed, pumping, scientific research