We demonstrated optical amplification at 650 nm in KMnF₃:Yb³⁺,Er³⁺@KMnF₃:Yb³⁺ nanoparticles doped polymer waveguides for the first time.

Ultra-small luminescent nanoparticles (NPs) are quite desirable for optoelectronic and biomedical applications. However, it is still a challenge to synthesize ultra-small NPs with high brightness owing to non-radiative energy losses caused by the surface defects as well as from vibrational deactivation ascribed to solvent molecules and ligands absorbed on the NPs. In this paper, we reported a strategy to improve up- and down-conversion luminescence of ultra-small BaLuF₅:Yb³⁺,Er³⁺ NPs by using multi-layer active-shells (containing Yb³⁺). Sub-10 nm BaLuF₅:Yb³⁺,Er³⁺@(X-shell, X = 1–5)BaLuF₅:Yb³⁺ NPs were synthesized via a high boiling solvent process through a layer-by-layer strategy. Up- and down-conversion fluorescence spectra of the NPs were recorded and analyzed by using a 980 nm laser diode as the excitation source. In comparison with optical properties of BaLuF₅:Yb³⁺,Er³⁺ NPs, the intensities of up- (∼545 nm) and down-conversion (∼1530 nm) fluorescence were enhanced by 52 and 9.8 times after coating 5-layer active-shells (BaLuF₅:Yb³⁺) on the BaLuF₅:Yb³⁺,Er³⁺ NPs, respectively. In addition, the intensities of up- and down-conversion fluorescence of the BaLuF₅:Yb³⁺,Er³⁺ NPs with multi-layer active-shells were 1.3 and 1.1 times larger than those of the BaLuF₅:Yb³⁺,Er³⁺ NPs with a one thick-layer active shell, respectively. These results showed that multi-layer active-shells could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb³⁺ ions inside the active-shells.

The demands for multiplexed biological detection have driven the development of fluorescence encoding nanoprobes. We have synthesized water-soluble dual mode emission core–shell rare earth nanoprobes (∼30 nm) for fluorescence encoding. The nanoprobes were composed of ytterbium (Yb), erbium (Er) and/or thulium (Tm) ions co-doped heterogeneous NaYF₄/NaLuF₄ nanocrystals as cores and amorphous SiO₂ embedded with europium (Eu) or terbium (Tb) complexes as shells. Excited by both infrared light and ultraviolet light, the nanoprobes exhibited dual characteristic emissions, which enable a novel spectral encoding strategy. The core nanocrystals exhibited tunable up-conversion emissions through various lanthanide ions doping. Combining these multiple upconversion emissions of core nanocrystals with downconversion emissions of shell containing rare earth complexes, a large amount of distinct fluorescence codes can be generated.

Hexagonal phase (β) sodium rare earth tetrafluorides (NaREF4, RE = Y, Gd, Lu, et al.) are considered the ideal matrices for lanthanide (Ln) ions doped upconversion (UC) luminescence materials, because they can provide favorable crystal lattice structures for the doped luminescent Ln ions to make intensive emissions. However, the cubic phase (α) NaREF4 always preferentially forms at low reaction temperature in short time as it is dynamically stable. Therefore, it is hard to obtain small sized β-NaREF4 via the traditional solvothermal method. In this paper, small sized β-NaYF4:Yb,Er nanoparticles were synthesized by a heterogeneous-core-induced method via the solvothermal reaction. The heterogeneous α-NaGdF4/β-NaYF4: Yb, Er core–shell structure was confirmed by the local elemental mapping. The formation mechanism of β-NaYF4:Yb,Er shell on the surface of α-NaGdF4 core was explained in detail. We reasoned that a hetero interface with a lower lattice symmetric structure was produced by cation exchanges between the core and shell, which caused the preferential growth of anisotropic hexagonal phase shell. The existence of this hetero interface has also been proven by observation of Gd3+ UC emission.

We propose a strip loaded amplifier employing SU-8 as the loaded waveguide and nanoparticles (NPs)-polymethyl methacrylate (PMMA) as the cladding layer. By leveraging the undoped SU-8 loaded waveguide, the polymer waveguide amplifier accomplished remarkably low transmission losses, reaching as low as 1.8 dB/cm at 1530 nm. We prepared NPs-PMMA nanocomposite by utilizing NaLu_0.1Y_0.7F₄: Er³⁺, Yb³⁺ @NaLuF₄ core-shell nanoparticles, which exhibited a significantly enhanced lifetime of 6.15 ms. An internal net gain of up to 17.7 dB was achieved on a strip loaded waveguide with a length as short as 0.5 cm when the on-chip pump power was 77 mW. Signal enhancement (SE) was measured at different wavelengths, revealing that the strip loaded waveguide exhibited broadband SE ranging from 1510 nm to 1570 nm, covering the C-band. To the best of our knowledge, this work has achieved the highest gain results reported thus far on a polymer matrix and provides an efficient method for optical amplification in passive devices on silicon and Si₃N₄ platforms, leveraging the ease of integration of polymer materials with diverse photonic platforms.