Bulk-like emission in the visible spectrum of colloidal LiYF4:Pr nanocrystals downsized to 10 nm.

Though Pr3+ doped LiYF4 (LiYF4:Pr3+) bulk crystals are a well-known laser gain material with several radiative transitions, their nanocrystal counterparts have not been investigated with regards to these. Through downsizing to the nanoscale, novel applications are expected, especially in composite photonic devices. For example, nanocrystals in stable colloidal form with narrow size distribution are highly desirable to reduce scattering in such composites. Herein, we synthesized monodispersed LiYF4:Pr3+ nanocrystals having a size of 10 nm resulting in colorless clear stable colloidal dispersions and conducted an extensive optical characterization for the first time. We observed unexpected yet intense emission with excited state lifetimes comparable to bulk crystals in the visible spectrum through excitation at 444 nm and 479 nm. In macroscopic bulk crystals, this emission is only exploitable through excitation of a different, subjacent energy level. A comprehensive comparison to the bulk crystals provides deeper insight into the excitation mechanism and performance of these nanocrystals. The presented results pave the way for developing application-oriented LiYF4:Pr3+ nanocrystals as emitters with tailored properties for quantum optics or biomedical applications.

Upconversion Nanocrystal Doped Polymer Fiber Thermometer.

In recent years, lanthanide-doped nanothermometers have been mainly used in thin films or dispersed in organic solvents. However, both approaches have disadvantages such as the short interaction lengths of the active material with the pump beam or complicated handling, which can directly affect the achievable temperature resolution. We investigated the usability of a polymer fiber doped with upconversion nanocrystals as a thermometer. The fiber was excited with a wavelength stabilized diode laser at a wavelength of 976 nm. Emission spectra were recorded in a temperature range from 10 to 35 ∘C and the thermal emission changes were measured. Additionally, the pump power was varied to study the effect of self-induced heating on the thermometer specifications. Our fiber sensor shows a maximal thermal sensitivity of 1.45%/K and the minimal thermal resolution is below 20 mK. These results demonstrate that polymer fibers doped with nanocrystals constitute an attractive alternative to conventional fluorescence thermometers, as they add a long pump interaction length while also being insensitive to strong electrical fields or inert to bio-chemical environments.

Towards Highly Efficient Polymer Fiber Laser Sources for Integrated Photonic Sensors.

Lab-on-a-Chip (LoC) devices combining microfluidic analyte provision with integrated optical analysis are highly desirable for several applications in biological or medical sciences. While the microfluidic approach is already broadly addressed, some work needs to be done regarding the integrated optics, especially provision of highly integrable laser sources. Polymer optical fiber (POF) lasers represent an alignment-free, rugged, and flexible technology platform. Additionally, POFs are intrinsically compatible to polymer microfluidic devices. Home-made Rhodamine B (RB)-doped POFs were characterized with experimental and numerical parameter studies on their lasing potential. High output energies of 1.65 mJ, high slope efficiencies of 56 % , and 50 % -lifetimes of ≥900 k shots were extracted from RB:POFs. Furthermore, RB:POFs show broad spectral tunability over several tens of nanometers. A route to optimize polymer fiber lasers is revealed, providing functionality for a broad range of LoC devices. Spectral tunability, high efficiencies, and output energies enable a broad field of LoC applications.