Investigation of Yb-doped LiLuF4 single crystals for optical cooling

Optical cooling of solids, relying on annihilation of lattice phonons via anti-Stokes fluorescence, is an emerging technology that is rapidly advancing. The development of high-quality Yb-doped fluoride single crystals definitely led to cryogenic and sub-100-K operations, and the potential for further improvements has not been exhausted by far. Among fluorides, so far the best results have been achieved with Yb-doped LiYF4 (YLF) single crystals, with a record cooling to 91 K of a stand-alone YLF:10%Yb. We report on preliminary investigation of optical cooling of an LiLuF4 (LLF) single crystal, an isomorph of YLF where yttrium is replaced by lutetium. Different samples of 5% Yb-doped LLF single crystals have been grown and optically characterized. Optical cooling was observed by exciting the Yb transition in single-pass at 1025 nm and the cooling efficiency curve has been measured detecting the heating/cooling temperature change as a function of pumping laser frequency

Tm-doped Crystals for mid-IR Optical Cryocoolers and Radiation Balanced Lasers

We report the complete characterization of various cooling grade Tm-doped crystals including the first demonstration of optical refrigeration in Tm:YLF crystals. Room temperature laser cooling efficiencies of 1% and 2% (mol) Tm:YLF, and 1% Tm:BYF crystals at different excitation polarizations are measured and their external quantum efficiency and background absorption are extracted. By performing detailed low-temperature spectroscopic analysis of the samples, global minimum achievable temperatures of 160 K to 110 K are estimated. The potential of Tm-doped crystals to realize mid-IR optical cryocoolers and radiation balanced lasers (RBLs) in the eye-safe region of the spectrum is discussed, and a promising 2-tone RBL in a tandem structure of Tm:YLF and Ho:YLF crystals is proposed.Comment: References were updated. (4 pages, 5 figures

BaYLuF8 drogato con ioni Tm3+: un nuovo materiale per laser nella regione dei 2micron

In questo lavoro di tesi è stato cresciuto, caratterizzato dal punto di vista spettroscopico e dimostrata l'emissione laser a 2micron di un monocristallo di BaYLuF8 drogato con ioni Tm3+, interessante per applicazioni laser come variante con Lutezio del BaY2F8

Cooling effect on fluoride crystals

In this work various Yb-doped fluoride single crystals have been investigated as active media for optical refrigeration. For each material, samples with varying doping levels have been grown by means of the Czochralski technique, optically charaterized through absorption, static and dynamic fluorescence measurements and tested for anti-Stokes cooling. The optical cooling efficiency has been measured by measuring the heating/cooling temperature change of samples as a function of pumping wavelength. A preliminary investigation of impurities by elemental analyses has been also carried out

Bulk Cooling Efficiency Measurements of Yb-Doped Fluoride Single Crystals and Energy Transfer-Assisted Anti-Stokes Cooling in Co-Doped Fluorides

Bulk Cooling Efficiency Measurements of Yb-Doped Fluoride Single Crystals and Energy Transfer-Assisted Anti-Stokes Cooling in Co-Doped Fluoride

Novel approach for solid state cryocoolers

Laser cooling in solids is based on anti-Stokes luminescence, via the annihilation of lattice phonons needed to compensate the energy of emitted photons, higher than absorbed ones. Usually the anti-Stokes process is obtained using a rare-earth active ion, like Yb. In this work we demonstrate a novel approach for optical cooling based not only to Yb anti- Stokes cycle but also to virtuous energy-transfer processes from the active ion, obtaining an increase of the cooling efficiency of a single crystal LiYF4 (YLF) doped Yb at 5at.% with a controlled co-doping of 0.0016% Thulium ions. A model for efficiency enhancement based on Yb-Tm energy transfer is also suggested

Chapter 11 Crystal Growth of Fluoride Single Crystals for Optical Refrigeration

Crystal Growth of Fluoride Single Crystals for Optical Refrigeratio

Czochralski crystal growth for laser cooling

In laser cooling of crystals in solid-state physics, it is really important to obtain crystals with a large size at a relatively fast growth rate and high-optical quality that is defect-free. To get that, one of the methods to grow crystals is the Czochralski technique. The Czochralski technique will be presented and, in particular, the furnaces in New Materials for Laser Applications Laboratories of Pisa for this application will be discussed. Afterward the parameters for the growth of crystal fluorides are depicted and it is shown how these parameters lead to build samples suitable for optical cooling. All processes that are necessary to avoid contamination inside crystals like OH-ion and how to avoid reduction of Yb3+to Yb2+will be given. Spectroscopy of all samples will be treated in order to obtain the cooling parameters λf and αb for each sample. Afterward, an efficiency model will be discussed and the data efficiency of cooling obtained by a sample's own crystals will be shown

Co-doping of LiYF4crystal: A virtuous effect of cooling efficiency

Anti-Stokes shift, provided by embedded rare earth (RE) ions in fluoride crystal host, is used to obtain a net cooling of a solid system. Yb3+is currently the RE ion that presents the best cooling performance when inserted into a suitable host, like yttrium lithium fluoride (YLF). Recently, a new approach to laser cooling has been proposed, in which an enhancement of the cooling efficiency is reached by co-doping with Yb3+and Tm3+. In this work, we compare, in terms of cooling efficiency, two samples 5%Yb:YLF and 5%Yb-0.0080%Tm:YLF, grown with the same starting material in order to avoid the difference in chemical composition of impurity. Some contaminants, like iron, are very detrimental for cooling efficiency and they make it difficult to compare grown crystals with different raw powders. We demonstrate that the presence of a small concentration of Tm3+ions permits it to decrease the background absorption and to increase the cooling efficiency