Fifty years of advances in solid-state laser materials

Photodarkening of Yb-doped aluminosilicate fibers by continuous wave 488 nm irradiation was investigated. The irradiation induced significant excess loss in the UV-visible spectroscopy (VIS) region in Yb-doped aluminosilicate fibers while pure aluminosilicate fibers showed negligible induced loss. Ultraviolet-VIS-near infrared spectroscopy revealed an absorption peak at 220 nm in unexposed Yb-doped aluminosilicate fiber preforms. The observed peak was attributed to Yb-associated oxygen deficiency centers (ODCs) and proposed as a precursor of the photodarkening. The proposed model was supported by measurements on oxygen loaded Yb-doped aluminosilicate fibers. In these, the photodarkening could be significantly reduced, which we attribute to a smaller number of ODCs following oxygen loading.

Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation

Harvesting of near infrared (NIR) light in the abundant and environmentally friendly solar spectrum is particularly significant to enhance the utilization rate of the cleanest energy on earth. Appreciating the unique nonlinear optical properties of upconversion materials for converting low-energy incident light into high-energy radiation, they become the most promising candidates for fabricating NIR light-active photocatalytic systems by integrating with semiconductors. The present review summarizes recent NIR light-active photocatalytic systems based on a sequence of NaYF4-based, fluoride-based, oxide-based and Ln3+ ion-doped semiconductor-based photocatalysts for degradation of organic molecules. In addition, we provide an in-depth analysis of various photocatalytic mechanisms and enhancement effects for efficient photo-redox performance of different upconversion semiconductor photocatalysts. We envision that this review can inspire multidisciplinary research interest in rational design and fabrication of efficient full-spectrum active (UV-visible-NIR) photocatalytic systems and their wider applications in solar energy conversion.

/pdf/nir-light-activated-upconversion-semiconductor-4ecf16qngr.pdf

NIR light-activated upconversion semiconductor photocatalysts

We present an overview of laser results we obtained with Yb-doped calcium fluoride and its isotype strontium fluoride. In order to study the laser performance in femtosecond and high power regimes, spectral and thermal properties are first discussed including the potential of these crystals at room and cryogenic temperatures. Experimental demonstrations of high-power and ultrashort pulse oscillators and amplifiers are presented and analyzed.

https://hal-iogs.archives-ouvertes.fr/hal-00658197/document

On Yb:CaF2 and Yb:SrF2 : Review of spectroscopic and thermal properties and their impact on femtosecond and high power laser performance

A detailed spectroscopic study of ${\text{Yb}}^{3+}:{\text{CaF}}_{2}$ is made by using laser selective excitation, low-temperature time-resolved fluorescence, and Raman techniques. Three dominant types of ${\text{Yb}}^{3+}$ isolated ion centers are identified at very low $(0.03\text{ }\text{at}\text{.}\text{ }%)$ dopant concentration: two of them with cubic and tetragonal symmetries and the third one not clearly identified but likely of trigonal symmetry. Fluorescence decays strongly differ from one site to another. The overall measured positions of the Stark sublevels of the ${^{2}F}_{7/2}$ and ${^{2}F}_{5/2}$ multiplets are satisfactorily explained by the crystal-field calculations. On the other hand, at higher dopant concentrations, only one dominant class of ${\text{Yb}}^{3+}$ centers occurs which corresponds to some hexameric cluster in which the ${\text{Yb}}^{3+}$ ions site in nearly tetragonal crystal-field environments and whose energy-level positions are discussed and compared with those previously reported in the literature.

Spectroscopy of Yb 3+ :CaF 2 : From isolated centers to clusters

Ca1−XYbXF2+X crystals were grown by two different methods: simple melting under CF4 atmosphere and the laser heated pedestal growth (LHPG) method under Ar atmosphere. Spectroscopic characterization (absorption, emission, Raman spectroscopy and decay curves) was carried out to identify Stark levels of Yb3+ transitions in the different crystallographic sites of the cubic structure in Ca1−XYbXF2+X crystals. The Yb3+ concentration dependence of the experimental decay time was analysed by using concentration gradient fibre in order to understand involved concentration quenching mechanisms. Under Yb3+ ion infrared pumping, self-trapping and up-conversion non-radiative energy transfer to unexpected rare earth impurities (Er3+, Tm3+) has been observed in the visible region and interpreted by a limited diffusion process within the Yb3+ doping ion subsystem towards impurities. The main parameters useful for a theoretical approach of laser potentiality have been given as τrad = 2.05 ms, N0 = 7.47 × 1021 cm−3 (32 mol%) and Nm = 6.39 × 1021 cm−3 (26.5 mol%).

https://iopscience.iop.org/article/10.1088/0953-8984/16/8/029/pdf

Crystal growth, Yb3+ spectroscopy, concentration quenching analysis and potentiality of laser emission in Ca1−XYbXF2+X

We investigate the prediction of the infrared laser emission in the Yb3+-doped cubic KY3F10 crystal that can be pumped by a laser diode. Bulky single crystals have been grown by the Czochralski technique at Tohoku University, and crystalline fibers have been grown by the laser heated pedestal growth technique at Claude Bernard/Lyon 1 University. Absorption and emission spectra were recorded to contribute to the determination of electronic and vibronic energy levels by using in addition the Raman spectrum and the barycenter law. Yb3+ concentration dependence of the 2F5/2 experimental decay time was analyzed using concentration gradient fibers in order to attempt to understand involved concentration quenching mechanisms. Under Yb3+ ion infrared pumping, self-trapping has been observed at low concentration, whereas upconversion nonradiative energy transfer to unexpected rare-earth impurities (Er3+,Tm3+) has been observed in the visible region and interpreted with a limited diffusion process within the Yb3+ doping ion subsystem toward impurities. Main parameters useful for a theoretical approach of laser potentiality have been given and compared with other fluoride crystals.

Spectroscopic properties, concentration quenching, and prediction of infrared laser emission of Yb3+-doped KY3F10 cubic crystal

Growth of Yb3+-doped KY3F10 concentration gradient crystal fiber by laser-heated pedestal growth (LHPG) technique

Yb3+ ions with various site symmetries have been observed in the absorption and emission spectra of Yb3+:CaF2 crystals, both γ-irradiated and annealed in hydrogen. The absorption intensity value is found to be much higher for the γ-irradiated crystal and strongly dependent on the gamma dose. The UV absorption spectra of γ-irradiated and H2-annealed CaF2:5 at.% Yb3+ crystals are quite similar. Yb2+ absorption bands are observed at 360, 315, 271, 260, 227 and 214 nm, which are called A, B, C, D, F and G bands, respectively. For γ-irradiated CaF2:30 at.% Yb3+, an additional band at 234 nm can be seen. It is suggested that only a negligible amount of Yb3+ ions are converted into Yb2+ under the γ-irradiation. The presence of Yb2+ is confirmed by the 565 and 540 nm luminescence under 357 nm excitation. It is also suggested that the excitation in the A, C, D and F absorption bands of Yb2+ gives rise to photo-ionization of Yb2+ ions and electrons in the conduction band to form the excited Yb3+ ions which emit IR Yb3+ luminescence.The UV absorption and emission spectra obtained for γ-irradiated and H2-annealed crystals have different structures. This suggests that different mechanisms are responsible for the creation of Yb2+ ions. γ-irradiation favours Yb2+ isolated centres by reduction of Yb3+ ions located at Ca2+ lattice sites, whereas annealing in hydrogen favours Yb2+ centres neighbouring Yb3+ ions when a Yb3+ ion pair captures a Compton electron. Also, γ-irradiation does not change the position of Yb3+ ions converted into Yb2+ in the CaF2 lattice. In the case of H2 annealing, a Yb3+ ion converted to Yb2+ is shifted to the Ca2+ position in the lattice.

https://iopscience.iop.org/article/10.1088/0953-8984/17/25/005/pdf

Optical study of Yb3+/Yb2+ conversion in CaF2 crystals

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Papers

Crystal growth, Yb3+ spectroscopy, concentration quenching analysis and potentiality of laser emission in Ca1−XYbXF2+X

Spectroscopic properties, concentration quenching, and prediction of infrared laser emission of Yb3+-doped KY3F10 cubic crystal

Growth of Yb3+-doped KY3F10 concentration gradient crystal fiber by laser-heated pedestal growth (LHPG) technique

Optical study of Yb3+/Yb2+ conversion in CaF2 crystals