Preparazione e caratterizzazione di materiali laser a stato solido e loro applicazione nei solar-pumped lasers

Responsabili di progetto

Valentina Biasini, Shadia El Rafie

Accordo

EGITTO - NRC - National Research Centre of Egypt

Bando

CNR-NRC 2016-2017

Dipartimento

Scienze chimiche e tecnologie dei materiali

Area tematica

Scienze chimiche e tecnologie dei materiali

Stato del progetto

Nuovo

Proposta di ricerca

Because of the ever-increasing environmental problems and the upcoming depletion of fossil fuels, the development of sustainable and renewable energy sources is urgently required, one of them being the use of solar energy. Several researchers have investigated the possibilities to directly convert incoherent broadband sunlight into coherent monochromatic laser radiation [1,2]. Compared to electrically powered lasers, solar laser is much simpler and more reliable due to the complete elimination of the electrical power generation and conditioning equipment. One important application of solar pumped lasers is the reduction of magnesium from magnesium oxide, offering a green and economic method for solar energy storage [3].
Among optically pumped lasers, the solid-state lasers are the most attractive for solar pumping because of their compactness, reliability and efficiency. The most important host material for solid state lasers is yttrium aluminum garnet (YAG, Y3Al5O12). Transparent ceramics represent highly promising host materials for solid state lasers; compared to single crystals, the production of ceramics provides a less time- and energy-demanding technology with higher control of doping homogeneity or spatial distribution. Besides, transparent ceramics can be produced in complex geometries. Transparent polycrystalline YAG is therefore currently of interest for use as a solid-state laser material [4].
Neodymium-doped YAG (Nd:YAG) has proved to be one of the best solid-state laser materials for various applications [5] due to its combination of high emission cross section with long spontaneous emission life-time, high damage threshold, good mechanical strength, high thermal conductivity and consequently low thermal distortion of the laser beam; however, the maximum reported absorption for Nd:YAG is low.
Absorption efficiency and optical pumping rate to Nd ions in the solar spectrum can be increased by co-doping Nd:YAG with Cr. According to Saiki et al. [6], Cr-codoped Nd:YAG can enhance absorption in the wavelength range near 400-500nm and 600-700 nm which is inside the solar spectrum. The sensitizer Cr3+ ions have broad absorption bands in the visible region. By the 4T2 to 4A2 transition of Cr3+ ions, energy is transferred from Cr3+ to Nd3+ ions [7]. At low repetition rates, the average output power of Cr:Nd:YAG was higher than that of Nd:YAG [1].
Ohkubo et al. [8] first used a Fresnel lens imaging condenser to focus sunlight as a pump light, and the maximum laser output power reached 80 W. Meanwhile, because Fresnel lenses are widely used as solar concentrators due to their small size, light weight, and low cost, as well as relatively high efficiency from sunlight to laser, it is important to analyze the physical process of solar-pumped lasers with Fresnel lenses.
A crucial factor in the production of transparent ceramics is the homogeneity of the sintered material - especially in terms of composition variations or porosity [9]. The presence of secondary phases can strongly affect the optical properties of the material both as scattering sources, which inhibit the propagation of light through the material, as well as point defects, which heat up during laser action and may lead even to the destruction of the material. The identification of segregated phases and their interaction within the system is of great importance for the optimization of the production process [10].
The project will deal with the production of laser gain media for solar-pumped lasers. The solar pump system will be designed and tested. Cr:Nd:YAG ceramics will be used as laser gain medium; the exact composition will be defined after a detailed literature research. The production process will be divided into the following steps:
- production of Cr:Nd:YAG nanopowder
- shaping of the powder
- thermal treatment of green samples leading to densification
- polishing of sintered materials.
Cr:Nd:YAG nanopowders will be produced and their morphology will be analysed by electron microscopy. The as-synthesized powders will then be shaped into discs 10 mm f and 4 mm thick and sintered in a clean high vacuum furnace, so that the porosity will be removed during the densification process.
The produced samples will be first thoroughly characterized both in terms of microstructural homogeneity and optical quality. Special attention will be paid to the presence of secondary phases and segregation on grain boundaries. The optimum thermal cycles which would minimize the possible segregation of secondary phases will be identified. Optical and laser properties of the materials will be evaluated and compared with the findings of microstructural analysis. This will provide important information about the effect of possible microstructural defects or inhomogeneity on the properties important for the desired applications.
The group at CNR ISTEC has a long experience in the production and characterization of transparent ceramics, especially YAG-based materials [4,9,10]. The main part of the work performed at ISTEC will include shaping and sintering of the nanopowders, polishing of sintered materials and characterization of microstructure and optical quality. The group at NRC will be involved in the production process by synthesizing the Cr:Nd:YAG nanopowders. Optical and laser characterization will be also performed and the good quality materials will be tested in a small-scale solar pumped laser system designed at NRC.
References:
[1] D. Liang et al., Opt. Express 19 (2011) 26399
[2] J.Almeida et al., Opt. Laser Technol. 44 (2012) 2115
[3] T.Yabe et al., J. Phys. Conf. Ser. 112 (2008) 042072
[4] J. Hosta?a et al., Opt. Mater. 35 (2013) 798
[5] A. Ikesue, Opt. Mater. 19 (2002) 183
[6] T. Saiki, et al., Opt. Comm. 282 (2009) 1358
[7] H. Yagi et al., Opt. Laser Technol. 39 (2007) 1295
[8] T. Ohkubo et al., Opt. Lett., 34 (2009) 175
[9] L. Esposito et al., Opt. Mater. 33 (2011) 713
[10] L. Esposito et al., J.Eur.Ceram.Soc. 32 (2012) 2273

Obiettivi della ricerca

The goal of the project is to develop, design, fabricate, test and optimize solid state laser material for the production of laser radiation.
The results will provide a background for the optimization of the production process of Cr:Nd:YAG ceramic with homogeneous dopant concentration from the production to full characterization, from microstructure to optical and laser properties.
Special attention will be paid to the presence of secondary phases and segregation on grain boundaries. This investigation will be performed by SEM-EDS for the identification of possible secondary phases. The optimum thermal cycles which would minimize the possible segregation of secondary phases will be identified.
Optical and laser properties of the materials will be evaluated and compared with the findings of microstructural analysis. This will provide important information about the effect of possible microstructural defects or inhomogeneity on the properties important for the desired applications.

Detailed objectives
1.Thorough literature analysis on fabrication and performance of transparent ceramics as solid state laser materials
2.Preliminary technical-economic comparison of selected procedure to fabricate solid state laser ceramics
3.Production, microstructural characterization and evaluation of optical properties of transparent ceramic gain media
4.Design of the pilot scale of solar pumped laser system.
5.Testing of laser production with solid state laser materials pumped by solar energy

Ultimo aggiornamento: 25/05/2025