National Institute For Lasers Physics

CONTRACT 35 / 06.10.2011; PROGRAMUL 'IDEI', PN-II-ID-PCE-2011-3-0822

TITLU: Procese de conversie a excitatiei in surse fotonice cu prospect pentru producere de energie sustenabila

TITLE: Quantum conversion processes of excitation in photon sources of prospect for sustainable energy production

FINANTARE : Unitatea Executiva pentru Finantarea Invatamantului Superior, a Cercetarii, Dezvoltarii şi Inovarii (UEFISCDI), Ministerul Educatiei, Cercetarii, Tineretului si Sportului, Romania
FUNDED BY: UEFISCDI, Ministry of Education, Research, Youth and Sport, Romania

DURATA PROIECTULUI: Ianuarie 2012 - Decembrie 2016
PROJECT DURATION: January 2012 - December 2016

CONDUCATOR PROIECT: Institutul National de Cercetare-Dezvoltare pentru Fizica Laserilor, Plasmei si Radiatiei, Magurele, Bucuresti, Romania
PROJECT LEADER: National Institute for Laser, Plasma and Radiation Physics, Magurele, Bucharest, Romania

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     One of the most challenging directions for scientific research is identification of new sustainable energy sources. The role of the Sun as the main long- or short-time stored, or as real-time source of energy suggests two main directions of research:- energy production by similar processes as in the Sun (nuclear fusion) and new or improved utilization of the solar energy, the transformation of solar energy in new forms of storable energy resources (such as the solar pumped lasers in the solar-magnesium-hydrogen cycle, etc). It is envisaged that the lasers could supply the pulse energy and peak power able to compress and ignite the D-T fuel to achieve nuclear fusion and the solar pumped lasers are also of interest for various technological applications.
    In both these directions the compact and rugged solid-state lasers based on inorganic materials (crystals, glasses, ceramics) doped with rare earths RE or transition metal ions could play an important role. Despite of sustained activity and of the remarkable success, a renewed interest for new or improved laser materials with increased pump absorption and its utilization in the lasing process, and with improved compositional, dimensional, optical, thermal and mechanical properties is observed in the literature.
    The aim of the project is investigation of fundamental and applied aspects of laser materials and quantum electronics processes of interest for sustainable energy production. The laser active materials proposed for investigation in this project are ceramics of cubic simple or complex oxides (ordered materials: garnets such as the yttrium aluminum garnet (YAG) or sesquioxides R₂O₃, intrinsic disordered garnets, mixed solutions) doped with RE³⁺ active ions, such as Nd³⁺ or Yb³⁺. In order to improve the pump absorption, co-doping with sensitizer ions able to absorb efficiently the radiation of the pumping sources (diode lasers, flashlamps, solar radiation) and to transfer nonradiatively the energy to active ions will be investigated. Special attention will be given to investigation of the possibilities to control the spectroscopic properties that determine the performances of laser emission in various regimes by proper selection of the composition of the host material and of doping. The modalities to prevent manifestation of parasitic losses such as thermal effects, the amplification of spontaneous emission (ASE) in case of large laser components or apparition of transient or permanent coloration (solarization) will be also investigated.
The experiments cover the preparation of various translucent or transparent ceramics (some of these in cooperation with Prof. A. Ikesue, World Lab Nagoya), high resolution spectroscopic static and dynamic measurements at different temperatures in order to obtain new data on energy levels, transition probabilities, thermal effects, energy transfer processes, etc, physico-mathematical modeling of the static and dynamic processes, optimization of absorption and emission efficiency, modeling of the laser processes. Selection of the actual composition and details or new aspects of investigation will be adapted to the results of each stage and to the new trends and results reported in literature.

OBJECTIVES
General objective: Investigation of new laser materials and quantum electronics processes of interest for sustainable energy production.

Specific objectives
I. Characterization of sensitization processes in systems with narrow-band emission.
II. Characterization of emission and sensitization processes in systems with broad-band emission.
III. Modeling of sensitized laser emission and evaluation of the potential for high energy or solar-pumped laser emission or for solar radiation converters.

Stage 1, 2012
Value: 685.000 lei
1. Preparation of undoped YAG translucent ceramic samples.
2. Sensitization of infrared emission under visible - near ir excitation.

1. Sensitization of infrared emission with blue-violet-ultraviolet excitation.
Stage 3, 2014
Value: 162.500 lei
1. Sensitization of emission in systems with intrinsic disorder.

Stage 5, 2016
Value: 292.119 lei (according to Act Aditional 2015)
1. Modeling of sensitized laser emission and evaluation of the potential for high energy or solar-pumped laser emission or for solar radiation converters.

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1. Selection of laser active and sensitizer ions and of host materials
The selection was based on data existing in literature and on preliminary investigation of spectroscopic properties of several systems:
- Laser active ions: Nd³⁺ and Yb³⁺;
- Sensitizer ions based on f-f transitions (Nd³⁺ for Yb³⁺), on d-d transitions (Cr³⁺ for Nd³⁺ ) or on d-f transitions (Ce³⁺ for Nd³⁺) for simple or complex (Ce-Cr-Nd, Cr-Nd-Yb) sensitiztion schemes;
- Host materials: simple (garnets such as YAG and cubic sesquioxides Sc₂O₃, Y₂O₃, Lu₂O₃), accidentally-disordered systems, solid-solution compositionally-disordered garnets (GSGG, YSAG), intrinsic compositionally disordered garnets (CLNGG family), complex mixed solution-intrinsic disordered systems (CLTNGG);
- Sm³⁺ In YAG and sesquioxides were investigated in order to assess its potential to suppress the amplification of spontaneous of Nd³⁺ or Yb³⁺ in these systems.

- The high-resolution spectroscopic properties and the de-excitation dynamics of the Nd-doped GSGG (Gd₃Sc₂Ga₃O₁₂) crystals and ceramics are similar;

- The concentration-dependent self-quenching of Nd³⁺ emission is weaker than in YAG, in agreement with the larger host lattice parameter;

- In this material accidental degeneracy at ~883 nm of two absorption transitions (Z₂®R₁ and Z₃®R₂) increases considerably the absorption efficiency and thermal stability of the direct pumping into the emitting level;

New spectroscopic data were obtained from investigation at different temperatures (10-300K) of disordered translucent ceramic of scandium-aluminum garnets Y₃Sc_xAl_5-xO₁₂ (x=0-2) doped with Nd³⁺, important systems for ultrashort (fs) laser pulses or emission at two wavelengths. We explored:

- The composition and temperature effects, revealed by line shifts and lineshape (widths) changes, associated with structural effects induced by larger Sc³⁺ ions replacing Al³⁺ in octahedral sites.

- Multicenter structure (reported for the first time on this system), was connected to the crystal field perturbations connected with the mixed occupancy of the first octahedral coordination sphere by Sc³⁺ and Al³⁺ ions and the inhomogeneous broadening determined by the perturbing effects of the farther octahedral coordination spheres.

- The necessity of electric charge compensation at substitution of the divalent host cation (Ca²⁺) by trivalent rare earth ions (Nd³⁺ or Yb³⁺) imposes modification of composition of the host material function on doping concentration;

- Modification of the composition of the cationic coordination spheres around the doping ion leads to composition-dependent multicenter structure of the optical spectra and to considerable inhomogeneous broadening of the absorption and emission bands;

- The multicenter structure shows that when doping with Nd³⁺ or Yb³⁺ the main centers have (4Nb⁵⁺) or (3Nb⁵⁺, 1Li⁺) ions in the first coordination sphere of octahedral sites around the RE³⁺ ions;

- Negligible intensity of the parasitic perturbed centers specific to the melt-grown crystals and similar spectroscopic properties of the main center Cr³⁺ center in ceramics and crystals;

- New spectral satellites, with relative intensities depending on Cr³⁺ concentration and emission kinetics different from the main center, that were tentatively assigned to Cr³⁺ pairs of different orders;

- The spectral characteristics in the 5d→4f Ce³⁺ emission of these perturbed Ce³⁺ centers were analyzed in terms of the effects of the structural changes induced by Ce³⁺ doping on the interaction with defects, such as residual antisites Y³⁺ in Al³⁺ octahedral sites in ceramics;

- The multisite structure of 5d→4f Ce³⁺ emission was correlated with the structure of the 4f-4f infrared absorption spectra of Ce³⁺;

4.2. Vibronics in Pr³⁺ and Sm³⁺ in YAG and quasi-resonant electron-phonon effects
New spectral data associated to vibronic sidebands in Pr³⁺ ³H₄→¹D₂ 10K absorption or Pr³⁺ ³P₀→³H₄, ¹D₂→³H₄ emission spectra of YAG crystals and ceramics were obtained, and mechanisms determining these spectral characteristics were analyzed. The asymmetry or splittings of some of Sm:YAG ⁴G_5/2→⁴H_5/2,7/2,9/2 emission lines at 10K in ceramics were assigned to quasi-resonant electron-phonon interaction between a vibronic and a pure electronic state, with T_2g Raman phonons involved.

4.3. Effects of electron-phonon interaction in Yb³⁺ in YAG and Y₂O₃ and electronic structure.
    The infrared Yb³⁺(4f¹³) spectra are generally assigned to electronic ²F_7/2 → ²F_5/2 transitions accompanied by relatively large vibronics, but an unambiguous separation of the Stark levels is difficult and there are a series of energy levels schemes proposed for Yb-in YAG and Y₂O₃.
    In contradiction with these models, recently, a new interpretation of the Yb spectra has been proposed (V.Solomonov et al, J. Lumin. 169 (2016) 151) that exclude arbitrary the vibronics from infrared Yb³⁺spectra and part of Yb peaks in YAG or Y₂O₃ ceramics are associated to Yb²⁺, with an assumed ground state 4f¹³6s. Elimination of ambiguities that can be induced by these interpretations is essential from fundamental point of view, as well as for the technological control of ceramics. Indeed, it is well known that Yb²⁺ is present in not adequately annealed ceramic samples, but the ground state is 4f¹⁴ with spectra in visible.
    New spectral data for Yb in YAG si Y₂O₃ ceramics, recorded in proper experimental conditions, and modeling in terms of electron-phonon coupling enabled a more accurate identification of the electronic levels for Yb in YAG and C₂ centers in Y₂O₃, providing arguments that infrared spectra of Yb-in YAG and in Y₂O₃ are due to exclusively to Yb³⁺. It was evidenced that the vibronic features extend the absorption and emission ranges beyond the limits delineated by the pure (ZP) electronic transitions, that allows the elucidation of the nature of some small features observed in Yb³⁺ spectra of YAG and Y₂O₃. Measurements of optical spectra of Yb- and (Nd,Yb)- Y₂O₃ ceramics under direct excitation in Yb³⁺ or via energy transfer from Nd³⁺ allow identification of Stark level structures of C_3i.

5.1.2. Nd³⁺ emission by mixed (Ce-Cr) sensitization
The strong and broad absorption bands at 340 and 465 nm of Ce³⁺ in YAG and the emission band at 540 nm would enable efficient sensitization of Nd³⁺ under solar pumping. Addition of Cr³⁺ contrites to further improvement of absorption and of sensitization process. Our study shows that in this case Ce³⁺ sensitizes Nd³⁺ both by direct energy transfer and by the chain Ce³⁺→Cr³⁺→Nd³⁺.

5.1.3. Sensitization of Yb³⁺ emission by Nd³⁺ In CLNGG
High-resolution spectroscopic investigation of compositionally disordered calcium niobium gallium lithium garnets - CLNGG doped with Nd³⁺ or Yb³⁺and modeling enabled the correlation of the broadening effects of the lines with the actual composition of the material, which are similar in laser crystals and ceramics. Our spectroscopic studies have shown that:
- For Yb:CLNGG, the emission band width shows potential for generation of pulses in the range of 50 fs, demonstrated experimentally subsequently by other research groups. Our investigation evidences that the Yb³⁺ emission linewidth in a solid solution of CLNGG with CLTGG (niobium replaced by tantalum) is with ~ 20% larger than for CLNGG and one could estimate pulses of ~40 fs;
- Such systems with intrinsic disorder could be appropriate for improvement of sensitization of Yb³⁺ emission by energy transfer from Nd³⁺ in (Nd, Yb) codoped ceramics, due to larger overlap of Nd³⁺ emission and Yb³⁺absorption than in ordered garnets, such as YAG;
- The sensitization of Yb³⁺ emission by Nd³⁺ in different co-doped (Nd, Yb):CLNGG crystalline or ceramic samples was evidenced in Yb³⁺ infrared emission spectra and decays under excitation in Nd³⁺ absorption bands. Our studies show that the sensitized emission spectra of Yb³⁺ contain contribution from all structural centers;
- The emission decay of Nd³⁺ accelerates in presence of Yb³⁺, whereas the rise-time of the characteristic Yb³⁺ emission kinetics when pumping into Nd³⁺ decreases and at high Yb concentrations it resembles the intrinsic decay of Yb³⁺;
- For large doping concentrations in CLNGG the Nd→Yb energy transfer, leads to a global efficiency of energy transfer close to ~100%, i.e. larger than the 93% transfer efficiency at the same Yb³⁺ concentration in YAG.

5.1.4. Nd³⁺ emission by mixed (Ce-Cr) sensitization
The efficient Cr³⁺→Nd³⁺ and Nd³⁺→Yb³⁺ energy transfer processes in YAG suggest a complex Cr³⁺→Nd³⁺→Yb³⁺ sensitization chain which would enable construction of efficient high energy Yb lasers under flash lamp pumping. Investigation of emission decay of these three ions at different doping concentrations in YAG ceramics evidences that contrary to expectation, the Cr³⁺ ion can also sensitize directly the emission of Yb³⁺, leading to further improvement of efficiency.

5.2. Sensitized laser emission in (Cr, Nd):YAG
    Modeling of sensitized laser emission shows that with properly selected concentrations for the sensitizer and activator ion the laser emission could be substantially improved. Sensitization modifies the small-signal gain to an extent dependent on the pump absorption by sensitizer and to the efficiency of the energy transfer to the active ion.
    In case of the Nd³⁺ emission in (Cr,Nd):YAG for solar pumped lasers, the performances are limited by the low doping concentration of Cr (typically 0.1 at.%) and of Nd (1 at.%). Examination of the spectroscopic properties for (Cr,Nd):YAG ceramics and calculations show that increasing Cr concentration to 0.7-1 at.% would improve considerably (to 5 times) the pump absorption. Increased Nd concentration enhances the energy transfer efficiency.
    Based on the energy transfer processes characteristics it was estimated that raising the Nd concentration could keep the threshold unchanged, but it will increase the slope efficiency by ~48%.

5.3. Heat generation in sensitized systems
Heat generation by parasitic non-radiative de-excitation can be a limiting factor for power scaling of the solar- or flashlamp pumped lasers. The macroscopic factors that characterize this process are the quantum defect between the pump and emitted quanta and the emission quantum efficiency of the laser ion. In case of sensitized emission additional factors are present.
Calculations show that in case of the Cr³⁺ sensitized Nd³⁺ emission in YAG the larger quantum defect increases the heat load coefficient that expresses the fraction of absorbed power transformed into heat under solar pumping compared with direct pumping of Nd relatively modestly, by ~4% for 1 at.% Nd and by ~17% for 2 at.% Nd. However, increasing the pump absorption would enhance the heat power: for 1 at. % Cr the heat power would be about 5-6 times larger than for 0.1 at.% Cr. Thus, the same factors that contribute to enhanced laser emission parameters for the sensitized systems would stimulate heat generation and thus special care to dissipate the generated heat and to control the distribution of the thermal field in the laser materials would be necessary.

● Raportul stiintific pe anul 2012 este disponibil aici: | 2012 |
● Raportul stiintific pe anul 2013 este disponibil aici: | 2013 |

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1. V. Lupei, A. Lupei, C. Gheorghe, L. Gheorghe, A. Achim, A. Ikesue, ”Crystal field disorder effects in the optical spectra of Nd³⁺ and Yb³⁺-doped CLNGG laser crystals and ceramics,”
J. Appl. Phys. 112, 063110 (2012).

1. V. Lupei, A. Lupei, C. Gheorghe, L. Gheorghe, A. Achim, and A. Ikesue, ”Nd³⁺ and Yb³⁺ in disordered garnet crystals and ceramics,” 8th International Conference on f-Elements” (ICFE8), 25-31 August 2012, Udine, Italia, presentation OPT 26P.
2. A. Lupei, C. Tiseanu, C. Gheorghe, and F. Voicu,^”Spectroscopic analysis of Sm³⁺ in C₂ and C_3i sites of Y₂O₃,^” 8th International Conference on f-Elements” (ICFE8), 25-31 August 2012, Udine, Italia, presentation OPT 23P.
3. A. Lupei, C. Tiseanu, and C. Gheorghe, ^”Electronic structure and energy transfer processes of Sm³⁺ in sesquioxides,” 3rd International Conference on the Physics of Optical Materials and Devices (ICOM 2012), 3 - 6 Sept. 2012, Belgrad, Serbia, Book of abstracts, ISBN: 978-86-7306-116-0, pg. 144.
4. C. Gheorghe, A. Lupei, F. Voicu, and C. Tiseanu, ”Sm³⁺ emission from different sites in Lu₂O₃ ceramics,” 3rd International Conference on Rare Earth Materials (REMAT) Advances in Synthesis, Studies and Applications, Wroclaw, Poland, 26-28 April 2013.
5. F. Voicu, A. Lupei, C. Gheorghe, C. Catalin, and M. Dumitru, ”Sm doped YAG and sesquioxides transparent ceramics,” International Conference "Modern Laser Applications" Third Edition, INDLAS 2013, 20-24 May 2013, Bran, Romania, presentation O11.
6. V. Lupei, “Selfquenching of Emission and Heat Generation in Nd Lasers Revisited,” Advanced Solid-State Lasers, 27 Oct. - 01 Nov. 2013, Paris, Franta, Poster AM4-A.13.
7. A. Lupei, V. Lupei, C. Gheorghe, A. Ikesue, and F. Voicu, “Thermal effects on Sm³⁺doped ceramic laser materials for ASE suppression,” Advanced Solid-State Lasers, 27 Oct. - 01 Nov. 2013, Paris, Franta, Poster AM4-A.02.
8. V. Lupei, A. Lupei, C. Gheorghe, and A Ikesue, “Sensitization processes of Nd³⁺ and Yb³⁺ doped YAG ceramics for broadband pumped lasers,” 9th Laser Ceramics Symposium (LCS), Dec. 2-6, 2013, Daejeon, Korea.
9. V. Lupei, A. Lupei, C. Gheorghe, A. Ikesue, and F. Voicu, “Suppression of Nd and Yb ASE by Sm absorption in ceramics,” 9th Laser Ceramics Symposium (LCS), Dec. 2-6, 2013, Daejeon, Korea.
10. V. Lupei and A. Lupei, "Nd:YAG at its 50^th anniversary: still to learn," 17th International Conference on Luminescence and Optical Spectroscopy of Condensed Matter (ICL2014), 13-18 July, 2014, Wroclaw, Poland; presentation I 31 (invited lesson).
11. A. Lupei, V. Lupei, C. Gheorghe, S. Hau, and A. Ikesue, “Perturbed centers in visible emission of Ce³⁺:YAG ceramic,” 17th International Conference on Luminescence and Optical Spectroscopy of Condensed Matter (ICL2014), 13-18 July, 2014, Wroclaw, Poland; presentation P 25 (poster presentation).

1. The approach, method of investigation and the obtained results were useful in consolidation of the conclusions and identification of the general trends of this field of research, exposed in the recent (June 2013) book ”Ceramic Lasers” by A. Ikesue (the inventor of transparent ceramic laser materials), Y. L. Yang and V. Lupei, Cambridge Univ. Press.

2. Chapter “Laser Materials: Relationship between Materials and Laser Properties” V. Lupei in “Reference Module in Materials Science and Engineering”, S. Hashmi (Ed), Oxford, Elsevier (2016).