NATIONAL INSTITUTE for LASER, PLASMA and RADIATION PHYSICS
  LABORATORY of SOLID-STATE QUANTUM ELECTRONICS

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
DIRECTOR DE PROIECT:     Dr. Voicu LUPEI
PROJECT MANAGER:         Dr. Voicu LUPEI
Email:                                 voicu.lupei@inflpr.ro
 

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

Contract value:                    1.500.000 lei

Valoare contract:                 1.500.000 lei

The research team:              LUPEI Voicu, PhD

Echipa de cercetare:            LUPEI Aurelia, PhD

                                          GHEORGHE Cristina Petruta, PhD

                                          ACHIM Alexandru, PhD

                                          VOICU Flavius Marian, PhD student

                                          HAU Stefania, PhD student

                                          CHIRCUS Laurentiu, subinginer

 
 

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http://rawmedcol.com/
     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.
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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.

Stage 2, 2013
Value: 221.230 lei

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 4, 2015
Value: 139.150 lei (according to Act Aditional 2015)

1. Sensitization of emission in solid-solution systems.

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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ReSULTS IN 2012-2015
The activities covered specific tasks of the planed stages and additional investigations connected to recent tendencies evidenced in literature, correlated to the project.

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
.

2. Preparation of ceramic samples
Undoped or doped translucent ceramics of garnet structure (YAG, Y3ScxAl5-xO12) and sesquioxides (Y2O3, Lu2O3, Sc2O3) with well developed grains of fairly uniform distribution of sizes have been produced by the technique of solid-state reaction. The process involves homogeneous mixing of raw materials, spray drying granulation, thermal synthesis, preliminary isostatic compression compacting, and sintering. XRD confirmed the garnet or sesquioxide single-phase of these ceramics. Several other ceramics (YAG, GSGG, CLNGG, CLNTGG) were obtained in a cooperation with World Lab. Co., Nagoya, Japan.

3. Spectroscopic properties of potential doping ions
3.1. Laser active ions
3.1.1. Ordered systems
A. Nd concentration and excitation intensity effects on quantum efficiency and heat generation in Nd:YAG.
    Emission decays of Nd-doped YAG crystals and ceramics are strongly influenced by the Nd concentration and the excitation intensity. The departures from exponential decay evidence the presence of self-quenching of emission by down- or upconversion of excitation from the emitting level due to direct and migration-assisted energy transfers. Modeling of emission decay shows that at weak pump intensities the self-quenching is dominated by down-conversion, but at high excitation the of down- and of upconversion are active, and evidence that:

- Non-radiative de-excitation processes reduce the emission quantum efficiency, as calculated with the energy transfer parameters inferred from the decays;

- Quantum efficiencies can be used for estimation of the heat generated by the non-radiative processes involved in self-quenching.
;
- Comparison of experimental data with the calculations confirm that upconversion alone cannot describe correctly the emission quantum efficiency and heat generation in Nd-doped laser materials.

B. Spectroscopic properties and laser emission under direct pumping of Nd-doped radiation-resistant gadolinium-scandium-gallium-garnet (GSGG) crystals.

This research has revealed that:

- 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 (Z2R1 and Z3R2) increases considerably the absorption efficiency and thermal stability of the direct pumping into the emitting level;

- Diode laser pumping in this transition has advantages compared to the traditional pumping at 808 nm in the level 4F3/2 were demonstrated for continuous-wave and active (acousto-optic) or passive (Cr4+:YAG as saturable absorber) Q-switched laser emission.

C. Pr3+ in YAG.
    Optical spectra of Pr3+ in  YAG crystals and ceramics show complex satellite structures around the zero-phonon main lines that are currently attributed to ions in unidentified minority sites. From the high resolution spectral data at different temperatures (10-300 K) especially in Pr3+ 3P03H4, 1D23H4 transitions, the main structural satellites that accompany main zero-phonon lines were assigned to Pr3+ nonequivalent centers, i.e. Pr3+ in Y3+ c-sites with crystal field perturbed by irregularities of lattice near it, such as antisites Y3+ (in octahedral sites occupied by Al3+ a) (up to 3 P lines, with negligible intensities in ceramics) or nearby Pr3+ ions (M n.n and n.n.n pair lines. Identification of these perturbed centers removed the ambiguity in identification of several lines that were previously attributed to transitions forbidden for the Pr3+ ions in the D2 symmetry of the site occupied by Pr3+ in YAG  ceramics and which forced the assumption of lowering of symmetry.

D. Sm3+ in Y2O3.
    Time-resolved optical emission and excitation spectroscopy of Sm3+ in Y2O3 ceramics enabled characterization of the strong electric-dipole transitions for the ions entering in the C2 sites and evidenced also magnetic-dipole transitions for the ions occupying the inversion C3i centers. A clear separation of spectra corresponding to the two classes is facilitated by the large differences in the emission lifetimes of 4G5/2 level, 1.48 and respectively 8.4 ms. The identified structure of the Stark levels evidences much stronger crystal field splittings in case of C3i than in C2 centers.

3.1.2. Laser ions in solid-solution disordered systems.
A.
Composition and temperature effects in Nd3+ spectra of scandium-aluminum garnets

New spectroscopic data were obtained from investigation at different temperatures (10-300K) of disordered translucent ceramic of scandium-aluminum garnets Y3ScxAl5-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 multicenter structure was analyzed by development of a statistical model based on random distribution of the Sc3+ and Al3+ ions in the garnet lattice. x=1 composition contains the larger contribution of the various centers and has largest bands width, the ceramic Y2ScAl4O12 shows the largest potential for short-pulse or tunable laser emission. It was also demonstrated that intercenter energy transfer cannot explain double wavelengths laser emission generation in this system.

3.1.3. Intrinsic compositional disordered systems.
A. Spectroscopic investigation of Nd3+ and Yb3+ in intrinsic disordered CLNGG crystals and ceramics.

It was demonstrated that in calcium lithium-niobium-gallium garnet (CLNGG):

- 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;

- The broad emission bands in these materials can be utilized for ultrashort laser emission by mode-locking, as demonstrated recently by other research groups.

3.2. Spectroscopic properties of ASE suppressing systems

3.2.1 Sm3+ in garnets and sesquioxides

    Investigation of spectroscopic characteristics of Sm3+ in YAG confirmed the prospect of this system for suppressing the ASE of Nd:YAG and its utility for side-pumped lasers with clad-core monolithic composite laser rods. Our investigation enabled a correlation of the Nd3+ emission with the Sm3+ absorption in YAG and Y2O3 at different temperatures. It was experimentally shown that the lines involved in ASE suppression show individually-selective temperature dependent shifts. It was found that in case of Nd3+, Sm3+ can act as suppressor of ASE in YAG at 300 K, but not at low temperatures, whereas in Y2O3 this ability manifests both at 300K and cryogenic temperatures. It was also found that Sm3+ cannot suppress ASE of Yb3+ in YAG, but this would be possible in Y2O3, regardless of temperature. The spectroscopic investigation of Sm3+ in YAG and sesquioxides suggests that this ion could be useful for reduction of solarization in case of broad-band pumped lasers.

3.3. Spectroscopic investigation of ions with d emission bands as potential sensitizer ions
3.3.1.Spectroscopic and dynamic emission properties of Cr3+ in YAG transparent ceramics

Investigation of the optical spectra of Cr3+ in YAG ceramics evidenced:

- 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;

- That the emission decay of Cr3+ accelerates and becomes non-exponential with increasing Cr concentration, behavior that was connected to energy transfer from the isolated Cr3+ ions to Cr3+ pairs and the reduction of emission quantum efficiency for different Cr concentations was estimated.

3.3.2 Multicenter structure in visible emission of Ce3+ in YAG ceramic

    Short (10ns) pulse 532 nm excitation evidenced unexpected
complex temperature dependent 5d→4f spectra and decays that were associated with perturbed Ce3+ centers:

- 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+;

- It was inferred that the unexpected presence of such antisites in ceramics could be favored by the expansion of the octahedral sites of garnets in vicinity of the large doping Ce3+ ions.

4. Electron-phonon effects in RE ions in YAG and sesquioxides spectra
4.1. Thermal shift of Sm3+ in YAG and Y2O3 ceramics
    The analysis of the thermal shift of Sm3+ zero-phonon optical lines in YAG and Y2O3 ceramics has shown that the Sm3+ lines in YAG present small red shifts, whereas in sesquioxides large blue shifts in absorption (up to ~9 cm−1 Y2O3), and blue or red shifts in emission are observed for the C2 centers, while the C3i magnetic-dipole allowed lines exhibit small red shift.
The data (especially blue shifts) cannot be explained by usual dynamic electron-phonon interaction theories and were analyzed in terms of the competition between the dynamic red and the static blue shifts produced especially by the thermal local structural changes, specific to every center and line.

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+ 3P03H4, 1D23H4 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 4G5/24H5/2,7/2,9/2 emission lines at 10K in ceramics were assigned to quasi-resonant electron-phonon interaction between a vibronic and a pure electronic state, with T2g 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 2F7/2  2F5/2 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 C3i.

5.Sensitization of infrared emission under visible - near IR and broad-band (blue, violet,  ultraviolet) excitation
5.1. Sensitization processes
5.1.1. Investigation of sensitization of Nd3+ emission by Cr3+

    By using a large variety of (NdxCry):YAG ceramic samples, the main characteristics of the sensitized process were revealed and allowed the selection of the optimal dopants concentrations within the restrictions imposed by parasitic processes inside the system of active ions. Thus:

- The emission dynamics investigation over larger Cr3+ or Nd3+ concentration ranges (that can be obtained only in ceramics), much superior to those used by other authors, evidenced that the decay of Cr3+ presents a very fast drop followed by non-exponential evolution, while the cinetics of Nd emission under Cr3+ excitation starts with a sudden jump, followed by a slower rise and a long decay;

- The sharp drop of the decay of Cr3+ and the fast jump of Nd3+ emission were attributed to a strong superexchange interaction that governs the direct energy transfer from Cr3+ to the first- and second near Nd3+ neighbors, whereas the slower non-exponential part to direct transfer to farther Nd3+ ions by electric dipole-dipole interactions;

- A weak contribution of the migration-assisted transfer, dependent on both the Cr and Nd concentrations was also evidenced;

- The transfer microparameters and the transfer efficiency, function of concentrations and temperature were evaluated. At the temperatures and Cr and Nd concentrations covered by this study the global de-excitation of Cr3+ remains slower than the intrinsic de-excitation of Nd3+

- The energy transfer efficiency, calculated with the energy transfer parameters inferred from the emission decay increases from 52% for 1 at.%Nd to 77% for 2 at.%Nd and is practically independent on Cr concentration
;
- The characteristics of the energy transfer indicate that that much higher doping concentrations than used earlier in the solar-pumped (Cr,Nd):YAG lasers would be necessary to optimize the sensitization process and the efficiency of these lasers.

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.

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

Raportul stiintific pe anul 2014 este disponibil aici:      | 2014 |
Short resume of the results (2012-2014), in English:    | here|
Raportul stiintific pe anul 2015 este disponibil aici:      | 2015 |


● 
Raportul stiintific pe anul 2016 este disponibil aici:      | 2016 |

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Valorification of results

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I. Papers in ISI journals

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)
.

2. V. Lupei, ”Pump intensity dependence of emission quantum efficiency in Nd-doped materials,” Rom. Reports Phys. 64, 1291-1306 (2012).
3. A. Lupei, C.Tiseanu, C.Gheorghe, and F.Voicu, ”Optical Spectroscopy of Sm3+ in C2 and C3i sites in Y2O3,” Appl. Phys. B. 108, 909-918 (2012).
4. A. Lupei , V. Lupei, and C. Gheorghe, “Thermal shifts of Sm3+ lines in YAG and cubic sesquioxide ceramics,” Optical Materials Express, 3 (10), 1641-1646 (2013).
5. A. Lupei, V. Lupei, and C. Gheorghe, Electronic structure of Sm3+ ions in YAG and cubic sesquioxide ceramics,” Optical Materials 36 (2), 419-426 (2013).
6.
C. Gheorghe, A. Lupei, F. M. Voicu, and C. Tiseanu, ”Emission properties and site occupation of Sm3+ ion doped Lu2O3 translucent ceramics”, J. Alloys and Comp. 588, 388-393 (2014).
7. V. Lupei, N. Pavel, and A. Lupei, ”Improved laser efficiency by direct diode laser pumping of the radiation-resistant Nd:Gadolinium-Scandium-Gallium Garnet,” Laser Physics 24 (4) 045801 (2014).
8. A.
Lupei, V. Lupei, C. Gheorghe, S. Hau, and A. Ikesue, "Multicenters in Ce3+ visible emission of YAG ceramics," Opt. Mat. 37, 727-733 (2014).
9. A. Lupei, V. Lupei, S. Hau, C. Gheorghe, and F. Voicu, "Structure and temperature effects on Nd3+ spectra in polycrystalline mixed scandium aluminum garnets Y3ScxAl5-xO12," Opt. Mat. 47, 465–472 (2015).
10. V. Lupei and A. Lupei, “Nd:YAG at its 50th anniversary:still to learn,” J. Luminesc. 169 (Part B), 426-439 (2016).
11. V. Lupei, A. Lupei, C. Gheorghe, and A. Ikesue, “Emission sensitization processes involving Nd3+ in YAG,” J. Lumin, 170(2), 594-601 (2016).
12. V. Lupei, A. Lupei, C. Gheorghe and A. Ikesue, “Spectroscopic and de-excitation properties of (Cr,Nd):YAG transparent ceramics,” Opt. Mat. Express 6(2), 552-557 (2016).
13. A. Lupei, V. Lupei, S. Hau, Vibronics in optical spectra of Yb3+ and Ce3+ in YAG and Y2O3 ceramics, Optical Materials (2016) http://dx.doi.org/10.1016/j.optmat.2016.06.024

II Papers In Non-ISI Journals

1.
V. Lupei, A. Lupei, “Concentration and pump intensity effects in the emission of Nd laser materials”, Studia Univ. Babes-Bolyai Cluj 60, 31-44 (2015).

III. communications in international Conferences

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 C3i 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 50th 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).


12. V. Lupei, A. Lupei, C. Gheorghe, A. Ikesue, ”Spectroscopic and de-excitation properties of (Cr, Nd):YAG ceramics,9th International Conference on F-Elements, 06 - 09 September 2015, UK, Oxford; (poster presentation).
13. V. Lupei, A. Lupei, S. Hau, C. Gheorghe, F. Voicu, ”Compositional disorder effects in the spectra of Nd3+ in Y3ScxAl5-xO12 ceramics,9th International Conference on F-Elements, 06 - 09 September 2015, UK, Oxford; (poster presentation).
14. V. Lupei, A. Lupei, C. Gheorghe, S. Hau, A. Ikesue, “Dynamics of Sensitization in (Cr,Nd,Yb):YAG Ceramics”, 9th International Conference on Dynamical Processes in Excited States of Solids (DPC’16), Tu 19 July P16 Poster Nr. 15.
15. A. Lupei, V. Lupei, S. Hau, C. Gheorghe, A. Ikesue, Electron-phonon interaction of Pr3+ and Sm3+ in YAG, DPC’16 , Tu 19July Poster Nr. P16.

III. UNPUBLISHED RESULTS
New results will be the subject of further publications; manuscripts are now in various stages of completion.

IV. OTHER IMPLICATIONS

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.

(http://www.cambridge.org/gb/knowledge/isbn/item5940233/?site_locale=en_GB).

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).

Laboratory of Solid-State Quantum Electronics