INSTITUTE for LASER, PLASMA and RADIATION PHYSICS
LABORATORY of SOLID-STATE QUANTUM ELECTRONICS
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
PROIECTULUI: Ianuarie 2012 -
PROJECT DURATION: January 2012 - December 2016
CONDUCATOR PROIECT: Institutul
Cercetare-Dezvoltare pentru Fizica Laserilor, Plasmei si Radiatiei,
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
General objective: Investigation of new laser materials and quantum electronics processes of interest for sustainable energy production.
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
of infrared emission with blue-violet-ultraviolet
Stage 3, 2014
Value: 162.500 lei
1. Sensitization of emission in systems with intrinsic disorder.
Value: 139.150 lei (according to Act Aditional 2015)
1. Sensitization of emission in solid-solution systems.
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.
_____________________________________________________________________________________________________________________________________________________ReSULTS IN 2012-2015
1. Selection of laser active and sensitizer ions
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.
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 (Z2®R1 and Z3®R2) 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.
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.
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.
3.2.1 Sm3+ in garnets and sesquioxidesInvestigation 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.
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.
- 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.
Vibronics in Pr3+ and Sm3+
in YAG and quasi-resonant electron-phonon
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 4G5/2→4H5/2,7/2,9/2 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.
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.
Nd3+ emission by mixed (Ce-Cr)
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+.
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.
Nd3+ emission by mixed (Ce-Cr)
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.
Sensitized laser emission in
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%.
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.
stiintific pe anul 2012 este disponibil aici:
● Raportul stiintific pe anul 2013 este disponibil aici: | 2013 |
stiintific pe anul 2014 este disponibil aici:
● 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 |
I. Papers in ISI journals
Lupei, A. Lupei, C.
Gheorghe, L. Gheorghe, A. Achim, A. Ikesue, ”Crystal field disorder
the optical spectra of Nd3+ and Yb3+-doped
crystals and ceramics,”
J. Appl. Phys. 112, 063110 (2012).
Lupei, A. Lupei, C. Gheorghe, L. Gheorghe, A. Achim, and A. Ikesue, ”Nd3+
in disordered garnet crystals
and ceramics,” 8th
International Conference on f-Elements” (ICFE8),
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).
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.
Relationship between Materials and Laser Properties” V.
in “Reference Module in Materials Science and Engineering”, S. Hashmi (Ed),
|Laboratory of Solid-State Quantum Electronics|