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RECHARGING PROCESSES OF ACTIVE IONS AND RADIATION DEFECTS

RECHARGING PROCESSES OF ACTIVE IONS AND RADIATION DEFECTS IN SOME LASER CRYSTALS DOPED WITH RE AND TM S.M. Kaczmarek 1 , G. Boulon 2 , T. Tsuboi 3 1 – Institute of Physics, Szczecin University of Technology, 48 Al. Piastow, 70-310 Szczecin

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RECHARGING PROCESSES OF ACTIVE IONS AND RADIATION DEFECTS

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  1. RECHARGING PROCESSES OF ACTIVE IONS AND RADIATION DEFECTS IN SOME LASER CRYSTALS DOPED WITH RE AND TM S.M. Kaczmarek1, G. Boulon2, T. Tsuboi3 1 – Institute of Physics, Szczecin University of Technology, 48 Al. Piastow, 70-310Szczecin 2 - Physical Chemistry of Luminescent Materials, Claude Bernard /Lyon 1 University, UMR, France 3 - Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-ku, Kyoto 603-8555 , Japan Solid state laser systems based in space are exposured to charged particles: electrons, protons, high energy cosmic rays, and bremsstralung photons. All these forms of radiation can damage the laser by ionizing constituent atoms in the gain medium.

  2. Content 1. Introduction 2. Garnets: YAG (pure, Nd, Cr, Er, Yb), YAP (Er, Pr, Nd), GGG (Nd, pure) 3. Galates: SrLaGa3O7 (Cr, Co, Dy), SrGdGa3O7 (Cr), Mg2SiO4 (Cr) 4. Perovskites: LiNbO3 (pure, Cr, Cu, Fe, Yb, Yb+Nd, Yb+Pr) 5. Fluorides: CaF2 (Yb, pure), LiLuF4 (Yb), YLiF4 (Yb), KY3F10 (Yb) BaY2F8 (Yb) 6. Li2B4O7 (pure, Mn, Co) single crystals and glasses (pure, Mn) 7. Conclusions

  3. YAG Nd:YAG

  4. -The shape of the additional absorption is almost of the same type for pure YAGand • doped with Nd for all types of the irradiation: g-rays, electrons and protons, • -Three at least color centers one can recognize: Fe3+ , Fe2+ and F-type with maxima at: 255, • 276, 300, 385 (440 pure YAG) nm, respectively. For crystals annealed in the air additional 586 • nm band is observed. With an increase of the g-dose from 102 to 107 Gy, fluencies of • electronsfrom 1014 to 1016 particles/cm2 and of protons from 1012 to 1016 particles/cm2, • values of AAbands became higher and higher. • The dependence of the additional absorption on the irradiation dose shows a tendency • to asaturation in case of g-rays. • -Protons fluency dependence of the additional absorption exhibits characteristic shape • with minimum at about 1014 protons/cm2. Such non-monotonic dependence is characteristic • for color centers rather than for Frenkel ones. For the latter centers, a monotonic, linear with • proton fluency dependence is observed. • -It seems that for electrons, ionization fraction is lower than for protons • -Annealing in the air leads to the increase in Fe3+ ions content in the crystals. Annealing in the • air at 673K for 3h seem to be high enough to receive • starting optical properties of g-irradiatedcrystals • -Annealing in hydrogen give almost the same shape • of the additional absorption as in case ofg-irradiation • - Lower susceptibility to electron and g-irradiation • reveal Nd:YVO4single crystals

  5. Nd:YAG laser • All forms of the irradiations: exposure to 60Co gamma rays, over threshold electrons (1 MeV) and high energy (20 MeV) protons and annealing in hydrogen create almost thesame damage centers which reduce optical output by absorbing of laser emission. • - Gamma irradiation lowers the slope efficiency of pulsed laser. After subsequent pulsesthe output energy of the laser increases to the level, which comes out from the thermalequilibrium of rod being the heated by pumping pulses, and, air cooled. This increase ofthe laser energy after subsequent pumping pulses suggests that UV contained in thepump spectrum causes heating up the rod and accelerates those relaxation processeswhich decrease the AA.

  6. Cr:YAG Er:YAG Cr,Tm,Ho:YAG

  7. Er:YAG Cr,Tm,Ho:YAG • -The obtained results point to the direct influence of the color centers on the processes of • formationof the inverse population of the energy levels of Er: YAG, Cr, Tm, Ho: YAG (positive) • lasers. Gamma irradiation leads to the formation of color centers which transfer energy of • excitation to excited laser level and also to an increase in active impurity concentration and thus • luminescence intensity. • The type of introduced CC strongly depends on the starting defect structure determined by • Growthconditions or annealing in the reducing or oxidizing atmosphere (see Cr:YAG AA spectra) • -Changes in the active dopant concentration are observed after all the types of irradiations: g-rays • electrons and protons in Cr:YAG and Cr,Tm,Ho:YAG crystals • -From AA of proton irradiated crystals there can be distinguished two dose ranges: (1) fluencies • less than 5*1014 cm-2 where recharging effects dominate and, (2) fluencies larger than 5*1014 cm-2 • where the presence of Frenkel defects is expected.

  8. Yb:YAG -Important in diode pumped high power laser systems: used sometimes in orbital space missions, ranging systems -Important for solar neutrino detection: a prompt electronplus a delayed gamma-signal is the signature of a neutrino event: scintillator is designed to work in the strong external fields of ionizing radiation - Due to both requirements it is important to study the ionizing effects in Yb:YAG crystals - The changes after g-irradiation are mainly related to the charge exchange Fe3+- Fe2+, F-type centers and Yb2+ ions arising as an effect of recharging of Yb3+ ions from pairs

  9. Er:YAP Nd:YAP Pr:YAP

  10. YAP -Important in developing of LD pumped lasers, promising as fast scintillators that exhibit very short fluorescence decay with time constant 1-100 ns, -Growth atmosphere (inert) leads to the presence of oxygen vacancies; there are present also uncontrolled dopants in the crystal and cation vacancies, -Changes after gamma and proton irradiations are mainly related to the charge exchange of Fe2+ , Fe3+ (234-260, 303-315 nm), cation vacancies and F-type centers (385 nm) [F+ →Vo+e-, F→Vo+2e-], -Annealing in the air at 673 K for 3h is enough to receive starting optical absorption of the crystal, annealing in the air at 1073 K introduce additional defects (430 nm band); annealing in the air at 1673 K introduce some additional defects (260, 358, 487 nm), annealing in hydrogen at 1473 K fully clear (bleaching) the crystal, -YAP crystals seem to be resistant to proton irradiation especially for doping with Er; saturation one can observe in the AA change as a function of proton fluency, -Increase in Pr3+ concentration from 0.5 to 3% leads to the three fold decrease in the value of AA, -Generally there are not observed distinct changes in the valence states of active dopants in the crystal. Nd:GGG -Three main centers arises after g-irradiation: 255, 340 and 465 nm being attributed to: the presence of Ga and O vacancies as well as Fe ions (255 nm), Ca2+F+ complex centers and hole O-centers (340 nm), and, F-centers (465 nm). Annealing in the air increase an amount of Fe3+ ions and new one 400 nm centers are creating. UV irradiation forms only first two centers but of the same intensity. Protons less influence the crystal than YAP and YAG.

  11. Nd:GGG

  12. SrLa(Gd)Ga3O7 5T2-5E 1223 nm, Co3+

  13. SLG, SGG - They appear to be promising active laser materials. They exhibit, however, strong changes in absorption and luminescence spectra under irradiation by ionizing particles. - Color centers, which appeared after g and proton irradiation (290 nm), shift the short-wave absorption edge towardsthe longer wavelengths by a few hundreds nm. They are probably attributed to the Ga2+ centers that are formed according to reaction Ga3++e-Ga2+ with a spin S=1/2, g|| = 1.9838(5)and g = 2.0453(5). The second type center arises in the AA spectrum at about 380 nm and is attributed to F-centers. - In Cr and Co doped SLG and SGG crystals beside the above CC, recharging of chromium and cobalt ions is observed after both types of the irradiation Forsterite and YAG:Cr - Gamma irradiation recharges both Cr3+ and Cr4+ ions, moreover, there arises color centers, observed between 380 nm and 570 nm, that may participate in energy transfer of any excitation to Cr4+ giving rise to Cr4+ emission. The g-irradiation leads to increase in intensity of excitation spectra.The 380 nm additional absorption band is assigned probably toCr6+ ions of 3d0 configuration or more probably to O-- hole centers and/or F-centers. The 570 nmband may be assigned to F+ color centers, - In the absorption spectrum of g-irradiated crystal we observe 275 nm additional band that may be interpreted as a valence change of Si4+ ions due to capture of electron coming from ionization of an O2- ion, - If conditions of optimal Cr doping content and optimal oxygen partial pressure can not be satisfied, one can deal with annealing in O2 to increase of Cr4+ emitting centers and, after that, with g-irradiation of the crystal - The observed behavior of the absorption spectrum of YAG:Ca, Crannealed in the air crystal under influence ofg-irradiation suggests that g-irradiation ionizes only Cr ions.

  14. Cr: Mg2SiO4 and Y3Al5O12

  15. LiNbO3 LN:Fe

  16. LN:Cu LN:Cu

  17. LN:Cr

  18. - OH- absorption do not exclude substitution of both octahedral sites: Nb and Li in all of the investigated crystals, especially in case of Pr doping, • - Annealing at 400oC and 800oC discover two different initial optical states, • - It had been observed rather unexpectedly that classical thermal annealing can lead to a decrease in optical homogeneity in the majority of cases. It may be attributed to generation of an internal electric field by the pyroelectric effect, and to the electrooptic effect involved thereafter. • The secondary electrons which are homogeneously generated by gamma or proton irradiation in the investigated crystals are believed to increase the optical homogeneity, also by canceling this field. Birefringence dispersion seems to be a good key parameter in manufacture of e.g. retardation plates, 2nd harmonic generators or polarizers, • - In the additional absorption of LINbO3 single crystals irradiated with gamma and protons therearises at least two additional bands peaked at about 384 (F-type color centers ) and 500 nm (Nb4+ - Nb4+ bipolarons ). After annealing process additional absorption arises near 650 nm (polarons Nb4+), • - One can observe changes in the concentration of TM active ions (Fe2+, Cu2+ and Cr3+) after the • Irradiations (recharging of active ions), • In fluency dependence of additional absorption at least three regions are seen. First one forfluencies below 1014 cm-2(recharging effects), second one for fluencies between 1014 and 5*1014 cm-2(mutual interaction of the cascades from different proton trajectories) and third oneover 5*1014 cm-2(Frenkel defects), • - Polarimetric measurements have shown that LN:Cu crystal exhibit strong susceptibility to proton irradiation. Even for such small fluencies as 1013 cm-2 the observed changes in polarimetric image and BRD coefficient are very significant.

  19. Cu:LiNbO3 (0.06at.%) Cu:LiNbO3 (0.07at.%) Annealed 1013 prot cm-2 1015 prot cm-2 1013 prot cm-2 Protons:Cu: LiNbO3 wafers

  20. LN:Yb LN codoped

  21. LN:Yb, Pr ZY ZX - In the co-doped crystals or crystals with large dopant concentration, two kinds of Yb3+ ions may be present, one is Yb3+ accompanied by nearby rare-earth ion – perturbed Yb3+, the other is Yb3+ located far from the rare-earth ion – isolated, - The same kind of the CC arises in LN crystals doped with RE ions (384 and 500 nm). Irradiation of the LN:Yb and LN:Yb, Pr crystals reveal IR AA suggesting the presence of Yb pairs, - Yb3+ ion is substituted for Li+ ion with small ionic radius of 0.74 nm, while Pr3+ ion with large ionic radius of 1.013 nm is substituted for Nb5+ ion with much smaller ionic radius of 0.64 nm, - The peak position of the sharp line cantered at 980 nm is different among LN crystals doped with rare-earths, its intensity strongly depends on the temperature, - From the angular variations of the EPR spectra it results Yb3+ ions of C1 symmetry arise in the crystal (170Yb, 173Yb), temperature dependence of EPR signal shows maximum at low temperatures (6K) suggesting pair presence of RE ions.

  22. Absorption and emission spectra after g-irradiation for Ca0.995Yb0.005F2.005 crystallized by simple melting

  23. Absorption spectra under hydrogen processing for Ca0.995Yb0.005F2.005 crystallized by simple melting

  24. EPR spectra: CaF2:Yb3+ 5%

  25. - Annealed in hydrogen and g-irradiated CaF2:Yb crystals show the presence of additional UV bands characteristic of Yb2+ absorption, - AA intensity value has been observed much higher for g-irradiated crystal and strongly dependent on the gamma dose, - Differentare mechanisms of Yb2+creating under g-irradiation and annealing in hydrogen. The latter favors Yb2+isolated centers by reduction of Yb3+ ions located at Ca2+ lattice sites whereas the former favors Yb2+ centers being neighboring to Yb3+ ions when one Yb3+ ion pair captures a Compton electron. As compared to the annealed crystal, g-irradiation does not change the position of Yb3+ ions being converted to Yb2+ one in CaF2 lattice. In case of the annealing in hydrogen the cluster is probably destroyed under the influence of temperature and Yb3+ ion being converted to Yb2+ one is shifted to lattice Ca2+ position. - Temperature dependence of EPR spectra shows agreement with the Curie law for most of the lines, - EPR spectra show Yb3+ as isolated ions, but temperature dependence of the linewidth suggests the presence of Yb3+ - Yb2+ interacting pairs after g-irradiation. Peak-to-peak linewidth changes continuously within 20 mT range for “as-grown” crystals, while reveals distinct increase above 25 K forg-irradiated ones. It suggests strong ferromagnetic coupling between neighbours Yb3+ and Yb2+ ions the latter being created due to Compton electron capture. So, Yb3+ co-exists with Yb2+ after the Yb3+-Yb2+ conversion under influenceof g-irradiation and/or annealing in hydrogen

  26. Absorption spectra under g-irradiation for other fluorides LLF:Yb, YLF:Yb BYF:Yb, KYF:Yb

  27. Absorption spectra under g-irradiation for other fluorides

  28. Fluorides • - g-irradiation introduce some radiation defects: • LLF – 315 nm (F-centre), 240 nm and 380 nm (perturbed Vkcenters, 520 nm (F2+ centers), • 600 nm (N2 center) • YLF – 260, 330, 440, 505 and 640 nm, additionally 520 nm • CaF2 – 280, 380, 430, 560, 760 nm • Doping with Yb generally reduce total induced absorption, the higher is Yb concentration, the • lower is induced absorption, the intensity of the F-center significantly decreases, new centre at • 340 nm (Yb2+ centre) arises – competition of Yb3+ ions with F vacancies in capturing free • electrons arising after g-ray irradiation. Yb2+ centers induced in LLF, YLF, CaF2 and BYF • crystals doped with Yb3+ are related to Yb3+. Only Yb2+ centers in KYF arise at the expense of • the Yb3+ isolated centers. • Yb: CaF2 – 214, 225, 237, 257, 272, 310, 360 nm • - Conversion from Yb3+ to Yb2+ under annealing in reducing atmosphere is observed only for • middle ytterbium concentrations (5-10%), when isolated Yb centers dominate over Yb pairs, • gamma induced bands we associated with accompanied Yb2+ centers disappear after • annealing in H2 at 903 h for 1h but isolated ones arises • - Curing influence of H2 annealing on point growth defects is clearly observed • - Excitation of induced Yb2+ bands give rise to photoionization of of Yb2+ ions and electrons • in the conduction band to form the excited Yb3+ions which emit IR Yb3+ luminescence.

  29. Li2B4O7:Mn glass Changes in the absorption and emission spectra at the course of time

  30. L5 a) b) c) EPR spectrum of LBO:Mn glass at room temperature: a – “as-grown” sample, b – irradiated with gamma, and c - annealed at 400 oC in the air for 4h, n = 9.389 GHz a) b) c) EPR spectrum of LBO: Mn crystal at room temperature along Z axis (Z||B): a) – a sample measured before annealing treatment, b) – after irradiation with g-rays, c) – after annealing in the air at 673 K for 4h

  31. LBO:Co crystal LBO:Mn crystal Pure glass Pure single crystal

  32. The gamma irradiation cures the LBO:Mn crystal from the point defects, giving additional L5 • EPR lineattributed to Mn0B (Mno substituting for Li+ in off-centre position), Vk centres (225 and • 370 nm AA bands), the same phenomenon is observed for LBO:Co crystal • The gamma irradiation of the LBO:Mn glass cures the glass from point defects (lithium or • oxygen vacancies) ionising Mn1+, Mn2+, and Mn3+ ions, leads to arising of the strong • additional absorption band (45 cm-1) on the FAE, centred at about 300 nm (B2+)and 575 nm • band assigned to Mn2+ , Mn3+ and F2+ centres, • In Mn2+ doped “as-grown” LBO single crystals and glasses there arises oxygen, and, Li+ • vacancies compensating Mn2+ substitution for Li+, CONCLUSIONS • - For given growth conditions (growth method, purity of the starting material, growth atmosphere, • technological parameters) some definite sub-system of point defects appears in the crystal (e.g. • active ions, vacancies, antisiteions, active ions, uncontrolled and controlled impurities or interstitial • defects). At the end of the growth it is electrically balanced andis left in a metastable state. Some • external factors, like irradiation or thermal processing, may leadto the transition of this • sub-systemfrom one metastable state to another. During this transition pointdefects may change • their chargestate. • Irradiation can induce numerous changes in the physical properties of a crystal ar a glass. • This may originatefrom atomic rearrangements which take place powered by the energy given up • when electronsand holes recombine non-radiatively, or could be induced by any sort of radiation • or particlebombardment capable of exciting electrons across the forbidden gap Eg into the • conduction band.

  33. - Different type of treatments (annealing in reducing or oxidizing atmosphere, irradiation) differ in • producing of characteristic defects. They may be color centers, polarons, trapped holes, • Frenkel defects, recharged active, lattice or uncontrolled ions. In the absorption spectrum they • may be observed even in infrared. The type of the radiation defects arising in the crystal and • glasses strongly depends on wether the material was obtained or next annealed at oxidizing • or reducing atmosphere • - Fluency dependence of the additional absorption exhibit characteristic shape with maximum at • about 1014 protons/cm2, minimum at about 1015 protons/cm2 and further sharp rise for higher • fluencies. Such non-monotonic dependence is characteristic for color centers, rather than for • Frenkel centers. For the latter ones, a monotonic, linear with proton fluency dependence is seen. • The probable reason of the decrease in the region 2*1014 -1015 protons/cm2 could be mutual • interaction of the cascades from different proton trajectories. • Irradiation and annealing treatments appear to be the effective tools of crystal change and • characterization. The observed in the absorption spectrum changes after ionizing radiation or • annealing treatment can have important influence on the performance of optoelectronic devices • applied in e.g. outer space. The obtained results point to the direct influence of color centers on • the processes of inverse population formation of many lasers.

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