RECHARGING PROCESSES OF ACTIVE IONS AND RADIATION DEFECTSIN SOME LASER CRYSTALS DOPED WITH RE AND TMS.M. Kaczmarek1, G. Boulon2, T. Tsuboi3

1 Institute of Physics, Szczecin University of Technology, 48 Al. Piastow, 70-310 Szczecin2 - 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.

Content

1. Introduction2. 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

YAGNd:YAG

- The shape of the additional absorption is almost of the same type for pure YAG and 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 586nm band is observed. With an increase of the g-dose from 102 to 107 Gy, fluencies of electrons from 1014 to 1016 particles/cm2 and of protons from 1012 to 1016 particles/cm2, values of AA bands became higher and higher. The dependence of the additional absorption on the irradiation dose shows a tendency to a saturation in case of g-rays. - Protons fluency dependence of the additional absorption exhibits characteristic shapewith minimum at about 1014 protons/cm2. Such non-monotonic dependence is characteristicfor color centers rather than for Frenkel ones. For the latter centers, a monotonic, linear withproton 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 theair at 673K for 3h seem to be high enough to receive starting optical properties of g-irradiated crystals- Annealing in hydrogen give almost the same shapeof the additional absorption as in case of g-irradiation- Lower susceptibility to electron and g-irradiationreveal Nd:YVO4 single crystals

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 the same damage centers which reduce optical output by absorbing of laser emission.

- Gamma irradiation lowers the slope efficiency of pulsed laser. After subsequent pulses the output energy of the laser increases to the level, which comes out from the thermal equilibrium of rod being the heated by pumping pulses, and, air cooled. This increase of the laser energy after subsequent pumping pulses suggests that UV contained in the pump spectrum causes heating up the rod and accelerates those relaxation processes which decrease the AA.

Nd:YAG laser

Er:YAGCr,Tm,Ho:YAGCr:YAG

- The obtained results point to the direct influence of the color centers on the processes of formation of 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 thusluminescence intensity. The type of introduced CC strongly depends on the starting defect structure determined by Growth conditions 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-rayselectrons 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) fluenciesless than 5*1014 cm-2 where recharging effects dominate and, (2) fluencies larger than 5*1014 cm-2where the presence of Frenkel defects is expected. Er:YAGCr,Tm,Ho:YAG

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 electron plus a delayed gamma-signal is the signatureof 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 thecharge exchange Fe3+- Fe2+, F-type centers and Yb2+ions arising as an effect of recharging of Yb3+ ions frompairs

Er:YAPNd:YAPPr:YAP

- Important in developing of LD pumped lasers, promising as fast scintillators that exhibit veryshort fluorescence decay with time constant 1-100 ns,- Growth atmosphere (inert) leads to the presence of oxygen vacancies; there are present alsouncontrolled 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-, FVo+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 at1673 K introduce some additional defects (260, 358, 487 nm), annealing in hydrogen at 1473 Kfully clear (bleaching) the crystal,- YAP crystals seem to be resistant to proton irradiation especially for doping with Er; saturationone 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 thecrystal.YAPNd: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 centersand hole O- centers (340 nm), and, F-centers (465 nm). Annealing in the air increase anamount of Fe3+ ions and new one 400 nm centers are creating. UV irradiation forms only firsttwo centers but of the same intensity. Protons less influence the crystal than YAP and YAG.

Nd:GGG

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

- 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-waveabsorption edge towards the longer wavelengths by a few hundreds nm. They are probablyattributed to the Ga2+ centers that are formed according to reaction Ga3+ + e- Ga2+ with a spinS=1/2, g|| = 1.9838(5) and g = 2.0453(5). The second type center arises in the AA spectrum atabout 380 nm and is attributed to F-centers.- In Cr and Co doped SLG and SGG crystals beside the above CC, recharging of chromium andcobalt ions is observed after both types of the irradiationSLG, SGGForsterite 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 excitationto Cr4+ giving rise to Cr4+ emission. The g-irradiation leads to increase in intensity of excitationspectra. The 380 nm additional absorption band is assigned probably to Cr6+ ions of 3d0 configuration or more probably to O-- hole centers and/or F-centers. The 570 nm band may beassigned to F+ color centers, - In the absorption spectrum of g-irradiated crystal we observe 275 nm additional band that maybe interpreted as a valence change of Si4+ ions due to capture of electron coming from ionizationof 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, Cr annealed in the air crystalunder influence of g-irradiation suggests that g-irradiation ionizes only Cr ions.

Cr: Mg2SiO4 and Y3Al5O12

LiNbO3LN:Fe

LN:CuLN:Cu

LN:Cr

- 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 there arises 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 theIrradiations (recharging of active ions),

In fluency dependence of additional absorption at least three regions are seen. First one for fluencies 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 one over 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.

Cu:LiNbO3 (0.06at.%)Cu:LiNbO3 (0.07at.%)Annealed1013 prot cm-21015 prot cm-21013 prot cm-2Protons: Cu: LiNbO3 wafers

LN:YbLN codoped

ZYZXLN:Yb, Pr- In the co-doped crystals or crystals with large dopant concentration, two kinds of Yb3+ ions maybe present, one is Yb3+ accompanied by nearby rare-earth ion perturbed Yb3+, the other isYb3+ 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). Irradiationof 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 ionicradius 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 thecrystal (170Yb , 173Yb ), temperature dependence of EPR signal shows maximum at lowtemperatures (6K) suggesting pair presence of RE ions.

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

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

EPR spectra: CaF2:Yb3+ 5%

- Annealed in hydrogen and g-irradiated CaF2:Yb crystals show the presence of additional UVbands characteristic of Yb2+ absorption,

- AA intensity value has been observed much higher for g-irradiated crystal and stronglydependent on the gamma dose,

- Different are 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 notchange 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 thelines,

- EPR spectra show Yb3+ as isolated ions, but temperature dependence of the linewidth suggeststhe 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 for g-irradiated ones. It suggests strong ferromagneticcoupling between neighbours Yb3+ and Yb2+ ions the latter being created due to Comptonelectron capture. So, Yb3+ co-exists with Yb2+ after the Yb3+ - Yb2+ conversion under influence of g-irradiation and/or annealing in hydrogen

LLF:Yb, YLF:YbBYF:Yb, KYF:YbAbsorption spectra under g-irradiation for other fluorides

Absorption spectra under g-irradiation for other fluorides

- g-irradiation introduce some radiation defects:LLF 315 nm (F-centre), 240 nm and 380 nm (perturbed Vk centers, 520 nm (F2+ centers),600 nm (N2 center)YLF 260, 330, 440, 505 and 640 nm, additionally 520 nmCaF2 280, 380, 430, 560, 760 nm

Doping with Yb generally reduce total induced absorption, the higher is Yb concentration, thelower is induced absorption, the intensity of the F-center significantly decreases, new centre at340 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 ofthe 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 formiddle ytterbium concentrations (5-10%), when isolated Yb centers dominate over Yb pairs,gamma induced bands we associated with accompanied Yb2+ centers disappear afterannealing 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 electronsin the conduction band to form the excited Yb3+ ions which emit IR Yb3+ luminescence.Fluorides

Li2B4O7:Mn glassChanges in the absorption and emission spectra at the course of time

EPR spectrum of LBO:Mn glass at room temperature: a as-grown sample, b irradiated withgamma, 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 measuredbefore annealing treatment, b) after irradiation with g-rays, c) after annealing in the air at 673 Kfor 4ha)b)c)

Pure glassPure single crystalLBO:Mn crystalLBO:Co crystal

- 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, antisite ions, active ions, uncontrolled and controlled impurities or interstitialdefects). At the end of the growth it is electrically balanced and is left in a metastable state. Someexternal factors, like irradiation or thermal processing, may lead to the transition of this sub-system from one metastable state to another. During this transition point defects may changetheir charge state.

Irradiation can induce numerous changes in the physical properties of a crystal ar a glass.This may originate from atomic rearrangements which take place powered by the energy given upwhen electrons and holes recombine non-radiatively, or could be induced by any sort of radiationor particle bombardment capable of exciting electrons across the forbidden gap Eg into theconduction band.The gamma irradiation cures the LBO:Mn crystal from the point defects, giving additional L5EPR line attributed to Mn0B (Mno substituting for Li+ in off-centre position), Vk centres (225 and370 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 strongadditional 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

- 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 theymay be observed even in infrared. The type of the radiation defects arising in the crystal andglasses strongly depends on wether the material was obtained or next annealed at oxidizingor reducing atmosphere

- Fluency dependence of the additional absorption exhibit characteristic shape with maximum atabout 1014 protons/cm2, minimum at about 1015 protons/cm2 and further sharp rise for higherfluencies. Such non-monotonic dependence is characteristic for color centers, rather than forFrenkel 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 mutualinteraction of the cascades from different proton trajectories.

Irradiation and annealing treatments appear to be the effective tools of crystal change andcharacterization. The observed in the absorption spectrum changes after ionizing radiation orannealing treatment can have important influence on the performance of optoelectronic devicesapplied in e.g. outer space. The obtained results point to the direct influence of color centers onthe processes of inverse population formation of many lasers.