RECHARGING PROCESSES OF ACTIVE IONS AND RADIATION DEFECTS

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, 70-310 Szczecin 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, YbNd, YbPr) 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
NdYAG
4

• - 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 586
• nm 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 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-irradiated
  crystals
• - Annealing in hydrogen give almost the same
  shape
• of the additional absorption as in case of
  g-irradiation

5
NdYAG 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 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.

6
CrYAG
ErYAG
Cr,Tm,HoYAG
7
ErYAG
Cr,Tm,HoYAG

• - 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
  thus
• luminescence 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 CrYAG AA spectra)
• - Changes in the active dopant concentration are
  observed after all the types of irradiations
  g-rays
• electrons and protons in CrYAG and Cr,Tm,HoYAG
  crystals
• - From AA of proton irradiated crystals there can
  be distinguished two dose ranges (1) fluencies
• less than 51014 cm-2 where recharging effects
  dominate and, (2) fluencies larger than 51014
  cm-2
• where the presence of Frenkel defects is
  expected.

8
YbYAG
- 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 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
YbYAG 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
ErYAP
NdYAP
PrYAP
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 ?Voe-, F?Vo2e-, - 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.
NdGGG
- 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), Ca2F 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
NdGGG
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 towards the 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 S1/2, 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 YAGCr
- 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 to Cr6 ions
of 3d0 configuration or more probably to O--
hole centers and/or F-centers. The 570 nm band
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 YAGCa, Cr annealed in the air crystal under
influence of g-irradiation suggests that
g-irradiation ionizes only Cr ions.
14
Cr Mg2SiO4 and Y3Al5O12
15
LiNbO3
LNFe
16
LNCu
LNCu
17
LNCr
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 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 the
• Irradiations (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 51014
  cm-2 (mutual interaction of the cascades from
  different proton trajectories) and third one over
  51014 cm-2 (Frenkel defects),
• - Polarimetric measurements have shown that LNCu
  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
CuLiNbO3 (0.06at.)
CuLiNbO3 (0.07at.)
Annealed
1013 prot cm-2
1015 prot cm-2
1013 prot cm-2
Protons Cu LiNbO3 wafers
20
LNYb
LN codoped
21
LNYb, 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 LNYb and LNYb, 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 CaF2Yb3 5
25
- Annealed in hydrogen and g-irradiated CaF2Yb
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, - 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 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 for g-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
LLFYb, YLFYb BYFYb, KYFYb
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 Vk centers, 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

29
Li2B4O7Mn glass
Changes in the absorption and emission spectra at
the course of time
30
a)
b)
c)
EPR spectrum of LBOMn 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 (ZB) 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
LBOCo crystal
LBOMn crystal
Pure glass
Pure single crystal
32

• The gamma irradiation cures the LBOMn crystal
  from the point defects, giving additional L5
• EPR line attributed to Mn0B (Mno substituting for
  Li in off-centre position), Vk centres (225 and
• 370 nm AA bands), the same phenomenon is observed
  for LBOCo crystal
• The gamma irradiation of the LBOMn 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, antisite ions, active
  ions, uncontrolled and controlled impurities or
  interstitial
• defects). At the end of the growth it is
  electrically balanced and is left in a metastable
  state. Some
• external 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 change
• their 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 up
• when electrons and holes recombine
  non-radiatively, or could be induced by any sort
  of radiation
• or particle bombardment 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
  21014 -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.