Multisites and energy transfer in Cr3+-Nd3+ codoped Y3Al5O12 and YAlO3 laser crystals

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Multisites and energy transfer in Cr3+-Nd3+ codoped

Y3Al5O12 and YAlO3 laser crystals

Jiri A. Mares, Wenjiang Nie, Georges Boulon

To cite this version:

Multisites

_{and energy}

transfer in

Cr3+-Nd3+

_codoped

Y3Al5O12

and

_YAlO3

laser

_crystals

Jiri A. Mares

_(1),

_Wenjiang

Nie

₍₂₎

and

_Georges

Boulon

₍₂₎

(1)

Institute of

_Physics,

Czechoslovak

_Academy

of Sciences, Na Slovance 2, 18040

_Prague

8,

Czechoslovakia

(2)

Laboratoire de

_{Physico-Chimie}

des Matériaux Luminescents

(*),

Université Claude Bernard,

Lyon

I, 69622 Villeurbanne Cedex, France

(Reçu

le 28 décembre 1989, révisé le 14 mars _1990,

_accepté

le 23 avril

₁₉₉₀₎

Résumé. 2014 On étudie

en détail les

_{propriétés spectroscopiques,}

la

_présence

de multisites relatifs à

l’ion Cr3+ et les transferts

_d’énergie

entre les ions Cr3+ et Nd3+ dans des cristaux lasers du type

YAG : Nd et YAP : Nd. Comme _pour le cristal YAG : Cr,

plusieurs

multisites ont en effet été mis en évidence avec YAP: Cr. Le transfert

_{d’énergie Cr3+}

~

Nd3+

est un _peu

_plus

efficace avec

YAP : Cr, Nd

qu’avec

YAG : Cr, Nd.

Abstract. 2014 Cr3+ multisites, Cr3+ ~ Nd3+

energy transfer and

spectroscopic properties

of Cr3+

codoped

YAG : Nd and YAP : Nd

_crystals

have been studied in detail. As in YAG : Cr, several multisites have been observed in YAP : Cr

_crystals.

The Cr3+ ~ Nd3+ energy transfer is

slightly

more efficient in YAP : Nd, Cr than in YAG : Nd, Cr.

Classification

Physics

Abstracts 78.50 - 78.55

1. Introduction.

A search for either new

_crystals

for tunable solid state lasers or more efficient

Nd3+

solid

state laser

_crystals

is associated with various transition metal ions

_doped

or

_codoped

Nd-crystals especially

with

_{crystals of garnet}

structure

_[1-8].

The most often used transition metal

ions are

Cr3+

and

Ti3+

_[4,

_9]

from which

Cr3+

is also used for sensitization of

Nd3+

fluorescence in

_garnets

_[8-10].

Both these ions are used as

_lasing

ions in various

_crystals

_[4,

11-13].

For

_example,

Cr3+

radiates either at fixed

_wavelength

in

_{ruby (A1203 :}

_{Cr3 + ,}

2E -->

_4A2

laser

_transition)

or at tunable

_wavelength

in the near IR in alexandrite

_{(BeAI 204 :}

Cr3 + ,

4T2 -> 4A2

transition)

while

Ti3 +

generates

tunable laser radiation in the range 700-1 000 nm

in

_sapphire

_[11].

The

3d3

electronic

_{configuration}

of

Cr3 +

ions is very stable

against

both oxidation and

reduction

_[4]

and

Cr3 +

_{occupies only}

octahedral

_crystal

field sites in

_garnet

crystals. Optical

and fluorescence

_properties

of

_{Cr3 + depend}

on

_crystal

field

_strength

and

_symmetry.

This means that

_(i)

narrow fluorescence lines are observed in

_strong

_crystal

fields

(2E -->

4A2

transition)

and

_(ii)

broad fluorescence bands are observed in medium or weak

_crystal

fields

(*)

Unité de Recherche Associée au CNRS 442.

(2T 2 ->. 4A2

transition).

These

unique

properties

of

Cr3 +

in various

_crystal

fields are used in

different

crystals

and materials such as mixed

_garnets

_[12],

aluminates

_(LMA

and

0-alumina),

glasses

and

glass

ceramics

_[13].

Another

_important

role of

Cr3 +

is that it can be used as a sensitizer for radiative or

nonradiative _energy transfer to

Nd3 +

or other ions in laser

_crystals

and

_{glasses [9,}

_10,

_14-19].

Various kinds of

Cr 3,

-->

Nd 3’

_energy transfers are now used in YAG :

_Nd,

Cr and YAP :

_Nd,

Cr rods

_{[ 17, 18].}

The

Cr 3,

-->

Nd 3’

_{energy transfer in YAG} was first observed

_by

Kiss and Duncan

_[19]

while the same transfer in YAP was observed in the middle of the

seventies

_{[20, 21].}

Both YAG and YAP activated with

Cr3 +

are

_crystals

where

Cr3 +

sites are

well defined and with

_strong

_crystal

fields. Our recent results

_[22]

show that

Cr3 +

multisites

are

_present

in YAG : Cr

_crystal,

_up to seven multisites

_(types

_R,

S and

_X).

YAG : Cr and YAP : Cr are

_crystals

with the

_{highest crystalline quality,}

_i.e.,

the various multisites have well

defined

_crystal

field

_strength

_Dq

and there is no continuous variation of

_crystal

fields as in

GGG-type

substituted

_garnets

_[23].

The favourable

_aspect

for the use of

Cr3 +

sensitization in

_garnets

is the presence of broad

and intense

_spin-allowed

pump bands in the visible and near UV

_(violet)

which is

advantageous

for

_{flashlamp pumping [4].}

The

_{processes’of}

transfer of excitation energy in

YAG :

Nd,

Cr and YAP :

Nd,

Cr

_crystals

are not

_simple

because besides

Cr3 +

-

Nd3 +

non

radiative energy transfer in both

crystals

[14,

16, 19,

21],

the radiative one is

_possible

in

YAP : Nd

[21 ],

energy

migration

among

Cr3+

ions sets in YAP : Cr at

_high

_temperatures

(above

400

_K)

_[20]

and energy transfers were also observed _among

Cr3+

and

Nd3+

multisites

in YAG and YAP

_[22,

_24,

_25].

The

_{spectroscopic}

studies of Cr

_codoped

YAG : Nd and YAP : Nd

_crystals

can contribute to a further detailed

_{understanding}

of fluorescence and

excitation processes in these

important

laser

_crystals,

e.g.,

by monitoring Cr3 +

fluorescence

decays

in these

_crystals

we can obtain results about

(i)

mechanisms of the

Cr3 + -->., Nd3 +

energy transfer and

(ii)

the distribution of

Cr3 +

ions

_(uniform

or

nonuniform)

_[26,

27].

Recently,

it has been shown that laser

_efficiency

of YAG :

Nd,

Cr

_crystal

is

improved

if this

crystal

is

_codoped

with

Lu3 +

too

_[28].

This is

_probably

caused

by

an effect of the three cations

(Nd3

+,

Cr3 +

and

_{LU3 + )}

on the elastic

_potential

energy which enhances the

Cr3 + -> Nd3 +

energy transfer.

Among

the main

_{properties concerning garnet-like}

structure laser

_crystals,

those

_dealing

with the overall

_{understanding}

of

physico-chemistry

are

_{especially important.}

An

_important

aspect

concerns the field of multisites

_{occupied by}

the activator ions inside the hosts as we

have shown in substituted GGG : Cr

[12],

GSGG : Cr

[23],

YAG : Cr and YAG :

Cr,

Nd

[22,

29,30].

Research in this field is very active and several contributions have come from other groups in Gd-Garnets

[31, 32]

and Y-Garnets

[33,

34].

The aim of this paper is to

_present

some of the newest data about

_{spectroscopy,}

energy

transfer and multisites in YAG :

_{Nd, Cr,}

YAP : Cr and YAP :

Nd,

Cr

_crystals.

We will

present

the

Cr3 +

fluorescence

_spectra

_together

with

Cr3+

fluorescence

_decays,

data about vibronic

interaction, Cr3 +

-

Nd3 + transfer

efficiencies,

possible

models of

Cr 3’

distributions

in these

_crystals

and

Cr3 +

multisites in YAP : Cr. 2.

_{Experimental.}

The

_{spectroscopic}

studies

_{(fluorescence}

and

_decays)

have been carried out on thin

(---

1-4

_{mm) optically}

_{pure YAP and YAG}

_single

_{crystals doped}

either

_singly

with Cr or

doubly

with Cr and Nd. All the

_crystals

were _grown

_{by Monokrystaly}

Turnov

(1).

The

(1)

Address:

_Monokrystaly

Turnov, Research Institute for

_{Single Crystals,}

Leninova 175, 51119

samples

for measurements had

_rectangular

_shapes

and were cut from facette-free

parts

of the

crystals.

The YAG :

_Nd,

Cr and YAP :

_Nd,

Cr

_samples

were cut from

_crystals

from which

laser rods were also manufactured. We chose the

_samples

from various

_crystals

with different

concentrations of

_impurities.

We studied three

samples

for each of the two

_{crystals (one}

doped only

with Cr and two

_doubly

_doped

with Cr and

Nd).

The

Cf3 +

and

Nd3 +

concentrations are

_given

in table I.

_{Generally, Cr3 + codoping}

of YAP : Nd is easier than for YAG : Nd. YAP : Nd

_crystals

_accept

_{higher Cr3 +}

concentrations without any deterioration of

the

_optical

and mechanical

_{properties [14].}

Table I. -

Cr3 +

and

Nd3 +

concentrations in the studied

singly

(Cr3 + )

and

_doubly

(Cr3 + , Nd 3+ )doped

YAG and YAP

single crystals

(some

of

the concentrations are not known

exactly,

these concentrations were

_estimated).

Fluorescence and

_decays

of the

_crystals

were studied under excitation

_by

several lasers into

the broad

Cr3 +

green

absorption

bands. The lasers used for excitations were either a tunable

pulsed dye

laser Molectron DL200

[35]

or a

_Quantel

YAG : Nd

_pulsed

laser with a tunable

dye

laser

[22].

The

Cr3 +

fluorescence was detected

_by

a RCA C31034 A

photomultiplier

and

processed by

a boxcar

integrator

ZWG BCI 280

_[35]

or with a

photon counting

_system

(Ortec).

These

_experimental

_set-ups

allow us to use site selective and time resolved

spectroscopy

methods

_{[22, 36].}

Some of the

Cr3

+ fluorescence

_spectra

were recorded

_by

means of an

_Optical

Multichannel

_Analyzer

ZWG OVA 284.

3. Results and discussion.

3.1

Cr3 +

ABSORPTION AND FLUORESCENCE SPECTRA OF YAG AND YAP (WITH OR WITHOUT

Nd)

CRYSTALS. -

Absorption

spectra

of

Cr 3+

ions in YAG or YAG : Nd are well known

[16,

19, 22,

37].

In addition to two broad

_absorption

bands in the visible

_{(4 A2 --> 4T2}

and

4A2 -> 4Tl

transitions

_peaking

at = 600 nm and 431.0 _nm,

_respectively

at room

temperature)

Gorban et al.

_[37]

have observed other

Cr3 +

absorption

bands in the UV

_(peaks

at =

276,

267,

243 and 235

_nm).

The

_absorption

_spectra

of

Cr3 +

in YAP : Cr or YAP :

Nd,

Cr are also known

_{[20, 38]}

and

_again

two broad bands

_{(4A2 -+}

_{4’T2, 4’T1}

transitions)

are observed in the visible range. Due to the different

crystal

fields in

_YAP,

the

Cr3 + absorption peaks

are

The

Cr3 +

fluorescence spectra of the measured YAG and YAP

_samples

are shown in

figures

1,

2 and 3. Besides the most intense

_RI

and

_R2

lines whose

_peaks

at

_{liquid nitrogen}

temperature

are

_{À 1}

₁ = 688.8 _nm,

À 2

= 687.9 nm for YAG : Cr and

_À

₌ 725.4 nm,

À 2

= 722.7 nm for YAP :

_Nd,

Cr

_(or

YAP : Cr

_also),

the other fluorescence lines and bands

(toward

longer

wavelengths)

have a vibronic

origin (with

the

exception

of the fluorescence

lines of YAP :

_Nd,

Cr

_peaking

at = 731

nm).

Phonon

_frequencies

are

_given

in table II

_(for

Fig.

1. - Fluorescence

spectrum of YAG : Nd, Cr

_(sample

n°

3)

at

_{liquid nitrogen}

_temperature

_(under

excitation Àex

= 580.9

nm).

Fig.

2. - Fluorescence

spectrum of YAG : Cr

_(sample

n°

₁₎

at

_{liquid nitrogen}

_temperature

_(under

Table II.

_{- Summary}

_{of phonon frequencies}

_{(il p}

in

cm-3

in YAG or YAG : Cr

crystals

which

were obtained

_{from different}

measurements at various

_{temperatures.}

Column 1 :

_{Ref. [22] ;}

column 2 : this

work,

see

_Fig.

2 ;

column. 3 :

this

work ;

columns 4 and 5 :

Ref.

[39].

YAG :

Cr) [22, 39]

and table III

(for

YAG :

_Nd,

_{Cr) [20].}

Cr3 +

fluorescence

_spectra

in both

crystals depend

on

_{temperatures.}

Fine resolved structures are observed at low

_temperatures

including

detailed

_phonon

sidebands. We have observed up to seven

Cr3 +

non

equivalent

sites in YAG : Cr

_crystal

at

_liquid

helium

temperature

[22].

Under laser excitation at 532 nm

within the

_{4A2 -.>}

_4T2

band,

the fluorescence

_spectrum

of YAP : Cr at 4.2 K consists of several

sharp lines,

and vibronic sidebands. The

sharp

lines are shown in

_figure

4

_{(2E -+ 4A2}

transitions)

and

_according

to our

_knowledge

this is the first observation

of Cr3 +

_{nonequivalent}

centres in YAP : Cr. The

_strongest

line is called

_{RI (its peak position}

at 4.2 K is = 724.05

nm)

Fig.

3. -

Cr3 +

fluorescence

spectrum of YAP : Nd, Cr

(sample

n°

3)

at

_{liquid nitrogen}

_temperature

(under

excitation _{A eX} = 580.9

nm).

Arrows indicate

positions

of

phonon-assisted

emission lines _see

Tab.

_III).

Fig.

4. 2013

Fluorescence spectrum of YAP : Cr at 4.2 K under excitation at 532 nm. Four

2E -+

_{4A 2}

transitions are observed due to

_{nonequivalent Cr3 +}

_centres.

724.6 nm for

_S2

and 724.8 nm for

S3.

The

position

of the

_RI

line is the same as that observed

by

Van der Ziel

_[40].

3+

Figures

5 and 6 show the whole

Cr3 +

fluorescence

_spectra

of YAP : Cr at 4.2 K.

_Figure

6

shows the time resolved

Cr3 +

fuorescence

_spectra

from which the difference between

Table III. -

Summary of phonon frequencies

_{(il p}

in

cm- 3

in

_{YAP : Nd, Cr}

and

YAP : Cr obtained

_{from fluorescence}

_spectra. Column 1 : this work

_(see

_Fig.

3) :

column 2 :

Ref [20].

,

(longer

times after

_excitation)

can be

_clearly

seen. The difference is also observed for

fluorescence

lifetimes,

T = 60 ms for the

_{RI line,}

T = 1 ms for

pair

emissions and

_again

T = 60 ms for vibronic sidebands. There is no

big

difference in lifetimes between

nonequiva-lent

Cr3 +

centres in YAP : Cr

_{(T =}

54-62 ms for

RI

and

_{SI-S3 lines).}

The measured lifetimes

are

_long

due to the inversion

_symmetry

centre

_{(Ci )}

like in the well known alexandrite

BeAI 204 :

Cr. The

_{phonon frequencies}

were estimated from

_phonon

sidebands and the main

ones at 4.2 K are

_given

in table II. Different noncoincident wavenumbers can be recorded

either in the

_absorption

or the fluorescence

spectra.

At room

_temperature

Cr3 +

fluorescence

_spectra

broaden as can be seen from

_figure

_7,

the

Cr3 +

fluorescence

_spectrum

of YAG : Cr is wider than that of YAP : Cr.

_Also,

_phonon

sidebands

(Stokes

and

antistokes)

are observed.

3.2

Cr3 +

FLUORESCENCE DECAYS OF YAG AND YAP CRYSTALS (WITH AND WITHOUT

_Nd).

c2 +

fluorescence

_decays

have been studied in detail for all six

_samples

_(see

Tab.

_I)

at

room and

liquid nitrogen

_{temperatures.}

The results of

Cr3 +

fluorescence

decays

at room

temperature

are shown in

figure

8

(YAG samples)

and

figure

9

(YAP samples).

The main

Fig.

5. - Fluorescence

specturm of YAP : Cr at 4.2 K under excitation at 532 nm.

Fig.

6. - Time resolved

Cr 3,

fluorescence

spectra of YAP : Cr at 4.2 K under excitation at 532 nm.

is observed for both

_crystals

if

Nd3 +

ions are

_present.

For YAG :

Nd,

Cr

samples

the

shortening

increases with an increase of the

Cr3 +

concentration.

_{(The Nd3 +}

content is

roughly

the

same).

This is not observed for YAP :

_Nd,

Cr

_samples

in the studied

concentration range

(for

the two YAP :

_Nd,

Cr

_{crystals) ; (2) Cr3 +}

fluorescence

_decays

of YAG : Cr is

_{purely exponential}

with lifetimes T = 1.8-1.9 ms at room

_temperature

_(see

Tab.

_IV).

Cr3+

fluorescence

_decays

of YAG :

_Nd,

Cr

_crystals

are

_{nonexponential, especially}

Fig.

7. -

Comparison

of fluorescence _spectra of YAG : Cr and YAP : Cr at 300 K under tunable

dye

laser excitation

_(Aex

= 580.9

nm).

Fig. 8.

Fig. 9.

Fig. 8.

-

Cr3 +

fluorescence

decay

curves

_Àem

= 687 _nm, R

lines)

of YAG : Cr and YAG : Nd, Cr

samples

at room _temperature. Dashed lines are

_{rough approximations}

of final _prts of

Cr3 + decays

in YAG : Nd, Cr

_samples.

Fig.

9. -

Cr 3,

fluorescence

decay

curves

_{(A em}

= 724 _nm, R

lines)

of YAP : Cr and YAP : Nd, Cr

decays

extend

_roughly

up to 5 ms

_(Fig.

_{8) beyond}

which the

_decays

return to an

_exponential

form with lifetimes close to the

Cr 3,

fluorescence lifetime of YAG : Cr

_crystal

with no

Nd3 +

ions

_(see

Tab.

_{IV) ; (3) Cr3 +}

fluorescence

_decays

of YAP : Cr and YAP :

_Nd,

Cr are

both

_{nonexponential}

in their initial

_parts

_roughly

up to 10 ms

_{(see Fig. 9). Beyond}

this

time,

the

Cr3 +

fluorescence

_decay

of YAP : Cr returns to an

exponential

form with lifetime

= 25 ms

(see

Tab.

IV) :

the

Cr3 +

fluorescence

decays

of YAP :

Nd,

Cr

_samples

behave

nonexponentially

also

_beyond

10 ms.

_Generally,

the

Cr3 +

fluorescence

_decays

of YAP : Cr and YAP :

Nd,

Cr are more

_complicated

in

_comparison

with those of YAG :

_{Nd, Cr ;}

₍₄₎

a

certain

_quantitative

characterisation of the

Cr3 + -> Nd3 +

energy transfer can be calculated

from the

_{following expressions :}

’

where _Ptr =

1/ T tr

and ’Tl tr

are the transfer rate and transfer

_{efficiency, respectively.}

TCr and TCr-Nd are the

Cr 3,

fluorescence lifetimes for Cr alone and for Cr with

Nd,

respectively

[15].

The transfer rates and efficiencies are

given

in table IV.

Here,

with

nonexponential

decays

Ptr

and ’Tltr

are defined

empirically by

using

the observed _Tlje for

Table IV. - Fluorescence

lifetimes, transfer

rates

_(Ptr)

and

_{efficiencies ( ntr) of}

the

cr3

+

-

Nd3 +

_energy

transfer

in the studied YAG and YAP

doped crystals

(T

lje is the

lifetime

when the

_intensity

decreases

to 1

_from

the initial

value) .

T cale is calculated

from

those parts

of

when the

_intensity

decreases

e

from

the initial

value).

t ca, Ic is

calculatedfrom

those parts

of

decays

which are

_exponential,

_{Ptr and ’ntr} are

explained

in

_equations

(1)

and

(2).

Data are

_only

given

at 77 and 300 K.

* _{These values have been calculated}

by taking

into account _{T lje} and not _{T calc} when the

_long

T cr-Nd and TCr. We see that the transfer efficiencies of YAP :

_Nd,

Cr

_crystals

are

_slightly

better

than those of YAG :

_Nd,

Cr.

Cr3 +

fluorescence

_decays

of YAG and YAP

crystals

are also studied at

_{liquid nitrogen}

temperature

(see

Tab. IV

_again).

The results are similar to those observed at room

temperature

with

_respect

to transfer rates and efficiencies but the

Cr3 +

fluorescence lifetimes increase for YAG : Cr and YAG :

_Nd,

Cr while the

Cr3 +

fluorescence lifetimes are

_similar,

for YAP :

Nd, Cr,

to the values at room

_temperature.

The transfer efficiencies of

YAP :

Nd,

Cr are better at low

_temperature.

The detailed form of

Cr3 +

_decay

curves at

_liquid

nitrogen

temperature

will be

_presented

in the next section

_(together

with some theoretical

models for the

_decays).

The main

_experimental

results are the

_{following : Cr3 + decay}

curves

of YAG :

Nd,

Cr,

YAP : Cr and YAP :

Nd,

Cr consist

_again

of two

_parts

from which

(i)

the initial one is

_{nonexponential (roughly}

_up to 5 ms for YAG :

_Nd,

Cr and _up to 10 ms for

YAP : Cr and YAP :

Nd,

Cr) ; (ii)

the final

parts

(after

the time values

_given

in

_(i))

are

exponential

with the same lifetime for YAG :

_Nd,

Cr and YAG : Cr

_crystals

but with different

lifetimes for YAP : Cr and YAP :

_Nd,

Cr

_crystals.

The

Cr3 +

fluorescence

decays

have also been studied for

_{Cr3 + nonequivalent}

centres in YAG : Cr

crystal (see

our _paper

_[22])

at

_liquid

helium

_{température :}

the

_{Cr3 + decays}

of

_{nonequivalent}

sites are

exponential

with various

lifetimes

_ranging

from 5.2 to 12.8 ms at 4.2 K under 532 nm laser excitation.

3.3 DlscussloN.

3.3.1

_{Spectroscopic}

and

_{crystallographic}

data.

_{YAG (Y3A15012)}

and

_{YAP (YAlO 3 )}

have different structures

_(garnet

or

_{perovskite, respectively)}

but the local structure of

Cr3 +

ions which

_{replace Al3 +}

ions is octahedral in both

_crystals.

The

_symmetry

is not

_purely

_octahedral,

there is a

trigonal

distorsion in YAG

_(C3

_i

_symmetry

_[22,

38]

while in YAP the _oxygen

coordination around

A13+

ions is

slightly

distorted. However it conserves the inversion centre

(Ci

symmetry).

Both

crystals belong

to the class of

crystals

with a

_strong

_crystal

field

(Dq /B

= 2.6 for YAG : Cr and 2.8 or 3.05 for YAP

_[3,

_20,

_31]).

This difference between

crystal

fields in YAG : Cr and YAP : Cr is caused

_by

_tight

_arrangement

in YAP

_{crystal (the}

shortest distances between

Al3 +

and

02 -

ions in octahedra are 0.190 nm in YAP and

0.195 nm in YAG

_{[36, 41]).}

The

_{nearest-neighbour}

aluminium distance is 0.38 nm in YAP and 0.52 nm in YAG

_[20].

In both

_crystals

the

Cr3 +

sites are well defined.

3.3.2

_{Nonequivalent Cr3 +}

centres. - We

_recently

showed that up to seven

_{nonequivalent}

Cr3 +

centres exist in YAG : Cr

_crystal

[22].

A similar effect has also been observed

_by

us for

YAP : Cr

crystal

for the first time

(see

Fig. 4).

The emission lines of

Cr3 +

_{nonequivalent}

centres in YAP : Cr are close to each other and the areas under the emission lines reflect the distribution of

_{nonequivalent}

Cr3+

centres. From their emission it is clear that most

Cr3 +

centres are « R » centres

_(around

90

_%)

while the concentration of

_Si,

_S2

and

_S3

centres

is smaller

_(due

to the

_overlapping

of

_Ri

and S centres it is not _easy to determine the exact

concentrations of S

centres).

The

_big

difference between YAG : Cr and YAP : Cr

_crystals

is _{the presence of}

_pairs

in

YAP : Cr

_{(see Figs.}

3 and

6).

The presence of

pairs

in YAP : Cr was studied

_by

Van der Ziel

[40]

but our own measurements show

_pair

emission

_by

time-resolved

_spectroscopy

and

fluorescence lifetime

_techniques.

No

_pair

emission was observed for the studied low

concentrated YAG : Cr

crystal [22] probably

due to the

_large

distance between two

nearest-neighbour Cr3 +

sites

(0.52 nm). Concerning

the nature of

_{Cr3 + pairs}

in YAP : Cr

_they

can

arise from various

_origins.

Up

to four

types

of

Cr3 +

_pairs

can exist in YAP : Cr

along

various

3.3.3 Phonon sidebands. - We also observe intense

phonon

sidebands of the

_RI

line for

both

_crystals

_{(see Figs. 2}

and

_{3). Up}

to 54 lattice-vibration modes with

_{antisymmetric}

symmetry

can induce electric

_{dipole-dipole}

sideband transitions

_[31].

We

_only

observed some

of them for YAG : Cr

_{(21 transitions)}

but Wall et al. have observed 44 transitions

[42].

In YAP : Cr we have observed 10

phonon-assisted

transitions at 77 K and 21 at 4.2 K. In YAP : Cr

_crystal

the

symmetry

of local sites can be influenced either

_by

the

_anisotropy

of YAP or

_by

_{the presence of}

Cr3 + pairs.

In YAG : Cr

_crystal

the

_majority

of

_interacting

phonons

in fluorescence sidebands agree with

phonons

of YAG lattice

(see

Tab. II and

_[39])

and this is an

_argument

for the

_hypothesis

that

_phonon

sidebands are

_{produced by}

lattice

phonons

and not

_by

local

_phonons.

3.3.4

_Energy

_transfer

_processes between the main sites

_of

both

Cr3 +

and

Nd3 +

ions. - The

energy transfer processes between

Cr3 +

and

Nd3 +

have been studied

extensively during

last years

[14, 30].

A new _way to the

_theory

of nonradiative energy transfer has been

proposed by

Rotman

_[27,

43,

44] (nonuniform

correlated

_placement).

For the

_{interpretation}

of

Cr3 +

donor fluorescence

_decay

of YAG :

Nd,

Cr

crystals,

we used the same

_assumptions

as

for

Ce 3+

-->

Nd3 +

nonradiative _energy transfer in YAG :

Nd,

Ce

[36],

i.e.,

that the

distribution of

_impurity

ions is random

(uniform distribution)

and that the diffusion among

Cr3 +

ions is

_negligible.

Under these

_assumptions,

we can use the

Inokuti-Hirayama

approximation [27, 36]

and the results of the calculations are

presented

in

figure

10 and in

table V. The

Cr3 +

-->

Nd3 +

_energy transfer is due to either

dipole-dipole

or

quadrupole-dipole couplings (quadrupole-quadrupole coupling

does not _{agree with the}

_experimental

curve).

Figure

10 shows that the

_long

time

_part

of

_{Cr3 + decay}

curve in YAG :

Nd,

Cr has

nearly

the

same

_slope

as the

_decay

curve of YAG : Cr without _any

Nd3 +

ions as

_expected

from the

Inokuti-Hirayama theory.

It means that the diffusion mechanism _among

Cr3 +

donor ions can

be

_neglected.

Three main reasons are

_{probably responsible}

for this observation :

i)

the very

weak concentration of donor ions in

_comparison

with the concentration of

acceptor

ions,

ii)

the

_{relatively high}

distance between two

_{nearest-neighbour}

octahedral sites

(0.52

nm)

and

_iii)

the occurrence of

4T2 --> 4A2

broad emission band

overlapping

the

2E -->

4A2

line above 77 K

leading

to the absence of resonance conditions between

Cr3 +

donors.

Cr 3+ --> Nd3 +

energy transfer in YAP :

Nd,

Cr

crystal

is more

_complicated

in

_comparison

with YAG :

Nd,

Cr

_(see

_Fig.

_9).

The

fitting

of

Cr3 +

fluorescence

_{decay according}

to the

Inokuti-Hirayama

approximation

is unsuccessful. The model of uniform distribution of

impurities

cannot be used for this

_crystal.

From a

_comparison

of the observed

Cr3 + decay

curves with Rotman’s

theory

of nonuniform

distribution,

the

Cr3 + decays

can be treated with

this

_theory

for enhanced concentrations

of Nd3 +

around

Cr3 +

donors

_(it

also follows from the presence of

pairs

in YAP :

Cr).

The

Cr3 +

decay

of YAP : Cr

crystal

is not a

_purely

exponential decay,

which can be

_{explained by}

energy transfer between various

Cr3 +

ions.

Regarding

the

_efficiency

of

Cr3 +

-->

Nd3 +

_energy

transfer,

it is

slightly

better in YAP than in

YAG

_(see

Tab.

_IV).

’

Table V. Critical distances

_of

the

Cr3 +

-->

Nd3 +

nonradiative _energy

transfer

at

liquid

Fig.

10. -

Cr3 +

fluorescence

decays

of YAG : Cr

_(sample

n° 1, the upper

curve)

and YAG : Nd, Cr

(sample

n° 3,

description

see lower left

_part)

at

_{liquid nitrogen}

_temperature. The

_fittings

of

Cr3 + decay

of YAG : Nd, Cr

_crystal

were carried out

_according

to

_{Inokuti-Hirayamas equation}

for

dipole-dipole

and

quadrupole-dipole

interaction. Dotted line is a

_{rough approximation}

of the final _part of

_{Cr3 + decay}

of YAG : Nd, Cr.

Further studies are in progress

_specially

for a better

_{understanding}

of

Cr3 +

and

Nd3 +

multisites in YAP in order to

_get

more data on the interaction between each kind of

sites

_using

the tunable and

_{pulsed dye-laser techniques}

that we have

_{previously applied}

to

YAG

[22,

29,

30].

4. Conclusion.

Four

_{nonequivalent}

Cr3 +

sites are observed in YAP : Cr

crystal

under laser excitation at

532 nm. Intense

Cr3+

_pair

emission at = 731 nm has been observed for YAP : Cr.

Cr3 +

-->

Nd 3’

_energy transfer processes are observed both for YAG :

_Nd,

Cr and

YAP :

_Nd,

Cr

_crystals.

The model of uniform distribution of

_impurities

can be used for

YAG :

_Nd,

Cr but not for YAP :

Nd,

Cr where the

_impurity

distribution is nonuniform

(enhanced

around

_{Cr3 + donors).}

Acknowledgments.

The authors are

_grateful

to J.

_Kvapil

and Jos.

_{Kvapil (Monokrystaly}

_Turnov,

_{Czechoslovakia)}

for

_supplying

them with YAG and YAP

samples.

One of us

_{(Jiri Mares)}

wishes to thank

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