Vibronic levels and zero-phonon lines of Cr3+ - doped yttrium aluminium garnet (Y3Al5O12)

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Vibronic levels and zero-phonon lines of Cr3+ - doped

yttrium aluminium garnet (Y3Al5O12)

W. Nie, G. Boulon, A. Monteil

To cite this version:

3309

LE

JOURNAL

DE

_PHYSIQUE

Short Communication

Vibronic

levels and

_zero-phonon

lines of

Cr3+ -

_doped

_yttrium

aluminium

_garnet

_(Y3Al5O12)

W.

Nie,

G. Boulon and A. Monteil

Physico-Chimie

des Matériaux

Luminescents,

UA 442 du

_CNRS,

Université

_Lyon

_I,

69622

Villeur-banne,

France

(Reçu

le 18

_septembre

1989,

accepté

le 20

_septembre

₁₉₈₉₎

Résumé. 2014 Les niveaux vibrationnels des deux états excités du

doublet 2E

et les raies à

_O-phonon

non seulement

de

_4A2

~

2E

mais _surtout des transitions

4A2

~

2T1

et

4A2

~

4T2

de l’ion

Cr3+

inséré comme

_dopant

dans la matrice cristalline de

_Y3Al5O12

_(appelé

_plus

couramment

_YAG)

ont été observés _par

_application

à 4.2 K de la

_technique

de

_{spectroscopie-laser,}

mettant en oeuvre

les _spectres d’excitation au _moyen des lasers à colorants accordables en

_longueur

d’onde et de haute résolution.

Abstract. 2014 Vibronic levels of 2E

doublets and

_zero-phonon

lines not

_only

of

_{4 A2}

~ 2E

but

mainly

of both

_{4A2 ~ 2T1}

and

_4A2

~

4T2

transitions of

Cr3+-doped

_Y3Al5O12

_(so-called

YAG)

have been observed

_{by high}

resolution

_dye-laser

excitation _spectra at

_liquid

helium

tempera-ture.

J.

_Phys.

France 50

₍₁₉₈₉₎

3309-3315

15 NOVEMBRE

1989,

PAGE 3309

Classification

Physics

Abstracts 78.50-78.55H

Introduction.

Progress

in the

_study

of solid-state laser materials stimulate

_{spectroscopic}

research,

such as

the

_knowledge

of the

_0-phonon

lines,

the

_{phonon frequencies}

of both

_ground

and excited states. The

_{phonon frequency}

of the

_ground

state can be obtained

_by

Raman or fluorescence

_techniques,

whereas that of excited states must be

_analyzed

with the aid of

_absorption

_spectrum.

_{Unfortunately}

problems

occur both with bad

_spectral

resolution and weak

_{intensity signals}

of usual broad band

spectrophotometers.

Moreover in _many cases

_depending

on the

_crystal

field

_{strength, absorption}

and emission

_spectra

are

_shifted,

and unstructured vibronic broad bands are observed. On the other

hand,

another cause of

_problems

is the

_quality

of

_inorganic

luminescent materials : in addi-tion to the

_unperturbed

main site

_spectrum

_perturbed

sites can be detected the

_spectra

of which

overlap

with the

_principal

one.

_Consequently

the

_{interpretations}

of

_{spectroscopic}

data are

3310

cated and

_large

_{inhomogeneous broadening}

can be seen.

_Among

all fluorescent metal transitions

ions,

Cr3+

ion is of

_special

interest for solid-state laser materials in

_ruby,

emerald, alexandrite,

garnets.

It _may be used either as a

_lasing

ion itself or a sensitizer ion for the

Cr3+ - Nd3+

and

Cr3+ -

Tm3+

_{energy transfer} mechanisms

_[1-3].

_Only

on the solid state laser materials with excel-lent

_{optical quality}

accurate

_{spectroscopic}

research can be

_performed.

In this

_work,

the main

_goal

is to show how new

_high

resolution

_dye

laser excitation

_spectra

allow us to

_get

new and accurate

data in this research field. First of all we have chosen one of the most famous laser hosts :

_yttrium

aluminium

_garnet

_{Y3A15012(also}

called

_YAG).

In the

_present

_paper,

we focus on the

electron-phonon

coupling

and the

_{zero-phonon 4A2}

---+-

2 Tl

and

4A2

-->

4T2

transitions at

Cr3+

ions in YAG.

The

Cr3+

_(d3)

ion

_spectroscopy

is

_{usually analysed}

in the octahedral coordination case where

two kinds of transitions _may occur : either

_t3

_{intraconfigurational}

transitions or

_t3g

-

t2geg

interconfigurational ones.

The former

_gives

rise to

_4A2

_{---+- 2E, 2T1, 2T2}

transitions with a weak

coupling (low Huang-Rhys

parameter)

and the latter

_gives

rise to

_4A2

-

4T2,

@

4Tl

transitions

with a

_stronger

_{coupling (higher Huang-Rhys parameter).}

The vibronic levels of the

_ground

state

4A2

have

_already

been obtained from emission

_spectra

at low

_temperatures

but those of the first excited

state 2 E

doublets cannot be studied

_by

fluores-cence

_{spectra-neither}

at low

_temperatures

since the emission occurs

_only

from the lowest vibronic level of the lowest excited

state 2 E

_(2A)

nor at

_high

_temperatures

because

_they

_overlap

with those of the

_ground

state.

In

_addition,

the

_{0-phonon 4A2}

---+-

4T2

and

4A2

---+-

2 Tl

transitions have _never been

precisely assigned

even some traces were observed in a excitation

_spectrum

_using

a

_lamp

_[4].

For Lack of these accurate

_experimental

_data,

the

_crystal

field calcula tions cannot be carried out.

_Here,

we

_give

new result

_only

on the dominant R site

_(C3;

_symmetry

with a

_{slight trigonal}

distortion characterized

_by

a

_strong

_crystal

field

_strength

at 4.2

_K).

The other data

_concerning

multisites we

have found in YAG

_[2]

will be

_published

later in a

_forthcoming

_paper.

Experimental.

The

_crystals

were

_prepared

_by

_Monocrystaly

_Thrnov,

Research Institute for

_{Single Crystals,}

Leninova

175,

51119

Turnov,

Czechoslovakia. The measurement was carried out on the

_sample

containing

2%

Cr in

_weight

concentration.

The

_absorption

_spectrum

was obtained on a VARIAN 2300

_{spectrophotometer}

in association with a

_liquid

He

_cryostat.

The excitation

_spectra

were made

_using

a

Quantel

Nd : YAG

_pulsed

laser with a tunable

_dye

laser

_(repetition

rate : 10

Hz,

time constant 15 ns,

spectral

width of the

dye

laser 0.1

_cm-l)

to scan the

_{required wavelength}

and a

_{photon counting}

_system

_(ORTEC)

to

accumulate the

_signals

with different

_delays

and different

_gatewidths.

In order to avoid the laser

beams,

the

_signals

were accumulated between laser

_{pulses (delay :}

1 _{pas ;}

_{gâte :}

80

_ms)

for

_figure

la. The excitation

_spectra

shown in

_figure

1 were obtained with the mixture of the rhodamin 640 and oxazine

720,

and that shown in

_figure

2a was

_got

with the

_dye

DCM.

Results.

Figure

la shows the excitation

_spectrum

of YAG :

Cr3+

at 4.2 K

_by

_monitoring

the lowest

component of 2E

-;

4A2

transition so called

RI

emission line. The

phonon

lines are resolved. A

_spread

_part

is shown in

_figure

1 b.

_Figure

la is less

_precise

than

_figure

1b since it is recorded in a

_{larger wavelength}

interval between the

_{recording points.}

We found that the energy

drifter-ence between i and i’ lines is the same

(about

19

_{cm’ 1 ) .}

This is

_equivalent

to that between the

RI

and

_{R2 lines, i.e.,}

the

_splitting

of

2E

levels

_[2].

These

_pair

lines with A

_(2E)

différence in

en-ergy must

_correspond

to the

_absorption

of vibronic levels of each of the two

2 E

excited states

Fig.1.-

a)

Excitation _spectra of YAG : Cr at 4.2 K. Monitored

_{wavelength :}

687.3 nm

_(Ri

_{line) ,}

_b)

a

_spreaded

3312

Fig.

2-

_a)

Excitation _spectrum of YAG : Cr at 4.2 K

_{by monitoring}

the

Ri

emission

_line,

_{b) absorption}

spectrum of YAG : Cr at 10 K.

Figure

2a shows

the

excitation

_spectrum

between 634-666 nm, Four

_absorption

lines at the

edge

of the

_4A2

---+-

.4T 2

broad band are observed. These lines coincide with that in the

_absorption

spectrum

at 10 K

_{(Fig. 2b).}

_{The first line situated just on the tail of the 4A2}

--+

4T2

broad band can

be attributed to the

_{zero-phonon 4A2}

--+

4T2

transition. The other three lines should

_correspond

to 4A2

---+-

2TI (2 A2) ,4 A2

---+-

2Ti

(2E (E)) and 4A2

-

2T1 (2E (2A))

transitions. All the

spectral

data is summarized in table I.

Table 1.-

_Zero-phonon

transitions

_{of the}

Cr3+R site in

YAG at 4.2 K.

Figure

3 shows the detailed

_absorption

_spectrum

between 640-720 nm. Besides the

_part

shown in

_figure

_2b,

the

Rl ,

R2

lines and their

_phonon

sideband are observed. Not like

_dye

laser excitation

spectra

(Fig. 1),

the

_phonon

lines are not resolved in the

_absorption

_spectrum

due to the bad

resolution of the

_{spectrophotometer.}

Discussion.

The

_phonon

assisted emission lines have been

_already

observed

_by

other authors

_[4,

_6].

At

Fig.

3.-

_Absorption

_spectrum of YAG : Cr at 10 K.

the vibronic levels of the

_{4A2 ground}

state can be calculatcd

_by

the _energy

difference

between the Rl line and its

_phonon

emission lines. There are extensive difficulties with

_respect

to a detailed

interpretation

of the

_{electron-phonon coupling}

at

Cr3+

_impurity

ions in YAG. First

_{of all,}

the k =

0

_{approximation}

which is well established for

_{single phonon}

_processes,

like the

_{IR-absorption}

or the Raman

_scattering,

can no

_longer

be

_applied

and the whole

_k-space

or at least the critical

_points

of the first Brillouin zone which exhibit

_{large phonon}

_densities,

should be considered.

_Second,

the

phonon

solutions of the

_unperturbed

lattice _may be modified

_by

the

_impurity

ions and localized modes _may contribute to the

_phonon

sidebands. In

_consequence,

there exists a

_large

number of vibrational modes. In

addition,

there is a

_possibility

of

_multiphonon

modes. The aim of the

present

work is to show a

_picture

on the vibronic levels of

2E4

excited states and to

_give

_definitively

attributions of the

_4A2

_,

4T2

and

4A2

----; 2T2 transitions.

Vibronic levels of excited states of

Cr3+

have never been studied.

_Using

a

_{powerful high}

res-olution

_dye

laser,

_{phonon energies coupling}

to 2E

doublets

of Cr3+

are observed

_{(Fig. 1).}

The

in-terpretation

of these

_phonon

assisted

_absorption

lines is based on the

_assumption

that the

_phonon

energies,

due to either diflerent vibrational modes or

_multiphonon

_modes,

_coupling

to

2E

excited

states should not be _very different from those

_coupling

to the

_4A2

_ground

state. This seems to be reasonable since

both 2E

and

_4A2

states

_belong

to the same electron

_{configuration}

_t2g.

Moreover

the

_phonon

wavenumbers of

2E

_(É)

should be

_pratically

identical with those of

2E

_(2Â)

because

2 E

_(É)

is

_only

19

cm-1

from 2 E

_{(2Â) .}

The first intense

_{phonon energies}

of the

_{4A2 ground}

state

have been

_reported

to be

_{124, 142,}

_162,

_184,196

cm-1

_[5,

_6].

The observed

_phonon

lines in

_figure

1 are due to the

_absorption

of the vibronic levels

of 2 E

doublets. The detailed

_{interpretation}

is

given

in table II. The

_reproducible

19

cm-1

between i and i’ lines and 5

cm-1

between i + 1 and

i’ lines are indicative. From reference

_[2],

we know that A

_{(2 E)}

= 19

cm-1

( 0 (2 E)

is the

2 E

splitting

of the

_principle

R

_site).

So if i

_corresponds

to

the 2 E

(2A)

level

_{(Ri), il}

must

_correspond

to

2 E

(É) .

The 5

cm-1

between i + 1 and i’ lines is the energy

gap

between the No. i’ vibronic level

of 2 E

_(É)

and No. i + 1

of 2 E

_(2À).

The

_principle

is

_{schematically}

_represented

in

_figure

lc. The line n° 2 in

_figure

lb

_corresponds

to the second vibronic level of the

2 E

_(2A)

level

_(Ri).

The

3314

7àble IL- Phonon wavenuinbers

_{of the}

_{split 2E excited states}

_{of Cr3+in}

YAG.

of

the 2 E

_(É)

level are

_pratically

identical with those of

the 2 E

(2Â)

level whereas

_weakly

different from those of the

_{4A2 ground}

state.

The 4A2

-

4T2

_zero-phonon

line

position

has been estimated to be 1000

cm- 1

above

2E

level

_{by temperature}

_dependence

of lifetime and

2E

radiative

_decay

measurements

_[4].

We found

it was 1043

cm-1

above

2E

_{(2Â) -}

Ri

and 1024

cm-1

above

2E

_{(Ë) -}

R2

_exactly.

The other three lines in

_figure

2a can

_easily

be attributed to

4A2

-->

2T1 absorptions.

We know that in

trigonal crystal

field T level will

_split

into A and E levels. On the other hand E will further

_split

due to the additional

_spin-orbit

interaction. From the line

_shapes,

two narrow lines

_clearly

show

"E" characteristics. The other one can be

_assigned

to the

_4A2

__.

2T1

(ZA2)

transition.

The

_crystal

field has been calculated with a

_Dq

value derived from the mean

_position

of

4A2

-

4T2

broad band and a

4A2

-

2Tl position approximate

[7].

We have

_got

the

_splitting

of the

_4A2

-

4T,

broad band

[5],

together

with the

Dq

calculated from the

4A2

--+

4T2

zero-phonon

line

_position

and

_4A2

____+

2 Ti positions

in the

_{present work,}

the

crystal

field

calcu-lation can more

_precisely

be _{carried out,} and the accurate

_{configuration}

coordinate curves could be drawn. It is worthwhile to mention that for such

_precise

_{spectroscopic}

data one has to take into

account the air index for the calculation of the wavenumbers from the

_{wavelengths. ’parking directly}

the

_reciprocals

of the

_wavelengths,

in the

_measuring

_spectral

domain,

it will lead to an increase of wavenumbers of

_{approximately}

4.5

cm-1.

We think such

_systematic

_experiments

are

_promising

for

_solving

the difficult

_problem

of

Summary.

By dye

laser excitation

_spectra

at 4.2 K the

_phonon

lines associated

with 2E

_(2A)

and2E

_(È)

excited states are observed. The

_{zero-phonon 4A2}

-

4Tz, 4A2

-->

2 Tl

(2A2)

@ 4A. 2

-2tel

_{(2 E (E)) ,}

4A2

-

2Ti

(2E

(2À))

_absorptions

are identified both on excitation and

absorp-tion

_spectra.

Acknowledgement

We would like to thank Dr. J. Mares who _very

_kindly

donated the YAG : Cr

_crystals.

References

[1]

MARES

J.,

JACQUIER

B.,

PEDRINI C. and BOULON

G.,

Czech. J.

_Phys.

B38

₍₁₉₈₈₎

802.

[2]

NIE

W.,

BOULON G. and

MONTEIL A., J.

Chem.

_Phys.

_Lett.,

to be

_published

_(1989).

[3]

NIE

W.,

KALISKY

_Y.,

PEDRINI

_C.,

MONTEIL A. and BOULON

G.,

Optical

and Quantum

Electronics,

to be

_{published (March 1990),}

[4]

HEHIR

_J.P,

HENRY

M.O.,

LARKIN J.P. and IMBUSCH

G.E,

J.

_Phys.

C 7

₍₁₉₇₄₎

2241.

[5]

NIE

W.,

BOULON G. and MARES

_J.,

Chem.

_Phys.

Lett.

_(to

be

_published).

[6]

DOUGLAS

_I.N.,

_Phys.

Status Solidi

(a)

9

₍₁₉₇₂₎

635.