Inhomogeneous profiles in re doped LaCl3 : satellites lines, energy transfer and up-conversion processes

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Inhomogeneous profiles in re doped LaCl3 : satellites lines, energy transfer and up-conversion processes

N. Pelletier-Allard, R. Pelletier

To cite this version:

N. Pelletier-Allard, R. Pelletier. Inhomogeneous profiles in re doped LaCl3 : satellites lines, energy transfer and up-conversion processes. Journal de Physique, 1982, 43 (2), pp.403-410.

�10.1051/jphys:01982004302040300�. �jpa-00209408�

Inhomogeneous profiles ^{in RE} doped LaCl3 :

satellites lines, energy transfer and up-conversion ^processes

N. Pelletier-Allard and R. Pelletier

Laboratoire Aimé-Cotton (*), ^CNRS II, Campus d’Orsay, ^Bâtiment 505, ⁹¹⁴⁰⁵ Orsay, ^France (Reçu ^le 17 juin 1981, ^{révisé le 28} septembre, accepte le 28 octobre 1981)

Résumé.

^-

La technique d’excitation monochromatique ^{de la} fluorescence a été appliquée ^{à la transition}

4I9/2 ^~ 4G5/2 de ^{Nd :} (LaxPr1-x)Cl3. ^Les variations de l’énergie ^{émise ont été observées sur} des transitions réab- sorbées. Cette méthode a permis d’attribuer les différentes raies satellites à de petits agrégats d’ions bien définis et l’existence d’une fluorescence d’up-conversion ^{à un} processus coopératif ^{mettant en} jeu deux ions excités iden-

tiques ^en interaction. Elle a conduit à une analyse précise ^du profil d’absorption inhomogène.

Abstract

^-

The technique of monochromatic excitation of the fluorescence has been applied ^{to the} 4I9/2 ~ 4G5/2

transition of Nd : (LaxPr1-x)Cl3. ^The variations of the emitted energy have been observed ^on reabsorbed tran-

sitions. This method has allowed the assignment ^{of the different} satellite lines to definite clusters of ions and the attribution of _energy up-conversion ^{to a} cooperative process involving ^{two identical} interacting excited ions.

This has led to an accurate analysis ^{of the} inhomogeneous absorption profile.

Classification

Physics ^Abstracts

71.70G - 61.70W

1. Introduction.

^-

Energy transfer in solids has been extensively investigated during the fifteen last years. Transfer occurs between both identical and different ions, ^with emission of anti-Stokes as well

as Stokes radiation. Various mechanisms are evi-

dently responsible ^{for this} energy transfer. The _expe- rimental methods are numerous, and _many attempts have been made to interpret the observations.

On the other hand the problem of the satellite lines has also given ^{rise to many studies, and there} again interpretations ^{are not} always ^consistent

We were led to consideration of these two pheno-

mena in order to interpret ^the hyperfine spectra ^of Nd : LaCl3 ^obtained using ^{a selective excitation}

induced fluorescence line narrowing technique [1].

Now we have studied these processes in ^{a more}

systematic way and present ^the experimental ^results

and the conclusion drawn from them.

2. Principle ^{of the} experiment.

^-

2 .1 EXCITATION.

- The technique ^used is monochromatic excitation of laser induced fluorescence. The absorption profile

of a line is scanned by ^{means of} continuous variation of the exciting wavelength, ^{and the} resulting changes

in the emitted fluorescence intensity ^{are observed.}

(*) Laboratoire associe a l’Universit6 de Paris-Sud.

If the laser is ^a monomode one, this is a very high

resolution technique ^as the width of the apparatus function is the width of the exciting laser line. However the very principle ^{of the method} requires ^that only

the inhomogeneous spectrum ^is obtained, ^{and the}

ultimate resolution thus depends ^{on both the} sample

and the excited transition.

For example ^even isotopes ^{of rare} earths substituted in _very low concentration ( ~ ^{5 x} 10- 4) ^{for lanthanum}

ions of lanthanum chloride _may give, ^at very low temperature ( ~ ⁴ K), inhomogeneous linewidths of about 0.06 cm-1 for transitions between the lowest energy levels of Stark multiplets. ^Such samples

excited under these conditions ^are then good ^tools

for these experiments.

2.2 REABSORPTION.

^-

The present study ^concerns

the satellite lines very ^near the main line, i.e. lines located in the wings ^{of a} line about 103 times more

intense. Therefore their profiles ^{are modified} by ^the

existence of an intense and rapidly decreasing ^back- ground - when they ^{are not} completely ^hidden.

Experiment has shown the important ^transition probabilities ^{of some} lines : in spite of the low concen-

tration of rare earth ions, ^{the CW 40 mW of a laser} whose beam has a few tenth ^mm diameter and whose

frequency ^{is tuned on the centre of the} absorption profile ^{of one of these lines, are} completely ^absorbed

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01982004302040300

404

Fig. ^1.

^-

Monochromatic excited fluorescence spectra. Excitation is in the 5 848 A absorption ^{line of Nd}

[41 9/2(5/2) ^- ~Gs~(l/2)]. Variations ^{in the} fluorescence intensity ^are recorded : a), b) ^{on the} up-converted ^Nd fluorescence 5 263 A line [~G~ j~(3/2) - 41 9/2(5/2)]. ^{The two spectra are obtained for two different positions of the laser} beam inside the

sample; c) ^{on the} up-converted ^Nd fluorescence 5 333 A line [4G7/2(3/2) ~ 419/2(5/2 ⁺ 3/2)].

by less than 1 mm depth ^{of the} crystal. ^{In these}

conditions the corresponding fluorescence lines are

strongly reabsorbed.

These self-reversed lines are obviously ^{relative to}

transitions to the ground ^{state. Their} reabsorption

rate can be modified by focusing the laser beam in the crystal ^at varying depth from the front surface

(fluorescence ^{is observed at} right angle ^{to the beam} direction).

When fluorescence is observed on strongly ^reabsorb-

ed lines, the satellite lines, ^{which are not} self-reversed,

i.e. much less reabsorbed, ^{are no} longer ^{hidden in}

the wings ^{of the} main line and a pure satellite spec- trum can be recorded

Figure ¹ illustrates these statements. The three spectra ^are records of the intensity variations of fluorescence lines relative to the same emitting level, the laser being ^{scanned over the same} absorption profile. Spectra a ^{and b} correspond ^to transitions to the ground ^{state, but are} recorded with two different

positions ^{of the incident beam inside the} sample, leading ^{to a} completely ^{or a} partially ^reabsorbed

main line. Spectrum ^c corresponds ^{to a transition}

to any level.

Comparison ^between spectra a ^and c, obtained in the same geometrical conditions, illustrates the interest of the method for an accurate study ^{of the}

weak satellites of an intense line.

2.3 OBSERVATION. - The study ^of the satellite lines has been carried on the most strongly ^reabsorbed

fluorescence lines. As stated before, they ^{all are}

relative to transitions to the ground level, but several types ^{can be} distinguished depending ^{on the} emitting

level :

-

Fluorescence level populated by ^{a cascade}

process (emitting ^{level energy} exciting energy).

-

Fluorescence level populated by ^an up-conver- sion process (emitting ^level energy > exciting energy).

-

Fluorescence level populated by ^{an interionic}

transfer process in the ^case of doubly doped samples (one ^{ion is} excited, and the other ^one observed).

The technique ^used evidently prevents ^{to observe} the resonant fluorescence from the directly ^excited level, ^{that is to} say ^to record the resonant excitation spectrum. As for the non-resonant excitation spectra (fluorescence ^{from the} directly excited level to any

level) they ^{are not} self-reversed However their observation has shown that, excluding ^{the satellite}

lines completely ^{folded in the main line,} they ^are

identical to the cascade spectra.

^,

3. Experimental set-up and results.

^-

3 .1 RECORD-

ING OF THE SPECTRA.

^-

At each scan of the absorp-

tion line we record simultaneously (Fig. 2) :

-

the intensity variation of a reabsorbed fluo-

rescence line (spectrum a). This line is isolated using ^a

monochromator with a wide slit;

-

the intensity ^{variation of the} whole fluorescence

(except ^the excitation spectral region) (spectrum b) ;

-

an iodine spectrum (spectrum c) ^obtained by recording ^the fluorescence emitted by ^a small iodine vapour cell placed ^{in the} path ^{of a} parasitic ^laser

beam. It removes any ^error due to an accidental laser jump and allows to connect the spectra ^obtained by successive scans of the laser cavity (one ^scan

covers about 1 cm’ 1).

Moreover the iodine spectrum ^{is used for a cali-} bration, the iodine lines being accurately ^known [2].

All apparatus ^are commercial :

-

The source is a 375 Spectra Physics ^CW single

mode tunable laser. The beam is focused on the

sample ^{with a 18 cm focal} length ^lens.

-

The monochromator is ^a Jobin-Yvon THR.

-

The cryogenic system ^{is a CF 204 Oxford}

Instruments continuous-flow cryostat ^{in which the}

sample ^{is cooled} by ^{a static} exchange gas. All the

spectra presented ^{here were recorded with a} sample

temperature around 4 K.

Fig. ^2.

^-

Monochromatic excited fluorescence spectra.

Excitation is in the 5 848 A absorption line of Nd Variations in the fluorescence intensity ^are recorded : a) ^on the cascade Nd fluorescence 8 751 A line [’FI,2(1/2) ^-+ 4I9~2(S~2)~ ;

b) ^{on the} whole visible _range. The 12 fluorescence spectrum is in c).

-

The RE doped LaCl3 samples ^were grown ^at the ^« Laboratoire de Physique Cristalline » in Orsay, using ^the Bridgman ^method. Samples ^of

3x4x5mm3

are cut with their faces parallel ^and perpendicular

to the crystallographic ^axis.

A great ^{number of} samples ^were used, ^{as their} hygroscopic ^{nature and their} fragility prohibit ^nume-

rous coolings.

3.2 INVESTIGATED SAMPLES AND OBSERVED TRAN- SITIONS.

^-

Three kinds of crystals ^{were studied :}

(0.000 5 148 Nd, ^{0.999 5} La)Cl3, (0.000 ^{5 Pr, 0.999 5} La)C13

and

(0.000 5 148 Nd, ^{0.000 5} Pr, ^{0.999 0} La)CI3 .

Two lines were excited : the 5 848 A line of Nd

corresponding ^{to the} %/2(5/2) ^- 4Gs/2(1/2) ^tran-

sition, and the 6011 A line of Pr corresponding ^to

the 3H4(2) ~ 1 D2(0) transition. In the latter case the

study ^was less extensive and less accurate, ^{as few}

levels can be observed in the visible region. ^Moreover

the existence of hyperfine ^{structure broadens the}

lines and reduces the spectral resolution. Still more

troublesome is the fact that the LaCl3 (purity ³ N)

used contains traces of Nd. Although its concentration is lower than 10- 6 and not detected by optical ^emission spectrography using total combustion in an inert

atmosphere, its intense fluorescence compared ^to

that of Pr has impeded ^a complete study.

Therefore we present ^here only the results obtained

by excitation of the 5 848 A line of 148Nd. A spectral region ^{of about ± 2 cm-1 was} scanned around the main line. The variation in intensity ^was observed for the different types ^of strongly reabsorbed fluorescence lines. A diagram ^{of the} corresponding transitions is

presented ⁱⁿ figure ^3.

Fig. ^3.

^-

^{Nd and Pr} transitions involved in the experi-

mental spectra.

-

Fluorescence after a cascade _process has been observed through ^{the 8 024 A line} corresponding ^to

the 4F 5/2(1/2) ^-~ 4I9j2(5/2) transition and through ^the

8 751 A line corresponding ^{to the} ~3/2(1/2) ^-~ 4I9i2(5/2)

transition.

-

Fluorescence after an up-conversion process has been observed on the 5 263 A line

(4G7/2(3/2) --+ 4I9,2(SI2) transition) .

-

Fluorescence after an interionic Nd -~ Pr trans-

fer process has been observed in the doubly doped

sample ^{on the 6011 A line of} Pr(iD2(0) -~ 3H4(2)

transition) ^{and on the 4 883 A line} (Ipo(o) ^-~ ~H~(2)

transition).

406

The recorded spectra ^are schematized in figure ^{4 :}

-

The lines, A, B, C, represent ^the satellites. This

was possible ^{due to} the fact that their profiles ^are

identical.

-

The axis of the main line

^-

completely ^reab-

sorbed in these spectra

^-

^{is shown} using ^{dots and}

dashes. To make the figure ^{clearer, these axes have}

been drawn the _same, though ^the change ^{of host}

introduced by ^0.05 % ^Pr produces ^a shift of 0.01 cm-1 toward higher energies.

4. Satellite lines.

^-

The existence of satellite lines has been observed _{in pure} as well as doped’salts.

They ^are usually grouped ^{within ± 20 cm-1 of the} strongest electronic transitions. Their intensities are

10- 3 to 10- 4 of the main line, they have, ^{in most}

cases, the width, ^the polarization, and the Zeeman effect of the main line.

The most commonly ^{used method for} studying ^them

is absorption spectroscopy. They have also been observed in fluorescence spectra, pulsed ^excitation spectra ^{and electron} paramagnetic ^resonance spectra.

As for their origin, ^we shall consider here only ^the interpretations for RE ions (most ^{often Pr or} Nd)

substituted in a crystal host. The first extensive study

was done by ^{Prinz and Cohen} [3] ^on several electronic transitions of Nd : LaCl3. They assign the presence of satellite lines to near-neighbour interactions and conclude the existence of clustering ^of impurity ^ions together ^{with the} distortion of neighbouring ^rare

earth sites.

Later on systematic investigations ^{were carried out} by progressively replacing the cations or anions of the host and by observing the appearance of new satellite lines [4, 5]. These studies lead to the conclusion that

an important ^cause of satellites is a change ⁱⁿ symme-

try producing shifts in the energy levels due ^to the presence of impurities.

ORIGIN OF THE OBSERVED LINES.

^-

a) ^Tentative assignment.

^-

^{As our} experimental method allows

suppression of the main line while preserving ^the

satellite lines, ^{and as it is a} high resolution spectroscopy technique, ^{we have} limited ourselves to lines within

± ² cm-1 of the main line, ^{~e. lines} usually completely

hidden in its inhomogeneous profile ^or partially ⁱⁿ

its wings.

All our results are consistent if :

- we assign ^{all satellite lines to the} electronic tran-

sition 419/2(5/2) ^-~ ~~5/2(1/2) ^{of Nd, each line corres-}

ponding ^to the excitation of ions with slightly ^different environments,

-

we attribute these differences to the fact that the Nd ions are near a non-La3 + ion, ^{meanwhile the Nd}

ions which give ^{rise to the main line} (that ^{is to} say most of them) ^{can be} considered as isolated in the

LaCl3 ^host and therefore equivalent. The weakness of the satellite lines with respect ^{to the} intensity ^of

the main line is consistent with the _very low proba- bility ^{for an} impurity ^{to be} neighbour ^{of another} impurity ^{in a} very dilutely doped sample.

Observation of the satellite spectrum ^{shows the} following characteristics :

-

All profiles ^are identical, and identical to that of the main line.

-

When LaCl3 ^is doped ^{with odd Nd} isotopes, ^the

satellite lines exhibit hyperfine ^structure (in ^{Nd :} LaCl3 crystals ^at least, ^{as we had no 143 or} 145N~

Pr : LaCl3 samples available).

-

All lines exhibit the same isotopic ^effect [6]. ^It

was not possible ^to study the effect of an external

magnetic field, ^{as the structure is too} tight ^{for a}

Zeeman spectrum ^{to be resolved} when each line is

split ^{into four} components.

Besides these qualitative considerations which show the identity ^{of the satellite} lines and of the main line,

Fig. ^4.

^-

Schematized satellite spectra. I, II observed from a cascade populated level; III, ^IV observed from an interionic transfer populated level; IV, ^V observed from an up-conversion process populated ^{level. The} samples ^{used and the observed}

transitions are noted on the left hand side.

the results arising ^{from the} study ^{of the} spectra pre- sented in figure ^{4 have allowed us to} ascertain the

origin ^{of the} satellites.

This figure ^shows that, ^with respect ^{to the cascade} fluorescence spectra, ^no additional line is introduced

by populating the Pr levels or the Nd levels higher

than ’G,/2’ ^{On the contrary some of the lines appear-}

ing in the cascade spectra, ^i.e. belonging ^to the excita- tion spectra, ^{are not} present ^{in the} fluorescence _spec-

tra due to an interionic transfer _process or to an up- conversion _process. In fact 3 types ^{of satellite lines}

can be distinguished :

-

the lines remaining ^{after an} interionic Nd -~ Pr transfer _process (noted A),

-

the lines remaining ^{after an} up-conversion pro-

cess on a Nd level (noted B),

-

the lines present ^{after a cascade} process only (noted C, C’, C").

b) ^{A lines.}

^-

Comparison of the 2 cascade fluo-

rescence spectra I and II of figure ^{4 obtained} respec-

tively with the Nd : LaCl3 ^and Nd, ^{Pr :} LaCl3 samples

allows us to conclude that the existence of A lines is due to the introduction of Pr in the crystal. ^{In the} hypothesis where satellite lines ^are electronic lines of Nd ions in regular sites with crystal ^fields slightly

different from the main crystal field, ^{one can} regard

the satellites as arising ^{from a} perturbation ^{of Nd ions}

due to the proximity ^{of a Pr ion.}

The fact that, concerning ^the excitation in the satel- lite lines, excitation in the A lines only induces 1 D2(0)

Pr fluorescence confirms this assumption ^{as it means}

that only ^{the ions} producing ^these particular ^satellite

lines participate in the Nd -~ Pr energy transfer.

Spectrum ^{III was obtained} by observation of intensity

fluctuation of the 6011 A reabsorbe,d ^{line of Pr.}

Observation of a non-reabsorbed 1D2(0) fluorescence line shows that excitation of the ions giving ^{rise to}

the main line also induces a Nd -~ Pr transfer.

These results are consistent with the fact that the transfer efficiency depends ^{at the same time on the}

number of excited ions and on the donor-acceptor

distance. There are very few Nd ions giving ^{rise to}

satellites A, but the Nd-Pr distance is short whereas for the ions giving ^{rise to} the main line and to the non-A satellites the interionic distance is large. ^{In these}

cases the efficiency difference is due to the 103 to 101 times higher concentration in ions giving ^{rise to the}

main line than in ions giving ^{rise to} the satellites.

c) ^{C lines.}

^-

^{C lines are the lines which} appear

only ^{in the cascade} fluorescence spectra. ^{Their main} characteristic is a sample dependence. Spectra ^{I and II}

of figure ^{3 are two} examples. ^The coincidences ^can

change ^{with other Nd :} LaCl3 ^or Nd, ^{Pr :} LaCl3 samples.

The results obtained from the study ^{of A} spectra have allowed confirmation that the satellite lines arise from the electronic 4I9~2(Sl2) ^~ 4Gsj2(1/2) transitions of ions for which the crystal ^{field is} slightly ^modified

by the presence of near perturbating ^{ions. We have}

then extended the argument ^{to the C} spectra.

The non-reproducibility ^of the C lines calls to mind the fact that many samples ^were investigated, ^{and that}

the lanthanum chlorides used to grow them were not

perfectly pure and ^moreover had different origins.

Some metals (such ^{as Si and} Ca) may be found in concentrations not much different from the concen-

trations of the deliberately introduced ^rare earth.

Under these conditions C satellite lines are attributed

to Nd ions ^near non-identified impurities. ^{The relati-} vely large shift from the main line (I shift I > 0.8 cm-1 )

can be explained by ^{the masses and ionic} radii of these

impurities ^{which are} very different from the masses

and ionic radii of the rare earths ions.

d) ^{B lines.}

^-

Contrary ^{to the C} spectra, identical B spectra ^{are recorded with} any sample. ^Moreover

the B lines appear ^to be totally independent ^{of the} possible presence of other impurity ^{ions. As a conse-}

quence and extending ^{once more the} argument applied

to the A and C lines, ^{the B satellite lines are attributed}

to weak clusters of Nd ions. It can be noticed that,

for these low concentrations, satellite lines due to Nd ions in RE clusters are concentrated within + ^0.6 cm-1 of the main line.

e) Conclusion.

^-

The fact that some ions are not involved in the interionic transfer _process has led to a better understanding ^{of the} origin of the satellite lines. We think that we have demonstrated that

^-

in the proximity of the main line at least

^-

satellite lines are due to the same electronic transition, ^and that the small shifts of the levels are caused by ^{a small} change ^{in the} surroundings ^{when a} nearby ^{ion of the}

host is replaced by ^{another one. These} perturbating

ions _may be impurities ^{and it is} possible ^{to eliminate}

them. However some satellite lines will remain, ^as long ^{as the same} crystal growing technique ^is used,

as they ^{are due to} clusters of the ions under study.

5. Up-conversion.

^-

^The up-conversion phenome-

non (transformation ^{of an} exciting photon ^{to one of} higher energy) ^{has been} investigated since the mid 1960’s. Interest in the subject ^{was related to the interest}

in quantum ^counter action, and the salts most studied,

were either _pure, or strongly doped ^{with 1 or 2 rare} earths, ^{one of them} being ^the sensitizer and the other the acceptor.

An extensive literature _survey ^was carried out by

Auzel [7]. Various mechanisms are proposed ^to explain ^the phenomena : ^{the sum of} photons ^{is pro-}

duced by ^{means of} cooperative processes ^or successive

absorption ^{with or} without internal relaxations; ^the

transfer between ions of a pair ^{is resonant or} phonon

assisted The number of photons involved in the _pro-

cess is determined ^from measurements of _{the up-} converted light intensity ^{as a function of the} exciting

power (lu ⁼ feW’), ^{but n is not} always ^an integer,

and the results obtained by different authors do not

necessarily agree.

408

Some authors have been interested in rare earth ions of low concentration in host crystals. ^{The inter-} pretations ^{are similar to those} for concentrated _crys- tals. The particular ^case of Pr(Nd) ⁱⁿ LaCl3 ^has already

been studied, ⁱⁿ particular by Zalucha et al. [8].

Depending ^{on the excited} level, they ^{attribute the}

existence of an up-conversion :

-

Either to an ETU (energy ^transfer up-conver-

sion) process : ^two nearby ^{excited ions} undergo ^an

energy transfer interaction leaving ^{one ion in a} higher

excited state, and the other in a lower state.

-

Or to a STEP process (sequential two-photon

excitation process) : the ion is excited in an initial step, ^{relaxes to a lower} multiplet whereupon ^{a second} photon ^causes the excitation to a higher multiplet.

The two mechanisms ^are distinguished by ^flux dependence ^and decay ^time experiments.

In fact, ⁱⁿ the different cases investigated, ^{all inter-} pretations ^can be reduced to two types ^of mechanism :

a successive absorption ^{of n} photons ^{on 1 centre}

^-

with or without intermediate relaxation processes

^-

or a cooperative process in which the excitation which

induces up-conversion ^{results from two} absorption

transitions on separate ions. The first problem ^{to be}

solved _appears to be a choice between these two mechanisms.

S .1 EXPERIMENTAL ^RESULTS.

^-

a) Spectroscopy experiments.

^-

Excitation of the 4Gs/2(1/2) ^{level of}

Nd in the two types ^of crystals ^{Nd :} LaCl3 ^and Nd,

Pr : LaCl3 always gives ^{rise to} up-conversion, ⁱⁿ spite

of the _very low concentration of rare earth ions.

Emission from all fluorescing ^{levels of} energy higher

than ’G,/2 ^{was observed, in Nd (levels} 4D3j2, 2G9/2~

4G 7/2) ^{as well as in Pr (level 3Po).}

This anti-Stokes fluorescence always exists when

exciting ^{radiation is} absorbed in the main line. But

figure ⁴ shows that excitation in only ^{some of the}

satellite lines induces an up-conversion process.

Moreover two kinds of anti-Stokes fluorescence have to be distinguished, depending ^{on the rare earth}

-

Nd or Pr

^-

to which the emitting ^levels belong.

The results can be seen by comparing spectra ^IV and V :

-

when excitation is in the B lines, ^the up-converted spectrum ^{is the same,} whether Pr is present ^{or not} More precisely, ^the up-conversion only ^{involves Nd,}

and no more lines _appear when Pr is added;

-

when excitation is in the A lines, ^no up-converted

fluorescence appears from Nd levels. As for the anti- Stokes fluorescence from 3Po ^Pr level, ^it weakly ^exists

when excitation is in some satellite lines (A2 lines),

not when excitation is in the others (At lines).

b) ^Flux dependence experiments.

^-

The number of

photons ^{involved in the energy} up-conversion process is in principle equal ^{to the} exponent ^{in the} expression

which relates the up-converted ^emission intensity ^to

the input light ^{flux. Therefore we} have carried out

appropriate measurements. Many samples ^{have been}

tested, ^{different Stark} levels of Nd and Pr have been

excited, ^{and laser} intensity ^{has been varied} by ^factors

up ^to 102.

At low _powers the curves representing ^{the flux} dependence ^{of the} up-converted fluorescence are qua-

dratic, ^{which is} characteristic of ^a two-photon pro- cess ; at high powers, saturation effects _appear. How-

ever these curves are not reproducible, ^the divergence being larger ^{than the} uncertainties in the measure- ments. Two main effects can be put ^{forward to} explain

this non-reproducibility :

-

the relative widths of the exciting ^{radiation and}

of the absorption profile;

-

the importance ^{of the} reabsorption ^which depends greatly ^{on the} position ^of the laser beam inside the sample ^{and which cannot be} completely

removed.

The existence of these two phenomena ^{makes dif-}

ficult to accurately determine the power density ^at

each laser wavelength actually ^absorbed Under these conditions it seems dangerous ^to give ^{credence to the}

results obtained ⁱⁿ those intensity measurements.

5.2 INTERPRETATION.

^-

a) Determination of ^the

mechanism.

^-

The study ^{of the} satellite lines had allowed us to assign ^{them to} electronic lines of non-

isolated Nd ions. The distinction between the A, B, ^C lines only depends ^{on the nature of} the perturbators :

Pr ions for the A lines, Nd ions for the B lines, ^un-

known impurity ions for the C lines.

The fact that the excitation in _any of these lines does not systematically ^{induce an} energy up-conver- sion process allows us to unambiguously distinguish

between the two mechanisms usually put ^{forward :} if there was successive absorption ^{of n} photons ^on

one centre, ^{with or} without internal relaxation on a

metastable level, ^the up-converted energy would be the _same, whether A, ^{B or C lines are} excited, ^{as the} corresponding concentrations are nearly ^identical.

As a matter of fact, only excitation of the B spectrum induces up-conversion ^{on Nd. On} another hand it does not induce any up-conversion ^on Pr. As the B lines are attributed to Nd ions in Nd clusters, ^it

can be assumed that the up-conversion ^mechanism requires ^the existence of a coupling ^{between two iden-}

tical excited ions. In the main line the weaker Nd-Nd

coupling ^is compensated by ^{a 103 to 104} higher ^con- centration, ^{whereas it is not in the A and C lines.}

These experimental facts led us to consider the

possibility ^{of a} process of cooperative excitation of

pairs ^of ions. The existence of pairs ^of excited ions

was demonstrated by the works of Varsanyi, ^Dieke

and Dorman [9] ^on PrCl3 and Nakazawa and Shio- noya [10] ^on YbP04. The much lower concentration of active ions in ^our samples ^is compensated by ^the

much more powerful excitation.

Such an assumption presents ^the advantage ^of accounting ^{for the} up-conversion phenomenon ^what-

ever level is excited As a matter of fact, up-conversion

appears ^to be a general phenomenon. ^We describe it here for a particular ^{case, but we have observed it}

in various hosts doped ^{with the various rare earths}

at our disposal. ^In all these salts we have observed it whatever Stark level was excited; ^{even excitation}

in some vibration bands _gave a non-zero up-converted

fluorescence.

In the case where excitation is in the A lines, ^the problem appears ^to be a little more complex :

-

the absence _{of any} up-conversion ^{on Nd levels}

is in agreement ^{with the fact that} only ^{one Nd ion}

is involved;

.

-

the existence of a weak up-conversion ^{effect on}

Pr 3Po level when some of the A lines (A2 lines) ^are excited, ^{leads us to} consider that two Pr ions are

involved These two ions being ^{excited via a Nd -~ Pr}

energy transfer process, the up-conversion ^mechanism

thus requires the existence of clusters of at least 2 Pr ions and 1 Nd ion. But the probability of such clusters is _very small, given ^the very low concentration in RE ions.

It seems more realistic to assume that this anti- Stokes fluorescence is a consequence of the Pr-Nd interaction. To account for :

-

the existence or non-existence of blue fluores-

cence depending ^on the A line which is excited,

-

the fact that this fluorescence is emitted by ^the

’Po ^{Pr level} only, ^{a detailed} knowledge of the energy spectra ^{of the various} quasi-molecules ^{should be}

necessary.

Lifetimes measurement would maybe ^{allow to}

infirm or to confirm this assumption. But, apart ^the difficult problem ^{of a monomode} pulsed intense exci-

tation, ^{the main} objection ^{to such} experiments ^{is the}

risk to get ^erroneous results due to the existence of a

radiation confinement

6. Absorption ^line profile ^{and ions} distribution.

^-

The technique of monochromatic excitation of the fluorescence observed on reabsorbed lines, applied ^to

the 41912(5/2) ^~ 4G5i2(1/2) transition of Nd : LaCl3,

has led to a good understanding ^{of the} inhomogeneous profile ^{of the 5 848 A line.}

The study ^of the energy transfer and anti-Stokes fluorescence phenomena by excitation of the satellite lines near the main line has allowed us to assign ^these

additional lines to electronic transitions in ions in

regular ^{sites, with a symmetry} slightly ^modified by

the proximity ^{of a} perturbing ^{ion. As a} consequence these satellite lines can be considered ^{as a} part ^{of the} inhomogeneous absorption profile of the line under

study.

In the case of a sample completely free from _any

impurity, ^{satellite lines exist,} quite ^{near the main} line, due to the accidental proximity ^{of two or more} doping

ions. The spectra ^we have obtained allow one to

differentiate between the two main causes for the

inhomogeneous broadening ^{of a line :}

-

A random distribution, ^{due for} example ^to

internal strains (as usually assumed), of the energy levels of ions in given surroundings. ^{On the} sample

studied here, ^the corresponding ^{width is about}

0.06 cm-1.

-

A juxtaposition ^{of lines} having ^such profiles.

These lines come from ions with different surround-

ings : ^{the main line} corresponds ^{to isolated} ions, satellite lines correspond ^{to more or less clustered}

ions. The observed profile ^{is the} envelope, of ^{all these}

lines.

’

The distribution of the satellite lines around the main line can be observed in spectra a ^{and b of} figure ¹

or in spectrum a ^of figure ^{2 as} only ^{B lines} appear ^on them. It allows one to explain ^the dissymmetry ^of

some profiles ^noticed by different authors in some more concentrated samples ^{where the} probability ^of clustering ^is larger; then the increase in the intensity

and of the number of the satellite lines, ^{induces an} apparent broadening ^{of the} absorption ^line.

It must be noticed that Flach et al. [11] ^had already

assumed that the non-Gaussian ’H4 --+ 3Po ^excitation

spectrum they ^{observed in Pr :} LaF3 ^{was due to} strongly concentration dependent ^{satellite lines that}

were folded into the broadened main absorption ^line.

In the same way Selzer et al. [12] express the hypothesis

that the wings ^{of the} Ri ^{line of} ruby correspond ^to

« abnormal ions » which could be weakly coupled pairs ^or clusters. The presence of such clusters giving

rise to the wings ^{of the} R1 absorption ^{line has also}

been proposed ^{as an} explanation ^{for anomalous} photon-echo ^results [13]. ^{The method we have used} here, together with the choice of the crystal (very ^low

concentration so that the number of the types ^{of sites} is limited, energy levels free from _any structure) ^has allowed us to verify ^these assumptions.

The problem ^{can also be} considered from a more

practical point ^{of view. In the} samples ^grown by

means of the Bridgman method, ^{the main} part ^{of the} introduced rare earth is statistically substituted with respect ^to the lanthanum ions. Nevertheless, ^{and in} spite ^{of the very small} quantity ^of impurities, ^{some of}

them are in small aggregates. The method has allowed

to show the existence of such aggregates, ^{and could}

be used to compare different growing techniques.

410

References

[1] PELLETIER-ALLARD, N., PELLETIER. R., ^J. Physique ⁴¹ (1980) ^{855. J.} Physique ⁴¹ (1980) ^861.

[2] GERSTENKORN, S., Luc, P., ^{Atlas du} spectre d’absorp-

tion de la molécule de l’iode 14 800-20 000 cm-1 (Editions ^du CNRS) ^1978.

[3] PRINZ, ^G. A., COHEN, E., Phys. ^{Rev. 165} (1968) ^335.

[4] ^{See for} example references in HÜFNER, S., Optical Spectra of Transparent Rare Earth Compounds (Academic Press) ^1978.

[5] FRICKE, ^W. Z., ^Z. Phys. B ³³ (1979) ^{255. Z.} Phys. ^{B 33} (1979) 261.

[6] PELLETIER-ALLARD, N., PELLETIER, R., ^{to be} published.

[7] AUZEL, F., ^Proc. of the ^{IEEE 61} (1973) ^758.

[8] ZALUCHA, ^D. J., SELL, ^J. A., FONG, ^F. K., ^{J. Chem.}

Phys. ⁶⁰ (1974) ^1660.

[9] VARSANYI, F., DIEKE, G. H., Phys. ^{Rev. Lett. 7} (1961)

442.

DIEKE, ^G. H., DORMAN, E., Phys. ^{Rev. Lett.11} (1963) ^17.

[10] NAKAZAWA, E., SHIONOYA, S., Phys. Rev. Lett. 25

(1970) 1710.

[11] FLACH, R., HAMILTON, ^D. S., SELZER, ^P. M., YEN,

W. H., Phys. ^{Rev. B 15} (1977) ^1248.

[12] SELZER, ^P. M., HUBER, ^D. L., BARNETT, ^B. B., YEN,

W. M., Phys. ^{Rev. B 17} (1978) ^4979.

[13] COMPAAN, A., Phys. ^{Rev. B 5} (1972) ^4450.