SPECTROSCOPIC STUDIES OF

~ d ~ + AND sm3+ ION IN

GLASSY MATRICES

3.1 Introduction

0 ptical characteristics of glasses for laser applications are greatly dominated by

the spectroscopic properties of rare earth ions doped in it. Characteristic nature

of glassy hosts such as short range order in structure, easy to fabricate in different

form and other various non-linear properties, make them a good lasing medium.

Glasses activated with trivalent rare earth ions emitting in the visible and MR region

are of current interest because of their potentiality as optical materials for high power

inertial continement fusion lasers and optical fibre amplifiers in telecommunications.

Due to its potentiality in laser applications, various efforts are being made to increase

the fluorescence efficiency of rare earth doped glassy medium. The optical spectra of

lanthanides in glasses are of much interest as many of the lanthanide ions exhibit

potential laser transitions in glassy matrices which provides optical homogeneity.

Also, the effect of chemical bonding and weak covalancy on the lanthanide spectra

attracted attention 'The fluorescence efficiency of an active ion in a material depends

on various spectroscopic parameters such as the absorption and emission cross-

sections, transition probabilities, lifetimes of the metastable levels, concentration of

the dopant ions, and also the effect of ligand field on them. A lot of theoretical as well

as experimental spectroscopic techniques are available to obtain these parameters for

a particular rare earth ion. The spectroscopic studes of hundreds of oxide glasses

were conducted in the last few decades. Search is now focused on glasses with higher

energy storage and extraction efficiencies, smaller refractive index non-linearity, and

higher optical damage threshold. In this chapter a detailed spectroscopic

investigations of trivalent neodymium ion in borate and samarium ions in phosphate

glassy matrices have been carried out with the help of the theoretical treatments

discussed in Chapter 2, and the results obtained are compared with experimental

findings.

Currently ~ d ' * is one of the most commonly known lasing ions whose spectroscopic

properties are very well investigated by many workers. Its efficiency as a good lasing

ion in the NIR region is well known. Spectroscopic properties of Ndl+ ion have been

reported in a variety of glassy rnatri~es ' .~.~.~ with considerable interest on their lasing

properties. Vanous spectroscopic quantities can be estimated from the absorption and

emission spectra with the help of the well-known Judd-Ofelt A numerical

calculation yielded the values of various interaction parameters and the variation of

these parameters with the alkali content is studied and the results are discussed in the

light of the ligand field strength, glass structure and coordination of the Ndl+ ion.

Also we have obtained various radiative pmrameten of ~ d " ion in Nd20fB203 glass

hosts and analyzed their dependence on glass compositions.

Muller et.al. ' have studied the spectroscopic properties of sm3+ in lithium borate-

tungstate glasses. Phosphate glass matrices with suitable compositions have been

found to be good host for differtht laser active ions X.9. However, spectroscopic

studies of ~ m " ion in phosphate glass matrix are relatively limited. In view of this,

we find it essential to report the spectroscopic properties of ~ m " ion in phosphate

glass matnx w~th different compositional network by analyzing the absorption and

emission spectra. 'The present study revealed the origin of variation in some

spectroscopic properties of ~ m " for laser action by compositional changes of the

glassy matrices

3.2 Experimental

Borate Class

For the preparation of borate glasses we have used BDH 99.9 % purity boric acid and

sodium carbonate. Neodymium oxide of 99.99% purity (Indian Rare Earth Ltd.,

Kochi.) was used as the dopant. Five different glass samples having the following

batch compositions (mol%) were prepared.

Glass A : 90B203 : 9.5Na2CO3 : 0.5Nd203

Glass B : 83B203 : 16.5Na2C03 : 0.5Nd203

Glass C : 73B203 : 26.5Na2CO~ : 0.5Nd203

Glass D: 64.5820,: 3 5 N a 2 a : 0.5Nd20,

Glass E : 69.5B203: 30Na2CO3 : 0.5Nd203

Phosphate Class

The phosphate glasses were prepared from NaH2 (P04)2 2H20, LiCO3, Na2C03 and

99.99% pure Sm20t (Indian Rare Earths Ltd.,Kochi). The following batch

compositions are taken for the sample preparation.

Glass A : 95.0 NaH2(P04)2 2H20: 1.5Li2C03.3.0 Na2CO3 : 0.5 Sm203

Glass B : 94.5 NaH2(P04)2 2H20:I .5Li?C01, 3.0 Na2CO3 : l .O Sm203

Glass C : 94.0 NaH2(P04)2 2H20:I .5 Li2CO3,3.0 Na2CO1 : 1.5 SmzOl

Glass D : 93.5 NaH2(P04)2 2H20:1.5Li~C03,3.0 NazCO3 : 2.0 Sm20~

Here B203 and NaH*(P04)* 2H20 act as glass formers while Na2CO1 and LizCO3

act as network modifiers. Calculated quantities of the chemicals were mixed in an

agate mortar and heated at about 900°C for five hours in silica crucible so that a

homogeneously m~xed melt is obtained. fhe melt was quickly transkcred on to a

preheated brass mould and pressed by a s~m~lar brass disc to obtain bubble free glass

disc of 2mm thickness. having diameter of 15mm. The prepared glass samples were

transparent and the glass discs thus obtained were annealed In a furnace at a

temperature of about 350°C for 3 hrs for the thermal and structural stability and then

taken out and subsequently polished well with water free lubricant. A photograph of

the prepared glass discs is given in Figure 3.1

Figure 3.1. Photograph of the (a) bId3+ doped Borate and (b) sm3+ doped Phosphate glass

Density of the glass samples was determined by Archimede7s principle using Xylene

as the immersion liquid. The refractive index was measured using Abbe's

refiactometer. The absorption spectra were recorded in the UV-VIS-NIR region on a

Hitachi U3410 spectrophotorneter using undoped glass sample as a blank. The time

resolved fluorescence measurements were taken on a SPEX Spectrofluororneter. All

the measurements were carried out at room temperature, -

3.3 Results and Discussion

3.3.1 Spectroscopic properties of ~ d ~ ' ions in borate glasses

Figure 3.2 shows the optical absorption spectra of Nd3+ ion in the five glass

compositions studied. Comparison of this spectra with standard wavelength chart of

Nd3+ ion identifies various spectral transitions of ~ d ' + ion." All the spectra are found

200 300 400 500 600 700 800 900 loo0 Wavelength (nm)

Figure 3.2 Optical absorption spectra of Nd'+ ion I'n the five borate glassy systems

to be almost identical in nature except for some differences in their absorbances. At

NalCO, lower than 26.5 mol% the spectra in all the three alkali borate glasses were

identical in their structure. it was also noticed that these spectra show relative change

in the band shape in the region of 583.3nm (41g/2+'~7ntransition), 738.8nm J

('1~,~~.+'~7,2 + Su12transition) and 813nm ( 4 1 ~ ~ n - + 4 ~ ~ 2 transition). From the spectra it is

clear that the splitting of the absorption band in the region 570d00nm is prominent 4 in the Glass E composition while for the other compositions the transition 4~9n+ G7,z

appears only as a shoulder.

The values of the various pamal derivatives and zero order energies that are evaluated

for the various experimentally observed levels are given in Table 3.1. The

theoretically computed energy levels of Nd3+ ion in each of these glass compositions

are given in Table 3.2 along with their corresponding RMS deviation. A comparison

of the energy levels of various absorption bands of NdJf ion in borate glasses with that

of sulphate glasses " shows that, borate glass systems have higher energies than

sulphate glasses and this difference indicates a contraction in the 4f3 configuration of

sulphate glass system whereas in the borate glasses there is an expansion of the 4p

configuration. The small RMS deviation between theoretical and experimental energy

values clearly shows the validity of the Taylor series expansion method in expressing

the intermediate coupling energy levels of Nd3' ion. The method is thus effective in

evaluating the energy values without diagonalising the 4e energy matrix, which is

rather a tedious procedure.

In Table 3.3 the values of the Slater-Condon parameters and spin-orbit interaction

parameters are presented for the five glass compositions. From the Table it is clear

that the variation of these parameters do not show a linear dependence with the alkali

content. Hence these variations can be explained by taking into account of the

interaction between trivalent Nd3+ ion and the alkali cation. The Slater-Condon

parameters (F parameters) generally represent the radial integral part of the

electrostatic interaction matrix elements of trivalent rare earth ion and can be

mathematically represented asi2

where k 2. 4. 6; r, and r.. represent the distanws from the nucleus to the nearer and

farther electron respectively; R(r, ) and R(r,) represent the normalized wave functions

of the I" and jh electron; 4, are constants. Similarly the spin-orbit interaction

parameter k r represents the radial integral part of the spin-orbit interaction matrix

element and is given by1'

where C,(r ) = hz12m2c2r aV(r )I& and V is a potential function for the spin-orbit

interaction. For a free ion these interaction parameters are constants. But, when the

ion is under the influence of another interacting field (ligand field or crystal field)

these parameters change due to the overlapping of the 4f wave function of rare earth

ion with that of the surrounding ligand ion. As a result of these overlapping effect, the

distance between the nucleus and the electron(r) changes slightly which in turn affect

the energy levels and spectroscopic parameters as is clear from Tables 3.2 and 3.3.

In glassy matrices the active ion sites are randomly dsordered unlike that of a crystal

matrix Hence each of these ions is under the influence of a field, due to the

surrounding cation which is found to vary from site to site. This non homogeneous

distribution of the active ion sites is further increased by the pairing and clustering of

~ d " ions in the borate matrix and is manifested from the fluorescence spectral data.

Thus evidences for the change in the environment of the ~ d " ion with increasing

NazO content are clearly shown in the alkali concentfation dependence of the ~ d "

absorption and emission spectra. It is because of this non homogenous nature of the

glassy systems, a smooth progression in the spectroscopic parameters cannot be

obtained as a function of glass composition for a given rare earth ion. Further the

parameten do not show an appreciable change with borate content. This may be due

to the fact that the interacting tield due to the boroxil ring system and the rare earth

ion is minimum.

The change in the boron wurdination i3 also found to be a critical factor affecting the

spectroscopic nature of ~ d l ~ ion in borate glassy systems. It is found that horate

glasses usually contain a mixture of BOX triangles and BO4 tetrahedra, depending on

composition. It is also noted that addition of alkali oxide will change the boron co-

ordination from 3 to 4. In the present glassy systems, analysis of the IR spectra clearly

shows the appearance of BO4 group (1065cm-I) at 30mol% and above the alkali

content (Figure 3.3). The present observations seem to agree well with those of Bray I4 " and of Krogh-Moe who found that the boron four cwrdination increases

smoothly with NazO concentration upto around 30moI% and more slowly until it

reaches a maximum at around 40mol% of NazO.

Optical absorption spectra of ~ d ' + doped alkali borate glasses have been analyzed

using Judd-Ofelt (J -0) theory of crystal field

induced electric dipole transitions. Analysis

yielded some of the important spectroscopic

parameters viz. radiative transition

probabilities, fluorescence branching radiative ratios and lifetimes, optical iF: gain of the principal fluorescence bansitions

3

originating from the 4 ~ 3 n metastable level. : i:

Variations of these parameters with glass .I-A-

a m m IS 10 5

compositions and their implications for Wave number ( 10' cm-I)

tailoring spectroscopic properties by Figure 3.3 Infrared spectra of alkali

compositional changes are discussed. To

understand the laser efficiency of the

borate (30 mol% of NazCO,) (Glass E).

material, the value of the spectroscopic

quality factor (Q) IS measured as a function of the fluoreswnce branching ratio and

the quantum efficiency of the '~xn+'llln emission component is obtained from the

measured and calculated lifetimes. Also, the stimulated emission cross-section

corresponding to this transition is obtained from the respective emission lineshapes.

The values of stimulated emission cross-section are comparable with that of other

glassy hosts used in solid state laser applications~

Figure 3.4 Variation of fluorescence branching ratios with Q-factor for the fluorescence transition 4 ~ 2 / 2 -+4~j(~=9/2, 1 112, 13R, 1512)

The quantum efficiency of the 4~1/,+411i/2 transition is found to be low in

comparison with other glassy matrices and is attributed to the high nonradiative

multiphonon relaxation rates in the borate matrix. In addition to this the intensity

parameters are used to evaluate the linestrengths for excited state absorption from the

metastable 4 ~ 3 1 2 manifold. Significant linestrengths are predicted at wavelengths near

1.044 and 1326prn. suggesting the possibility of radiative depumping of the 4 ~ 3 1 z

upper laser level via the stimulated em~ssion field.

According to Carnall et.al I S the observed absorption band intensities were

determined in terms of oscillator strengh (fq) from the Equation 2.72. According to

Judd-Ofelt theory, oscillator strenbah of an intracofigurational transition within the 4f

shell of a trivalent rare earth ion can be obtained from the Equation 2.85 Using the

matrix elements given in the litentureih and the experimental oscillator strengths

(f,,,). the intcnsitv paranleters ( C L j . ) have k r r evaluated by a least square

programming !hng the Ri parameiers the values of electr~c dipole line strengths

(Sd) are evaluated from the Equation 2.82 '". Even though all the bansitions have

appreciable electric dipole contribution some transitions satisfying the selection rules

AJ = 0, +I; AL = 0; AS = 0; and Al = 0 will have a small magnetic dipole contribution

and the line strength due to which is given by5.I7

The matrix elements 4i"Y 1 ~ + 2 d I YJ > were evaluated following Carnal1 et.all'.

The other three important radiative parameters viz radiative transition probability

(A), radiative lifetime (TR) and fluorescence branching ratio (P) can be evaluated

using the respective expressions Is. 'The stimulated emission cross-section,oE can be

determined from the emission lineshape using l9

Where 4 is the peak fluorescence wavelength and A is the radiative transition

probability for the m i t i o n Since the emission band is found to be slightly

asymmetric, an effective linewidth A& was determined using the expression17

where I, is the peak fluorescence intensity corresponding to Xp. Knowing the values

of the absorption (on) and emission (oej cross-sections, optical gain coefficient a, can

be obtained from the equation '"

where N is the ~onic concentration and P is the ratio of the excited to ground state

population density Using a and the length of the exciting material L. optical gain G is

obtained from the expression

Following the procedure of Weber and Moos 2122 the non-radiative transition

probability due to multiphonon emission, W, is given by

h 6J where ng =[ex&-- I)].'. Here &K is the occupancy of the effective phonon

k7'

modes and AE is the energy gap between the emitting level and the adjacent lower

level. Also for borate matrix B= 2 . 9 ~ a =3.8xl0"cm and h o = 1400cm.' For

Nd3+ ion, contribution due to non-radiative multiphonon relaxation is considerable for

the emission channel '~nn-+~l l jn . for which the energy gap is calculated to be

5400cm-' .

All the absorption spectra reported were similar, the only difference being a small

change in the relative band intensities. Because of the inhomogeneous line

broadening, the Stark component is poorly resolved. Table 3.4 presents all the

observed transitions with their experimental and theoretical oscillator strengths. The

oscillator strength values obtained in the present analysis are well in agreement with

the reported values of Nd3+ ion in other glassy matrices. The transition 4 1 ~ ~ + 4 ~ s a is

hypersensitive in nature2" because of its very large oscillator strength and hence is

excluded from the calculation of J - 0 parameten. Also the transition 4 1 ~ ~ - - + 4 ~ , , 2 , " ~ > ~

are found to be overlapping in nature. Hence in the fitting procedure these bands were

tmted as a single band with an appropriate combination of the respective reduced

matrix elements as done by ~ r u ~ k e " The average wavelengths were taken to be the

center of gravity of the absorption bands. The small rms. deviation between

experimental and theoretical oscillator strengths testifies the validity ot 'J-0 model in

predict~ng the radtative properties of ~d'+ion. The intensity parameters (2,.

determined from the least square fit of the measured absorption hand Intensities are

presented in 'l'ahle 3.5 I 'he obtained values range among those observed for other

borate glasses"

From the J - 0 parameters, the spontaneous emission probability (A) the radiative

lifetime (TK ) and fluorescence

branching ratio (P) have been

theoretically computed for the principal 4 tluorescence transitions, ~zn+?j ( J =

9i2, 1 2 1312, 1512) and are - = m

summarized in Table 3.6. - X u . - Y1

2 F -

The fluorescence spectrum for the five

glass compositions is given in Figure

3.5 corresponding to the transition 0.95 1.05 1.15 4 4 Wavelength (pn) Fzn+ Irrc The results in Table 3.6

Figure 3.5 Fluorescence spectra of clearly show that the transition Nd'+ ion (4~3n+ 4111n) in probability and hence the branching five glass compositions

4 ratio is maximum for the '~3n-t 1110

transition while minlmum for the 'Fln-tJllsn transition in all the glass compositions

studied. This is not surprising since the same result can be observed in all the Nd+'

activated hosts' Because of the higher value of transition probability, according to

Fuchtbauer-Ladenburg (Equation 3.4) the stimulated emission cross-section is

expected to be maximum for the ' F , ~ + ' I I I ~ transition. Supporting this argument.

analysis of the experimental fluorescence spectrum clearly shows that stimulated 4 4

emission cross-section and optical gain are always maximum for the Fzn+ l l l n

transition. In Table 3 7 we report the stimulated emission cross-section, effective

linewidth and optical gain for this transition in the five glass compositions. Results

show that galn and stimulated em~ssion cross-section vary in the sequence Glass

1:-(;lass A -.<ililssk3. .<ila.isC' -(;lassl). A general comparisol~ 01. the important

radiative proptrtles and J - 0 parameters of some of the common glass hosts is given in

'l'able 3.8

Table 3. t Zero order energy values (Eo,) and parhal derivatives of ~ d ' + ion for various transitions observed.

rable 3 .2 . Theoret~cally calculated and experimentally observed energy levels (cm-I) for various glass compositions

Table 3.3. Slater-Condon, Spin orbit interaction and Racah parameten of ~ d " in the borate glasses.

Table 3.4. Experimental and calculated oscillator strength of various absorption transitions.

RMS f 2.4x10h, _f 1 . 9 ~ 1 0 ~ , f 2 . 3 x l 0 ~ ( ) . f 2 . 1 9 ~ 1 0 ~ . _ f 2 . 1 5 ~ 1 0 ~

Table 3.5. Calculated J - 0 parameten and Qfactor for various glass compositions.

Table 3.7. Stimulated emission parameters obtained for the principle Fluorescence transition 4~zn341g1n.

~ ~ - 1 ~ ] - ~ l a s s B I Glass C I Glass D I Glass E

Table 3 . 6 . Theoretically computed radiative transition parameters for the fluorescence transitions from 4 ~ 3 , 2 excited state.

1 Transitions Glass A Glass B Glass C

- cm2 0.01 0.73 1.6 0.5

- Sdx lo'p

Sd x1q20

T

(ps) A cs-')

10 202 892 495

T

(ps) A (s-'1

P (%)

P (%)

625 0.6 12.6 55.7 30.9

cm' 0.19 1.42 3.01 0.84

20 390 1677 831

342 0.68 13.3 57.4 28.4

Table 3 8 Comparison of the J - 0 parameters (R2, RJ, G), peak wavelength (h), effective emission line width ( h b ) , radiative lifetime (r) and stimulated emission cross-section (oE) in various glassy matrices.

4 m

CLS: Gallium Lanthaniun Sulphide,ALS: Aluminium Lanthanum Sulphide

Table 3.9. Values of the excited state absorption transition line strength (S,) absorption cross-section (a,) in five different glass matrices

A close examination of the emission transition matrix elements of ~ d " ion reveals

that the 4~zn+41~s/2,41~x/2 transitions are almost independent of the matrix elements

< 4 ~ j n ( l ~ " ~ ~ 4 1 ~ ~ ~ , ~ 1 ~ > (h=2.4) and hence of the J-0 parameters Qand a. On the other

hand the transitions "~2,2+~190. I I R are dependent only on and Q, since <4~312/1~'/1

'I 9n,1 IR> are zero. In otherwords all the four transitions from the 'Fin excited state of

Nd3+ ion are independent of the parameter and hence a convenient way of

analysing the strength of each transition is to plot a graph between the fluorescence

branching ratio and the ratio of to Q,, the Q-factor as shown in Figure 3.4. From 4 the figure it is clear that to maximise the fluorescence intensity of the 4~31z+ I l l f i

4 transition, one wants %a. In contrast, to maximise the intensity of 4 ~ 3 1 2 + 1912

transition one wants Q,+. in practice it has been observed that WQ, varies only in 1,24 - the relatively narrow range of 0.7-1.1 in most of the glassy matrices except for the

borate glasses. In borate glasses Q-factor varies over a still relatively smaller range of

0.2-0.77 which is more in line with the range of 0.32-0.55 obtained in the present

study. The variation of the Q-factor which are functions of and Q, from one glass

composition to another is because of the dependence of these parameters on

refractive index through the factor (n2+2)l 9n as well as on the odd crystal field

parameters (Ak,) and Slater-Condon parameters (Fk). The large mabmitude of the P values are quite comparable to those reported previously24~i. Since and Q,

exhibits only small changes with composition for the five glasses studied, variations

in the values of p, crlc and G are almost negligible. In fact the G values are expressed

in units of dBtcm which is a logarithmic scale and hence the variations appears to be

not negligible. tiowever, in the real sense the changes are negligible if the gain is

expressed in units of cm'.

4 Measured fluorescence decay time of the F112 level shows that generally it varies in

the range SO-5Sps. Hecause of the very small fluorescence decay time and

comparat~vely h~gh radiative lifetime, which varies in the range 331-625ps, the

quantum efficiency of the 'F~,? emission is found to be vev small. 'l'he variation of

the radrat~ve lifetime i rR) with compos~tion can be accounted by the dependence of

the radiative transition probability (A) on refractive index, as A varies with n(n2 1 2b2!9

in the relevant expression for A In. The quantum efficiency is found to vary in the

range 8-15.2%. In fact the same result can be observed in any kind of borate

glasses 25 It is the only oxide glass whose average quantum efficiency is substantially

less than unity. The very low values of the quantum efficiency for the 4 ~ 3 / 2 emission is

due to the various nonradiative processes viz. multiphonon relaxation, cross-

relaxation and upconversionl. In the present case the nonradiative decay rate due to

multiphonon relaxation is calculated to be 3272s.' which is smaller than the maximum

radiative decay rate of 3021i' observed in Glass A composition. In addition to this

the other two nonradiative processes viz. cross-relaxation and upconversion can

considerably reduce the fluorescence lifetime of ~ d ' + ion. Our lifetime studies show

that the sum of the n o d a t i v e decay rates due to these two processes amounts to

13508s-~. The overall effect of these non-radiative processes is to quench the 4 fluorescence lifetime of the F3n emission and thereby decrease its quantum

efficiency.

The radiative parameters viz. radiative lifetime ( r ~ ) , stimulated emission cross-

section (01;) and optical gain (G) are found to vary from-one composition to another.

Analysis shows that the radiative lifetimqr~) of the 4 ~ 3 R state varies as

GlassD>GlassA>GlassC>GIassB>Glass E whereas the fluorescence branching ratios

are found to be almost constant. The peak wavelength(hp) is also found to be constant

whereas the stimulated emission cross-section(o1:) and optical gain for the '~3,7-+~11 I D

emission transition vanes as Glass E>Glass A>Glass B>GlassC>Glass D. Variation

of the above mentioned radiative parameters with composition in the present

investigation can be accounted by the e h t of the alkali cations that surrounds the

Nd3+ ion. It IS well known that borate glasses generally have lower expansion

coefficient and higher densities than other oxide glasses. These evidences show that

there is stronger bonding and denser packing in the borate. The electrostatic

interaction in the Nd-0-B system is comparatively stronger. Thus the lower decay

time, broader emiss~on bands and variation in the emission spectrum may be partly

explained on the basis of the greater interaction of Nd" with the surrounding borate

network. It was found that the Nd-O hand hecomes more covalent with increasing

Na20 content in the glasses. In general, the smaller the anionic field strength, the

smaller the Stark splitting and the narrower the effective linewidth. For a given

network former, the effective linewidth generally increases with increasing charge

and decreasing size of the modifying cations".

Clear evidences for the change in the environment of the Nd" ion with increasing

Na20 content are shown in the alkali concentration dependence of the Nd3+ absorption

and emission spectra. There is a change in the relative absorption band intensities in

the 580 and 750nm regions and also a change in the splitting of the former band. It is

highly probable that the ~ d ' + ion is occupying more than a single type of site, and its

large ionic size allows a coordination number 6 to 12. Because of its triple charge

and large ionic size, ~ d " ion probably occupies a network modifier site. It is usual

that multiple sites could result in a non-exponential fluorescence decay as the ions in

each type of site may have a different decay time. The exponential decay of the 'F~/z

emission indicates that the ~ d " ion occupies predominantly one type of site.

Several authors have discussed the observed trends of the J-0 parameters with Z6.27.28 compositional changes . In general it has been found that the addition of small

amounts of modifying ions to a particular system does not greatly affect the J-0

parameters. However the addition of large amounts of Na ions as in the present glass

system causes a decrease in R2 which is in conformity with the results reported

previously27. The large values have been correlated with- the increasing

asymmetry of the local environment of the rare earth ion2*. The rigidity of the

network surroun&ng the rare earth Ion was suggested to influence the magnitude of

f& and Q, '" lzumitani et.alu' observed that f& and Q> increase with Na content

and it was contirmed earlier workers" and was related to ionic packing ratios. In the

present system. however. the f& and C 2 ; values do not change consistently and do

not vary together in the same manner as sccn in most other glass systems. In general

our results show that the iL values appear to vary randomly with composition. Such a

random varlatlon in the 1;) values c a n only ht: explained if one assumes a preferential

placement of the ~ d " tons in the vlclnlty of a part~cular mod~lier.

The J-0 model is also effective in evaluating the intensities of excited state absorption

transitions originating from 4 ~ 3 ~ 2 metastable level corresponding to those wavelengths

at which appreciable radiation is present within the laser resonator. Figure 3.6 shows a

partial energy level diagram of the excited state manifolds of Nd3+ and several

relevant transitions. The first three transitions on the left corresponds to the three laser

bands at 1.32, 1.04 and 0.84pm. The energy spacing between the '~312 and the

in&cated terminal J manifolds are in all cases close enough to the energies of the

possible laser hansitions viz. ' F J ~ - + ' I Y ~ , ~ I ~ . I ~ ~ . The two transitions on the right of

Figure 3.6 are excited state absorption of the 0.813pm and of the 0.5257pm doubled

radiation of 1 . 0 4 4 ~ respectively

Knowing the various transition

matrix elements the values of the

linestrengths and absorption cross-

sections are evaluated and

summarized in Table 3.9. A

comparison of these results with

those presented in Table 3.6 shows

that the total linestrength in 4 absorption from the Fj12 to the

' ~ 1 9 n is nearly equal to the total

tluorescence line strength of the 4 ~ j n - + ~ l l l n transitions. This result is

consistent with the experimental

observations made by Vance on

~ d ' + i n soda lime glass. ~h~ Figure 3.6 Partial energy level diagram showing various excited state

element .- 4 ~ 3 ~ 2 ~ ~ l ~ ' ~ ~ L ( ; I .,$ for the absorption transition from the .t

excited state absorption transition F ~ Q state.

4 F , ~ + ? G is found to be zero and

hence ~ndependent of thc 01 parameter In the absence of C12 parameter, the strength

of the 'FIIZ-+'(~ I,,/: trans~t~on is solely be determ~ned by Ch and Q, parameters both of

whlch are found to be almost independent for ~ d " on It 1s the 522 parameter of ~ d "

ion, which varies greatly from host to host, and this explains the reason for the

comparatively same values for the '~3n+'G1912 and 4~112+4~lln transition line

strengths. In fact a calculation of the integrated cross-sections for these transitions

clearly shows that the cross-section for the 4 ~ 3 n + 4 ~ l ~ n transition is always higher

than that of the 'FXD+' Gm transition.

Table 3.9 also shows that the absorption line strength for the transition from the 4F3n

to the 4 ~ 7 n level is about 115 the fluorescence linestrength at 1 . 3 2 6 ~ . This excited

state absorption transition satisfies the selection rule AJ = X? and derives most of the

line strengths from the R2 parameter and is independent of the parameter. The

transition strength for the excited state absorption transition 4 ~ , n - + 2 ~ l n is negligibly

small compared to that of the 4~20-+419n absorption transition. Similarly for 4 F ~ ~ - + ~ D , ~ ~ transition also the linestrength is small. Thus by a theoretical approach

using the Judd-Ofelt model one can quantitatively evaluate the ' ~ 3 n excited state

absorption transition intensities of Nd3* ion in a given mamx.

Application of Judd-Ofelt theory proved to be effective in explaining all the radiative

properties of Nd3' ion in Nd201 containing borate glass and their variation with alkali

content. Comparison of the present results with other Nd" doped glassy hosts shows

fairly good agreement. The high electrostatic interaction in the Nd-0-B system

results in the low value of the ' F J ~ fluorescence decay time and high value of the

effective fluorescence linewidth. Both these factors substantially reduce the quantum \

efficiency and stimulated emission cross-section of the principal ihorescence

transitions from the 4 ~ 3 n excited state. The non-exponential character of the, 'F3n

fluorescence decay suggests that ~ d " ion probably occspies a network modifier site.

In addition, the large variation in the J - 0 parameters with compositional changes may

suggest a possibility of obtaining enhanced lasing properties by compositional

manipulatrons i'heoretical computations based on J - 0 theory also reveals that for the

excited state absorption from the 'F,,? state. transition linestrength is maximum for the

4F1:2 +'P~,: tnnsltlon Even though the quantum efficiency IS very small in all the

five g l a - composltlons, the 4~tt2-+ '11~n emission cross-section and optical gain are

comparatively large in glass E composition and hence this appears to be a possible

candidate for effective lasing action.

3.3.2 Spectroscopic properties of sm3' ions in phosphate glasses

All the absorphon spectra are found to be almost similar in appearance except for the

band intensity. Therefore a representative absorption spectrum comspond~ng to the

glass sample with 1.5 mol% of sm3+ is shown in Figure 3.7. The absorption spectrum

show ten absorption bands corresponding to transitions 3 6 ~ ~ 3 ~ , %In, %3n, 6 F5n, 6~7n, %yn. 'FI IR, 41 I In. 'KI I R + ~ L ~ ~ R , 6 ~ 7 / 2 . The estimated absorption coefficient

of sm3+ bands in different sample follows a linear relationship with the sm3+

concentrations. This figure indicates that sm3+wntent of the glass is the same as that

of the starling materials. This is in conformity with the results reported for other

Zirconium-fluoride glasses'2.

1350 2500 Wavelength (nm)

k'lgure 3 7 Absorption Spectrum of ~ m " Ion in phosphate glass

Compositional studies of the phosphate glass have been achieved by using the theory

of crystal field-induced electric dipole transition between 4f-4f ~tate".'~. The

intensity of the absorption bands is determined in terms of oscillator strength f.,

using the expression

where E ( V ) is the molar extinction coefficient at the wave number (v cm-I) the

symbols have the usual meaning. According to Judd-Ofelt theory the oscillator

strength for electric dipole transition between 4 f states of rare earth ions is given by

the Equation 2.85. The results are summarised in Table 3.10.

Using the values of f,, we have computed the three Judd-Ofelt intensity parameters

(Qls) by least square analysis. The values of the reduced matrix element of a state

determine the integrated absorption coefficient. It was reported that matrix element of

majority rare earth ions are only significantly dependent on environment in which

they are situated. The factor, which is responsible for the variation of intensity of

transitions, is these three intensity parameters. This effect gets relaxed to some

extent when rare earth ion forms a covalent bond with host matrix2'. From the

calculated values of Q electric line strengths (Sd) and magnetic dipole line strength

(&) are calculated using the expression 2.81and 2.94 respectively.

The various spectroscopic parameters calculated are given in Table 3.1 1. The value

Q2 for ~ m " In the present glass lies, in general, between values observed for

crystalline oxide and fluoride glasses The value of 0 2 is strongly enhanced by

covalent bonding in glasses which is equivalent to the dynamic polarization of the

ligands by the quadrupole moment of the transitions. R;. values are much higher in 1.1 phosphate glasses than in tluondes The reason for such a behavior is the large

number of occupation of non-equivalent sltes and lower symmetry in which the rare

earth Ions are located 'l'he etrect manifests in the form of large line width of the

opt~cal spectrd 'l'k "Hit! 3 ' ~ 7 ~ ~ IS found to be hypersensitive by its higher value of

oscillator strength. Its intensity may vary significantly with environment due to

strong 4f-5d mixing. This hypersensitivity comes into play with strong covalancy of

the bonds, or with strong local variation of the dielectric constant 36.

Figure 3.8 shows the fluorescence spectra of sm3+ in phosphate glasses. The

observed emissions correspond to transitions with higher probabilities. From the

spectra we have identified four emission transitions 4~5/2+6~5,2,7/2:)fl.~ IR.

The intensity of the emission

lines varies slightly with sm3+ ion concentration in the glass - samples. This indicates a matrix =!

m - independent emission per ion for .- 0 all the four transitions that leads 2

S 0 -

to the assumphon that the

concentration dependence should

result from changes in the 430 530 630 730 Wavelength (nm)

absorption alone. sm3+ Figure 3.8 Fluorescence Spectrum of ~ m "

fluorescence lines are parity ion in phosphate glass forbidden, but they can be

observed by the violation of

parity for dipole transitions. This is the special case only with selection rule AJ = 0,

+1 ". In this case the transitions ' G ~ ~ + ~ H ~ ~ , ~ ~ are dipole in nature and the other two

are higher multipole transitions. The break down of the above selection rule must be

due to the hybridization of the rare earth 4f -state with the nearest neighbour shell ".'.

Fluorescence proprties of the glasses are analysed by Judd-Ofelt model applied to

dipole and multipole transitions. 'I'he radiative transition probabilities (A) for

different transitions are calculated using the Equation 2.98

By combln~ng these values for all the levels, the radiative llfetlme ( r ~ ) is evaluated

from the relatlon

Table 3 10. Measured values of energy levels, oscillator strength(lo4), electrical dipole ~ t r e n ~ t h ( l ~ . ~ ~ c m . ~ ) , of sm3* ion in phosphate glass with three different mol % of Sm:03.((i). 0.75 mol%, (ii). 1.0 mol%, ( I ~ I ) 1 25 rnol ?'o, (jv).1.5molQh)

~ ~ -~

I I 2 1 3 ! 4 I

. . - ~~ ~ 7 I.e\ els v f S d ; v

,. . .

G- : 30058 255.02 ' 8.31 ' 3051

v

30056 . . - - ~ ---- ,

F

267.64

Sed

8.91

f ' S d v I I

, 251.81 i 8.01 I 30049

4 6 3 ! K : ; , 21381 20.01 1.53 24390 20.80 ' 5.51 124392 i I

9.61 1 f

247.81

i

1.51

1.67

2.81

1.89

1.73

0.53

0.73

1.30

4 - L!;:: I I ;x----

I 20513 ' 9 1 3 ; 1.17 j 20491 -~,-. P 1

"FI]:: ) I 0 6 2 8 2 . 3 1 11.31 (10621

T---T~G~ F.1: 2.03 2.32 I 9089

21.50 582

m m

24389

20502

10631

9081

8089

7111

6667

6412

5260

7.97

2.67

2.31

2.21

1.80

0.98

1.81

1.29

I 21.51 ,

8.97

2.63

2.47

2.01

1.93

1.28

1.72

1.32

2.01

1.72

3.01

1.90

1.85

0.46

0.81

0.3 1

i

8.38 1.92 - 2.51 1 . 5 3

2.38 1 2.41

20509

10630

9085

8094

7109

6679

6407

5259

' ? f77 -18097 ' 1 . W I 1 7 5 i8091 11.89 11.98

' 5 , : 7 1 0 0 I 8 1 ' L 6 2 1::; 7 1,: ' 6694 '0.97 I 0.32 r--t-

FI': 6409 1 1.53 / 0.75 ! 6404

h 5255 1.19 , 0.29 I 5252 --

"2 " 2 I

1.10 10.42 1 1.62 0.82

1.21 I 0.28 I

Table 3 1 I Measured values of energy levels, transition probability A (S), total transition probability A-I (S), radiative l ~ f e time T ~ ~ ~ I ~ s ) and Judd-Ofelt intensity parameters Qxs(10-'~ cm2) sm3+ ion in phosphate glass with three different mol 4.0 of Sm203. (i). 0.75 mol%,(ii). 1.0 mol0/0, (iii). 1.25mol%,(iv). 1.5 mol%)

C- I Levels I A I p I A P A -r~ ~ ~~

P I

G-,: 753 5 0 27 ! A, =

----,--

I 1 - L17: t = 7 = --- , m- ~ 111::'415.3 10.15 1360.2 353.4

Table 3.12. Calculated life time(ps), half band width (nm), emission cross section

(1 0-20cm-2) of identified emission bands for sm3+ doped phosphate glass

KK - I/Al{YIJ)

where AI ('4'1) - EA(YJ, 'YJ' )

The calculated values of radiative transition probabilities and lifetimes are listed in

Table 3.11. The concentration dependence of the fluorescence intensity of sm3+ in the

phosphate glass for the three fluorescence bands is given in Figure 3.9. At lower

concentrations, intensity increases linearly but at concentration above 1.0 mol% a fall

in the intensity is observed due to the fluorescence quenching by the nearest

neighbour ion interaction.

b~gure 7 9 Vanatlon of'lntens~ty of em~ss~on bands Vs concentration o f ~ m " ton in Phosphate glass

'The variation In intenslt). I'or the 706nm band is observed to be almost negligible and

therefore not glven in thc figure. The time resolved fluorescence decay profiles have

been analyzed to extract life time values. 'The meawred life time of the excited level

may be written as l/r -- WR + WNK where Wll is the radiative transfer rate of the

level and WNK is the multiphonon relaxation rate of the host lattice and its value is

found to be 1.26~10'. For the lifetime measurements the desired level was directly

excited. This is especially important for slow nonradiative rates. For laser active

transition, the corresponding PR value characterizes the laser power of the transitions.

In the present samples we found such a possible transition '~sn3'111n with PK assuming the maximum value 0.16 (The transition + '(370 is hypersensitive).

However, the fluorescence spectrum shows a maximum fluorescence eficiency for

the 4 ~ s / 2 + 6 ~ 7 n such a behavior has been attributed to the nonradiative relaxation

from the level.

The stimulated emission cross section (0) for 4~ 5 ~ 3 ( ' ~ 7 ~ has been calculated and has

the same order of magnitude for other rare earth ions in phosphate glasses1. We have

used the Equation 3.5, here lip corresponds to the peak position of emission line and

ALn. is the effective line width, which is used because of asymmetry of emission

bands. The results are summarized in Table 3.12. The ' ~ 5 1 2 3 ' ~ 7 0 transition is

found to have high cross-section and hence high optical gain.

3.4 Conclusion

The spectroscopic parameters derived from the absorption spectra show variation as a

function of alkali oxide concentrations. Such a variation can be attributed to the

overlapping of the 4f wave function of the rare earth ion with that of the surrounding

ligand ion. Also non homogeneous environment experienced by the rare earth ion

with the alkali content is responsible lor the uneven variation of the parameters. It can

be noted that the coordination change takes place when the alkali content reaches a

value of around 30% On the other hand the parameters do not seem to be affected by

the increas~ng borate content. Also the application of Judd-Ofclt theory proved to be

effective in explasn~ng all the radiattve properties of Nd" ion in Nd20, containing

borate glass and thetr variat~on wrth alkali content. Comparison of the present results

with other Nd" doped glassy hosts shows fairly good agreement. The extremely high

electrostatic interaction in the Nd-0-B system results in the low value of the 4 ~ 3 n

fluorescence decay time and high value of the effective fluorescence line width. Both

of these factors substantially reduce the quantum efficiency and stimulated emission

cross-section of the principal fluorescence transitions from the 4 ~ 3 ~ excited state. The

non-exponential character of the 'F3n fluorescence decay suggests that ~ d ' + ion

probably occupy a network modifier site. Theoretical computations based on J-0

theory also reveal that for the excited state absorption from the 4 ~ 3 ~ state, transition

line strength is maximum for the 4 ~ 1 ~ - + Z ~ ~ ~ transition. In conclusion even though the

quantum efficiency is extremely small in all the five glass compositions, the 4 F ~ ~ + ~ I I I ~ emission cross-section and optical gain are comparatively largest in glass

E and hence this could be regarded as a possible composition.

The absorption spectra of sm7* in phosphate glass matrix have been analysed for

spectral and Judd-Ofelt parameters. From the optical absorption spectra of sm3+ in

phosphate glass various d a t i v e properties such as transition probabilities, radiative

life time, branching ratios were determined. The higher value of n2 compared to

crystalline lattice supports the presence of ion occupying sites with non centro

symmetric potential. Radiative lifetimes of the excited states are detennined and used

to obtain nonradiative transition rates. The fluorescence studies confirmed the

quenching mechanism at high rare earth ion concentration and low phonon relaxation

rate. It is concluded that the transition 4 ~ 5 ~ + 'H~/z has the potential in realising

optical amplilicatlon.

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