Indi an Journal or Pure & Applied Physics Vol. 40, August 2002, pp,577-587

Absorption and emission spectral studies of Sm3+ and Dy3+ doped

alkali fluoroborate ~asses R R 'Reddy ,' Y Nazeer Ahal~l~e(( pr Abdul Azeen;, KfRam3 Gopa l &T V R/Rao -

Department of Physics, SK Uni vers it y, An ant apur 515003

and

S Buddhudu & N Sooraj Hussain

Department or Physics, SV Uni versity, Tirupati, AP

Received 23 Jul y 200 1; revised 22 December 200 1; accepted 8 March 2002

Absorption and photoluminescence spectra of Sm~+ and Dy'+ doped alkali flu oroborate glasses of the co mposit ion 90,5 8 20., + 4 AIF + 5 RF + 0.5 LnF3 (R= Li , Na, K and Ln = Sm, Dy) are reported. On exc itation , with 402 nm, the Sm3+ glasses were found to be orange fluorescent in colour and richness of that colour was high in the Na+ glass, compared to Li + and K+ glasses. Similarly, on exci tati on with 398 nm, the Dy'+ glasses were fluorescent yellow and that colour was richer in the K+ glass.

1 Introduction

Gl asses have severa l convenient and useful advantages compared to different crystalline mate rial s . Although there are a series of inorganic glasses available, the re ex ist certain problems in achieving higher luminescence effic iency by the lanthanides as the fluorescing centers. Since 1975, the situati on was somewhat different , with the emergence of a few halide opt ical glasses w ith suffi c ientl y high luminescent yie ld by the dopant rare-earth ions. From that time onwards , a considerable progress has been made in the opti cal c haracte ri zation of g lasses based on flu orides, chlorides and iodides l

.x.

Optical spec troscopy ha~ been used as an important too l to study the nature of g lasses for the past man y years. The re lative impac t o f spectroscopy has probably been greate r in the analys is of glasses than those of c rysta ls . Investigation of the absorption and luminescence properti es o f rare-earth ions doped into diffe rent g lasses have found diffe rent appli cations in the fie ld s of lase rs. te leco mmunicat ions and al so in the producti on of a w ide vari e ty of optical component s (as windows, pri sms, optica l fibre cabl es) . T he development and success ful use o f rare-earth compounds as solid state laser mate ri a ls and the ir utility in modern tec hno logy as opti ca l devices (up

converters , phosphors, optical fibre amplifiers, etc.) stimulated interest in rare-earth spect roscopy. In thi s direction , a great amount of investigati on has been carried out to optimize new g lass matrices containing rare-earth ions. Optimizat ion of new or improved optical quality g lasses doped with rare­earth ions have been characterized by absorpti on and emiss ion transition probabilities which are influenced by the li gand fi e ld of the surrounding rare-earth ions.

Study of the optical properti es of the rare-earth ions in g lasses provides fundamental data that inclades tran sition 'positions and cross-sections, transition probab il ities ; radiat ive and non-radiati ve decay rates, branching ratios , etc . for the excited states . These data are essential to des ign opti cal devi ces such as lase rs. co lour d isplay ', up converters and fibe r amplifie rs . 1n order to identi fy new optical devices fo r specific utili ty , or devices with enhanced performance, active work is be ing carri ed out by se lec ting appropriate new hosts doped with rare-earth ions . The aim of the prese nt study i ( I ) to systt'matize the ludd-Ofell parameters for Sm3

+ and Dy3+ ions , (2) to dete rmine the radiati ve properties for s ignifi cant le ve ls. Many workers~ II>

have emphas ised the importance o f ra re-earth spectra and its applications . Ana lys is and unders tanding of the ra re-earth spectra ga i n ~ d

furthe r momentum through the s imultaneous

sn INDI AN J PURE & APPL PHYS, VOL 40, AUGUST 2002

developments of theoretica l models, whic h were reviewed broadly in Wybourne's book '7 The object of the present paper is to report the production and the optica l properties of alkal i f1 uoroboratc glasses doped wit h two rare-earth ions (S I11 '+ and Oy'+), suitab ly added with the required amou nts of the alka li flu orides (RF, where R= Li , Na and K) as the glass network modifiers.

2 Experimental Detai ls

The chemi ca ls (99.99 %) used in the glass preparati on were HlBO" AIF1, LiF, SmF, and DyF1. Fo ll owing are the chemi cal compos it ions of the alkal i f1uoroborate glasses: 90.5B20 ,+4AIF, +5RF+ O.5LnF, (where R = Li, Na and K; Ln = Sm and Oy) . The detai Is of the preparation of the glass techniques were already reported in literature ,x-2' .

The glass melting temperature was in the range of 650-900 QC and the melts were quenched in between two well-poli shed plates, in order to obta in smooth­surfaced bubble-free glasses. The synthes ized glasses are all in ci rcular shapes with a uniform thickness of 0.297 cm and having a diameter in the order of 2-3 cm.

For convenience, three glasses have been labeled as Glass A(LiF), Glass B(NaF) and Glass C(KF). Tab le I provides the chemical compos ition (mol%) of these glasses (A, B and C).

T able I - The chemical composi ti ons (mol %) o f Sl1y1+_ and Dy'+ doped alkal i Iluoroborate glasses

Glass B20., AIF, L iF NaF KF SmF,/DyF, A 90.5 4 5 0.5

B 90.5 4 5 0.5

C 90.5 4 5 0.5

By employing Archimedes principle, the glass densiti es were measured with Xylene as an immersion liquid . The refracti ve indices of these glasses have been measured at A = 589.3 nm on an Abbe refractometer with the monobromonapthalene as the contact layer between the sample and pri sm of the refractometer by using sodium vapo r lamp as the source. The refractive index and density of the sm1+- and Dy1+_ doped glasses thus measured are given in Table 2.

The absorption spectra of Sm1+ and Dy'+ doped glasses were recorded on Hitachi··U3400 spectrophotometer in the wavelength range 390-590 nm and 350-850 nm. Figs I and 2 show the

measured absorp ti on spectra of Sm1+ and Dy'+ doped li thium f1uoroborate glasses, respectively. The spec tra fo r the other glasses are quite similar (not shown). The emiss ion spectra of the e glasses have been recorded on an F-30 I 0 Hitac hi fluorescence spectrophotometer at the Central Instruments Laboratory, Un ivers ity of Hyderabad . The spectral fea tures for Sm'+ and Dy\+ in lithium' f1 uoroborate glasses are shown ill Figs 3 and 4, resp cti vely. The emtSS lon spectra for the other glasses are quite similar.

On excitation, wi th 402 nm, the Sm'+ glasses were fo und to be orange fluorescent in colour, whose ri chness was hi gh in the Na+ gla<;s, compared to Li + and K+ glasses. Similarly, on exc itation with 398 nm, the Dy'+ glasses were flu orescent ye ll ow and that co lour was richer in the K+ glass.

3 Results and Discussion

3.1 Glass density and refractive index

The densiti es of the glass samples were determined by the Archimedes method, in which the glass was weighed in air and then in xy lene at room temperature. The density (p) was then ca lcul ated fro m the foll owing formula :

HI p=--0.865

I V- II'I

where w is the weight of the glass sample in air, H' I

is the weight of the glass sample in xyl ene and 0.865 is the density of xy lene at room temperature (30 QC).

The concentration , C of the Ln ion in the glass (mol/l ) was found from the express ion:

c=L~IOOO mw x

where y is the Ln sa lt mass, x is the total mass, d is the density of the glass and I11W is the molecul ar weight of the Ln salt.

The concentrati on in mol/I can be converted into ionslcm' by mUltiplying it wi th a fac tor of N A I 1000, NA is the Avagadro's number.

Refractive indices of the Dy'+-doped f1u oroborate glasses studied JI1 the present investi gation were measured uSlllg an Abbe refractometer at the wavelength of sod ium ye ll ow line (589.3 nm). The contact liquid used was:

REDDY et al. :FLUO BORATE GLASSES 579

I-Bromonaphtha lin (C IO H7Br) . The va ri ati ons in the refrac tive indices de te rmined at diffe rent wavelength s (by using diffe rent fi lte rs for mercury source) were found to be neglig ible and within the ex perimental e rrors. Therefore, the re frac ti ve indices of the Dy'+: f1u oroborate glasses were dete rmined onl y at the wavelength of sodium light. But in the case of Sm\+ : f1u oroborate g lasses, by empl oy ing an Abbe refrac tomete r, with detachable

lamp facilit y re fract ive indi ces 1I e- (at 656 .3 nm), lIiat 589.3 nm) and II I (at 486. 1 nm) have been obta ined .

3.2 Physical properties

From the es timated values o f both dens iti es(d) and refracti ve indices(lld) ' othe r re lated phys ica l propert ies, such as average mo lecu la r we ight (M), atomic vo lume (V), ra re-earth ion conce ntrati on(N),

1.35.-----------------__________________________________________________________ --.

1.3

1.25

!1 'c :::J

€ 1 .2 .e .2;-'j!! Q)

Cl (ij

1 .15

,g a.

0

1 . 1

1.05

1 .

390 400 41 0 4 20 430 «0 450 460 470 460 490

W a velength (nm)

Fi g. I -- Absorption spectrum of Sm'+: 90.3 BP.\ + 4 AIF + 5 LiF glass

1 .25 6 H 1512 ~ 4 M 19f2

1 .2

~ 1 .15.

c::: :::J .ri 1 . 1

~ ~ 1 .05 'm c: Q) "0

CI3 1 <.> :a 0 0 .95

0 .9

0 .85 350 450 550 650 750 850

Wavelength (nm)

Fig. 2 -- Absorption spectrum of Dy'+: 90.3 8 20 , + 4 AIF + 5 LiF glass

580 INDIAN J PURE & APPL PHYS, VOL 40, AUGUST 2002

molar refracti vity (RM ) , polaron radius ( rp),

interi onic di stance (r j), electroni c polari zability (a e),

and fi eld strength (F) of the glasses have been computed by using the relevant ex press ions shown below. The Sm and Dy ion concentrati on is obtained from21' :

N(ions)/cm' =

(Avagadro's numher)x(glass density) (mol % of RE)x---------

(glass average molecu lar we ight)

From the evaluati on of the magnitude of N, one can determine the three other related physica l properti es as described below27

:

~ ' in c:

$ .£ 300

:'l c:

'" ~ :!! o

.E ~ 2 0 0

'" a; rr

100

ot=====~==~~------------~~--~~~====~====j 5 00 550 600

W a velength (nm)

6 5 0

Fig. 3 - Photoluminescence spectrum of Sm' +: 90.3 B20., + 4 AIF + 5 LiF glass

7 00

350 r------- - ------- - ------------------- -------,

300

250

100

50

0 ,~--------------------~-----~~==~~5:5:0--------~600 4 00 4 5 0 500

W a vele ngth (nm)

Fi g. 4 - Photo luminescence spectrum or D/ ' : 90.3 BeQ .. + 4 Air + 5 LiF gla"

REDDY et al.: FLUOBORATE GLASSES 581

Polaron radius, r p (A)= 1/2(O/6N) 1/3

Inter nuclear distance, r j (A) = ( I IN) 1/3

Field strength , F (cm2) = (Zl rp 2)

calcul ated from the followi ng relationsh ip2x:

? -

R = (n;i - I) M /1/ ( . 2 2) d nil +

The molar refractivity (RM) for each glass was where nJ is the refrac tive index of the glass at 589.3

Table 2 - Physical properti es of Sm3+_ and Dy'+- doped alkali flu oroborate glasses

Physica l propert ies Glass A Glass B Glass C

Sm3+ Dy'+ Sm3+ Dy'+ S m3+ Dy'+ M (g) 6 1.649 1 61.7099 62.4516 62.5 124 63.257 1 63.3 179 D(g/ cm·3) 1.8099 2. 1248 1.9447 2. 11 49 1.9983 1.7495 . nd (589.3 nl11 ) 1.4324 1.4324 1.4296 1.4296 1.43 10 1.43 10 Vd - I 0.0 176 0.0 175 0.0175 Ni lO l3 esu) 1.0355 1.025 3 1.0304 Y ( lOi S cm2/w) 0.3027 0.3003 0.30 15 RM (cm··1) 8.8420 7.5390 8.289 1 7.6294 8. 1940 9.3683 V (g/cm3 /atom) 5. 1453 4.387 1 4.85 10 4.4650 4.78 18 5.4670 N( I 022 ions/cm3

) 0 .8841 1.0369 0.9378 1.0 188 0.95 13 0.832 1 ex" ( I 0.24 cm3) 3.5064 2.9897 3.2872 3.0255 3.2495 3.7152 1'1 CA) 4.836 1 4.5858 4.742 1 4.6 128 4.71 94 4.9348 rp (A) 1.9486 1.8478 1.9 107 1.8586 1.90 16 1.9884 F( I 016 cm·2) 0 .7901 0.8786 0.82 17 0.8685 0.8296 0.7588

M, average molecu lar weight; V, atomic vo lume; Vd- I, glass dispersive power; 11 2, non-linear refracti ve index; y, non-linear refracti ve index coeffici ent ; N, rare earth ion concentration; RM , molar refractivity; rp, Pol aron radius; r j, interioni c distance; ex", electron ic pol ari zability; F, fi eld strength

Table 3 - Line strengths (Sed X 10211 cm2), spectral intensiti es ([cd X 106) and Judd-Ofe lt intensity (D Ax I 0211 cm2) parameters of

Sm3+_ and Dy'+- doped alkali tluoroborate glasses

Absorpti on Glass A Glass B Glass C

Transitions Sed

Sm'+ I~xp t~al Sed fexp 1',,,1 Sed fexp ( "I

6H s/2 ~ 4F712 0 .7697 4.42 16 4.42 16 0.8 157 4.6764 4 .6757 0.7504 4 .3065 4.3088 ~ 6

P S/2 0.2362 1.3647 1.3 112 0.2950 1.6724 1.3865 0.2388 1.3572 1.2777

~ 4113/2 0 .2038 0.7567 1.0 138 0.2301 0.8429 1.0721 0.2034 0.7533 0.9880

~ 4111 12 0 .0838 0.94 17 0.4037 0.0931 1.0664 0.4269 0.0834 0.93 15 0.3934

D2 3783. 1635 3998.4384 3686.1 528

D4 8.9823 112160 9.0803

D6 7.5559 8.6 170 7.7250

D41n6 I. 1887 1.3016 1.1754

Dy3+

2.2257 5.1 928 4.62 11 6H ISI2 ~ 4M IW2 1. 852 1 4.3229 4.3952 1.7486 4.3808 6.6394

~ 411312 0.6684 2.4877 1.49 19 0.7587 2.5238 2.2537 1.1 28 1 3.6690 1.5680

~ 411 S/2 1.0039 1.4459 1.9227 1.1 990 1.8927 2.9044 1. 8 158 2.9352 2.02 15

~ 6F,12 0.0590 0.0932 0.0676 0.0965 0.28 16 0.1021 0.1204 0.497 1 0.0711

~ 6F s/2 0.3340 0.3646 0.3584 0.5464 0.5776 0.5414 0.6812 0 .7004 0.3768

D2 124.6292 146.28 14 226.2773

D4 102.6246 92.0882 11 6.5025

D6 1.9676 1.5827 1.9733

DJD6 52.1572 58.1842 59.0394

582 INDIAN J PURE & APPL PHYS, VOL 40, AUGUST 2002

nm; M is the average molecular weight and d is the density of the glass.

The molecular electronic polarizability factor (a) was determined using the formula2~:

1 (11,7 -I) a = -----'---"----'-

(40N/3) (,,3+2)

where nel is the refractive index at 589.3 nm and N is the Sm>;- and Dy>+ - ion concentration. From the average molecular weight of the glass, the densities and the total number ions and the mean atomic volume (in grnIcm~/atom) can be obtained.

Non-linear properties - The non-linear parametrization was considered necessary to understand the optical quality of the glasses. These properties were estimated from the glass refractive indices obtained at three wavelengths [11.0 Ad and Arl. Boling et aUo suggested a theoretical formulation to predict the non-linear refractive index (fl2) changes in optical glasses, according to which 112 must be small enough to minimize the environment effects on the dopant ion. According to the article published by Glass" of Lawrence Livermore Laboratories (1975), the prepared material could be declared as optically active, based on the magnitudes of their dispersive power, defining the popularly known Abbe number (vd) having the values between 50 and 100. The mathematical expression for the Abbe number is:

and non-linear refractive index [1l2] is1 1:

Once this 112 is estimated, it becomes easy to determine the non-linear refractive index coefficient (y) as given below12

:

where C is the velocity of light.

The estimated physical properties of these glasses have been presented in Table 2. It is

observed from Table 2 that the atomic volume, molar refractivity, electronic polarizability, polaron radius, and interionic distance are higher in Glass A than in the other two glasses (B and C) in the case of Sm>+. In the case of Dyl+, all the above parameters are high in Glass C than in the other two glasses (A and B).

3.3 Absorption spectra

The measured absorption spectra of the Sm>+ and Dy'+ glasses (Figs I and 2) show four absorption bands in Sm1+ and five absorption bands in Dyl+.

3.4 Spectral intensities

The most difficult problem in rare-earth spectroscopy is the measurement of intensities of absorption bands. The intensities of spectral lines are measured in terms of oscillator strengths using the relation:

Cp = 4.32 x 10·~ J E (v) dv

where E (v) is the molar extinction coefficient at the wave number (cml) . If the absorption band takes a Gaussian shape, the oscillator strength can be calculated by the half-width method when :

Cp = 4.32 x IO·~ L fly

where fly (cml) is the width of the band at half the peak intensity.

Quite often, the bands in glasses do not show a Gaussian shape. In such cases J E (v) dv should be evaluated by measuring the area under the curve, which is reliable though tedious. Further, the bands in glasses exhibit inhomogeneous broadening and the half-width method gives erroneous results. In the present work, the intensities of all the bands are measured by the area method .

The observed and calculated oscillator strengths for Sm~+- and Dy>+- doped alkali fluoroborate glasses are shown in Table 3. It is interesting to note that, the oscillator strengths of the observed bands of the Sm>+- doped sodium fluoroborate glass are relatively higher than those of lithium fluoroborate and potassium fluoroborate glasses. This implies that, the non-symmetric component of the electric field acting on the Sm1+ ions in the sodium

REDDY el al.: FLUOBORATE GLASSES 5R3

f1u oroborate glasses is relat ive ly stronger than the lithium and potassi um f1uoroborate glasses . Simi larly, in the case of Dy'+- doped alkali f1u oroborates, the osc ill ator strengths of the observed bands are higher in the case of potass ium f1u oroborate glasses compared with other two alkal i f1u oroborate glasses.

3.5 Judd-Ofelt parameters

The osci ll ator strength of a band is theoretica ll y represented by the formul a.13:

where T2, T~ and T(, are ca lled as Judd-Oflet intensity parameters and ( II if II r, ( II V~ II )2 and ( II V" II r are the squares of the unit tensor operates ca lcul ated in the intermediate coupling approximation . Since these square reduced matrix elements are host invariant , the va lues reported by Subrahmani am el

a/ :'~, Carnall et aC' were used by the authors. Us ing the measured oscill ator strengths for j~' I" the wave numbers of the respect ive bands and the values of d I if II )", ( II u' II r and ( II u' II r are derived by the least squares method .

Since T" factors obtai ned from least square f it analys is do not invo lve the effect of glass refrac ti ve index II , they are transformed into Q / throu gh the fo ll owi ng expression'(':

where II is the refractive index of the glass, (2J+ I )

the degeneracy of the grou nd -state of the ion, IT the Planck constant and the other symbols have their u. ual significa nce. The Q" parameter obtained for the Smh and Dl+ions in alkal i fluoroborate glasses are presented in Table 3.

A glance at Table:) reveals that, the Judd-O nct parameter (n,) is higher in value compared to the other two factors ( n~ :!I1d Q r, ) in all glibses. For the observed absorpti on bands , the values of the un it tensor operators ! I V~ I were taken from the li terature (Carnell et a/.J<) on the absorption spect ra of RE ions (aquo). The tabulated absorption resu lt<; show thaI. the spec tra l intenslt lcs are more prominent in Glass B, in the Sm 1

> - J oped alkali fluoroboral t.: glasses, than in Gla:.;ses A or C. But , in

the case of Dyl+_ doped alka li f1uoroborate glasses the spectral intensities (j~xp x l O r,) are more in Gl ass C, than in Glasses A or B.

Jacobs & Weber17 have reported th at, the rati o (QiQr,) wou ld be known as the spectroscopic qua lity factor to characteri ze the glass concerned. The va lue of (Qi Q r,) for the glasses studied are given in Table 3. Based on the magn itude. of the spectroscop ic quality factor (QiQ r,) , it is fou nd that, the Sm-Glass Band Dy-Glass C appear to be better opti cal glasses.

3.6 Radiative properties

The absorption spectral resu lts have been utili zed to understand the radiative propert ies of the fluorescent levels of the Sm and Dy glasses. For the Sm '+, Dyl+ ions , the foll owing are the emission leve ls:

~ P5/2 ~ ('F,n, ~Fj/ 2 ' 6F7I2 , "H'/2 ' "H7/2 , (' F 'JI2 , "FII/2 . flH 11/2/'H un , 6Hw~

~ F7/2 ~ 6F1I2 , ('F;,2, ('FI /2 , r, F 7n , ('H7/2,('E '12 ' 6H1/2, 6F11I2 ,

"H, It!, !l HIJ/~' Ill-I 1:\;2, (lH 15/:!

The J-O parameters listed in Tab le 3 have been used to dekmline the spontaneous emiSS IOn probab ility of eac h of these 14 fluo rescent levels fo r Sm" and 30 fhlorescen t levels for Dl ' from the ex press ion17

:

where Sod is the electri c-di pole lille-\t rcngth_ 1/ is the re frac tive index. v is the energy of the transit ion (Cln- I) and the other fac tors have their standarJ mea ning. To evaluate the S,,! va lues for these e ml ss lon-s t a te~ . the squared reduced matrix elements have been co llected from Ca rnall e l {f/. l' si nce they are host invarian t. The tota l transitioJl probabil ity of emission-state was obtained 0 11 taki ng the summation of ind ividual probabili ty va lues from thi s state to its lower lying states:

584 INDIAN 1 PURE & APPL PHYS, VOL 40, AUGUST 2002

Table 4 - Radiative properties (A, AT, PR (% ), l R) of the different emission levels of Sm:1+_ and O/-doped alkali tluoroborate glasses

Tran ~ iti()n Energies Glass A Glass B Glass C v (em·l) A (S· I) PR(% )

(for Sm:1+) A (S· I) PR( % ) A (5.1) PR (%)

~G5I~ ~ (,Em 8842 1319 2 1386 ~ 1281 2 L.

~ (,F512 10890 9549 16 10034 17 9277 17 ~ "FJ12 11 394 1763 3 1852 3 171 3 3 ~ (, 1-19/2 15690 44281 76 46538 76 43021 76 ~ (, 1-1 7/2 16896 1943 3 2300 4 1920 ]

~ (, 1-1 512 17930 1386 3 1458 2 1346 2

AT(s· l) 6024 1 63568 58558

l R ( ~ s) 16.59 15 .73 17.07

~G7/2 ~ ('FY12 10972 474 I 498 I 460 I 12152 11 231 19 11 800 19 10910 19 ~ ('F712 13020 80 1 I 842 I 778 I

~ ('F512 13524 5090 8 5349 8 4945 9 ~ ('FJ/2 16494 39725 67 41741 67 38593 66 ~ (, 1-1 11/2 20060 2228 4 2348 4 2366 4 ~ (' H'I2 59549 62578 58052

AT(s· l) 16.79 15.98 17.22 l R (ps ) 12345 ]700 4 3887 4 3595 4

4FJ12 ~ (,FJ12 17847 84355 92 88629 92 81949 92 ~ (, 1-1 7/2 18881 36 10 4 3793 4 3507 4 ~ (' 1-1 '12 91655 96309 8905 1

AT(s· l) 10.91 10.38 11.22 l R ( ~ s)

(For 0 /+) ~ FYI2 ~ (,F5/2 8757 109 I 124 I 189

~ (, 1-1 7/2 12005 300 3 278 2 363 2 ~ (,F9/2 12062 216 2 20 1 2 265 I

~ ('F" 12

13375 330 3 345 3 497 3

~(, I-I Y12 13422 234 2 242 2 347 2

~(' 1-I 11 /2 15276 908 S 1026 8 1559 9

~ (' 1-I 1J/2 17602 8085 73 8982 75 13471 76 21053 883 R 824 7 1044 6

~ (, 1-1 1512 11 065 12022 17735 AT(s· l) 90.37 83. 18 56.39 l R (~s )

(' P '/2 ~ (,FV2 14357 13374 2 15008 2 22736 3 ~ (,F5I2 15153 49 194 8 54649 8 823 ~ (,F7/2 16554 110802 17 121765 19 182013 22 ~ (' 1-1 '12 1735 1 510 1 I 4550 I 5773 I ~ (, 1-1 7/2 1840 1 20 11 2 3 17961 3 22789 3 ~ (,F9/2 18458 174863 27 189779 30 28 1358 34 ~(,F I I/2 19771 40937 6 36522 5 46342 5 ~ 61-1Y12 198 18 4 11 97 7 3689 1 6 469 18 6 ~(, 1-I 11 /2 21672 106508 17 95 161 15 120740 15 ~ (,F IJl2 23998 77294 12 69203 II 87799 II

AT(s· l) 639382 641489 81729 1 l R (ps) 1.56 1.56 1.22

4F712 ~ (,F1/2 12042 66 1 589 748 ~ (' FJ/2 12589 1026 I 915 11 61 ~ (,F5I2 13385 1510 2 1462 2 1992 I ~ ('F712 14786 2876 3 2833 3 39 14 3 ~(, 1-I 512 15583 629 590 774 ~ (' 1-1 7/2 16633 947 978 I 1378 ~ "FY12 16690 10448 12 11 367 13 16879 14 ~(,F I I /2 18003 20226 23 22830 26 34713 28 ~ (' 1-19/2 18050 1538 2 1511 2 2072 2 ~(,1-I11 /2 19904 15489 17 16404 18 23915 19

~ (, 1-I 1J/2 22230 312 1 4 2769 3 35 11 3 ~ (, 1-1 15/2

25681 28645 33 25627 29 32514 26

AT(s· l) 87 11 6 87875 123571

l R (ps) 11.47 11.37 8.09

REDDY et al.: FLUOBORATE GLASSES 585

AT=LA

The radiat ive lifetime l R of an exc ited state is given by'7:

I T R = -

AT

with es timation of A and AT, the branching ratio ~R% was calculated from17

:

The calculated values of transition probability (A ), total transition probability (AT)' radi ati ve lifetime (l R) and branching ratio ( ~R%) are presented in Table 4.

Looking at the radiative parameters (A, Ar) of the measured emission transitions of SmJ+ glasses, the following order has uniformly been observed:

4G 5I2 ~ (,H512 (563nm)

4G512 ~ (,Hm (599nm) Glass C < Glass A < Glass B

4G 5/2 ~ ('HW2 (644nm)

and life times(l R)

Glass C > Glass A > Glass B

Due to the non-avai lability of experimental faci lities, the authors cou ld not compare the predicted l R values with the experimental va lues . The magnitudes of the Branching rati o, ~ R (Tab le 4) for sm1+ - doped al ka Ii fI uoroborate gl ass are max imum for 4G 512 ~ (,HW2, 4G712 ~ 6H11I2 and 4 F3I~ ~

f>H712 transitions, which indicate favourable laser action. Similar trends are observed in all three Sm1+­doped f1u oroborate glasses studied. The predicted ~R va lues are in fair agreement with the BK values of

I 'X phosphate ami f1u orozirconate g asses ' .

Similarl y. the measured emiss ion trans itions of DyJ+ glasses show the fo ll owing trends:

JFm ~ 6HI.112 (455 11m)

4F.)12 ~ ('H 1'<12 (484 11m) Glass A < Glass B < Glass C

4F.)12 ~ 6H I112 (575 11m)

and the life times (l R)

Glass A> Glass B > Glass C

It is pertinent to mention here that , the Branching rati o, ~R for Dy'+- doped alkali f1uoroborate glass for the transitions 4Fw2 ~ 6Hu/l> ('P5l2 ~ ('F.m, 4F712 ~ 6FI1I2 and 4F 7I1 ~ f>H 15/2 are maximum. Favourable las ing action may be observed for the above transitions. ~R magnitude observed in the above glass are found to be similar in a ll three Dy-1+_ doped f1uoroborate glasses studied in the present work . Based on the magnitudes of ~R and life times, Satyanarayana ef a/.3~ have also drawn the same conclusion .

Tab le 5 - Emission level wavelength (/1.1' nm), effective bandwidths(L'.Ap), stimulatead emi ssion cross-sections (a Er

x 1022 cm2) for the measured fluorescent transitions of SmJ+. and Dy'+. doped alkali flu oroborate g l asse~

Transition Para· Glass A Glass B Gl ass C meter

SmJ+ 4G5I2 ~ (' H."2 AI' 56 1 562 562

L'.Ap 12 12 12

a\ 73.98 78 .69 72.56 "G"2 ~ (, 1-1 7/2 AI' 595 596 596

L'.Ap 20 16 20 ~p 7.90 11.77 7.84

JG5I2 ~ 61-1W2 AI' 64 1 642 642 L'.Ar 14 14 14 a Ep 3453.44 3666.48 3382.76

Dy·1+ JFm --7 "1-1 1.112 "I' 454 455 455

L'.Ap 10 S t: a\ 85.75 96.3 1 12 1.89

JF'l/2 ~ (' H 1.</2 AI' 482 482 482 L'.Ap 16 14 16

aR 19.26 20.62 22.82 I' JF.,,~ ~ 6HI1I2 AI' 573 573 5TJ

L'.Ap 14 14 18 a Lp 402.62 449.05 522 .79

The measured emi ss ion peak wavelength (/1.1').

effec ti ve half bandwidth (.6.AI')' stimu lated emission cross-section (0'\ ) are presented in Table 5. The procedures employed in eva luating these emi ss ion characteri stic parameters are si mi lar to those ado pted by Cases et ({I. -'(,AII and Reisfe ld el 01. 41 .

Table 5 reveals that the measu red crEI" va lues of SmJ+- doped alka li flu oroborate glasses are maximum for 4G 5/2 ~ (' H'l/2 level and show the fo llowing order:

Glass B > Glass A > Glass C

5R6 INDl AN J PURE & APPL PHYS, VOL 40, AUGUST 2002

For Dy3+ doped alkali flu oroborate glasses st imulated emission cross secti on (crE

p) -lF~12 ~ f> H L112

and trend is as follows :

Glass C > Glass B > Glass A

Similar conclusions have been observed in the case of fluoroborate glasses reported by Aruna el

al.-l ~ .

4 Conclusions

The studi es on the absorption and lu mi nescence spectra (Tabid 3 to 5) in the present report demonstrate the va li dity of the app li cati on of the Judd-Orelt theory in evaluating the opti cal quali ty of the material s inves ti gated. Amongst the three Sm'+­doped alka li fluoroborate glasses, Glass-B with 5%M NaF has proven its superiority over the other two glasses. Where as in the case of Dy'+ doped alkali fluoroborate glasses, Glass-C with 5%M KF is appears to be better than Glass-A and Glass-B.

Acknowledgements

The authors are very much thankful to the Indian Space Research Organization (ISRO), Bangalore, Department of Space, and Government of India, for their financi al support in the form of a research project.

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