ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1314–1319
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Optical properties of Sm3+-doped CaF2 bismuth borate glasses A.A. Ali Glass Research Department, National Research Center, Dokki, Cairo, Egypt
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 January 2009 Received in revised form 11 May 2009 Accepted 18 June 2009 Available online 25 June 2009
Sm3+-doped calcium ﬂuoride bismuth borate glasses were prepared and characterized optically and the oscillator strengths and Judd–Ofelt parameters for the glass containing 1.5 mol% of Sm2O3 were calculated. Density and optical absorption, transmission and the emission spectra were measured. The values of Judd–Ofelt parameters suggested an increase in the degree of asymmetry the local ligand ﬁeld at Sm3+ sites. The optical band gap energy, band tailing parameter and Urbach’s energy were calculated for all glass samples. It was found that with increasing the concentration of Sm2O3 content the values of the optical band gap energy decrease whereas Urbach’s energy increases. Absorption and excitation spectra indicate that commercial UV and blue laser diodes, blue and bluish-green LEDs and Ar+ optical laser are powerful excitation sources for Sm3+ visible ﬂuorescence in the glass. & 2009 Elsevier B.V. All rights reserved.
Keywords: Rare-earth oxide Optical properties Bismuth borate glasses
1. Introduction Boric oxide, B2O3, is one of the signiﬁcant glass formers and ﬂux materials. Melts with compositions rich in B2O3 exhibit rather high viscosity and tend to form glasses. In crystalline form, on the other hand, borates of various compositions are of great importance because of their interesting linear and nonlinear optical characteristics . The boron atom usually coordinates with either three or four oxygen atoms forming [BO3]3 or [BO4]5 structural units. Furthermore, these two fundamental units can be arbitrarily combined in different ways to form different BxOy structural groups . Among these borates, especial the monoclinic bismuth borate, BiB3O6, in particular shows up remarkably large linear and nonlinear optical coefﬁcients [3,4]. Calculations indicate that this can be mainly attributed to the contribution of the [BiO4]5 anionic group [5,6]. Glasses based on heavy metal oxides, e.g. Bi2O3, PbO and Ga2O3 have wide spread applications in the ﬁeld of glass ceramics, layers for optical and electronic devices, thermal and mechanical sensors, reﬂecting windows, etc. [7,8]. Because of small ﬁeld strength of Bi3+ ions, bismuth oxide cannot be considered as network former; however, in combination with B2O3, glass formation is possible in a relatively large composition range . The large glass formation region in bismuth borate glasses has been attributed to the high polarizability of the Bi3+ cations, which makes bismuth glasses suitable as nonlinear optical/photonic material with high-nonlinear optical susceptibility . The study of optical absorption spectra in
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solids provides essential information about the band structure and the energy gap in the crystalline and non-crystalline materials. This technique is based on the principle that a photon with energy greater than the band gap energy will be absorbed. Analysis of the absorption spectra in the lower energy part gives information about the atomic vibrations, while that of the higher energy part of spectrum contributed to our knowledge about the electronic state in the atoms . The possible use of glasses doped with rare-earth ions as solid state lasers, solar concentrators, phosphors, etc., has promoted considerable interest in the study of optical absorption and ﬂuorescence properties of rareearth ions in glasses. The investigations of absorption and luminescent properties of the Sm3+ ions have indicated that the optical properties of Sm3+ ion are dependent on glass composition, thus opening up the possibility of engineering applicationfriendly compositions [12–15]. Refractive index is one of the characterizing important properties of optical glasses. Therefore, a large number of researchers have carried out investigations to establish the relation between refractive index and glass composition . The present work intends to give a description of the optical properties of Sm2O3-doped CaF2–Bi2O3–B2O3 glasses.
2. Experimental 2.1. Glass preparation Reagent grade H3BO3, Bi2O3, CaF2 and Sm2O3 were used as starting materials for preparing glasses having the compositions 20Bi2O3 60BO3 (20x) CaF2xSm2O3 mole%, (0rxr1.5). For each composition, the raw materials in the powder form were mixed
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using a mortar. Each mixture was melted in an alumina crucible at 1000 K for 30–40 min in air. The glass melts were stirred occasionally with an alumina rod to achieve good homogeneity. The highly viscous melt was cast into a cylindrically shaped splitmold of mild steel. The glass produced was annealed at 400 1C in a second furnace for 1 h after which the furnace was switched off and the glass was allowed to cool gradually in situ for 24 h. 2.2. Density The densities of these glass samples were determined by the Archimedes method using xylene as an immersion liquid at room temperature. The density (r) was then calculated from the following formula:
w rx w w1
where w is the weight of the glass sample in air, w1 is the weight of the glass sample when immersed in xylene of density (rx) 0.865 gm/cm3 at room temperature (30 1C). The reported values of density represent the average of 3 determinations using different samples for each composition. 2.3. Optical absorption and emission The optical absorption spectra of the polished samples (thickness 3.670.1 mm) were recorded at room temperature using a recording spectrophotometer in the range 200–2000 nm (JASCO Corp., V-570, Rel-00, Japan). Excitation and ﬂuorescence spectra of the samples were measured at room temperature using a Spectroﬂuorometer Jasco FP777 (made in Japan). The excitation wavelengths are 400 and 600 nm.
3. Results and discussion 3.1. Physical properties The densities (r) of the glass samples prepared in the present study are given in Table 1 with probable error of 70.001. The molar volume (Vm) of the glass samples was calculated using the molecular weight (M) and density (r) with the following relation: Vm ¼
The number density of Sm3+ ions can be calculated by , N¼
2 r An Wc M
where N is the number density of rare-earth ions, r is the glass density, Wc is the weight percent concentration of rare-earth oxide, M is the molecular weight of rare-earth oxide and Av is Avogadro’s number. From the evaluation of the magnitude of N, we could determine the other three related physical properties as
described below : ˚ ¼ 1=2ðp=6NÞ1=3 Polaron radius; rp ðAÞ
˚ ¼ ð1=NÞ1=3 Internuclear distance; ri ðAÞ
As shown in Table 1 the number density of Sm3+ ions increases with increasing the Sm2O3 concentration while both of polaron ˚ and inter-nuclear distance, ri (A) ˚ decrease. radius, rp (A) 3.2. Optical absorption and transmission The absorption spectra of glasses with and without Sm2O3 are shown in Fig. 1(a) and (b). As shown in Fig. 1a, the absorption spectra of the base glass in the range from 190 to 500 nm reveal ﬁve absorption peaks at 228, 278, 316, 346 and 380 nm, which may be attributed to the strong absorption of the host glass in the ultraviolet range, due to presence of Bi3+ ions. The absorption spectra of all glass samples in a range from 500 to 2000 nm are shown in Fig. 1b. The spectra reveal that base glass has no peaks in this region. The absorption bands of Sm3+ are arranged into two groups, the ﬁrst one is the ‘‘low energy’’ group which corresponds to transitions up to 10700 cm1, (E935 nm), whereas the other is the ‘‘high-energy’’ group which corresponds to transitions in the energy range 17600–32000 cm1, (E935–313 nm) . For bismuth borate glasses, the absorption bands of Sm3+ those are located at wavelengths shorter than 450 nm, i.e. in the highenergy region, could not be distinguished. This indicated that they are overlapped by strong ultraviolet absorption by Bi3+ ions and accordingly the Sm3+ peaks are not well resolved. As shown in Fig. 1b, the absorption spectra of all glass samples containing Sm2O3 have many absorption bands with different intensities. All the transitions of Sm3+ are electric dipolar with signiﬁcant intensities in the observed frequency range and arise from the transition of electrons from ground state to different excited states. Assignments of the bands for the excited states from the ground states of Sm3+ are also indicated in Fig. 1b. The absorption bands at 936, 1072, 1222, 1364, 1468 and 1960 nm are due to transitions 6 H5/2-6F11/2, 6H5/2-6F9/2, 6H5/2-6F7/2, 6H5/2-6F5/2, 6F3/2, 6H5/26 F1/2, 6H15/2 and 6H5/2-6H13/2, respectively. The intensity of all bands increases with increasing Sm2O3 content from 0.5 to 1.5 mol%. The absorption band overlapping between 6F5/2, 6F3/2 and 6F1/2, 6H15/2 are observed here due to presence of Bi2O3. Fig. 2 shows the transmission spectra of the base glass doped with different concentrations of Sm2O3. This ﬁgure shows that increasing the concentration of Sm3+ ions leads to increase in the cut off wavelength. It is known that the addition of Bi2O3 to any glass former (P2O5 or B2O3) narrows the forbidden band of the glass and less energy is needed to excite electrons from valence band to conduction band, thus causing a red shift of the ultraviolet cut off . 3.2.1. Oscillator strengths and Judd–Ofelt parameters The most difﬁcult problem in rare-earth ions spectroscopy is the measurement of intensities of absorption bands. The intensity
Table 1 Glass composition in mol%, density (d), inter-nuclear radius (ri), polaron radius (rp), cut off wavelength (l cut off), optical band gap (Eg), band tailing parameter (B) and Urbach energy (DE). Glass no.
˚ inter nuclear distance ri (A)
˚ polaron radius rp (A)
Wavelength (l) cut off
1 2 3
60 60 60
20 20 20
19.5 19 18.5
0.5 1 1.5
4.205 4.257 4.32605
0.833 1.67 2.527
22.897 18.15 15.81
9.227 7.318 6.37
407 424 441
2.709 2.549 2.402
14.6 14.9 13.1
0.2231 0.2372 0.2571
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6 base 0.5 Sm2O3 1.0 Sm2O3 1.5 Sm2O3
Intensity in arb units
5 4 3 2 1 0
Intensity in arb units
1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
200 300 400 Wavelength (nm) 6
base 0.5 Sm2O3
F1/2 , H15/2
400 600 800 1000 1200 1400 1600 1800 2000 2200 Wavelength (nm) Fig. 1. (a) Optical absorption spectra of Sm2O3 pure and containing glasses (200–500 nm) and (b) Optical absorption spectra of Sm2O3 pure and containing glasses (500–2500 nm).
where e(n) is the molar extinction coefﬁcient at the wavenumber n (cm–1). If the absorption band takes a Gaussian shape, the oscillator strength can be calculated by using the half-width method : X fexp ¼ 4:32 109 Dn; ð7Þ where Dn (cm1) is the width of the band at half the peak intensity evaluated by measuring the area under curve that is reliable though tedious. Moreover, the absorption bands observed in the spectra of different glasses reveal inhomogeneous broadening and the data calculated by half-width method gives erroneous results. In the present work, we studied the oscillator strengths of the bands of glass containing 1.5 mol% Sm2O3, and the intensities of all the bands are measured by the area method. The two separated bands, (the fourth and ﬁfth bands at 1364 and 1468 nm, respectively) are combined in the analysis to one single band with barycenter at 6937 cm1. This is sensible, because Sm3+ has four multiplets in this region (3F1/2, 6H13/2, 6 F3/2 and 6F5/2) which we cannot deal with each of them separately. The oscillator strengths of the observed bands of Sm3+ are relatively much higher than those found for other glass systems containing a similar concentration of Sm3+, i.e. around 1.5 mol%. We believe that this behaviour is a consequence of the asymmetric component of the electric ﬁeld acting on the Sm3+ ion in the environment of calcium ﬂuorobismuth borate glasses. For this reason, the asymmetric component is relatively stronger than that of other glasses. The presence of F ions, which is strongly electronegative, may induce an asymmetric electric ﬁeld that is more pronounced on the Sm3+ ions. According to the Judd–Ofelt theory [20,21], the oscillator strength of a transition between an initial J manifold (S ,L)J and a ﬁnal J0 manifold (S0 ,L0 )J0 is given by fcal ððS; LÞJ; ðS0 ; L0 ÞJ 0 Þ ¼
8p2 mcu ðn2 þ 2Þ2 ½Sed ððS; LÞJ; ðS; LÞJ 0 Þ 9n 3hð2J þ 1Þ
where (2J+1) is the degeneracy of the ground state of the Sm3+ ions, n is the refractive index of the medium, m is the mass of the electron, n is the mean energy of the transition in cm1, c velocity of light, h Planck’s constant and Sed is the electric dipole strength, and are given by following equation: X Ol j/ðS; LÞJjjU ðlÞ jjðS0 ; L0 ÞJ0 Sj2 ð9Þ Sed ½ðS; LÞJ; ðS0 ; L0 ÞJ 0 ¼
40 0.5 Sm2O3 1.0 Sm2O3
Wavelength λ (nm) Fig. 2. Optical transmission spectra of Sm2O3 containing glasses.
of spectral lines is measured in terms of oscillator strengths using the relation : Z fexp ¼ 4:32 109 eðnÞ dn ð6Þ
where JU(l)J(2) are the squared reduced matrix elements of a unit tensor operator evaluated in the intermediate coupling approximation. The values of these matrix elements reported by Carnall et al. , have been used because these elements are the host invariant. Substituting the oscillator strengths calculated from the absorption spectra for fcal and using the values of the reduced matrix elements and other parameters, make possible the calculation of Ol (l ¼ 2, 4, 6) by the least-squares method. The rms deviation between the experimental and calculated oscillator strengths is deﬁned as sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ Pk 2 i¼1 ðfmeas fcalc Þ ð10Þ Dfrms ¼ K3 where K is the number of absorption bands taken into account. The Judd–Ofelt theory [20,21] is the most useful theory in estimating the probability of the forced electric dipole transitions of rare-earth ions in various environments. The Judd–Ofelt intensity parameters Ol were derived from the electric dipole contributions of the experimental oscillator strengths using a least-squares ﬁtting approach. The measured and calculated oscillator strengths, and Judd–Ofelt intensity parameters of Sm3+ in CaF2 bismuth borate glass are presented in Table 2. The
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Table 2 Oscillator strengths and Judd–Ofelt parameters of glass containing 1.5 mol% Sm2O3. Transition from the ground state 6H5/2-
Oscillator strength (f 106)
H13/2 F1/2, 6H15/2, 6F3/2 and 6F5/2 F7/2 6 F9/2 6 F11/2 drms 106 O2 1020 (cm2) O4 1020 (cm2) O6 1020 (cm2)
1960 1442 1222 1072 936 0.59853 15.31409 9.09472 9.22277
1.288 18.7 12.28 8.534 1.463
0.855 18.7 12.25 8.79 1.44
O2 1020 (cm2)
O4 1020 (cm2)
O6 1020 (cm2)
8Li2O–7BaO–14La2O3–70B2O3–1Sm2O3 3Sm2O3–30Bi2O3–67B2O3 53.33PbO–13.33PbF2–33.33B2O3–xSm2O3 Sm3+ in LKBBT glass Sm3+ ion-doped Li2O–MO–B2O3 Sm3+:CaO–Li2O–10B2O3–BaO Sm3+:CaO–Li2O–15B2O3–BaO Sm3+:CaF2–Bi2O3–B2O3
6.81 3.639 3.15 4.73 5.13 15.45 15.37 15.3141
4.43 5.66 3.63 2.78 4.91 9.23 3.15 9.09472
2.58 4.468 2.34 1.77 4.03 14.1 6.64 9.22277
       Present work
measures of the ﬁtting is given by the root-mean square deviation drms between the measured and the calculated oscillator strengths using Eq. (10) are also shown in Table 2. The Judd–Ofelt intensity parameters Ol are used to extract information on the interaction of Sm3+ ions with the surrounding glass network as a function of glass composition. The calculated values of O2, O4 and O6 for Sm3+ in our glass system are 15.31409 1020, 9.09472 1020 and 9.22277 1020, respectively. Ol parameters follow the trend O24O64O4 and O4/O6 ¼ 0.98. The parameter O2 in Sm3+-doped glass has been found to be associated with the symmetry of the ligand ﬁeld in the Sm3+ site . The value of O2 is higher than those for Li2O–BaO–La2O3–B2O3 (LBLB) glass , bismuth borate , lead borate glass (PbB) , alkali barium–bismuth–tellurite glass (LKBBT)  and Li2O–MO–B2O3 (LiMB)  but is close to those in CaO–Li2O–15B2O3–BaO (CaLBBa) . The parameter O2 is associated to the covalency of the Ln–O bond as well as the asymmetry around the Ln3+ ion site, while O4 and O6 parameters are long-range parameters that can be related to the bulk properties of the glass such as viscosity and basicity of the matrix and O6 to the 6s electron density of Ln3+ ions [29–31]. The higher value of the O2 parameter suggests that the symmetry of the site occupied by Sm3+ in the present glass is lower than those in the glasses referred to above, indicating higher mixing of the opposite parity electronic conﬁgurations, which is responsible for the spectral intensities and also exhibits lower covalency compared to the other hosts. It is known that  at higher concentrations Bi2O3 enters into glasses as a network former, which leads to sharp variation of Judd–Ofelt parameters. The degree of covalency of Sm–O bond can be determined by the shift of the hypersensitive absorption bands to larger wavelengths due to nephelauxetic effect. The lower the symmetry in the vicinity of the Ln3+ ion the higher will be the value of the O2 parameter. The values of JO parameters suggest that the asymmetry of the site occupied by Sm3+ ions in the present glass is higher than that of glasses compared in Table 2, except for the CaLiBBa glass  due to presence of Bi2O3. It is known that, the s-bonds are formed between the 2p orbital of the ligands, such as oxygen and ﬂuorine ions, and the 6s
orbital of a RE ion. The overlap between the ﬁlled 2p orbital and the empty 6s orbital leads to a s-electron donation from the ligands to the RE ions. The covalency between the ligands and RE ions increases as the 6s electron density increases, and thus the value of O6 increases . The higher value of O6 indicates that the higher covalency of Sm–O bond. 3.3. Optical band gap The optical absorption spectra for all the samples were recorded (Fig. 1a and b), and the absorption coefﬁcient a(n) near the edge of each spectra was calculated using the relation : 1 Io : ð11Þ aðuÞ ¼ ln It d where d is the thickness of each sample, and ln (Io/It) corresponds to absorbance, A where Io and It are the intensities of incident and transmitted light, respectively. For amorphous materials, the relation between a(u) and the phonon energy of the incident radiation hu is given by the following relation :
ahu ¼ Bðhu Eg Þr
ð12Þ 1 2
and 13 where r is the index, which can have values 2, 3, corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions, respectively. B is a constant called band tailing parameter, Eg is the optical band gap energy and hn is the incident photon energy. In various glass systems, Eq. (12) depicts a straight line for r ¼ 2. Fig. 3 represents the Tauc’s plot [(ahn)1/2 vs. hn] for different samples . The values of Eg were calculated from the linear region of these curves which were extrapolated to meet the hn axis at (ahn)1/2 ¼ 0 and are listed in Table 1 for all the compositions. The values of the band tailing parameter, B, obtained from the slope of the curves of Fig. 3 are also given in Table 1. As shown in Table 1 the value of Eg lies between 2.7 and 2.4 and decreases with increasing the concentration of Sm2O3. This can be understood in terms of the structural changes that are taking place in the glass with increasing the concentration of Sm2O3. It is known that the changes induced in glass structure as a result of the introduction
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of rare-earth ions include the formation of greater number of nonbridging oxygens  which accordingly decreases Eg. It is generally accepted that the location of absorption edge depends on the oxygen bond strength in the glass forming network . Introduction of RE changes the oxygen bonding in glass forming network and any change of oxygen bonding in glass network such as the formation of non-bridging oxygen changes the absorption characteristics. This explains why the optical band gap decreases with increasing Sm2O3 content. Therefore, it may be concluded that whatever is the mechanism of transition, the glass system under study behaves as an indirect gap semiconductor. The values of a(n), that are lying between 102–104 cm–1 are deﬁned as Urbach’s exponential tail region,  and are given by hu ð13Þ aðuÞ ¼ C exp DE where C is a constant and DE is the Urbach’s energy interpreted as the optical transitions between localized tail states adjacent to the valence band and the extended states in the conduction band above the mobility edge. The values of DE were calculated by
taking the reciprocals of the slopes of the linear portion of the ln
a(n) vs. hn curves, Fig. 4, and are included in Table 1. The region [a(n)r102 cm–1] involves low energy absorption and is considered to be due to the optical transitions between localized states. The values of the Urbach energy lie between 0.2231 and 0.2571 eV. It has been reported that the exponential tails observed in various materials with different structures have the same physical origin which can be attributed to the phonon-assisted indirect electronic transition [39,40].
3.4. Emission Sm3+ containing glass emits bright reddish-orange light under blue and UV light excitations. The emission spectrum of 1.5 mol% Sm2O3-doped CaF2 bismuth borate glass is shown in Fig. 5. The reddish-orange light from Sm3+ is composed of 564.5, 600 and 646.5 nm emission bands, corresponding to the 4G5/2-6HJ ðJ ¼ 52 ; 72 and 92Þ transitions, respectively. The ratio of radiation energies for the three emission bands  can be replaced by the
300 0.5 Sm2O3
(α hυ)0.5 (cm-1eV)0.5))
1.0 Sm2O3 1.5 Sm2O3
200 150 100
50 2 0
2.6 2.8 3.0 hυ (eV )
Wavelength (nm) Fig. 5. Emission spectrum of 1.5 mol% Sm2O3 glass under 400 nm excitation.
Fig. 3. (ahn)1/2 vs. hn for Sm2O3 containing glasses.
ln () (cm-1)
3.0 2.5 2.0 1.5
0.5 0.0 2.0
hυ (eV) Fig. 4. ln a(n) vs. hn curves for Sm2O3 containing glasses.
420 440 460 Wavelength (nm)
Fig. 6. Excitation spectrum for 600 nm emission of 1.5 mol% Sm2O3 glass.
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ratio of emission intensities integrated along wavenumber (u0 ), i.e. Z u0 2 Z u00 2 Z u2 Z u2 rblue ðuÞdu : rred ðuÞdu : rNIR ðuÞdu ¼ Iblue ðuÞdu u1
: u0 1
Ired ðuÞdu :
R u2 where r(u0 ) is energy density. The integrated intensity ð u1 IðuÞduÞ ratio among the green, reddish-orange and red bands turned to be 24:37:7. It is obvious that the 600 nm emission band is the most intense and its width is more than one-half of the total energy, so the luminescence color of Sm3+ in our glass is reddish-orange. The excitation spectrum for 600 nm main emission of the 1.5 mol% Sm2O3 containing glass is given in Fig. 6. The spectrum is composed of seven excitation bands peaking at 366, 378, 404, 417.5, 440.5, 464 and 476.5 nm, and these peaks are due to the 4f–4f inner shell transitions of Sm3. The efﬁcient excited wavelength range of Sm3+ in Ca–bismuth borate glass covers the whole long-wavelength UV, blue and bluish-green spectral ranges well. It indicates that commercial UV and blue laser diodes, blue and bluish-green LEDs and Ar+ optical laser are powerful pumping sources in Sm3+-doped CaF2-bismuth borate glass. 4. Conclusions CaF2–bismuth borate glasses doped with different concentrations of Sm2O3 (0.5, 1 and 1.5 mol%) have been prepared and their optical absorption, transmission and emission were measured. 1. The Judd–Ofelt intensity parameters, O (t ¼ 2, 4 and 6), for the glass containing 1.5 mol% were calculated from absorption spectrum and the values obtained are 15.31409 1020, 9.09472 1020 and 9.22277 1020, respectively. 2. The values of Judd–Ofelt parameters are as high as those obtained for CaLiBBa glass . 3. The values of different optical and physical parameters, viz., density, optical band gap, Urbach energy, band tailing parameter, density, inter-nuclear distance, Polaron radius and cut off wavelength were calculated. 4. The optical band gap, polaron radius and inter-nuclear distance were found to decrease with increasing concentration of Sm2O3 whereas density and Urbach energy were found to increase. 5. Intense reddish-orange ﬂorescence has been observed when the glass sample was excited by UV and blue radiation. Broad excitation wavelength range from UV to bluish-green shows that commercial UV and blue laser diodes, blue and bluishgreen LEDs and Ar+ optical laser are powerful pumping sources in Sm3+-doped CaF2 bismuth borate glass.
Acknowledgements The author is thankful to Prof. Dr. Karl Gatterer, Institut ¨ fur Physikalische und Theoretische Chemie, Technische ¨ Graz, for his kind help in calculation of Judd–Ofelt Universitat, parameters. References                                        
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