Judd–Ofelt analysis of Spectroscopic properties of Nd +3 :SiO 2 Prepared via Sol-Gel

Doped and undoped nano particular silicate dioxide was prepared via sol–gel method under varying conditions. The optical properties of prepared samples were investigated by a variety of techniques, including X-ray diffraction, UV-Visible spectroscopy, FTIR spectroscopy and fluorescence spectroscopy. The peak of the fluorescence spectrum was recorded at the wavelength around to 1048nm, which it is close to known fluorescence peaks of Nd:YAG crystal in NIR region. A Judd-Ofelt analysis is performed to calculate the spectroscopic properties of Nd 3+ ions embedded in SiO 2 and compare it with spectroscopic properties of Nd:YAG crystal. The conclusions behind this study show that the doped silicate samples have a high peak emission cross-section σ p , which gives an acceptable indication in the direction of using Sol-Gel technique to prepare Nd:SiO 2 as a solid state laser active medium.


INTRODUCTION
or more than 50 years, so many research is covered a large number of rareearth active ions doped with glass as a host material.Neodymium Nd 3+ ion, is one of an interest primary rare earth ion of for most commercial application of glass lasers [1].So many methods have been developed to synthesis Nd 3+ doped with silica, including plasma-enhanced chemical -vapor-deposition, ion implantation, flame hydrolysis, and ion exchange.In recently, a growing interest has been focused on a wet chemical process to prepare of transparent monoliths which containing Nd 3+ (using Neodymium (III) acetylacetonate hydrat [2][3][4]).
The sol-gel process has been widely shown to be a suitable process to fabrication of a optical materials with various configurations, such as monoliths, coatings, fibbers and films for optical device applications [5,6].The formation of oxide particles in a liquid phase, leads to make the structure of sol-gel materials inherently porous.Silicon alkoxides generally react slowly with water, using of acid and base catalysts makes the reaction process (hydrolysis and condensation) to become faster [7,8].

Experimental Samples Preparation
The doped and un-doped samples were synthesized by sol-gel method from tetraethylorthosilicate (TEOS) (Aldrich 98%), Ethanol (EtOH 99.9%) from GCC, hydrochloric acid (HCl, 34.5%) from BDH and neodymium (III) acetylacetonate hydrate (Aldrich).Deionized water was used for the hydrolysis of (TEOS) and preparation of pure and doped SiO 2 sol.The performed of reaction process was done at a cooler water jacket (the solution was cooled by a cooled water (10-15°C)).The amount of each chemical in this procedure was TEOS:H 2 O:EtOH:HCl= 1:1:10:0.1 in molar ratio.All solution was prepared as follows: 1 mole of tetraethylorthosilicate (TEOS) and ethanol (EtOH) were mixed and stirred for 10 min.At the stirring time, 0.1 M catalysts in water were added drop wise to the solution until water to TEOS molar ratio of R= 2 are attained.A neodymium (III) acetylacetonate hydrate was solved in ethanol and used to mixing with TEOS.All solutions were then leaved to stir for about 2 hours further at room temperature aged for 24 hours before use.
A first drying occurred (at temperature 60 o C and for about 2 hour) after aging for 24 hour.Then samples without covers left in room temperature in order to permit solvent evaporation through the drying process.The doping rate of samples with Nd 3+ is equal to 5% wt.figure (1) show Snapshot for some prepared doped samples with different shape.

Samples characterization
Structural characterizations of the doped and un-doped SiO 2 samples were done by X-ray diffraction (XRD).θ-2θ scans were recorded using ITAL-STRUCTURE diffractometer equipped.Shimadzu FTIR spectrometer was used to obtain the Mid IR spectra for the prepared samples (using KBr pellets of the samples).
Absorption spectra at room temperature were obtained with TupCen UV-VIS Spectrometer.SolarLab mono chromatore equipment has been used to obtain the emission spectra (at room temperature).As excitation source we used the 808nm 1W Laser diode.Furthermore standard measurements were obtained via measuring the absorption and emission spectra were to the Nd:YAG crystal.

Results
X-ray diffraction analyses show that both of prepared samples (doped and undoped) have amorphous structure [9].The FTIR spectrum of doped and un-doped samples was in the range of 4000-400 cm -1 illustrated in figure (2).The characteristics vibrational bands of silica were found in the FTIR spectra.Where; the absorption bands at about 1100 cm -1 , 472 cm -1 and 808 cm -1 which attribute to bending, asymmetric stretching and symmetric stretching vibrations of Si-O-Si groups respectively.The band at around 960 cm -1 could be attribute to stretching vibration of silanol (Si-OH) groups [10,11], indicates the limited number of these groups in the silica network [10].The band at around 960cm -1 for dopant sample is weak when it compare with the same band for pure sample that means the condensation reaction of doped sample is nearly complete more than from condensation reaction of pure sample.Two other bands at around 1600 cm -1 and 3400 cm -1 were appeared and attribute to the characteristics vibration of O-H bond in water molecules [12].These two bands are indicating that the drying process does not completely trap the water molecules from the prepared samples (silica Xerogel network).Therefore, the drying process of sample needs to be done at temperatures higher than 500 ºC to obtain sol-gel silica glass [9].The room temperature UV-VIS-NIR absorption spectra of the Nd 3+ doped sample are presented in Figure (3).For comparison, other spectrum of Nd:YAG crystal is also given in Figure (3).It is clearly seen that all absorption bands of Nd 3+ in SiO 2 sample are somehow close similar to absorption bands of Nd 3+ in YAG crystal hosts [13][14][15][16][17].
The absorption peaks band width of ND:SiO 2 are widely in comparison with absorption peaks ND:YAG.That's because the prepared SiO 2 samples have amorphous structure.One of most important result could be concluded from the absorption spectra is that the optical pumping for prepared dopant sample could be done in similar way of Nd:YAG optical pumping.
The fluorescence property to the prepared sample is one of an important performance indicator for the sample to be useful for glass laser applications.To determine the optical characteristics of the samples, photoluminescence measurements were carried out using the 808nm/1W Laser Diode for excitation.The obtained spectrum is shown in Fig. 4 for both of Nd:SiO 2 and Nd:YAG crystal.From Nd:YAG fluorescence spectrum it could observed a weak and broad peak around 1065nm with band width of 11nm at full width half maximum (FWHM).While the Nd:SiO 2 fluorescence spectrum have a weak and broad peak at around 1048nm with band width of 18nm at FWHM.These two peaks correspond to the transition between the levels 4 F 3/2 -4 I 11/2 of Nd 3+ ion [13][14][15][16][17].
The absorption and fluorescence spectrums are well resolved so that almost every stark components corresponding to different manifold of Nd 3+ are observed and tabulated in Table 1 and Table 2 for absorption and fluorescence spectra respectively.From the integrated absorption cross section, the so-called line strength, S m , can be found by Eq. 1 [18]: Where J is the total angular momentum of the initial ground manifold, found from the 2S+1 L J designation.σ(λ) is the absorption cross section as a function of wavelength λ.The absorption bands were chosen to determine the phenomenological Judd-Ofelt parameters [18][19].The J-O parameters for Nd:YAG crystals and Nd:SiO 2 are given in Table (1).These phenomenological J-O parameters were subsequently utilized to determine emission line strengths S ed corresponding to the transitions from the upper multiplet manifolds 2S + 1 L J to the corresponding lower lying multiplet manifolds 2S' + 1 L J' of Nd 3+ in YAG and TiO2.The S ed are determined by Eq. 2 [18]: where M ij are components of a 3 x N matrix for the square matrix elements of U (2) , U (4) and U (6) .The Ω i are components of a 1 x 3 matrix for the Judd-Ofelt parameters  The radiative transition probabilities A(J;J'), are given in Eq. ( 3) [18], were obtained with the line strength for the excited 4 F3/2 to 4 I J' manifold for Nd 3+ (J; J ′ ) = 64 π 4 3h (2J+1)λ 3 [ n(n 2 +2) 2  9 ]S ed … Where [n(n 2 +2) 2 /9] is the local field correction for Nd 3+ in the initial J manifold.J' is the final manifold.n is the refractive index at the wavelength (λ) of the transition.The efficiency of a laser transition is evaluated by considering stimulated emission cross-section (σ em (λ)).In our case σ em (λ) between 4 F 3 → 4 I J' was determined from the emission spectrum using Fuchtbauer-Ladenburg method [23]: Where: λ p is the wavelength of the peak emission, c is the speed of light in vacuums, and n(λ p ) is the refractive index at each emission peak wavelength.Δλ eff is an effective linewidth.

Conclusion
The transparent and unbroken sample of Nd 3+ doped Nanosilica is successfully prepared by wet chemical synthesis method.The optical properties of prepared doped samples are close similar to optical properties of Nd:YAG crystal.The Nd 3+ doped Nanosilica network have a high peak emission cross-section σ p .This suggests that it could be use Sol-Gel technique to prepare of Nd:SiO 2 as solid state Laser active medium.

Figure ( 1 )
Figure (1) Snapshot for some prepared doped samples with different shape.

Ω 2 ,
Ω 4 and Ω 6 .N represents the number of transitions to fit.The square matrix element does not depend on host materials[21][22].

Figure ( 3 )
Figure (3) Absorption spectrums for (A) SiO 2 doped with 5%wt Nd +3 (B) Nd:YAG Crystal Nd:YAG are close to known parameters of Nd:YAG crystal[13][14][15].That's gives a good indication about the accuracy of parameters measurements to doped silica.Also it can see that Nd:SiO 2 parameters are somewhat close to parameters of Nd:YAG.That's means the spectroscopic properties of prepared doped sample are close similar to spectroscopic properties of Nd:YAG crystal.This result gives further more good indication in direction of using Sol-Gel technique to prepare of Nd:SiO 2 samples as solid state Laser active medium.

Table ( 1) Measured absorption Line Strengths of Nd 3+ in YAG crystal and SiO 2 .
According to Table (1) and (2), the measured and calculated parameters to