Third-order nonlinearities and other properties of molybdenum lead-pyrophosphate glass
Danilo Manzani1*, Valentin Besse2, Mariana Napoli1, Georges Boudebs2, Sidney J. L.
Ribeiro1 and Cid B. de Araújo3
1 Institute of Chemistry, São Paulo State University, UNESP, CP 355, Araraquara, SP, 14801-970, Brazil.
2 LUNAM Universite, LPhiA, Laboratoire de Photoniques d’Angers, EA 4464, Universite d’Angers, 2 Boulevard Lavoisier, 49045, Angers Cedex 01, France.
3 Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife, PE, Brazil.
Corresponding Author
*Dr. Danilo Manzani
São Paulo State University – UNESP - Institute of Chemistry CP 355 - Araraquara, SP – Brazil
E-mail: [email protected]
Abstract
Glasses in the binary system (100-x)Pb2P2O7-xMoO3, with x = 10 – 80 mol %, were synthesized by conventional melt-quenching technique. Thermal analysis, linear optical absorption, refractive index measurements, Raman scattering and nonlinear (NL) optical experiments were performed to characterize the samples. The dependence of MoO3 content on thermal, structural and optical properties was investigated. Molybdenum oxide increases both the glass transitions temperature and thermal stability against devitrification up to 50 mol % due to formations of P-O-Mo linkages and the glass network connectivity increases. The nonlinear optical properties were studied at 1064 nm and 532 nm with pulses
of ≈17 ps. The NL refractive index measured was
n2 +10-19 m2/W at both wavelengths for samples with different relative concentrations of the constituent compounds. At 1064 nm
we determined that the two-photon absorption coefficient, α2 , is smaller than the
minimum that we can measure ( α2 <0.01 cm/GW) while at 532 nm we measured α2≈ 0.25 cm/GW. The nonlinear response of the samples is attributed to contributions from the lone electron pairs of Pb2+, MoO3 clusters, and to Mo5+ and Mo4+ ions.
Keywords: molybdenum-lead pyrophosphate glass; thermal properties; light scattering;
nonlinear refractive index; nonlinear absorption.
1.Introduction
Phosphate glasses are receiving large attention of many research groups due to their unique glass-forming tendency and because their potential applications as laser hosts for rare-earth elements, optical fibers, and Raman lasers, among other uses [1-12]. Some glasses compositions contain lead oxide because the phosphorous-oxygen-lead bonds (P-O- Pb) that appear in the glass structure contribute to improve the chemical durability, to reduce the melting temperature and to increase the nonlinear (NL) optical properties.
Recently tungsten lead-phosphate glasses prepared in the binary system Pb2P2O7-WO3
was investigated showing interesting thermal and optical properties [2]. Incorporation of tungsten oxide in a lead-pyrophosphate network produces chemically stable glasses with high glass transition temperatures and high resistance against devitrification [2]. It was also demonstrated interesting optical properties such as high NL refractive indices [3]. The optical properties were correlated with the glass network formed by insertion of tungsten octahedral WO6 inside the lead-phosphate covalent network to form strong P-O-W bonds.
The occurrence of the highly polarizable W-O-W bond at high WO3 contents is pointed out to be responsible of the dominant optical properties. In this sense and considering the structural similarity among WO3 and MoO3, tungsten has been substituted by molybdenum and glasses were studied in the binary system Pb2P2O7-MoO3.
A large effort has been dedicated by several authors to characterize the structure of phosphate glasses using techniques such as Raman scattering, infrared absorption, electron paramagnetic resonance (EPR), X ray diffraction and thermal measurements [4-11].
Phosphate glasses compositions including heavy-metal oxides present high refractive index (higher than 1.8), wide transparency window from infrared to blue-green region and high
third-order susceptibility, χ(3). For example, the structure and the optical properties of NaPO3-Nb2O5 glass network were studied few years ago and the formation of NbO6 clusters was observed as well as the increase of linear and nonlinear refractive indexes attributed to the high polarizable Nb-O-Nb bonds [4]. Another example is the tungstate fluorophosphate
glass (NaPO3-BaF2-WO3) that shows NL refractive index, n2, of ≈ 10-15 cm2/W and two-
photon absorption coefficient, α2, of 0.4 cm/GW at 532 nm [13]. The third order susceptibility is enhanced in function of WO3 concentration due to the high polarizability associated to W-O-W bonds. Higher infrared nonlinearity of NaPO3-BaF2-WO3 glasseswere obtained replacing BaF2 by Bi2O3, which has higher optical polarizability [3,14]. The NaPO3–WO3–Bi2O3 glasses contain different amounts of bismuth oxide were investigated at
1064 nm and 800 nm and the values of n2 ≈ 10-15 cm2/W and α2 ≤ 0.03 cm/GW were determined. Our previous work identified the nonlinear response of tungsten-lead- pyrophosphate glass to adequate the composition for nonlinear optical fiber fabrication and for application as all-optical switching devices [15]. In addition, the Pb2P2O7-WO3 glasses have higher thermal stability against crystallization than NaPO3–WO3–Bi2O3 glasses, in other words higher T (Tx–Tg), and the glasses are easily obtained free of strains and colorless to yellowish, regardless the melting and mold temperatures. Nonlinear refraction and absorption were studied for samples containing different amounts of lead oxide and tungsten oxide. Experiments made at 1064 nm using a 17 ps laser revealed values of n2 and
α2 that correspond to figures-of-merit appropriate for all-optical switching.
In this sense, the present work we report the production of a new molybdenum-lead- pyrophosphate (Pb2P2O7-MoO3) glasses and shows the thermal and structural characterizations of the samples using differential scanning calorimetry (DSC) and Raman
light scattering, respectively. The nonlinear parameters n2
n2 and
a2 were measured for samples with different relative concentrations of MoO3 at 1064 nm and at 532 nm.
2.Experimental part
The glasses samples were synthesized by the conventional melt-quenching method using the raw materials MoO3 (Aldrich 99.8%) and lead orthophosphate PbHPO4, precipitated by add pure orthophosphate acid (Merck 85% - P.A.–A.C.S) into an aqueous (deionized water) lead acetate solution (Merck P.A–A.C.S, solubility degree in water at 25°C = 44.31g/100 mL) at room temperature, as already described in [2]. Above 350°C, the lead hydrogen orthophosphate (PbHPO4) changes phase to form the lead pyrophosphate (Pb2P2O7) structure [2]. The raw materials were stoichiometrically weighted considering the compositions rule (100-x)Pb2P2O7-xMoO3 for x = 10, 20, 30, 40, 50, 60, 70 and 80 mol % and then loaded into a platinum crucible. First, the batches were heated at 200 °C for 1 h to reduce adsorbed water and gases, followed by melting at a temperature ranging from 950 to 1100 °C, depending on the MoO3 content, during 40 minutes to ensure homogenization and fining. Finally, the melt was cooled inside a metal mould preheated at 20 °C below the glass transition temperature, Tg, and then annealed at these temperatures for 2 h to minimize mechanical stress resulting from thermal gradients upon cooling.
Characteristic temperatures (Tg for glass transition and Tx for onset of crystallization) were obtained from differential scanning calorimetry from 300 to 600 ºC, which were
carried out under N2 atmosphere at a heating rate of 10 ºC/min, using a TA instruments DSC 2910 calorimeter, with a maximum error of ±2 ºC for Tg and Tx. Thermal stability against crystallization was evaluated from the stability parameter T = Tx – Tg and the results are presented in Table1.
Table 1
Raman spectroscopy was used to evaluate structural changes due to the molybdenum oxide insertion into the lead pyrophosphate network. The spectra were recorded from 100 to 1500 cm-1 range, obtained from the bulk samples with a Jobin-Yvon Horiba LABRAM- HR-800 micro Raman spectrometer equipped with a He/Ne laser excitation source (638 nm). The Raman intensity data were recorded at intervals of 1 cm-1.
Linear optical absorption was performed on bulk glasses samples with parallel and optically polished surfaces. The absorption spectra were measured using a commercial spectrophotometer operating from 400 to 2400 nm and the linear refractive indices were measured using an ellipsometer.
The excitation source for the NL experiments was a linearly polarized mode-locked Nd:
YAG laser (1064 nm; 17 ps; 10 Hz) and its second harmonic at 532 nm. The measurements were performed using a Z-scan setup integrated in a 4f-system as described in the next section.
3. Results and discussions
Glasses samples obtained in the binary system (100-x)Pb2P2O7-xMoO3 with different molybdenum oxide concentration from 10 to 80 molar %, show an intense reddish color (up to 40 mol%) and black colors for higher concentrations, as illustrated in Figure 1. The
samples were optically homogeneous to naked eyes and free of strains due to the high melt temperatures, large melting time, and low cooling rates used.
Figure 1
Figure 2 shows the absorption spectra of the samples A to E used for nonlinear optical measurements. Based on those results it is clear that the strong coloration observed is related to the broad and intense absorption band at around 850 nm that raises in function of molybdenum oxide concentration, and due to red shift of the glasses absorptions edges located at the visible region. Note the sample containing 10 mol% (sample A) presents a large transparency window from 470 nm to 2400 nm, however the optical absorption of MoO3 based glasses has been attributed to molybdenum reduced species Mo5+ and Mo4+
formed during the melting [18].
According to some EPR results for other molybdenum containing glasses, these ions are expected to be mainly in the Mo6+ (d0) state into the glass network [17]. However, for samples containing more than 10% of MoO3, the absorption edge gradually is red shifted due to the broadening and increased intensity of the absorption band between 500 and 600 nm, which is overlapped by the growth and broadening of the near infrared band centered at 850 nm. This behavior is attributed to the presence of Mo5+ (d1) ions, and the absorptions are assigned to the spin forbidden 2A2→2T2, 2T1, and 2E transitions at 620 nm, 930 nm and 990 nm; which are strong enough to hamper visible and near infrared transmission, resulting in dark samples [16]. The absorption bands in the visible and near infrared are associated with reduced molybdenum species Mo5+ and Mo4+, are attributed to two distinct absorptions mechanisms: the absorption due to d-d electronic transitions and polaron absorption, which can be defined as the transfer of an electron from a reduced species (Mo5+) to an oxidized species (Mo6+) after one photon absorption [16, 18]. The reason for
this reduction tendency is not well understood in glasses but is probably related to the formation of oxygen vacancies in the vitreous network and subsequent reduction of molybdenum ions to maintain the electric neutrality. The reduced species are responsible for the well-known dark color of molybdenum-containing glasses as already discussed in [18].
Figure 2
The structural evolution of the vitreous network as a function of the composition shows that molybdenum can be present in MoO6 clusters that act as modifier between covalent bonds and as MoO4 playing the role of intermediate between phosphate PO4 inside the covalent network [19].
Table 1 exhibits the values obtained for Tg and Tx, as well as the thermal stability parameter, ΔT, for MoO3 concentrations varying from 10 to 80 mol %. The value of Tg
increases up to MoO3 concentration of 50 mol% (sample E) going from 388 to 422 °C, and then decreases for higher concentrations up to 365 ºC. This anomalous behavior, as similarly observed for tungsten-bismuth phosphate glasses [14], suggests that for concentrations higher than 50 mol%, the inclusion of molybdenum oxide into the glass structure increases the connectivity (increasing Tg) due to formation the formation and insertion of MoO6 octahedrons into the phosphate chains. As a result, the MoO3
incorporation improves the thermal stability against devitrification of the glass samples since the crystallization peak is shifted to higher temperatures and is not observed until 600°C for the sample containing 50 mol% of MoO3. Above 50 mol% of MoO3, Tg and T starts to decrease due to the phosphate network opening and formation of MoO6 clusters, as showed by the formation of Mo-O-Mo linkages identified by Raman spectroscopy
Figure 3 shows the Raman spectra for all samples. The spectra exhibit several characteristic features that are useful to monitor the structural and optical dependence of the glass composition. Typical vibration of phosphate units are observed at 720 cm-1, assigned to symmetric stretching of P-O-P linkage [20], and at 1018 cm-1 and 1190 cm-1, attributed to symmetric terminal stretching of P-O bonds of metaphosphate (Q2) tetrahedral and to symmetric stretching of the two non-bridging oxygen atoms of pyrophosphate (Q1) tetrahedra (P2O7)4-, respectively, which progressively decrease with MoO3 concentration.
This behavior suggests a progressive break of pyrophosphate units and a probable insertion of MoOn polyhedral between PO4 units as P-O-Mo bonds. Another band starts increasing in intensity at around 390 cm-1 for samples up to 50 mol% of MoO3, can be attributed to the stretching vibrations of P-O-Mo linkages, which indicate the insertion of MoOn
polyhedrons through covalent bonds into the lead phosphate chains, resulting in an increase of the glass network connectivity. For MoO3 concentrations above 50 mol %, the band intensity related to P-O-Mo bond decreases and a new band at around 310 cm-1, assigned to the stretching vibration of Mo-O-Mo linkages, grows in the samples containing 70 and 80 mol% of MoO3, which suggest a clustering formation of MoOn polyhedral into the glass structure [20]. The behavior observed for samples A to E can be correlated with the increase of Tg values indicating higher connectivity of the network up to sample E. In addition, two dominant bands centered at 870 cm-1 and 930 cm-1, attributed to Mo-O terminal bonds (Mo- O- and Mo=O) and related to either four-, five-, or six-coordinate Mo atoms, increase in intensity and strongly suggests that MoOn polyhedral are incorporated in the glass network for all compositions.
Figure 3
From a general point of view, the Raman results depict a logical structural evolution with MoO3 incorporation: MoOn polyhedra are inserted between PO4 tetrahedra as P-O-Mo bonds and increase the degree of polymerization of the glass, increasing the glass network connectivity up to 50 mol% probably because the formation of Mo-O-P linkages. This assumption is consistent with thermal results that pointed out an increase of Tg and glass stability against devitrification at this concentration range. For higher MoO3 content, MoOn
polyhedra start to link together, destabilizing the glass network as reflected by decrease of both Tg and T due to MoOn cluster formations.
Since other phosphate glasses studied before present high potential to be used in nonlinear photonics, we also investigated the third-order nonlinearity of Pb2P2O7-MoO3 to evaluate its potential. The NL refractive indices of several samples were measured using the Z-scan integrated in a 4f-system technique [22, 23] and the schematic of the setup used is presented in Figure 4. The 4f-system is composed of two equal focal-lengths (20 cm) convergent lenses. The image receiver at the output of the 4f-system was a 1000 x 1018 pixels cooled CCD camera (-30 °C) operating with a fixed gain. The sample was moved in the focal region along the beam propagation direction (Z axis). Open- and closed-aperture Z-scan transmittances were numerically processed from the acquired images by integrating over all the pixels in the first case and over a circular numerical filter in the second case (corresponding to a linear aperture transmittance S = 0.73 to optimize the sensitivity and the signal-to-noise ratio [23]. The incident intensity was adjusted by a polarizing system at the entry of the setup.
Figure 4
Figure 5 shows closed-aperture profiles for samples A and E, measured at 1064 and 532
nm, that indicate self-focusing nonlinearity (
n2>0) at both wavelengths; the open-aperture profiles are shown in the insets of Figure 5. Figures 5(a) and 5(b) show the results for the experiments at 1064 nm; the laser intensity was 7.4 GW/cm2. The samples did not exhibit
nonlinear absorption indicating that α2 <0.01 cm/GW (the minimum value that our apparatus can measure) as presented in Table 2. Notice that although the amounts of
reduced Mo species are increasing with MoO3 concentration, the value of n2 does not
change much. Therefore the behavior of n2 indicates that the nonlinear response is mainly dominated by the [MoO3] clusters and by the lead-pyrophosphate which presents large nonlinearity due to the 6s lone electron pairs of Pb [24]. Figure 5(c) shows the Z-scan profiles corresponding to sample E at 532 nm. The two-photon absorption coefficient
measured was α2 ≈0.2 cm/GW for all samples and the value of n2 increases as the MoO3 concentration is increased. The laser intensity used in this case was 3 GW/cm2 and we observed that for higher intensities the samples present photo induced an effect that
changes their optical transmittance. The behavior of n2 with the increase of the MoO3
concentration and the occurrence of photo induced effects indicates that the Mo5+ and/or Mo4+ ions also contribute for the nonlinear behavior at 532 nm.
Figure 5
The NL parameters measured for both wavelengths are summarized in Table 2. The
values of n2 measured in this work have the same order of magnitude as in the Pb2P2O7- WO3 glass [15] and they are one order of magnitude larger as in silica [25]. The fact that n2 has the same order of magnitude for Pb2P2O7-WO3 and Pb2P2O7-MoO3 is reasonable once W and Mo atoms have polarizabilities of the same order of magnitude. In the case of molybdenum-lead-pyrophosphate glasses, the MoO6 units act similarly as WO6 units in tungsten-lead-pyrophosphate glasses, where the main contribution for the nonlinear response is attributed to the distorted octahedral structural units of MoO6 within the glass network.
Table 2
4. Conclusions
In summary, we characterized the third-order nonlinearity of a new binary molybdenum-lead-pyrophosphate glass in the near infrared and other properties were also explored by thermal analysis, Raman and absorption spectroscopies. By replacing the lead- pyrophosphate precursor by MoO3 up to 80 mol%, we observed an increase of glass transition temperature up to 50 mol% and an increase of thermal stability against crystallization up to 60 mol% of molybdenum oxide. The glass structure changes were monitored by Raman spectroscopy and showed a break process of phosphate chains and the
raising of molybdenum related bands. Finally, we remark that the n2 values reported, the infrared transparency window and the thermal characteristics of our samples are similar to the characteristics pointed as adequate for holey fiber production in [26]. The large
transparency window from the visible to the infrared of the sample with smaller MoO3
concentration (sample A) combined with its large n2 may be useful for Kerr-lens mode locking in lasers.
Acknowledgments
The authors acknowledge financial support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq), the Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE) and the grant number 2010/20776-5 from São Paulo Research Foundation - FAPESP. The work was performed in the framework of the Photonics National Institute (INCT de Fotônica) project. Financial support from the
"Région Pays de la Loire" for the senior foreign research chair contracted to C. B. de Araújo is also acknowledged.
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Figure caption
Figure 1. Photographs of the glass samples in the binary system (100-x) Pb2P2O7-xMoO3. Figure 2. Optical absorption of the samples. Samples’ thickness: (A) 1.68 mm; (B) 1.81 mm; (C) 1.62 mm; (D) 1.70 mm; (E) 1.54 mm.
Figure 3. Raman spectra of the samples.
Figure 4. Schematic of the 4f - coherent system for the Z-scan measurements (f1 = f2). The sample d is scanned in the focal region. The labels refer to: lenses (L1-L3); mirrors (M1, M2); beam splitters (BS1, BS2).
Figure 5. Closed-aperture Z-scan profiles: (a) sample A and (b) sample E, measured at 1064 nm. (c) Sample E – laser wavelength: 532 nm. The insets show the open-aperture Z- scan profiles.
Table caption
Table 1. Samples’ compositions, characteristic temperatures (Tg forglass transition; Tx for the onset of crystallization) and thermal stability parameter ΔT=Tx-Tg.
Table 2. Linear and nonlinear parameters of the samples: n0 is the linear refractive
index, n2 is the nonlinear refractive index and α2 is the nonlinear absorption coefficient.
