Correlation between the nonlinear refractive index and structure of germanium-based chalcogenide glasses

Materials Research Bulletin 42 (2007) 2107–2116 www.elsevier.com/locate/matresbu

Correlation between the nonlinear refractive index and structure of germanium-based chalcogenide glasses L. Petit a,*, N. Carlie a, A. Humeau b, G. Boudebs b, H. Jain c, A.C. Miller d, K. Richardson a a School of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA Laboratoire des Proprietes Optiques des Materiaux et Applications, UMR CNRS 6163, Universite d’Angers, 2 Boulevard Lavoisier, 49045 Angers Cedex 01, France c Center for Optical Technologies and Department of Materials Science and Engineering, Lehigh University, 5 E Packer Avenue, Bethlehem, PA 18015, USA d Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, PA 180015-1539, USA

b

Received 25 September 2006; received in revised form 16 January 2007; accepted 24 January 2007 Available online 30 January 2007

Abstract The nonlinear refractive index (n2) of new germanium-based sulfo-selenide glasses has been measured at 1064 nm using Z-scan technique, with picosecond pulses emitted by a 10 Hz Q-switched mode-locked Nd:Yag laser. We have determined the impact of the progressive replacement of S by Se on the nonlinear properties of these glasses. The value of n2 strongly increases with the substitution of S by Se, up to 350 times the n2 for fused silica. X-ray photoelectron spectroscopy (XPS) indicates that the increase of Ge–Se bond units in the glass network is responsible for the increase of n2. The suitability of these glasses for optical switching at telecommunication wavelengths based on such nonlinear properties has been also confirmed. # 2007 Elsevier Ltd. All rights reserved. PACS : Germanium-based sulfo-selenide glass; XPS spectroscopy; Nonlinear refractive index Keywords: A. Amorphous materials; A. Chalcogenides; C. Raman spectroscopy; D. Optical properties

1. Introduction Optical glasses with large non-resonant nonlinear refractive index are good candidate materials for all-optical switching devices and may also be used to enhance the performance of mode-locked solid-state lasers [1,2]. Glasses have advantages compared to semiconductors, semiconductor-doped glasses, and organic materials because of their fast response times, negligible linear loss, and small two-photon absorption (TPA) in the wavelength range of interest. In addition, glasses are mechanically strong and compatible with waveguide fabrication procedures [3,4]. Several research groups have demonstrated that the nonlinear refractive indices of oxide glasses can be increased by the addition of heavy-metal cations, such as Pb, Bi, and Tl [5]. The studies of sulfide glasses reported particularly large non-resonant optical nonlinearities [6,7]. Similarly, selenides have been identified as candidate materials for nonlinear optical applications. Because of its large * Corresponding author. Tel.: +1 864 656 1259; fax: +1 864 656 1099. E-mail address: [email protected] (L. Petit). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.01.013

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atomic radius compared to oxygen in oxide glasses and sulfur in sulfide glasses, selenium is believed to be the key to the nonlinear optical properties in selenide glasses. The introduction of selenium to the As–S system increases the nonlinear refractive index (n2) up to 400 times the value for fused silica [8]. Such large n2 for glasses with small As/(S + Se) molar ratios has been correlated to the presence of covalent, homopolar Se–Se bonds in the glass structure; it could not be attributed to any red shift in the absorption edge or to a resonant effect. Germanium-based chalcogenide glasses with heavy-metal species such as antimony are promising materials for photonic devices as their phonon energies are expected to be lower than those of fluoride glasses. Our effort aims to develop glasses in the Ge–Sb–S system with a low Sb concentration and with an excess of S to present good physical stability and high nonlinear index suitable for use in novel optical applications. In order to increase the nonlinear refractive index, sulfur has been replaced progressively by selenium. Recently, we reported results of a systematic study examining the relationship of the physical properties to the structure of the glasses in the system Ge0.23Sb0.07S0.70xSex where the S was substituted by Se with x = 0–0.70 [9]. We showed that with increasing Se, the density increases, the glass transition temperature decreases and the absorption band gap shifts to longer wavelengths in the infrared region. These variations in physical properties have been correlated to the glass structure modification when S is replaced by Se. In our preliminary study, we reported that the replacement of 10% of S by Se in this glass system increases the nonlinear refractive index [10]. We have attributed the variation of n2 to the total number of electron lone pairs and to the position of the absorption band gap, which are influenced by the presence of GeS4 units and/or covalent, homopolar Se–Se bonds in the glass structure. In this paper, we report results of a complete study of nonlinear optical properties of these new sulfo-selenide glasses. X-ray photoelectron spectroscopy (XPS) has been used to examine the glass structure modification when S is progressively replaced by Se. A correlation is established between the molecular units in the glass network and the nonlinear optical properties, allowing a better appreciation of the effect of the modification of glass network on the nonlinear index variation. The figure of merit of these glasses has been also estimated to verify their suitability for optical switching at telecommunication wavelengths. 2. Experiment procedures 2.1. Glass preparation Ge0.23Sb0.07S0.70xSex glasses with x = 0, 0.05, 0.10, 0.20, 0.50 and 0.70 were prepared in 10 g batches from high purity elements (99.999% Ge from Aldrich, 99.9% Sb from Alpha, 99.999% S and Se from Cerac). Starting materials were weighed and batched inside a nitrogen-purged glove box and vacuum sealed into quartz ampoules. Prior to sealing and melting, the ampoule and batch were pre-heated at 100 8C for 4 h to remove surface moisture from the quartz ampoule and the batch raw materials. The ampoule was then sealed and heated for 24 h to between 800 and 950 8C, depending on the glass composition. A rocking furnace was used to rock the ampoule to increase the homogeneity of the melt. Once homogenized, the melt-containing ampoule was air-quenched to room temperature. To avoid fracture of the tube and glass ingot, the ampoules were subsequently returned to the furnace for annealing for 15 h at 40 8C below the glass transition temperature, Tg. As the selenium was introduced to the glass composition, the sample color changed from orange to dark. The compositions were verified by energy dispersive spectroscopy (EDS) and were found to be same as the initial batch composition. No loss of sulfur and selenium was observed within the accuracy of the measurement (2%). 2.2. X-ray photoelectron spectroscopy The investigated samples, fractured in situ in the analysis chamber with a hard tool, were studied using a Scienta ESCA-300 spectrometer at a vacuum of 109 Torr. A monochromatic Al Ka X-ray (1486.6 eV) source was used for the analysis; a low energy electron flood gun was used to minimize surface charging. To avoid any contamination, the fresh surface of the samples was analyzed just after being fractured. Survey spectra were recorded from 0 to 1250 eV with pass energy of 150 eV using 1 eV/0.3 s steps in 1 scan. The low resolution XPS scan from the sample with x = 0, taken as an example, is shown in Fig. 1. The core-level peaks and X-ray induced Auger lines from the constituent elements are easily identified and marked on the spectrum. Individual high

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Fig. 1. Low resolution XPS spectrum obtained from the glass with x = 0.

resolution Ge(3d), Sb(3d), Sb(4d), S(2p), Se(3p) and Se(3d) spectra were recorded with a pass energy of 150 eV, 0.05 eV/0.3 s steps, and scans numbering 2–10, depending on the intensity of the peak, to reach a satisfactory signal/noise ratio. Amorphous glasses in the system Ge–Sb–S–Se are semiconductors and their surface becomes somewhat charged with the ejection of photoelectrons. This charging was compensated by flooding the surface with low energy electrons for the duration of the experiment. To compare the spectra accurately, the binding energy drift due to any uncompensated charging was further corrected by adopting a common reference. Specifically, the spectra for the different illumination states were shifted in energy such that the steep rise of the top edge of the valence band coincided with the zero of the binding energy (BE). Each core-level spectrum was fitted to obtain sets of doublets representing the spin–orbit splitting of d and p electron core-level. The goodness of fitting determined the minimum required number of doublets. The full width at half maximum (FWHM) was assumed to be the same for the peaks within one doublet, but was allowed to be different for other doublets of the same core-level spectrum. ESCA Analysis software from Scienta Instrument was used to facilitate peak decomposition [11]. Mix between the Gaussian and Lorentzian in the Voigt function was chosen the same for all doublets of a given core-level. Fitting procedure gave mix values close to 90% Gaussian and 10% Lorentzian. The peak ranges were chosen carefully and consistently for different glass compositions. With these constraints, the uncertainty in peak position and area of each component were 0.05 eV and 2%, respectively. 2.3. Nonlinear measurement The nonlinear refractive index was measured using Z-scan method [12]. Excitation was provided by a Nd:YAG laser delivering linearly polarized 15 ps single pulses at l = 1064 nm with 10 Hz repetition rate. Other experimental parameters using classical Z-scan method were: f (focal length of the focusing lens) = 20 cm; d (distance from the beam waist plane to the camera) = 26 cm. The beam waist at the focal plane was v0 = 30 mm giving a Rayleigh range z0 = pv20 /l  2.6 mm. This value was larger than the sample thickness (typically 1 mm). The photoreceptor was a 1000  1018 pixels cooled camera (Hamamatsu C4880) with a fixed linear gain. The camera pixels had 4095 gray levels and each pixel was 12 mm  12 mm. Two sets of acquisitions (in the linear and the nonlinear regime) were carried out for the measurement in order to correct for inhomogenities and surface imperfections in the sample. Open and closed Z-scan normalized transmittance were numerically processed from the acquired images by integrating over all the pixels in the first case and over a circular numerical filter (with radius equal to 1 mm) in the second case.

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3. Results 3.1. Nonlinear refractive index measurement The Z-scan test-bed was first calibrated with the CS2 reference liquid, which was placed in a 1 mm-thick fused silica cell of good optical quality. The measured nonlinear index was 3.2  1018 m2/W at 1064 nm, which is within 7% of that found in the literature [12], well within the 15% error bar of the measurement (Table 1). To avoid the contribution of any permanent photo-refractive index change or damage for each sample, the measurements were repeated at least twice, illuminating the sample on the same position with different laser intensities. In Table 1 are listed the nonlinear indices (n2) and the nonlinear absorption coefficient (b) at 1064 nm as well as the band gap wavelength defined as the wavelength for which the linear absorption coefficient is 10 cm1 (lgap). Note that the nonlinear index increases slowly up to x = 0.10 and strongly for higher Se content. The increase of the nonlinear refractive index with the progressive replacement of S by Se is consistent with prior results [7,10]. To be considered as a good candidate for ultra fast optical switch in an optical fiber configuration with a peak power of 1 W and an attenuation of 0.5 dB/m, the glass must have a high nonlinear refractive index (about 400 times higher than that of silica) and a small value of the nonlinear absorption (b). The values of the figure of merit (FOM) of the investigated glasses for this application at 1064 nm, defined by F¼

2bl n2

are listed in Table 1, where lower values are preferred. The values of F are lower than 1 for the S-rich glasses. So the glasses with x < 0.20 can be considered good candidates for the realization of an optical switch at this wavelength. But the interesting wavelengths for telecommunication optical fibers network are located around 1550 nm. Quemard et al. [13] have measured the nonlinear refractive index of As40Se60 and Ge20As40Se40 glasses at 1064 nm and also at 1430 nm. They have demonstrated that the value of the figure of merit of these glasses is higher than 1 at 1064 nm but <1 at 1430 nm. For this reason, the glasses with x = 0.50 and 0.70 also can be considered good candidates for optical switching application. Compared to these results, we can also expect a decrease of the nonlinear absorption inducing a FOM <1 at 1430 nm or at 1550 nm which is the target telecommunication wavelength. 3.2. XPS analysis 3.2.1. High resolution XPS spectrum of Sb(3d) and Sb(4d)–Ge(3d) The core-level 3d spectrum for Sb is shown in Fig. 2(a) for the pure sulfide and selenide glasses (with x = 0 and 0.70). For both spectra, the signal consists of a single peak due to the absence of convolution of the 3d5/2 peaks and 3d3/2 peaks of all the chemical environments. Their binding energies (BEs), fixed at 538.3 and 538.7 eV, respectively, were chosen based on the referenced Sb(3d) line for Sb2Se3 and Sb2S3 [14]. These peak binding energies and their widths did not show variation with composition. For the sulfo-selenide glasses, at least two single peaks were needed to properly fit the spectra of the glasses except for the glass with x = 0.20. For this particular glass, an additional single Table 1 Nonlinear characteristics of investigated germanium-based glasses measured by Z-scan technique at l = 1064 nm in function of Se content Glass composition CS2 [12] CS2 x=0 x = 0.05 x = 0.10 x = 0.20 x = 0.50 x = 0.70

Total electronic lone pairs (cm3)

n2 (1018 m2/W) (15%)

5.46E+22 5.49E+22 5.51E+22 5.29E+22 5.08E+22 5.00E+22

3 3.2 1.66 1.9 1.9 2.7 6.8 10.3

n2/nsilica

b (cm/GW) (15%)

lgap (nm)a

F

55 63 63 90 227 343

– – <0.1 <0.1 <0.1 0.12 0.6 2.4

530 605 643 683 718 745

<1.3 <1.1 <1.1 0.9 1.9 5.0

n2 is the nonlinear refractive index; b is the nonlinear absorption; F is the figure of merit at 1064 nm. a lgap, the band gap wavelength defined as the wavelength for which the linear absorption coefficient a is 10 cm1.

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Fig. 2. High resolution XPS spectrum of Sb(3d3/2) (a) and of Ge(3d)–Sb(4d) levels (b) for the investigated glasses.

peak at 539.14 eV was added to properly fit the spectrum. The relative fractions of Sb species in each of the different environments are given in Table 2. As expected, the intensity of the peak corresponding to Sb3+(Se) increases with increasing Se content. A similar consistent variation in Sb3+(S)/Sb3+(Se) content has been confirmed from the Sb(4d) spectra for the investigated glasses. In this region of spectrum, the Sb(4d) peaks overlap with Ge(3d) peaks as seen in Fig. 2(b). The Sb(4d)–Ge(3d) spectra of the pure sulfide and selenide glasses have been decomposed into one doublet and one single peak. The doublet observed at 33.36–34.60 eV in the spectrum of the pure sulfide glass and at 33.1 and 34.3 eV in the spectrum of the pure selenide glass are the results of the convolution of the 4d5/2 peaks and 4d3/2 peaks and have been attributed, respectively, to Sb3+(S) and Sb3+(Se) [15]. The core-level 3d spectrum for Ge consists of a single peak located at 31.33 eV in the spectrum of the pure sulfide glass and at 31.0 eV in the spectrum of the pure selenide glass. These single peaks are due to the absence of convolution of the 3d5/2 peaks and 3d3/2 peaks of all the chemical environments. They correspond to Ge4+(S), in accordance with our previous study [16] and to Ge4+(Se) [17]. The Sb(4d)–Ge(3d) spectra of the sulfo-selenide glasses have been fitted to two doublets and two single peaks except for the glass with x = 0.20. In order to obtain acceptable quality of the fitting, three doublets and three single peaks were necessary to fit the spectrum of the latter glass. The relative fractions of these species are summarized in Table 3. It is shown that the relative fractions of Ge4+(Se) and Sb3+(Se) units increase with an increase of Se content. One can also notice that the relative fractions of Sb species obtained from the fitting of the Sb(3d) spectra and of the Sb(4d)–Ge(3d) spectra are in good agreement. Table 2 Ratios of the different Sb3+(S), Sb3+(Se) and Sb3+(O) components in the fitted Sb(3d3/2) spectrum Atomic percent of Se (x)

0 0.05 0.10 0.20 0.50 0.70

Relative fraction of various species Sb3+(S)

Sb3+(Se)

100 89 64 19 16 0

0 11 36 65 84 100

Sb3+(O)

16

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Table 3 Ratios of the different Sb3+(S), Sb3+(Se) and Sb3+(O) components and Ge4+(S), Ge4+(Se), Ge4+(O) components in the fitted Sb(4d)–Ge(3d) spectrum Atomic percent of Se (x)

Relative fraction of various species Ge(3d)

Sb(4d)

4+

0 0.05 0.10 0.20 0.50 0.70

4+

Ge (S)

Ge (Se)

100 86 70 58 38 0

0 14 30 31 62 100

4+

Ge (O)

11

Sb3+(S)

Sb3+(Se)

100 82 67 18 12 0

0 18 33 58 88 100

Sb3+(O)

24

3.2.2. High resolution XPS spectra of Se(3d) and S(2p)–Se(3p) Se(3d), Se(3p) and S(2p) peaks were deconvoluted using the parameters summarized in Table 4, where D is the doublet separation for d or p orbits, DR is the area ratio for the doublets, FWHM is the full width at half maximum of the peaks. The parameters were chosen by using pure elemental data as reference and optimum fitting. As the parameters were chosen, the spectra were fitted with as few peaks as possible. Then through self-consistency of all fitting results and our understanding of the glass system, we assigned the peaks to different chemical environments. The core-level 3d spectrum for Se is shown in Fig. 3(a). The spectrum of the pure selenide glass exhibits a large peak in the range 52 and 57 eV which has been deconvoluted into two doublets. These doublets are due to the convolution of the 3d5/2 and 3d3/2 peaks of all the chemical environments. The one located at 55.26–54.42 eV Table 4 XPS data fitting parameters

S(2p) Se(3p) Se(3d)

D (eV)

DR

FWHM (eV)

1.18 5.72 0.85

0.5 0.4 0.66

0.95 1.9 0.93

D is the doublets binding energy separation for p and d orbits; DR the area ratio for the doublets; FWHM the full width at half maximum of the peaks.

Fig. 3. High resolution XPS spectrum of Se(3d) (a) and of S(2p) and Se(3p) levels (b) for the investigated glasses.

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Table 5 Ratios of the different S components when connected to Se, S, X and O in the fitted Se(3d) spectrum Atomic percent of Se (x)

Relative fraction of Se when connected to

0 0.05 0.10 0.20 0.50 0.70

S

Se

X = Ge, Sb

O

67 47 28 26

7 11 24 22 20

26 42 32 52 80

16

corresponds to Se bonded to Se (corresponding to homopolar Se–Se bond) and the one at 54.98–54.11 eV to Se bonded to Ge or Sb (corresponding to heteropolar Se–X bond with X = Ge or Sb). For the sulfo-selenide glasses, another doublet at 55.66–54.82 eV was needed in the fitting. Because of its higher bonding energy compared to the other doublets, this doublet has been assigned to Se bonded to S (Se–S). As seen in the fitting of the Sb(3d) and Sb(4d)– Ge(3d) (Fig. 2(a) and (b)), another doublet at higher binding energy (56.88–56.00 eV) was required to fit properly the spectrum of the glass with x = 0.20. The relative fractions of these species are summarized in Table 5. In the pure selenide glass, 80% of the Se ions are expected to be bonded to Ge or Sb whereas 20% seem to be organized in the glass network into homopolar bonds. When the concentration of Se decreases, the number of homopolar and heteropolar bonds decreases whereas the number of Se–S bonds increases. The S(2p) and Se(3p) spectra of the investigated glasses with their fitting are presented in Fig. 3(b). The spectra of the pure sulfide and selenide glasses have been deconvoluted into two doublets. The S(2p) doublets are located in the range 162–163 eV with a separation of 1.18 eV between 2p3/2 and 2p1/2 components. The doublet observed at 163.79– 162.7 eV in the glass with x = 0 has been assigned to S bonded to S (homopolar S–S bonds) and the one at 162.9– 161.83 to S connected to Ge or Sb (heteropolar S–X bonds with X = Ge or Sb). The relative fractions of these species are listed in Table 6. It is shown that the network is formed by 75% of S–S bonds and 25% of S–X with X = Ge or Sb. The Se(3p) components are located between 160 and 166 eV with a peak separation of 5.72 eV between 3p3/2 and 3p1/2. The Se(3p3/2) peak overlaps with the peaks of S(2p). In the spectrum of the pure selenide glass (x = 0.70), the doublet at 166.7–160.95 eV has been assigned to Se bonded to Se (homopolar Se–Se bonds) and the one at 166.4– 160.95 eV to Se connected to Ge or Sb (heteropolar Se–X bonds with X = Ge or Sb). As presented in Table 6, the relative fractions of Se–Se and Se–X are in agreement with the one estimated from the fitting of the Se(3d) spectrum of this glass. The combined S(2p) and Se(3p) spectra of the sulfo-selenide glasses have been fitted with six doublets except for the glass with x = 0.20. The new doublet at 163.2–162 eV, at lower BE than the doublet corresponding to S–S, has been assigned to S bonded to Se (S–Se) as this heteropolar bond is less covalent than the homopolar S–S bond. The new doublet at 167–161.25 eV, at higher BE than the doublet assigned to Se–Se, has been related to Se bonded to S (Se–S) because of its higher electronegativity compared to that of S–S bond. The spectrum of the glass with x = 0.20 has been fitted using two more doublets at 164.8–163.72 and 168–162.25 eV. The relative fractions of these species are summarized in Table 6. When the concentration of Se increases, the number of S–Se bonds increases with the progressive decrease of the number of S–S and S–X bonds. Table 6 Ratios of the different Se and S components when connected to Se, S, X and O in the fitted S(2p)–Se(3p) spectrum Atomic percent of Se (x)

0 0.05 0.10 0.20 0.50 0.70

Relative fraction of Se when connected to S

Se

X = Ge, Sb

67 47 24 26

7 11 22 22 20

26 42 38 52 80

Relative fraction of S when connected to O

17

S

Se

X = Ge, Sb

4 9 10 48

25 26 24 22 7

75 70 67 53 45

O

16

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4. Discussion The aim of this study was to establish a relation between the nonlinear properties and the structure of glasses in the system Ge–Sb–S/Se. Before discussing the variation of nonlinear refractive indices of the glasses, a brief description of the glass structure characteristics as a function of Se content is presented. A detailed study of the Raman spectra of the glasses as a function of Se content has been published already [9]. It was shown that the basic structural units of the pure sulfide glass are GeS4 tetrahedra and SbS3 pyramids which are weakly coupled through two-atom –S–S– bridging groups because of the excess of S in the glass network. Furthermore, the study has shown that: (i) When x increases up to 0.20, the substitution of S by Se leads to a decrease of S–S homopolar bonds in ring structure, and the Ge–Sbridging bond in GeS4. Few two edge-sharing Ge2S4S2/2 and S3Ge–S–GeS3 units, GeSe3S units and Se chain are expected to be part of the network. We have suggested that it is the S atom that remains in the tetrahedral GeS4 units during substitution of Se, while the Se tends to form homopolar Se–Se or heteroatomic Se–S bonds. (ii) For 0.50  x > 0.20, the glass network is believed to be mainly formed by Se in chains and rings, and by Ge–Se bonds in the tetrahedral mixed GeSSe3 structural units. Some GeSe4 units are also expected to be part of the network. Two edge-sharing Ge2S4S2/2 and S3Ge–S–GeS3 units are expected to be still present in the network of the glass with x = 0.50. (iii) When x increases from 0.50 to 0.70, it appears that corner-sharing GeSe4/2 units through bridging Se–Se entities and edge-sharing Ge2Se8/2 bi-tetrahedra form the glass network along with Sen in rings, Se–Se in chains and SbSe3 units, due to a low concentration of Sb. Here, the GeS4 glass network has been progressively replaced by a GeSe4-dominated network. Table 1 lists the absorption band gap position and the total number of electronic lone pairs in the investigated glasses which have been calculated assuming one electronic lone pair per Sb ions, two per S and Se and none per Ge ions. Fig. 4 presents the evolution of the total number of electronic lone pairs and the absorption band gap position as a function of n2 of the investigated glasses. Note that an increase of n2 associated with the progressive replacement of S by Se may be related to the decrease of the total number of electronic lone pair and to the red shift of the absorption band gap. This is in good agreement with Harbold et al. who have shown that n2 is not solely dependent on the lone electron pairs concentration but also on the energy gap which is also modified by the change in lone electron pairs concentration [18]. In our preliminary study, we attributed the red shift of the absorption band gap with the progressive replacement of S for Se in the Ge0.23Sb0.07S0.70xSex glass system to the structural modification: the decrease of the number of corner-sharing GeS4/2 and SbS3 units and the increase of the number of corner-sharing GeSe4/2 and SbSe3 units, Ge–Se–Ge and Se–Se homopolar bonds [9].

Fig. 4. Evolution of the total number of electronic lone pairs and absorption band gap position in function of n2.

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Table 7 Density and total number of Ge, Sb, S and Se ions/cm3 in the investigated glasses Atomic percent of Se (x)

Density, r (g/cm3) (0.02)

Ge ions/cm3

Sb ions/cm3

S ions/cm3

Se ions/cm3

0 0.05 0.10 0.20 0.50 0.70

2.94 3.10 3.26 3.41 4.08 4.55

8.54E+21 8.58E+21 8.62E+21 8.28E+21 7.94E+21 7.83E+21

2.6E+21 2.61E+21 2.62E+21 2.52E+21 2.42E+21 2.38E+21

2.60E+22 2.43E+22 2.25E+22 1.80E+22 6.91E+21 0.00E+00

0.00E+00 1.87E+21 3.75E+21 7.20E+21 1.73E+22 2.38E+22

In order to verify which structural units in the glass structure may be related to the variation of the nonlinear refractive index of the investigated glasses, XPS spectra of the samples have been measured and the relative concentrations of Ge, Sb, S and Se species are given in Tables 2–6. The total number of Ge, Sb, S and Se ions in the glasses have been calculated using the density and the molar weight of the glasses and are presented in Table 7. As seen in Figs. 2 and 3, more single peaks and doublets have been used in order to properly fit the different spectra of the glass with x = 0.20. As all the new components in the fitting are located in higher BE than the other peaks, it is possible to relate these new single peaks and doublets to new bonds with O, which are less covalent than the bonds with S or Se. The single peak at around 539.14 eV added in the Sb(3d) spectrum (Fig. 2(a)) has been related to Sb3+(O), based on our separate measurements on standard Sb2O3 samples and confirmed by the referenced Sb(3d3/2) line in Sb2O3 [19]. The added single peak at 32.3 eV in the Ge(3d)–Sb(4d) spectrum in Fig. 2(b) has been related to the presence of Ge4+(O) and the doublet at 33.9 and 35.1 eV to Sb3+(O). The new doublet at 56.88–56.00 eV in the Se(3d) spectrum (Fig. 3(a)) has been related to Se bonded to O. The doublets at 164.8–163.72 and 168–162.25 eV in the S(2p)–Se(3p) spectrum (Fig. 3(b)) have been assigned, respectively, to S bonded to O and Se connected to O. The relative fractions of the species are summarized in Tables 2, 3, 5 and 6. Less than 20% of Sb, S and Se and 10% of Ge are expected to be connected to O. As the spectra were measured at the fresh surface of the samples, the presence of oxygen only in the glass with x = 0.20 is probably due to oxygen contamination during the batch preparation. From Tables 5–7, we have verified that the number of S bonded to Se estimated from the fitting of the S(2p) spectra corresponds to the number of Se connected to S from the fitting of the Se(3d) spectra. Fig. 5(a) and (b) presents the variation of the number of S and Se, respectively, when connected to S, Se and X with X = Ge or Sb. When the concentration of Se increases, there is a progressive increase of S–Se, Se–Se and Se–X bonds and a corresponding decrease of S–S and S–X bonds with X = Ge and Sb. In Fig. 5(b), it is clearly shown that the increase of n2 correlates with the increase of heteropolar Se–X bond with X = Ge or Sb. From Tables 3 and 7, the numbers of Ge and Sb atoms bonded only to Se ions have been estimated and are presented in Fig. 6 as a function of the Se content in the glass network. Also presented in this figure is the variation of the nonlinear refractive index of these glasses as a function of the Se content. As can be seen in Fig. 6, there is a correlation between the increase of the n2 and the increase of the number of Ge and Sb connecting Se. Furthermore, note that the

Fig. 5. Evolution of the nonlinear refractive index and of the number of S and Se when connected to S, Se and X with X = Ge or Sb in function of x.

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Fig. 6. Evolution of the nonlinear refractive index and of the number of Ge and Sb when connected to Se in function of x.

variation of n2 is more directly related to the increase of the number of Ge–Se bonds in the glass network than the number of Sb–Se bonds. 5. Conclusion In this paper, we have shown that the introduction of selenium to the Ge–Sb–S glass system increases the nonresonant n2 up to 350 times the value for fused silica. We have demonstrated that this evolution is not simply related to the formation of homopolar Se–Se bonds. Using XPS, we have shown that the increase of nonlinear refractive index of these new sulfo-selenide glasses is not related to the total number of electronic lone pairs either, but is related to the increase of Ge–Se bonds number in the glass network. From the calculation of the figure of merit of the investigated glasses, we have concluded that only the S-rich glasses (x below than 0.20) are suitable for application at 1064 nm but all the sulfo-selenide glasses could be good candidates for optical switching at higher telecommunication wavelengths. Acknowledgment The authors acknowledge the support of the National Science Foundation NSF-DMR 0312081. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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