Second-order nonlinear optical properties of Ge-Ga-Ag-S glass irradiated by electron beam

TAO Hai-zheng(陶海征), DONG Guo-ping(董国平), XIAO Hai-yan(肖海燕), LIN Chang-gui(林常规), ZHAO Xiu-jian（赵修建）

Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Ministry of Education, Wuhan 430070, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Ge-Ga-Ag-S chalcogenide glasses with the composition Ge₃₀Ga₃Ag₄S₆₃ were obtained by the conventional melt-quenching method. According to the visible-infrared and infrared spectra, Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass possesses wide transmittance window from 510 nm in the visible region up to 11.5 μm in the infrared region. And the present glass has better glass-forming ability (the difference between glass transition temperature and the peak temperature of crystallization is larger than 100 ℃). Utilizing maker-fringe technique, a prominent second-harmonic generation was observed in Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass after irradiated by an electron beam (Accelerating voltage: 25 kV; Irradiating current: 15 nA; Irradiating time: 10 min). And the mechanism of second-harmonic generation in the Ge-Ga-Ag-S system glasses was discussed.

Key words: chalcogenide glasses; Ge₃₀Ga₃Ag₄S₆₃; second-harmonic generation; microscopic mechanism

1 Introduction

Nonlinear optical phenomena can be described by the nonlinear polarization P, which is driven by interactions between the electric field and the nonlinear susceptibilities present in the material:

P=ε₀(χ⁽¹⁾E+χ⁽²⁾E²+χ⁽³⁾E³+…)                     (1) 

where ε₀ is the permittivity in free space; χ⁽¹⁾ is the linear susceptibility; χ⁽²⁾ is the second order susceptibility associated with processes such as sum and difference frequency generation; χ⁽³⁾ is the third order susceptibility related to effects such as the intensity dependent refractive index; E is the applied optical electric field[1]. For the materials with centro-symmetry, χ⁽²⁾  is necessarily zero and only third order and higher nonlinear processes are expected.

However, the observation of photo-induced second-harmonic generation(SHG) in germano-silicate optical fibers by ?STERBERG and MARGULIS in 1986[2] indicates the presence of an effective second order nonlinearity χ⁽²⁾ despite the fact that it is centro-symmetric. This phenomenon initiated great interest in scientific and industrial circle, due to many merits of glasses compared with crystals such as structural controllability, the atomic composition being continuously tailored, good machining performance. Much research has been done about the SHG in oxide glasses poled by various means such as thermal electric poling, electron beam poling, UV poling[3-5]. Recently, significant interest has arisen in the nonlinear optical properties of chalcogenide glasses because of their rather larger nonlinear optical susceptibility compared with the oxide ones[6].

In the present study, clear second-harmonic generation of the Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass poled by an electron beam was reported, and its microscopic mechanism was discussed in this paper.

The samples of Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass were prepared from particular high-purity element (Ge, Ga, Ag, S, all of 5N) using the 10 mm inner-diameter silica tube by the well-established melt-quenching techniques in our Lab. And the annealed glassy specimen was cut and then optically polished for the measurements of optical properties. For details see our previous works [7, 8].

The chemical compositions of the samples were analyzed using an electron probe microanalyzer (EPMA) (Type: JXA-8800R). Homogeneity and amorphous characteristics of the prepared materials were confirmed by optical and electron microscopy and X-ray diffraction with CuK_α radiation.

The optical absorption spectra were recorded with a UV-Vis-near IR spectrophotometer (Shimadzu UV-1601) in the region of 220-1 100 nm using the resolution of 0.5 nm and the transmission spectra using a fourier transform infrared spectrometer (Model: Nicolet 60-SXB FTIR) in the region of 4 000-400 cm^-1 using 4 cm^-1 resolution.

The Raman measurements were performed in a 180? backscattering geometry using an inVia Laser Confocal Raman Microscope made by Renishaw PLC. The 632.8 nm line of a He-Ne laser was used and the spectral resolution was of the order of 1 cm^-1. For details see our previous paper [9].

Measurement of second-harmonic generation (SHG) was carried out by using Maker fringe technique (i.e., detecting the second-harmonic intensity of a fundamental wave of a pulsed Nd³⁺: YAG laser at a wavelength of

1 064 nm as a function of the angle of the incident light) [3].

The prepared Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glassy specimens were optically homogeneous to the eye and its amorphous characteristic was confirmed according to the methods given above. According to the results of the EPMA analysis, the difference in compositions between the batch and the prepared glassy samples was within reasonable range. So, for the sake of clarity, the glasses will be labeled by the batch composition in the following text.

Characteristic temperatures, t_g (glass transition temperature)=386 ℃ and t_x (the peak temperature of crystallization) =487 ℃, were determined by differential scanning calorimeter (NETZSCH STA 449C) at a heating rate of 10 K/min. The difference between t_x and t_g is larger than 100 ℃, indicating its high fiber-drawing capability that must be considered when preparing the optoelectronic facilities such as optical switching devices as the optical fibers provide larger interaction lengths.

The visible-near IR absorption spectrum of the Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glassy sample was represented in Fig.1. The visible absorption edge, λ_vis, defined as a wavelength at which 10% of the transmission at longer wavelength is obtained, is at about 510 nm for this sample (0.9 mm in thickness). The λ_vis of the glasses is ascribed to the electrical transition between valence bands and conduction bands.

Fig.1 Visible-near IR absorption and IR transmission (insert) spectra of Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glassy specimen

In addition, the long-wavelength cut-off edge λ_IR (defined as a wavelength at which 10% of the transmission at shorter wavelength is obtained) of this glassy sample is at about 11.5 μm (Fig.1). The distinct absorption band at about 2 500 cm^-1 is due to the fundamental vibration of -SH that was attributed to the hydroxide contamination during the glass preparation [10]. Great efforts should be made to remove these impurities when considering the practical applications of these chalcogenide glasses.

Fig.2. presents the Maker fringe pattern of the Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glassy specimen after irradiated by an electron beam (Accelerating voltage: 25 kV; Irradiating current: 15 nA; Irrating time: 10 min). To study the microstructural origin of SHG in the present Ge₃₀Ga₃Ag₄S₆₃  chalcogenide glass irradiated by an electron beam, RAMAN scattering measurement of the as-prepared and irradiated glassy sample was conducted (Fig.3). However, not any clear evolution was detected.

4 Discussion

RAMAN spectroscopy is now, as before, a very valuable non-destructive tool for research about the microstructure of amorphous materials, because very few unit cells of a definite structure are required for its detection, whereas in the case of diffraction methods, coherence lengths of about 150-200 nm are necessary [11]. According to the RAMAN spectrum of the Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass, the microstructuralunits of the present glass mainly include tetrahedra

Fig.2 Dependence of second-harmonic intensity on angle of incidence for Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glassy specimen irridiated by electron beam (Accelerating voltage: 25 kV; Irradiating current: 15 nA; Irradiating time: 10 min)

[GeS₄](340 cm^-1), ethane-like units S₃Ge-GeS₃(250 cm^-1) and less quantity of tetrahedra [GaS₄](345 cm^-1), ethane-like units S₃Ga-GaS₃ (270 cm^-1)[12-14]. However, no clear vibrational promicence related to the added Ag was detected. According to Refs.[15], it is reasonable to infer that the Ag atoms are homogeneously distributed in glassy net in the form of S atoms as the nearest neighour coordination, which act as the charge compensator. Certainly, further analysis is necessary to asertain this deduction.

Up to now, there are mainly two mechanisms, which were presented to elucidate the microscopic origin of SHG in various oxide glasses treated by thermal/electric poling, electron-beam poling, UV poling and so on. The first one is related to the creation of an electrostatic field E_DC originated from the poling treatment. The coupling of E_DC and the third-order susceptibility (χ⁽³⁾) of the glass leads to an effective χ_eff⁽²⁾ through equation χ_eff⁽²⁾∝χ⁽³⁾?E_DC. The second one is correlated with the reorientations of dipoles (polar bonds) or hyperpolarizable entities during the poling treatment that is considered to be the possible microscopic mechanism of the induced macroscopic second-order nonlinearity in poled glasses. According to these mechanisms, the establishment of second-order nonlinearity in poled Ge₃₀Ga₃Ag₄S₆₃ glass irradiated by an electron beam can be explained reasonably[16]. However, further analysis about which mechanism plays the main role is rather difficult.

Compared with oxide glasses, chalgenide glasses have weak bonds and accordingly larger structural variability. It should be feasible to find out the mechanism of SHG in chalcogenide glasses through the probing of microstructural change related to the irradiating by an electron beam. However, no spectral

Fig.3 Room temperature Raman spectra of as-prepared Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glassy sample and irradiated one by electron beam (Accelerating voltage: 25 kV; Irradiating current: 15 nA; Irradiating time: 10 min). For clarity, spectra are shifted vertically

evolution was found out distinctly about the RAMAN probing of the as-prepared and irradiated Ge₃₀Ga₃Ag₄S₆₃ glass specimen.

To better understand the microscopic mechanism of SHG in chalcogenide glasses poled by various means, further studies are now under way, utilizing other tools probing the microstructure such as EPMA, AFM.

5 Conclusions

Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass was prepared by the well-established melt-quenching technique. The present Ge₃₀Ga₃Ag₄S₆₃ chalcogenide glass has wide transmission window from 0.51 to 11.5 μm given by the visible-near infrared and infrared spectra and better glass-forming capability (the difference of t_g (386 ℃) and t_x (487 ℃) is larger than 100 ℃) ascertained by the DSC/TG curves. Clear second harmonic generation has been observed in the present glass poled by an electron beam. This phenomenon can be reasonably explained by two mechanisms. While the present study can’t exclude any one of these two mechanisms and further study is required.

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(Edited by CHEN Ai-hua)

Foundation item: Project (50125205) supported by the National Natural Science Foundation of China; Project (SYSJJ2004-14) by Key Laboratory of Silicate Materials Science and Engineering (Wuhan University of Technology) Ministry of Education

Corresponding author: TAO Hai-zheng; Tel: +86-27-87669729; Fax: +86-27-87669729; E-mail: thz@mail.whut.edu.cn