Optical limiting of niobic tellurite glass induced by self-trapped exciton absorption of the AgCl nanocrystal dopant

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1 Chinese Science Bulletin 2009 SCIENCE IN CHINA PRESS Springer Optical limiting of niobic tellurite glass induced by self-trapped exciton absorption of the AgCl nanocrystal dopant ZHAO ZhenYu 1, LIN Jian 2, JIA TianQin 1, SUN ZhenRong 1 & WANG ZuGeng 1 1 State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, Shanghai , China; 2 School of Materials Science and Engineering, Tongji University, Shanghai , China Niobic tellurite glass doped by silver chloride nanocrystal was prepared with the melting-quenching and heat treatment method, and the self-trapped exciton absorption band of the silver chloride nanocrystal was observed at 532 nm in the UV-visible absorption spectrum. The glass structure characteristics were investigated by Raman spectroscopy, and the mechanism of self-trapped exciton was analyzed by Jahn-Teller model. Its optical limiting was measured with 532 nm picosecond laser pulses, and the corresponding nonlinear absorption coefficient was measured with open-aperture Z-scan. The experimental results showed that optical limiting at 532 nm was attributed to free carrier absorption between the self-trapped state and the continuum band. optical limiting, self-trapped exciton, silver chloride, nanocrystal When the grain-size of a semiconductor crystal diminishes to its Bohr s radius, it will exhibit quantum confinement effect (QC) [1,2]. With good optical properties, such as large nonlinear susceptibility and high photoluminescence efficiency, semiconductor nanocrystals (NCs) become excellent potential materials for all-optical switching and quantum dot laser [3]. However, little attention had been paid on the optical limiting induced by the exciton absorption until the aforementioned phenomena were observed in the CdS and CdTe nanocrystals [4,5]. Glass is a good host matrix for nanocrystals due to its advantages of low cost and high reliability. The exciton absorption of II-VI compound nanocrystal doped glass, such as CdS, CdSe, CdTe, ZnSe and PbS, has been investigated [6 12], but few on I-VII compound nanocrystal doped glass has been reported. Recently, Lin et al. [13 15] investigated the nucleation and growth of AgCl nanocrystals in the tellurite glassy matrix and their third order nonlinear optical susceptibility. However, the self-trapped exciton (STE) absorption band was ignored due to the low concentration of the AgCl nanocrystals [13]. AgCl is the indirect band gap semiconductor, and the top valence band locates at point L. Its interband transition is a phonon assisted process, and the lattice deformation has strong effects on the hole and electron of the free excitons. The absorption band of free excitons will redshift (the Stokes shift) [16], and the self-trapped exciton will be generated. Till now, the self-trapped exciton of the AgCl nanocrystals has only been observed in the AgCl doped alkali halide host matrix [17,18], and there has been no report on the glass host matrix. In this work, the AgCl nanocrystal doped niobic tellurite glass was prepared with the melting-quenching and heat treatment method, the self-trapped exciton absorption band of the AgCl nanocrystal was discussed and analyzed, and fi- Received September 22, 2008; accepted December 22, 2008 doi: /s Corresponding author ( zrsun@phy.ecnu.edu.cn) Supported by Shanghai Leading Academic Discipline Project (Grant No. B408), National Basic Research Program of China (Grant Nos. 2006CB806006, 2006CB921105), Program for Changjiang Scholars and Innovative Research Team in University, Doctoral Program of High Education (Grant No ) and Project sponsored by Shanghai Science and Technology Committee (Grant Nos. 06DJ14008, 07dz22025, 06QH14003) csb.scichina.com Chinese Science Bulletin April 2009 vol. 54 no

2 nally the optical limiting was investigated. 1 Experimental The glass substrate admixtures included 80 wt% TeO 2 (99.99% Aldrich) and 20 wt% Nb 2 O 5 (99.99% Aldrich), and the AgCl (99.99% Aldrich) dopant was 1 wt%. The batches were melted in gold crucibles at 800 in the box furnace for 15 min, and the melted glass was poured onto a heated iron plate and then annealed at 300 for 1 h. The doped samples were instantaneously thermally treated inside a Muffle furnace for min, and then the AgCl nanocrystals formed during the heat treatment process. The resultant glass samples were cut into 2.5 mm slides and polished for optical limiting and Z-scan experiments. Their absorption spectra were measured by a Unico UV-2802S spectrometer, and the glass structure was characterized by a Jobin-Yvon T64000 Micro-Raman spectrometer. The Continuum PY-61 Q-switch modelocked picosecond laser at 35 ps/10 Hz was used for optical limiting and Z-scan measurement. 2 Results and discussion The glass substrate and the AgCl doped glass appear slightly yellow. Because of the Cl color center in the glass matrix, the doped samples became darker with the heat treatment going on. The AgCl defects may lead to the lattice deformation of the AgCl nanocrystal, which will give rise to the self-trapped exciton band [16,18]. As shown in Figure 1, the absorption tails will gradually shift to red-side with the heat treatment going on. It indicates that there is a distinct exciton band at the vicinity of 532 nm in the AgCl doped glass samples thermally treated for 60, 90 and 120 min, and no exciton band can be observed in the glass sample thermally treated for less than 30 min due to the low concentration of the AgCl nanocrystals. As shown in Figure 2, the Raman spectra show the glassy structure, including the covalent bond vibration of Te, O and Nb atoms [19 21]. The peak at 445 cm 1 refers to the symmetric bending vibrational mode of the Te-O-Te linkages at the corner-sharing sites. The symmetric stretching vibrational mode of the Te-O ax bonds in the trigonal bipyramidal [TeO 4 ] 4 units occurs at 660 cm 1. The peak at 760 cm 1 refers to the Te-O symmetric stretching vibrational mode in the ligand unit [TeO 3+1 ] 4. The 890 cm -1 vibrational mode refers to the Nb-O bond in the [NbO 4 ] 3 units. The niobic tellurite oxide ligand, interconnected in the chains, forms the glass network structure, and Nb 2 O 5 plays a role in transferring the [TeO 4 ] 4 unit to the [TeO 3 ] 2 unit via the [TeO 3+1 ] 4 unit. The AgCl particles will be prepared in the ligand interstice during the heat treatment period, which results in the formation of the AgCl nanocrystals. In the glass structure, the cations Te 4+ and Nb 5+ possess much smaller ionic radius than that of the anion O 2, and it can influence the proportion of the Ag + ion to the Cl ion on the interface between the glass ligands and the AgCl nanocrystals. The lattice defects on the surface of the AgCl nanocrystals can induce the lattice deformation of nanocrystal, so electron or hole will be trapped into a deep state. In the AgCl nanocrystals, the Ag + state can be trapped into the Ag 2+ state, and it results in a self- OPTICS ARTICLES Figure 1 UV-visible absorption spectra for the glass samples. Figure 2 Raman spectra for the glass samples. ZHAO ZhenYu et al. Chinese Science Bulletin April 2009 vol. 54 no

3 trapped hole (STH). A STH can bind a free electron and form the self-trapped exciton. The bound energy of the self-trapped exciton is lower than that of the free exciton, so the self-trapped exciton absorption band appears near 532 nm (as shown in Figure 1). The nanocrystal density and size will increase with the heat treatment going on, and the broad self-trapped exciton absorption band is attributed to the inhomogeneous size distribution of the AgCl nanocrystals. Figure 3 shows the optical limiting for the glass samples at 532 nm. The incident optical flux increases from 0 to 0.65 mj, and the samples will be damaged when the optical flux is beyond 0.65 mj. The transmission behaviors of the glass without the heat treatment will approach saturation until the incident optical flux is up to 0.6 mj, while the transmittance of the heat treated samples evidently decreases. The glass sample without the heat treatment exhibits a linear transmission versus the incident energy, while the heat treated glass samples exhibit a distinct optical limiting behavior. Clearly, self-trapped excitons absorb the incident energy, and the transmitted optical energy decreases dramatically as the self-trapped exciton absorption increases. In order to explore the contribution to their optical limiting, their nonlinear absorption was measured with open-aperture Z-scan experiment at 532 nm (as shown in Figure 4). Figure 4 shows that all samples have a decrease trend in the normalized transmission at higher incident optical flux, and the transmission valley decreases with the heat treatment going on. It is wellknown that two photon absorption (TPA) [4] and reversed saturable absorption (RSA) [21] can lead to the abovementioned phenomena. For the semiconductor materials, TPA and free carrier absorption (FCA, namely reversed saturable absorption (RSA)) dominate the nonlinear absorption [22 24]. For the glass samples without heat treatment, there is no self-trapped exciton absorption band around 532 nm, so the incident laser will induce TPA, corresponding to the deep valley in the Z-scan experiment (as shown in Figure 4(a) (c)). For the heat treated glass samples, the self-trapped exciton will be generated and increase with the prolongation of the heat treatment. The 532 nm incident laser will bring the resonant electronic transition from the ground states to the trapped states, and then the electron can be further jumped to the continuum band, namely the free carrier absorption. The exciton absorption and two-photon absorption were schematically shown in Figure 5, and the linear absorption coefficient and FCA cross section were calculated and shown in Table 1 by the following equations [23,24] :. I z = α + σn I (1) ( ), ( )( ) T = T T0 Fσ 0 /4 h ω, (2) where I is the incident intensity, α is the linear absorption coefficient, σ is the FCA cross-section, ħω is the photon energy, N is the free carrier density, t is time, z is Figure 3 Optical limiting at 532 nm for the glass samples ZHAO ZhenYu et al. Chinese Science Bulletin April 2009 vol. 54 no

4 ARTICLES Figure 4 Open-aperture Z-scan for the glass samples excited with the 532 nm picosecond laser pulses. (a) The undoped glass; (b) the 1% AgCl doped glass; (c) the sample heat-treated for 30 min; (d) the sample heat-treated for 60 min; (e) the sample heat-treated for 90 min; (f) the sample heat-treated for 120 min. much larger FCA cross-section than the untreated samples. The FCA at 532 nm increases much more than TPA for the heat treated glass samples, so it can be inferred that their self-trapped exciton absorption will dominate the nonlinear optical absorption and result in their optical limiting. Figure 5 Schematic diagram for the exciton absorption and two-photon absorption. the distance, T 0 is the initial transmittance of samples and F 0 is the peak incident intensity. In order to explore the two photon absorption coefficients, their nonlinear absorption was measured with open-aperture Z-scan experiment at 800 nm (as shown in Figure 6). The linear absorption coefficient and TPA coefficients were calculated and shown in Table 1 by the following equations: I z = α + βi I (3) ( ), ( β ) ( β ) T = ln 1 + I L I L, (4) 0 0 where β is TPA coefficient. As shown in Figures 4, 6 and Table 1, their TPA coefficients were calculated to be about ( ) MW/cm 1. The FCA cross-section of the untreated glass samples was calculated to be cm 2, which is more than 200 times larger than that of the samples thermally treated for 120 min. It is obvious that the heat treated samples for 120 min have Figure 6 Open-aperture Z-scan for the glass samples excited with the 800 nm laser pulses. 3 Conclusions OPTICS In conclusion, the AgCl nanocrystal doped 80TeO 2-20Nb 2 O 5 glass samples exhibit self-trapped exciton absorption band in the visible region, and it can be attributed to the lattice defects of the AgCl nanocrystal in the ZHAO ZhenYu et al. Chinese Science Bulletin April 2009 vol. 54 no

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