Intensity-dependent optical absorption property of composite films of nanoparticulate transition metal oxides and transparent oxides

Transcription

1 ISBN Intensity-dependent optical absorption property of coposite fils of nanoparticulate transition etal oxides and transparent oxides M. Ando', K. Kaadk, K. Ohtk, P. Audebert', J.-F. Deloui:, K. Nakatani2C% J. Delaire2 lphotonics Research Institute, NationalInstitute of Advanced Science and Technology, Osaka, Japan. 2Laboratoire de Photophysique et de Photochiie Supra- et Macroolkdaires Ecole Norale Supe'rieure de Cachan, Cachan Cedex, France. Abstract Two kinds of nanoparticulate transition etal oxide fils with large nonlinear absorption, C34 fil without dispersed in glass, and coposite fil of C34 nanoparticles and Si2-Zr-2 glass, were prepared by RF sputtering technique and sol-gelletal salt pyrolysis technique, respectively. Both C34 fil and Co34/[Si2-Zr2] coposite fil showed large nonlinear (intensity-dependent) optical absorption in the visible-near IR wavelength region. In both fils, strong laser irradiation at wavelengths of 355 and 532 n brought about large increase of absorption (induced absorption) at wavelengths around n and decrease of absorption (bleaching) at around Large induced absorption suggests the potentiality of those fils for optical power liiting the wide wavelength range. Daage threshold of the fils against strong laser irradiation was at least thee ties higher in the Co34/[Si2-Zr2] coposite fil copared with C34 fil not dispersed in glass. This iproved durability against strong laser irradiation observed in the coposite fil is considered to be due to the diinution of increase of teperature in the fils on laser irradiation because of the presence of the unheated transparent oxide atrix surrounding the nanoparticles. Those results show that the coposite structure of nanoparticulate transition etal oxide and transparent oxide is useful for iproving optical power liiting aterials.

2 254 ISBN High Pr/~fornnce Stctwes and Coposites 1 Introduction Over the past several decades nonlinear optical responses of aterials have been a topic of uch interest due to their potential applicability to photonic devices such as all-optical switching and optical power liiting. In particular, optical power liiting has attracted uch interest because of the growing needs of instantaneous sensing and liiting of unpreferably strong laser irradiation for protecting optical devices and huan eyes fro daage. Transition etal oxides are attracting growing interest as a new class of candidate as optical power liiting aterials based on their large optical nonlinearity for two reasons. One reason is that generally transition etal oxides have advantages fro the viewpoints of theral and cheical stability and echanical strength. Another reason is that soe transition etal oxides are theoretically predicted to show large optical nonlinearity reflecting strong correlation of electrons [l]. Recently we have found that the fils of nanoparticulate transition etal oxides such as Co34, V25 CuO and Fe23 show considerably large third-order optical nonlinearity [2,3]. Furtherore, we have found that V25 fils show considerably large intensity-dependent optical absorption possibly useful for optical liiting [4]. In this paper, we investigated the nonlinear optical absorption property of nanoparticulate transition etal oxide such as C34 with and without dispersion in transparent oxide such as silica-zirconia (Si2-Zr2) glass. Cobalt oxide (co34) was selected because it shows particularly large third- order nonlinear susceptibility (~(~ and 1) figure of erit (x(~)/ix,where a is linear absorption coefficient) aong transition etal oxides [2]. We report here that both c34 fil without dispersed in glass and Co3Oq/[Si2-Zr2 glass] coposite fil show strong nonlinear absorption, particularly large induced absorption in the wavelength range around The coposite fil showed higher daage threshold against strong laser irradiation copared with the c34 fil not dispersed in glass. 2 Experiental 2.1 Preparation of c34fil without dispersed in glass by RF sputtering technique Cobalt oxide (Co34) fil was prepared on glass plate substrate (18 x 18 x.1 ) by the deposition of sputtered species eitted fro a disc target (75 diaeter) of C34 in a agnetron equipent (Ueno Vacuu Co.; RF power=15 W at MHz; plasa gas: 5x1s3 Torr of Ar-2 ixture (go:2 in volue ratio)). During deposition, the glass substrate was kept heated at 5 C. 2.2 Preparation of Co34/[Si2-Zr2 glass] coposite fil by sol-geueta1salt pyrolysis cobined technique Coposite fil of Co3~/[!32-Zr2 glass] was prepared as by a new technique [5] which cobined sol-gel technique and etal salt pyrolysis technique as follows. Mixture of two kinds of alkoxydes (DEDMS: diethoxydiethylsilane and TEMS: triethoxyethylsilane) was hydrolyzed first

3 ISBN High Perfor,ance Stctures and C'oposites 255 in organo-aqueous ixed solution. Another alkoxyde (zirconiu propoxide) was added and hydrolyzed in the solution. Hoogeneous sol was obtained after hydrolysis. Mixture of the sol and toluene solution of cobalt salt (cobalt 2- ethyhexanoate) was spin-coated on glass plate substrate (18 x 18 x.1 ), and then pyrolyzed at 38 C. Coposite fil of Co34/[Si2-Zr2 glass] with 4 ass % of C34was obtained. 2.3 Structural analysis and easureent of linear absorption property of the fils Structure of the C34 fil and the Co34/[Si2-Zr2 glass] coposite fil was analyzed by X-ray diffraction (XRD) and by transission electron icroscopy (TEM). Linear optical absorption spectra of the fils were easured in the visible-near IR region with a Shiadzu UV-3lOOPC spectroeter. 2.4 Measureent of nonlinear absorption property of the fils Nonlinear absorption property of the C34 fil and the Co34/[Si2-Zr2 glass] coposite fil were easured by use of a 1 -pulsed laser for excitation. The light source for excitation was a frequency-tripled and doubled Q-switched, ode-locked Nd3+:YAG laser with two wavelengths (355 n and 532 n) and a pulse duration of 1 ns. The applied peak power density was norally.6 MW/c2. The light source for the analyzing light was a Xe lap. The intensity of the transitted analyzing light fro the fil saple was easured in the wavelength range of 4-85 n. Preliinary single bea intensity-dependent transission easureent was also perfored on the CO-34 fil not dispersed in glass using the 35 pspulsed laser of 532 n wavelength. The incident light intensity on the fil saples was varied, and the transitted light intensity was easured as a function of the incident light intensity. 3 Results and discussion 3.1 Structure and linear absorption property of the fils The obtained c34 fil when not dispersed in glass showed XRD patterns characteristic of spinel C34 structure, which was centrosyetric crystal structure. The ean crystal size of the c34 fil was estiated to be IO n fro the XRD line widths by using Scherrer's equation. The ean crystal size of C34 in the Co34/[Si2-Zr2 glass] coposite fil was siilar to that in the c34 fil not dispersed glass fro XRD line widths. TEM showed that the coposite fil was coposed of c34 nanoparticles with a diaeter range of 5-15 dispersed Si2-Zr2 glass. We consider that the foration of c34 nanoparticles in Si2-Zr2 glass atrix is due to the fact that the diffusion coefficient of gel is uch saller than that of cobalt 2-ethylhexanoate in the elting condition. Two broad absorption bands centered at 41 n and 75 n were observed in the optical absorption spectru of C34 fil not dispersed in glass. These absorption bands centered at 41 n and 75 n are assigned to

4 256 ISBN High Perfonnance 3ructwes dcoposites absorptions characteristic to the Co3+ and Co2+ centers in the octahedral and tetrahedral sublattices in the c34 spinel, respectively [6,7]. The absorption edge of the C34 fil was about 1 n. The Co3Oq/[Si2-ZrO2 glass] coposite fil showed absorption spectru, in which two absorption bands characteristic to the Co3+ center and the Co2+ center and the absorption edge shifted to the shorter wavelength range copared with the c34 fil by 5-1 n. This blue-shift suggests that electrons are confined in the dispersed c34 nanoparticles in the coposite fil, however, electrons in the c34 fil not dispersed in glass are ore delocalized in the polycrystalline c34 fil. 3.2 Nonlinear absorption property of the fils Preliinary single bea intensity-dependent transission easureent at a wavelength of 532 n using the 35 ps-pulsed laser showed that the optical absorbance of the C34 fil increases with increasing the incident light intensity, indicating that the C34 fil shows strong induced absorption at 532 n. This absorbance change occurred reversibly and reproducibly. Measureent of nonlinear absorption using the 1 ns-pulsed laser showed the wavelength dependence of induced absorption change. The induced absorption changes for the C34 fil excited at a wavelength of 355 and of 532 n are depicted in Figures 1 and 2, respectively, showing the relation between AA ([absorbance on laser irradiation]-[absorbance before laser irradiation]) and the analyzing light wavelength. It is seen that when excited at 355 n, the C34 fil shows decrease in absorbance (bleaching) at wavelengths of 4-54 and large increase in absorbance (induced absorption) at n (Figure 1). The wavelength range for decrease and increase in absorbance by the 532- excitation was 4-48 n and n, respectively (Figure 2). Figures 3 and 4 show the AA spectra of the Co34/[Si2-Zr2 glass] coposite fil excited at 355 n and 532 n, respectively. In the case of the 355-n excitation, the Co34/[Si2-Zr2 glass] coposite fil showed bleaching at 4-45 and large induced absorption at On the other hand, when excited at 532 n, the Co34/[Si glass] coposite fil showed bleaching at 4-46 n and large induced absorption at n, respectively. The AA values increased quickly within the excitation pulse width and reached the axia shown in Figures 1-4, and reversibly recovered to zero at 2-3 p after excitation. The AA spectra and their tie responses reflect the photophysical process after excitation. The fast rise of absorbance in the c34 fil and Co34/[Si2-Zr2] fil is advantageous for optical power liiting application and suggests that the induced absorption process would contribute to the third-order optical nonlinearity of c34 with fast response ties [2]. The coparison between Figures 1 and 2 and between Figures 3 and 4 shows that the excitation at 355 n and the excitation at 532 n give any

5 ISBN , X P) C -e D cd. c & cd c E. 2 Figure 1: Changes absorbance(m)of the c34 filwithout dispersed in glass on excitation with a 1ns-pulsed laser at a wavelength of X P) c cd -2 9 cc. C Figure 2: Changes absorbance(m)of the c34 fil without dispersed in glass on excitation with a 1-pulsed laser at a wavelength of 532 n.

6 258ISBN High Pet-foance Structwes and ('oposites 2 x. E M U S i Figure 3: Changes in absorbance (AA) of the Co34/[Si2-Zr2 glass] coposite fil on excitation with a 1ns-pulsed laser at a wavelength of 355. c e, D cd S L S: D cd. C Figure 4: Changes in absorbance (AA) of the Co3Oq/[Si2-Zr2 glass] coposite fil on excitation with a 1-pulsed laser at a wavelength of 532.

7 ISBN High Pe~oanceStr-tctures and Coposites 259 siilarities in the AA spectra. The wavelengths of absorption edge of the C34 fil and Co34/[Si2-Zr2] fil (9-1 ) are longer than the two excitation wavelengths, and the absorption of the Co3+ center (41 ) is between the two excitation wavelengths. This suggests that the excitation at 355 run and 532 would bring about the excitation of electrons in the valence band of seiconducting Co34, rather than the excitation of the Co3+ center. The fact that the AA spectra of the C34 fil (Figures 1 and 2) are siilar to the AA spectra of the Co34/[Si2-Zr2] coposite fil (Figures 3 and 4) shows that both the C34 nanoparticles in glass with confined electrons and the polycrystalline c34 fil with ore delocalized electrons have potential for optical power liiting. Concerning the daage threshold against strong laser irradiation, we found that the CO~O~/[S~O~-Z~O~] coposite fil was superior to the c34 fil. Daage of the C34 fil not dispersed in glass by1 -pulsed laser started to appear at intensity higher than.8 MW/c2. On the other hand, no daage was observed in the C341 Si2-Zr21 coposite fil against 1 ns- pulsed laser at least up to 2 MWk!i. This iproved durability against strong laser irradiation in the Co3Oq/[Si2-Zr2 glass] coposite fil is considered to be due to the diinution of the increase of the teperature in the fil on laser irradiation, because of the effective theral conduction fro the transition etal oxide heated by laser to the unheated transparent oxide atrix surrounding the nanoparticles. Those results show that the coposite structure of nanoparticulate Cog4 and transparent glass is again useful for increasing the daage threshold in optical power liiting aterials. References Hanaura, E., Tanabe, Y. & Fiebig, M. Nonlinear optical responses of strongly correlated electronic systes. Physics and Cheistryof Transition-MetalOxides, ed. Fukuyaa, H. 8~Nagaosa, N., Springer- Verlag: Berlin and Heidelberg, pp , Ando, M., Kadono, K., Haruta, M., Sakaguchi, T. & Miya, M. Large third-order optical nonlinearities in transition-etal oxides. Nature, 374, pp , Ando, M., Kadono, K., Kaada, K. & Ohta, K. Third-order nonlinear optical responses of nanoparticulate Fe23 and CuO fils. Nonlinear Optics, 24, pp ,2. Ando, M., Kadono, K., Kaada, K., Ohta, K., Delouis, J.-F., Nakatani, K. &L Delaire, J. Nonlinear optical responses of spin-coated vanadiu oxide fils. Materials Research Society Syposiu Proceedings, 637, pp. E E5.19.6,21. Audebert, P,,Ando, M., Kaada, K. & Ohta, K. to be published. Belova, I. D., Roginskaya, Yu. E., Shifrina, R. R., Gagarin, S. G., Plekhanov, Yu. V. & Venevtsev, Yu. N. Co(II1) ions high-spin configuration in nonstoichioetric c34 fils. Solid State Coun., 47, pp , Cook, J. G. & Van Der Meer, M. P. The optical properties of sputtered c34 fils. Thin Solid Fils, 144, pp , 1986.