Nonlinear optical absorption in Bi 3 TiNbO 9 thin films using Z-scan technique

Transcription

1 Appl Phys A (2009) 96: DOI /s z Nonlinear optical absorption in Bi 3 TiNbO 9 thin films using Z-scan technique Bin Yang Hengzhi Chen Mingfu Zhang Feiyan Wang Kokwai Cheah Wenwu Cao Received: 26 January 2009 / Accepted: 30 January 2009 / Published online: 19 February 2009 Springer-Verlag 2009 Abstract Bi 3 TiNbO 9 (BTN) thin films with layered perovskite structure were fabricated on fused silica by pulsed laser deposition. The XRD pattern revealed that the films are single-phase perovskite and highly (00l) textured. Their fundamental optical constants, such as band gap, linear refractive index, and linear absorption coefficient, were obtained by optical transmittance measurements. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The nonlinear optical absorption of the films was investigated by single beam Z- scan method at a wavelength of 800 nm with laser duration of 80 fs. We obtained the nonlinear absorption coefficient β = m/w. The results show that the BTN thin films are promising for applications in absorbing-type optical devices. PACS k Dy Fg Ci B. Yang ( ) H. Chen W. Cao Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin , People s Republic of China binyang@hit.edu.cn Fax: M. Zhang Center for Composite Materials, Harbin Institute of Technology, Harbin , People s Republic of China F. Wang K. Cheah Department of Physics, Hong Kong Baptist University, Hong Kong SAR, China W. Cao Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, USA 1 Introduction Nonlinear optical thin films are being developed and studied for future photonic applications because of their high optical nonlinearity, fast response time, and good compatibility with fabrication of waveguide and integrated optic devices. Some ferroelectric thin films with perovskite structure are regarded to be among the most promising materials due to their unique physical characteristics, such as large spontaneous polarization, high dielectric constant, high optical transparency, electro-optic effect, and remarkable optical nonlinearity [1 3]. In addition, recent research exhibited that most of bismuth-base layered ferroelectric thin films have excellent nonlinear optical properties. This kind of materials with Aurivillius phase can be described as (Bi 2 O 2 ) 2+ (A m 1 B m O 3m+1 ) 2, where A represents Bi, Ba, Pb, Sr, Ca, K, Na, and rare-earth elements, B represents Ti, Ta, Nb, W, Mo, Fe, etc., and m = 2, 3, 4,... represents the number of BO 6 octahedra between two neighboring Bi 2 O 2 layers [1]. Thin films of ferroelectric oxides, such as Bi 3.75 Nd 0.25 Ti 3 O 12,SrBi 2 Nb 2 O 9,Bi 3.25 La 0.75 Ti 3 O 12, and Bi 2 Nd 2 Ti 3 O 12, shown a large nonlinear refractive index n 2 and a nonlinear absorption coefficient β [4 7]. As one of bismuth-base layered ferroelectric, Bi 3 TiNbO 9 (BTN) has good ferroelectric properties and excellent fatigue free properties [8], and its optical waveguide property has been studied by Yang et al. [9]. Understanding optical properties of BTN films is important for evaluating its potential as active electro-optical material. So, BTN thin film may be a new multifunctional material. And this thin film could be potentially used in spaceflight because of its relatively high Curie point (T c = 940 C). Using the transmittance measurement and the Z-scan technique with femtosecond laser pulses, we have investigated the optical transmittance and third-order optical non-

2 1018 B. Yang et al. linearities of BTN thin films grown on silica substrates by the pulse laser deposition (PLD) technique. 2 Experiment BTN targets used in PLD were prepared by a conventional solid-state reaction technique with starting materials Bi 2 O 3, TiO 2, and Nb 2 O 5. Excess 20 mol% Bi 2 O 3 was added to compensate for the Bi evaporation. The powder was mixed by ball milling for 12 h and then preheated at 700 C for 3 h. In order to obtain dense BTN ceramics, 5% polyvinyl alcohol solution prepared by immersion in a water bath at 90 C was added to the mixed powders at the ratio of 1:95 (polyvinyl alcohol solution to the mixed powders). The screened uniform mixture of the powder was pressed under a pressure of 10 MPa to form pellets of 2 cm diameter. Finally, the pellets were sintered at 1000 C for 2 h in a conventional box furnace. Dense yellowish pellets we acquired through this procedure. The films were fabricated on double-sided polished fused silica substrates by pulsed laser ablation in an oxygen chamber with a pressure of 200 mtorr. A KrF excimer laser (LPX205i, Lambda Physik, 248 nm wavelength, 30 ns pulse width and 5 Hz repetition rate) was focused on the surface of a rotating target. The deposition temperature and laser fluence were optimized at 600 C and 2.0 J/cm 2, respectively. After deposition, all the films were annealed in situ for 30 min with 0.5 atm O 2 pressure. The microstructure and the morphology of the as grown films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The optical properties of the thin films were measured by a Lambada2S UV/VIS spectrophotometer. Measurements for nonlinear absorption coefficient were conducted by the single-beam Z-scan technique using an 80 fs pulse of the Ti:sapphire laser operating at a wavelength of 800 nm with a repetition rate of 1 khz. The focal length of the lens was 200 mm. A typical peak power density was 1 GW/cm 2. The sample transmission was monitored by an energy ratiometer. 3 Results and discussion The BTN thin films fabricated by PLD were studied by the X-ray diffraction. Figure 1 shows the XRD patterns of the BTN thin films deposited on fused silica substrates under oxygen pressure. Even though fused silica has no lattice parameter relation with the Aurivillius BTN unit cell, a highly (00l) oriented texture has been obtained. This means that the intrinsic properties of BTN films, such as differences in surface energies for different planes of the unit cell and strong Fig. 1 X-ray diffraction pattern of BTN thin film deposited on fused silica substrate Fig. 2 Optical transmittance of BTN thin film on fused silica substrate. The inset is a plot of (hνα) 2 vs. hν for the BTN thin film interaction between the octahedrons, could lead to preferred orientation. All X-ray peaks can be indexed by the BTN tetragonal phase. Although there is excess Bi 2 O 3 in the target, no Bi 2 O 3 peaks were observed. It means that the excess 20% mol of bismuth ions in starting materials compensate for the Bi evaporation during the preparation of BTN targets and the thin films. The degree of (hkl) plane orientation was roughly estimated by measuring the ratio of the (hkl) planes to other planes [10]. The relative intensity of BTN (008) is 0.95, indicating that the BTN thin films fabricated on fused silica substrates have a highly c-axis oriented texture. The optical properties of the as-prepared BTN thin films on the silica substrate were investigated by the optical transmittance spectra. Shown in Fig. 2 is the optical transmittance of the films. The films are colorless and highly transparent with a transmittance between 60% and 96% in the vis NIR

3 Nonlinear optical absorption in Bi 3 TiNbO 9 thin films using Z-scan technique 1019 wavelength region. It is clear that the transmittance drop is due to interband absorption in the thin films. The oscillations in transmittance come from the interference due to reflection from the top surface of the film and the interface between the film and substrate. The well oscillating optical transmittance indicates that the film has a flat surface and a uniform thickness. The transparency of the films drops rapidly at nm and decreases to zero at approximately nm. The optical bandgap energy E g of the films is estimated to be ev from the graph of (hνα) 2 vs. hν base on the linear absorption coefficient α and the band gap E g : (hνα) 2 = const(hνα E g ), where hν is the incident light energy. For the single-layer weakly absorbing films on a transparent substrate, the linear refractive index n 0, absorption coefficient α, and film thickness can be obtained from the transmittance curve using the envelope method [11, 12]. The linear refractive index n 0 of the thin films at 800 nm was determined to be from Fig. 2. We also obtained the linear absorption coefficient α of the thin films at 800 nm to be cm 1. The thickness of the films calculated in this way is about nm. Figure 3 shows a dispersion curve of the BTN thin films. Open circles represent data points obtained by transmittance measurements. The refractive index of the BTN thin film decreases abruptly as the wavelength increased and approaches unit, showing a typical shape of a dispersion curve near an electronic interband transition. The dispersion data in the interband-transition region are modeled base on a single electronic oscillator. This theory assumes that the material is composed of a series of independent oscillators which are set into forced vibrations by incident radiation. According to the single electronic oscillator model proposed by Didomenico and Wemple [13], the dispersion of the refraction index is given by the well-known Sellmeier relation: n 2 S 0 λ 2 0 = (λ 0 /λ) 2, (1) where λ 0 is the average oscillator position and S 0 is an average oscillator strength. By fitting the refractive-index data to (1), the values of λ 0 and S 0 were found to be 269 nm and m 2, respectively. The energy of the oscillator given by ε 0 = hc/eλ 0 (c is the speed of light, h is Plank s constant, and e is the electronic charge) is calculated to be 4.61 ev. As shown in Fig. 3, the dispersion curve fits very well to the experimental data. Figure 4 shows the cross-section SEM image of the BTN films. The film and the substrate are easily distinguished in the cross-section SEM image, indicating that there is no obvious interdiffusion between the fused silica and the BTN Fig. 3 (Color online) Refractive index as a function of wavelength and the dispersion curve of BTN thin film on fused silica substrate Fig. 4 SEM cross-section micrograph of BTN thin film on fused silica substrate

4 1020 B. Yang et al. large nonlinear absorption observed here results from the BTN film. The nonlinear absorption coefficients of Bi 3.75 Nd 0.25 Ti 3 O 12,SrBi 2 Nb 2 O 9,Bi 3.25 La 0.75 Ti 3 O 12, and Bi 2 Nd 2 Ti 3 O 12 thin films are , , , and , respectively [4 7]. The high nonlinear absorption ( m/w) of BTN film compares favorably with the nonlinearities of these materials. This indicates that the new BTN film is promising for applications in nonlinear optical devices. We have reported that BTN thin film has good ferroelectric properties, excellent fatigue free properties, and optical waveguide property [8, 9]. So, BTN thin film is a new multifunctional material. Fig. 5 (Color online) Open-aperture Z-scan data of BTN thin film using 80 fs pulses at 800 nm. The symbols are the measured data, and the solid line is the theoretical fitting film. The thickness of this film is about 700 nm, which is in agreement with the calculated from optical measurements. A Z-scan result on BTN film with open-aperture (S = 1) is shown in Fig. 5, where the curve is symmetric with respect to the focus point (z = 0). The open-aperture Z-scan plotted in Fig. 5 comprises a normalized transmittance valley, indicating the presence of nonlinear absorption in the films. There are two kinds of nonlinear absorption: multiphoton absorption and free carrier absorption. Because the width of incident laser pulse (80 fs) is much smaller than the recombination time of free carriers in the films (nanosecond or longer), the free carrier absorption effect is negligible. In a semiconductor, the contribution to nonlinear absorption from the N-photon (N 3) absorption is much smaller than the two-photon absorption (TPA) [14]. Thus, the nonlinear optical absorption in BTN thin films is attributed to TPA. Since the bandgap in BTN (3.395 ev) is larger than the excitation energy of the laser (2hν = 3.1 ev), TPA is not attributed to a direct transition process. However, the TPA can occur at 800 nm in BTN thin films from the strong laser pulse with intermediate levels in the forbidden gap induced by impurities [15]. The TPA coefficient β can be deduced from the normalized transmittance T for open aperture using (2)[16] T = ( βi 0 L eff ) m (1 + z 2 /z0 2, for βi 0 L eff )m (m + 1) 3/2 1 + z 2 /z0 2 < 1, (2) m=0 where L eff =[1 exp( αl)]/α is the effective film thickness, L is the film thickness, α is the linear absorption coefficient, I 0 is the laser intensity at the focal point, and z 0 = 2πω0 2 /λ is the Rayleigh range of the beam. The TPA coefficient β is equal to m/w. Because the silica substrate has a very small nonlinear optical property, the 4 Conclusions BTN thin films with layered perovskite structure were prepared on fused silica substrate by the pulsed laser deposition method. The optical constants were determined from the transmittance spectra using the envelope method. The optical bandgap energy was found to be ev. The dispersion in the refractive index was fitted by the Sellmeier dispersion relation and described by an electronic oscillator model. The average position λ 0, average strength S 0, and energy ε 0 of the oscillator are 269 nm, m 2, and 4.61 ev, respectively. Thickness of the thin films was measured by SEM to be about 700 nm. Its nonlinear optical properties were obtained using the Z-scan technique at a wavelength of 800 nm with a laser duration of 80 fs. The TPA coefficient of BTN thin films was determined to be m/w. These good optical properties show that BTN thin film is a promising material in absorbing-type optical applications. Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No ). References 1. C.A. Paz de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott, J.F. Scott, Nature 374, 627 (1995) 2. W.F. Zhang, Y.B. Huang, M.S. Zhang, Z.G. Liu, Appl. Phys. Lett. 76, 1003 (2000) 3. Y. Noguchi, M. Miyayama, T. Kudo, Phys. Rev. B 63, (2001) 4. Y.H. Wang, B. Gu, G.D. Xu, Y.Y. Zhu, Appl. Phys. Lett. 84, 1686 (2004) 5. K. Chen, H. Gu, J. Zou, W. Li, H. Yi, Mater. Lett. 61, 3701 (2007) 6. F.W. Shi, X.J. Meng, G.S. Wang, J.L. Sun, T. Lin, J.H. Ma, Y.W. Li, J.H. Chu, Thin Solid Films 496, 333 (2006) 7. B. Gu, Y.H. Wang, X.C. Peng, J.P. Ding, J.L. He, H.T. Wang, Appl. Phys. Lett. 85, 3687 (2004) 8. B. Yang, X.J. Zhang, S.T. Zhang, X.Y. Chen, Z.C. Wu, Y.F. Chen, Y.Y. Zhu, Z.G. Liu, N.B. Ming, Appl. Phys. Lett. 79, 4559 (2001)

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