Dielectric behavior of CaCu3Ti4O12 ceramics in the terahertz range Liang Wu,1 Furi Ling,1,* Ting Liu,1 Jinsong Liu,1 Yebin Xu,1 and Jianquan Yao1,2 1
2
Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China *
[email protected]
Abstract: The dielectric properties of 1050 °C/12h sintered CaCu3Ti4O12 (CCTO) ceramics have been investigated by using terahertz time domain spectroscopy in the frequency range of 0.2-1.6 THz at room temperature. When applying an external optical field, an obvious variation of dielectric constant was observed and reached up to 7%. However, the dielectric loss does not change appreciably. From the results, we found the change of refractive index has a linear relationship on scale with the applied light intensity. These findings were attributed to the change of spontaneous polarization in the ceramic caused by the excited free carriers. ©2011 Optical Society of America OCIS codes: (130.0250) Optoelectrics; (130.0260) Ferroelectrics; (300.6495) Spectroscopy, terahertz.
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(C) 2011 OSA
Received 19 Jan 2011; revised 17 Feb 2011; accepted 17 Feb 2011; published 2 Mar 2011
14 March 2011 / Vol. 19, No. 6 / OPTICS EXPRESS 5118
1. Introduction Recently, CaCu3Ti4O12 (CCTO), a complex cubic perovskite material (space group Im3), has drawn the attention of many researchers because of its colossal permittivity and remarkable dielectric properties [1]. The compound shows giant dielectric constant in the order of 105 at room temperature and is weakly temperature dependent between dc and MHz. With decreasing the temperature, a Debye-like relaxation exists in the dielectric constant accompanied with a steplike decrease. The temperature at which the steplike decrease in dielectric constant takes place is found to be strongly depends on the measuring frequency and follows an Arrhenius behavior. High dielectric constant is generally associated with intrinsic ferroelectric or relaxor ferroelectric properties [2]. From the measurements of high-resolution x-ray, neutron powder diffraction, and Raman phonon, however, no evidence of any structural phase transition has been found in CCTO. A huge amount of work has been carried out in an attempt to understand the origin of the dielectric properties of CCTO [2–10]. Both intrinsic [2–5] as well as extrinsic mechanisms have been proposed. With the help of first principles calculation [6], it is gradually realized that the giant dielectric constant should originate from extrinsic effects. Among the extrinsic effects, an internal barrier layer capacitance (IBLC) mechanism is widely accepted, which is strongly supported by impedance spectroscopic (IS) results [7,8], as well as by the dependence of measured dielectric properties upon processing conditions and grain size [9]. The measurement of impedance spectroscopy also shows that the compound when prepared in air at temperature greater than 1000°C would consist of semiconducting grains and insulating grain boundaries [1,2]. Fang et al. reported a dielectric relaxation of CCTO at room temperature with an activation energy of 390.3meV, which was attributed to the MW-type relaxation associated with grain boundaries [10]. In view of the above works, most observations have been carried out at low frequency range (
