Linear and Nonlinear Optical Properties of Ferroelectric Thin Films

Abstract

Nowadays, nonlinear optical materials play a crucial role in the technology of photonics, nano-photonics, and bio-photonics. Among a wide variety of materials, thin films have the additional design advantage of small volume and good compatibility with fabrication of waveguide and integrated nonlinear photonics devices over solutions and single crystals. Interestingly, most ferroelectric thin films exhibit notable physical characteristics of large spontaneous polarization, high dielectric constant, high optical transparency, and large nonlinear response with respect to the electromagnetic radiation in the optical range. During the past decade, ferroelectric thin films such as Bi2Nd2Ti3O12, Ba0.6Sr0.4TiO3, and Bi3.25La0.75Ti3O12 have been received intense interest due to their high optical transparency and remarkable optical nonlinearity for potential applications on nonlinear photonic devices (Gu et al., 2004, Liu et al., 2006, Shi et al., 2006, Shin et al., 2007). Moreover, most of these investigations have been performed under the excitation of nanosecond and picosecond laser pulses. Owing to today’s fast advance of laser sources with ultrashort pulse duration, the femtosecond nonlinear optical response has been detected in several ferroelectric thin films. Enlightened by a recent report on the third-order nonlinear optical response presented in CaCu3Ti4O12 at a wavelength of 532 nm (Ning et al., 2009), observations indicate that the optical nonlinearities depend strongly on the pulse duration of the excitation laser, although many reports declared that the observed nonlinear effect is an instantaneous optical nonlinearity in the nanosecond and picosecond regimes. Apparently, it is imperative to gain an insight on the underlying physical mechanisms for the observed optical nonlinearities at different time scales. In this chapter, the linear transmittance spectrum and Z-scan technique are used to characterize the linear and nonlinear optical properties of ferroelectric thin films, respectively. Two methodologies are based on measurements of the sample’s transmittance under weak or intense light excitation. The nonlinear optical properties of representative ferroelectric films in nanosecond, picosecond, and femtosecond regimes are presented. In particular, the underlying mechanisms for the observed optical nonlinearities are also discussed in details.