Theoretical study on the optical properties of polyvinylidene fluoride crystal

We report our first-principles studies on the linear and non-linear optical properties of ferroelectric polyvinylidene fluoride crystal. Calculated values of the refractive indices agree well with experiments. As regards the second-harmonic-generation coefficients, theoretical values are larger than experimental ones. Possible reasons for the discrepancy are discussed. Polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (TrFE) have been studied extensively due to their unique piezoelectric, pyroelectric, and ferroelectric properties [1–4]. They are widely used in industry as low-cost electromechanical devices such as transducers and detectors. Furthermore, the ferroelectric phases of PVDF copolymer crystals also have good non-linear optical (NLO) properties [2, 3]. They have quite large optical windows (1200–200 nm),which makes them good candidates for use as frequency conversion materials working in the ultraviolet region. They can also be used in optical memory devices [5]. In addition, the NLO quality of this material can be significantly improved by hosting molecules of high NLO susceptibility in the amorphous region, which is a great advantage of polymers [6]. Experimentally, there are many reports on the determination of the optical constants of PVDF [7–10]. However, the crystalline polymers used in experiments often consist of crystalline regions comprising microcrystals and non-crystalline areas between the microcrystals [11], and the orientations of microcrystals in the samples used in experiments generally have a broad distribution. Thus those experimental values differ substantially from each other. As regards the theoretical side, to our knowledge, there has been only very limited investigation of the optical properties of thismaterial. Cakmak and Wang [12] and Spector and Stein [13] have given reports on the refractive indices of PVDF. However, their quantum chemistry calculations strongly depend on the choices of bond parameters, which results in a large uncer-