tailieunhanh - Ferroelectrics Physical Effects Part 14

Tham khảo tài liệu 'ferroelectrics physical effects part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 510 Ferroelectrics - Physical Effects electrostriction is a phenomenon that the inhomogeneous optical field produces a force on the molecules or atoms comprising a system resulting in an increase of the refractive index locally. This effect has the characteristic response time of nanosecond order. When the electron occurs the real transition from the ground state of a system to a excited state by absorbing the single photon or two indential photons electrons will occupy real excited states for a finite period of time. This process is called a population redistribution and mainly contributes the whole refractive nonlinearity of ferroelectric films in the picosecond regime. The electronic Kerr effect arises from a distortion of the electron cloud about atom or molecule by the optical field. This process is very fast with typical response time of tens of femtoseconds. The electronic Kerr effect is the main mechanism of the refractive nonlinearity in the femtosecond time scale. Accumulative nonlinearity caused by the defect For many cases the observed absorptive nonlinearity of ferroelectric thin films is the two-photon absorption type process. Moreover the measured two-photon absorption coefficient strongly depends on the laser pulse duration see Table 1 . This two-photon type nonlinearity originates from two-photon as well as two-step absorptions. The two-step absorption is attributed to the introduction of electronic levels within the energy bandgap due to the defects Liu et al. 2006 Ambika et al. 2009 Yang et al. 2009 . The photodynamic process in ferroelectrics with impurities is illustrated in Fig. 1. Electrons in the ground state could be promoted to the excited state and impurity states based on two-and one-photon absorption respectively. The electrons in impurity states may be promoted to the excited state by absorbing another identical photon resulting in two-step two-photon absorption. At the same time one-photon absorption by impurity levels populates .