Molecular Origin of the Nonlinear Optical Response Trong chương 3, chúng tôi trình bày một lý thuyết chung của cơ học lượng tử nhạy cảm quang học phi tuyến. Tính toán này dựa trên lý thuyết nhiễu loạn phụ thuộc thời gian và dẫn đến dự đoán rõ ràng cho sự phụ thuộc tần số hoàn toàn của các tuyến tính và phi tuyến tính nhạy cảm quang. Thật không may, tuy nhiên, những biểu hiện cơ học lượng tử thường quá phức tạp để được sử dụng cho các tính toán thực tế. Trong chương này chúng ta xem. | Chapter 5 Molecular Origin of the Nonlinear Optical Response In Chapter 3 we presented a general quantum-mechanical theory of the nonlinear optical susceptibility. This calculation was based on time-dependent perturbation theory and led to explicit predictions for the complete frequency dependence of the linear and nonlinear optical susceptibilities. Unfortunately however these quantum-mechanical expressions are typically far too complicated to be of use for practical calculations. In this chapter we review some of the simpler approaches that have been implemented to develop an understanding of the nonlinear optical characteristics of various materials. Many of these approaches are based on understanding the optical properties at the molecular level. In the present chapter we also present brief descriptions of the nonlinear optical characteristics of conjugated polymers chiral molecules and liquid crystals. . Nonlinear Susceptibilities Calculated Using Time-Independent Perturbation Theory One approach to the practical calculation of nonlinear optical susceptibilities is based on the use of time-independent perturbation theory see . Jha and Bloembergen 1968 or Ducuing 1977 . The motivation for using this approach is that time-independent perturbation theory is usually much easier to implement than time-dependent perturbation theory. The justification of the use of this approach is that one is often interested in the study of nonlinear optical interactions in the highly nonresonant limit 0 where is the optical frequency and 0 is the resonance frequency of the material system 253 254 5 Molecular Origin of the Nonlinear Optical Response in order to avoid absorption losses. For m m0 the optical field can to good approximation be taken to be a quasi-static quantity. To see how this method proceeds let us represent the polarization of a material system in the usual form P Ễ0X 1 Ẽ Ễ0X 2 Ẽ2 Ễ0X 3 Ẽ3 . We can then calculate the energy stored in polarizing the .
