tailieunhanh - Nonlinear Optics - Chapter 3

Quantum-Mechanical Theory of the Nonlinear Optical Susceptibility Trong chương này, chúng tôi sử dụng pháp luật của cơ học lượng tử để lấy được biểu hiện rõ ràng cho tính nhạy cảm quang học phi tuyến. Động lực cho việc thu thập các biểu thức ít nhất là gấp ba lần: (1) các biểu thức hiển thị các hình thức chức năng của tính nhạy cảm quang học phi tuyến và do đó hiển thị như thế nào nhạy cảm phụ thuộc vào thông số vật liệu chẳng hạn như những khoảnh khắc chuyển tiếp lưỡng cực và các. | Chapter 3 Quantum-Mechanical Theory of the Nonlinear Optical Susceptibility . Introduction In this chapter we use the laws of quantum mechanics to derive explicit expressions for the nonlinear optical susceptibility. The motivation for obtaining these expressions is at least threefold 1 these expressions display the functional form of the nonlinear optical susceptibility and hence show how the susceptibility depends on material parameters such as dipole transition moments and atomic energy levels 2 these expressions display the internal symmetries of the susceptibility and 3 these expressions can be used to make predictions of the numerical values of the nonlinear susceptibilities. These numerical predictions are particularly reliable for the case of atomic vapors because the atomic parameters such as atomic energy levels and dipole transition moments that appear in the quantum-mechanical expressions are often known with high accuracy. In addition since the energy levels of free atoms are very sharp as opposed to the case of most solids where allowed energies have the form of broad bands it is possible to obtain very large values of the nonlinear susceptibility through the technique of resonance enhancement. The idea behind resonance enhancement of the nonlinear optical susceptibility is shown schematically in Fig. for the case of third-harmonic generation. In part a of this figure we show the process of third-harmonic generation in terms of the virtual levels introduced in Chapter 1. In part b we also show real atomic levels indicated by solid horizontal lines. If one of the real atomic levels is nearly coincident with one of the virtual levels of the indicated process the coupling between the radiation and the atom is particularly strong and the nonlinear optical susceptibility becomes large. 135 136 3 Quantum-Mechanical Theory of the Nonlinear Optical Susceptibility a b ------------- Figure Third-harmonic generation described in terms of virtual .