tailieunhanh - Nonlinear Optics - Chapter 10

Stimulated Raman Scattering and Stimulated Rayleigh-Wing Scattering Hiệu ứng tự phát Raman đã được phát hiện bởi CV Raman vào năm 1928. Để quan sát hiệu ứng này, một chùm ánh sáng chiếu sáng một mẫu nguyên liệu (có thể là rắn, lỏng, hoặc khí), và ánh sáng tán xạ được quan sát thấy spectroscopically, như minh họa trong hình. . Nói chung, ánh sáng tán xạ có tần số khác nhau từ những nguồn kích thích. Những thành phần mới chuyển sang tần số thấp hơn được gọi là thành phần Stokes, và chuyển sang tần số cao hơn. | Chapter 10 Stimulated Raman Scattering and Stimulated Rayleigh-Wing Scattering . The Spontaneous Raman Effect The spontaneous Raman effect was discovered by . Raman in 1928. To observe this effect a beam of light illuminates a material sample which can be a solid liquid or gas and the scattered light is observed spectroscopically as illustrated in Fig. . In general the scattered light contains frequencies different from those of the excitation source. Those new components shifted to lower frequencies are called Stokes components and those shifted to higher frequencies are called anti-Stokes components. The Stokes components are typically orders of magnitude more intense than the anti-Stokes components. These properties of Raman scattering can be understood through use of the energy level diagrams shown in Fig. . Raman Stokes scattering consists incident light frequency co Raman scattering medium scattered light frequencies co and co s 3. Figure Spontaneous Raman scattering. 473 474 10 Stimulated Raman Scattering and Rayleigh-Wing Scattering Figure Energy level diagrams describing a Raman Stokes scattering and b Raman anti-Stokes scattering. of a transition from the ground state g to the final state n by means of a virtual intermediate level associated with excited state n . Raman anti-Stokes scattering entails a transition from level n to level g with n serving as the intermediate level. The anti-Stokes lines are typically much weaker than the Stokes lines because in thermal equilibrium the population of level n is smaller than the population in level g by the Boltzmann factor exp -htóng kT . The Raman effect has important spectroscopic applications because transitions that are one-photon forbidden can often be studied using Raman scattering. For example the Raman transitions illustrated in Fig. can occur only if the matrix elements g r nz and nz r n are both nonzero and this fact implies for a material system that possesses