Lect.9 _激光.pptVIP

* Laser (激光) : Light Amplification by Stimulated Emission of Radiation 光， 放大， 激发， 发射， 辐射 E 1 E 2 h u Absorption （吸收） h u Spontaneous emission （自发发射） E 2 E 1 h u Stimulated emission （受激发射） In h u Out h u E 2 E 1 Three possible processes in photon-electron interactions ( hv = E2 - E1 ) 在受激发射过程中，光子的数目得到了放大。 受激发射 受激发射光子 v.s. 入射光子： 同能量: hv = E2 - E1 同相位 同偏振方向 受激发射 吸收 — Population inversion （粒子数翻转） Absorption（吸收） E 1 E 2 h u h u Stimulated emission （受激发射） In h u Out h u E 2 E 1 E 1 E 3 E 2 h u 32 (b) E 1 h u 1 3 E 2 Metastable state (亚稳态) (a) E 3 hu13=E3-E1 hu32=E3-E2 (a) Atoms in the ground state (基态) are pumped up to the energy level E3 by incoming photons of energy hu13=E3-E1. (b) Atoms at E3 rapidly decay to the metastable state (亚稳态) at energy level E2 by emitting photons or lattice vibrations: hu32=E3-E2. 气体激光器——原理 泵浦 E 1 E 3 E 2 (c) E 1 E 3 E 2 h u 21 h u 21 Coherent photons (相干光子) OUT IN (d) hu21=E2-E1 (c) As the states at E2 are long-lived, they quickly become populated and there is a population inversion between E2 and E1. (d) A random photon (from a spontaneous decay) of energy hu21=E2-E1 can initiate stimulated emission. Photons from this stimulated emission can themselves further stimulate emissions leading to an avalanche of stimulated emissions and coherent photons being emitted. E 1 E 3 E 2 h u 32 (b) E 1 h u 1 3 E 2 Metastable state (亚稳态) (a) E 3 hu13=E3-E1 hu32=E3-E2 E 1 E 3 E 2 (c) E 1 E 3 E 2 h u 21 h u 21 Coherent photons (相干光子) OUT IN (d) hu21=E2-E1 Current regulated HV power supply （高压电源） Flat mirror (Reflectivity = 0.999) (平面镜) Concave mirror (Reflectivity = 0.985) (凹面镜) gas Laser beam Very thin tube 气体激光器 示意图 Ar atom ground state Ar + ion ground state 4 p levels 4 s 0 15.75 eV 488 nm 514.5 nm 72 nm Energy Pumping 氩离子（Ar-ion）激光器 (488 nm emission). 氦氖（He-Ne）激光器 (632.8 nm emission). (1 s 2 ) (1 s 1 2 s 1 ) 0 20.61 eV He (2 p 6 ) G r o u n d s t a t e s (2 p 5 5 s 1 ) Ne (2 p 5