laser2-研深圳大学-高等激光物理学期末复习课件

1、Unit 2 Interaction between Light and Matter,Interaction between Light and Matter Einstein coefficients Lifetime of excited states Lineshapes and linewidths of optical transitions Linewidth broadening Natural linewidth Homogeneous linewidths Inhomogeneous linewidths Doppler broadening #,Based on Plan

2、cks theory of the blackbody radiation 1905, 1921; 1916, Einstein developed the quantum description of absorption and emission of radiation. introduced the notion of spontaneous and stimulated emission opened the way for the concept of light amplification. #,Interaction between Light and Matter,Inter

3、action between Light and Matter,Spontaneous emission Stimulated emission Absorption #,Einstein coefficients,Spontaneous emission : A21 Stimulated emission : W21=B21 Absorption: W12=B12 n2 /n1 : Boltzmann statistics rate equation #,Einstein coefficients,The ratios of the coefficients The A coefficien

4、t for a given system - measure or calculate the B coefficients -Use the formulas to calculate the coefficients for stimulated emission and absorption are equal. -Stimulated emission and absorption are the same processes going in the opposite direction. #,Einstein coefficients,Spontaneous emission /

5、Stimulated emission (1) A21/W21 - T (2) A21/W21 - hv R, hv (1) R1 (2) R 1 , hv kT Spontaneous emission is far more probable than stimulated emission e.g - a light bulb (blackbody) (a) T=2000K ; visible wavelengths- ( 5x1014Hz). R 1 (R 105) (b) visible wavelengths, R 1; then T=30,000K (3) R 1 #,Einst

6、ein coefficients,Spontaneous emission / Stimulated emission (3) R 1 hv kT W21 Stimulated emission can become significant if the frequency is low enough. e.g. - Microwave amplifiers Frequency - 1010Hz Room temperature, T=300K. R - 0.015 #,Lifetime of excited states,spontaneous lifetime. metastable st

7、ates. non-radiative transitions 1/teff = 1/trad + 1/tnonrad #,Lineshapes and linewidths ofoptical transitions,linewidth - the range of frequencies-FWHM - the full width at half maximum of the spectrum. Lineshape- the profile of the frequency spread lineshape function g(). The linewidth and lineshape

8、 are two concepts that play an important role in the physics of lasers.,lineshape function g(),. Linewidth ,Linewidth broadening,Natural linewidth Other processes can increase the linewidth, but the linewidth cannot be smaller than the natural linewidth homogeneous broadening all the atoms emit or a

9、bsorb radiation in the same way. inhomogeneous broadening Inhomogeneous broadening occurs when the atoms in a system interact with the radiation differently. ,Linewidth broadening,Natural linewidth the linewidth cannot be smaller than the natural linewidth the uncertainty principle The shorter the u

10、ncertainty of this time, the larger the uncertainty of the energy E : the uncertainty in the energy ,Homogeneous broadening,Collisions broadening. homogeneous broadening all the atoms of the given system interact in an identical way with the EM radiation. homogeneous broadening all the atoms emit or

11、 absorb radiation in the same way. homogeneous broadening no distinction between the atoms. e.g. the finite lifetime of the transitions homogeneous broadening Lorentzian lineshape #,Homogeneous broadening,homogeneous broadening Lorentzian lineshape Lorentzian lineshape function of a homogeneously br

12、oadened transition #,Inhomogeneous broadening,Doppler broadening - Inhomogeneous broadening Inhomogeneous broadening occurs when the atoms in a system (gas, liquid, solid) interact with the radiation differently. Inhomogeneous broadening is the envelope of the many-many homogeneous broadenings comin

13、g from each individual atom. Theoretically, all transitions are homogeneously broadened and also inhomogeneously broadened. #,Inhomogeneous broadening,Doppler broadening -The Gaussian lineshape #,Transition probability,1. = 0 Spontaneous emission : A21 (1-1) Stimulated emission : W21=B21 (1-2) Stimulated absorption : W12=B12 2. 0 A21() =A21g() ； A21 ； Laser field Continuum light field #,Unit 2 Interaction between Light and Matter,Learning outcomes discuss spontaneous and stimulated emission, and absorption of radiation by atomic systems calculate the ratio of spontaneou