laser2-研深圳大学-高等激光物理学期末复习课件.pptx

1、Unit 2 Interaction between Light and Matter,Interaction between Light and MatterEinstein coefficientsLifetime of excited statesLineshapes and linewidths of optical transitionsLinewidth broadeningNatural linewidthHomogeneous linewidths Inhomogeneous linewidthsDoppler broadening #,Based on Plancks the

2、ory of the blackbody radiation1905, 1921; 1916, Einstein developed the quantum description of absorption and emission of radiation. introduced the notion of spontaneous and stimulated emissionopened the way for the concept of light amplification.#,Interaction between Light and Matter,Interaction bet

3、ween Light and Matter,Spontaneous emission Stimulated emissionAbsorption#,Einstein coefficients,Spontaneous emission : A21 Stimulated emission : W21=B21 Absorption: W12=B12 n2 /n1 : Boltzmann statisticsrate equation #,Einstein coefficients,The ratios of the coefficientsThe A coefficient for a given

4、system - measure or calculate the B coefficients -Use the formulas to calculatethe 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 / Stimulated emiss

5、ion (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 #,Einstein coefficients

6、,Spontaneous emission / Stimulated emission (3) R 1 hv kT W21Stimulated 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 states. non-radiati

7、ve transitions1/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 are two concepts t

8、hat 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 linewidthhomogeneous broadening all the atoms emit or absorb radiation in th

9、e 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 linewidththe uncertainty principle The shorter the uncertainty of this time

10、, 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 absorb radiation in th

11、e same way. homogeneous broadening no distinction between the atoms. e.g. the finite lifetime of the transitionshomogeneous broadening Lorentzian lineshape #,Homogeneous broadening,homogeneous broadening Lorentzian lineshape Lorentzian lineshape function of a homogeneously broadened transition#,Inho

12、mogeneous broadening,Doppler broadening - Inhomogeneous broadeningInhomogeneous 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 coming from each individual ato

13、m.Theoretically, all transitions are homogeneously broadened and also inhomogeneously broadened. #,Inhomogeneous broadening,Doppler broadening -The Gaussian lineshape #,Transition probability,1. = 0Spontaneous emission : A21 (1-1)Stimulated emission : W21=B21 (1-2)Stimulated absorption : W12=B12 2.

14、0 A21() =A21g() ； A21 ；Laser fieldContinuum light field #,Unit 2 Interaction between Light and Matter,Learning outcomesdiscuss spontaneous and stimulated emission, and absorption of radiation by atomic systemscalculate the ratio of spontaneous to stimulated emission rates in blackbodiesdiscuss natural linewidth and homogeneous and inhomogeneous linewidthsdiscuss Doppler broadening , broadenings due to nonradiative decay, dephasing collisions and so on.#,