tailieunhanh - 2 - Interaction of Electrons and Photons

This chapter provides the basis for the discussion in the following chapters by summarizing the fundamental concepts and the quantum theory concerning the interaction between electrons and photons in a form that is convenient for theoretical analysis of semiconductor lasers [1–9]. First, quantization of electromagnetic fields of optical waves is outlined, and the concept of a photon is clarified. Quantum theory expressions for coherent states are also given. Then the quantum theory of electron–photon interactions and the general characteristics of optical transitions are explained. . | 2 Interaction of Electrons and Photons This chapter provides the basis for the discussion in the following chapters by summarizing the fundamental concepts and the quantum theory concerning the interaction between electrons and photons in a form that is convenient for theoretical analysis of semiconductor lasers 1-9 . First quantization of electromagnetic fields of optical waves is outlined and the concept of a photon is clarified. Quantum theory expressions for coherent states are also given. Then the quantum theory of electron-photon interactions and the general characteristics of optical transitions are explained. Fundamental mathematical expressions for absorption spontaneous emission and stimulated emission of photons are deduced and the possibility of optical wave amplification in population-inverted states is shown. QUANTIZATION OF OPTICAL WAVES AND PHOTONS Expression of Optical Waves by Mode Expansion The electric field E and magnetic field H of optical waves together with the electric flux density D magnetic flux density B current density J and charge density p generally satisfy the Maxwell equations @B V x E - JD p V x H J @ V B 0 The electromagnetic fields can be expressed using a vector potential A and a scalar potential 0. For cases where there is no free charge in the medium p 0 J 0 in particular we can put 0 0 and accordingly E and H Copyright 2004 Marcel Dekker Inc. can be described by using only A as E - H J x A dt ßo We express A by a superposition of sinusoidal wave components of various angular frequencies m as A r t I YXam t Am r a m t Am r am t am exp -ia mt Then E can be written as E r t 2 X am t Em r aX t Em r i m Am r Let nr be the refractive index of a medium at angular frequency then the components of D and E are correlated by D n2e0E and Em r satisfies the Helmholtz wave equation r2Em n 2Em 0 J-Em 0 where c 1 e0 0 1 2 is the light velocity in vacuum. Am also satisfies the same .

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