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Unformatted text preview: Chapter 6 Interaction of Light and Matter Atomic or molecular gases in low concentration show sharp energy eigen spectra. This was shown for the hydrogen atom. Usually, there are in f nitely many energy eigenstates in an atomic, molecular or solidstate medium and the spectral lines are associated with allowed transitions between two of these energy eigenstates. For many physical considerations it is already su ﬃ cient to take only two of these possible energy eigenstates into account, for exam ple those which are related to the laser transition. The pumping of the laser can be later described by phenomenological relaxation processes into the up per laser level and out of the lower laser level. The resulting simple model is often called a twolevel atom, which is mathematically also equivalent to a spin 1/2 particle in an external magnetic f eld, because the spin can only be parallel or antiparallel to the f eld, i.e. it has two energy levels and energy eigenstates [4]. The interaction of the twolevel atom with the electric f eld of an electromagnetic wave is described by the Bloch equations. 6.1 The TwoLevel Model An atom with only two energy eigenvalues is described by a twodimensional state space spanned by the two energy eigenstates  e i and  g i . The two states constitute a complete orthonormal system. The corresponding energy eigenvalues are E e and E g , see Fig. 6.1. In the position, i.e. xrepresentation, these states correspond to the wave functions ψ g ( x ) = h x  g i , and ψ e ( x ) = h x  e i . (6.1) 271 272 CHAPTER 6. INTERACTION OF LIGHT AND MATTER Figure 6.1: Twolevel atom The Hamiltonian operator of the twolevel atom is in the energy representa tion H A = E e  e ih e  + E g  e ih g  . (6.2) In this twodimensional state space only 2 × 2 = 4 linearly independent linear operators are possible. A possible choice for an operator base in this space is 1 =  e ih e  +  g ih g  , (6.3) σ z =  e ih e  −  g ih g  , (6.4) σ + =  e ih g  , (6.5) σ − =  e ih e  . (6.6) The nonHermitian operators σ ± could be replaced by the Hermitian oper ators σ x,y σ x = σ + + σ − , (6.7) σ y = − j σ + + j σ − . (6.8) The physical meaning of these operators becomes obvious, if we look at the action when applied to an arbitrary state  ψ i = c g  g i + c e  e i . (6.9) We obtain σ +  ψ i = c g  e i , (6.10) σ −  ψ i = c e  g i , (6.11) σ z  ψ i = c e  e i − c g  g i . (6.12) 273 6.1. THE TWOLEVEL MODEL The operator σ + generates a transition from the ground to the excited state, and σ − does the opposite. In contrast to σ + and σ − , σ z is a Hermitian operator, and its expectation value is an observable physical quantity with expectation value h ψ  σ z  ψ i =  c e  2 −  c g  2 = w, (6.13) the inversion w of the atom, since  c e  2 and  c g  2 are the probabilities for f nding the atom in state  e i or  g i upon a corresponding measurement. If we consider...
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 Spring '09
 Zahn
 Electromagnet, dipole moment, density operator, twolevel atom

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