Lasers pp 225-261 | Cite as

Quantum Theory of Interaction of Radiation Field with Matter

  • K. Thyagarajan
  • Ajoy Ghatak
Part of the Graduate Texts in Physics book series (GTP)


In this chapter we show that the electromagnetic field can be considered as an infinite set of harmonic oscillators, each corresponding to a particular value of the frequency, wave vector, and a particular state of polarization. Comparing with the quantum mechanical treatment of harmonic oscillators, we replace the generalized coordinates and generalized momenta by operators. By imposing the commutation relations between the canonical variables, it is shown that the energy of each oscillator can increase or decrease by integral multiples of a certain quantum of energy; this quantum of energy is known as the photon. Having quantized the field, we show that the state which corresponds to a given number of photons (also referred to as the number state) for a particular mode does not correspond to the classical plane wave. Indeed, we show that the eigenstates of the annihilation operator (which are known as the coherent states) resemble the classical plane wave for large intensities.


Harmonic Oscillator Coherent State Radiation Field Beam Splitter Spontaneous Emission 
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Copyright information

© Springer Science+Business Media, LLC 2011

Authors and Affiliations

  1. 1.Department of PhysicsIndian Institute of TechnologyNew DelhiIndia

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