Perturbing Hamiltonian optical absorption in semiconductors

[dyn]

Hi
I am looking at the problem of optical absorption in direct gap semiconductors. It seems like the perturbing Hamiltonian is an oscillating perturbation , ie. an electromagnetic wave. Why can't the problem be treated as the absorption of a single particle , ie. a photon ?

 

[DrDu]

Of course it can, but it is more complicated to treat the electromagnetic field quantum mechanically than classically.