Parametric process (optics)

A parametric process is an optical process in which light interacts with matter in such a way as to leave the quantum state of the material unchanged. As a direct consequence of this there can be no net transfer of energy, momentum, or angular momentum between the optical field and the physical system. In contrast a non-parametric process is a process in which any part of the quantum state of the system changes.

Since a parametric process prohibits a net change in the energy state of the system, parametric processes are considered to be 'instantaneous' processes. This can be seen as follows: if an atom absorbs a photon with energy E, the atom's energy will increase by ΔE = E. Since we are assuming this is a parametric process, the quantum state cannot change and thus this energy state must be a virtual state. By the Heisenberg Uncertainty Principle we know that ΔEΔt~ħ/2, thus the lifetime of a parametric process is roughly Δt~ħ/2ΔE, which is appreciably small for any non-zero ΔE.

In a linear optical system the dielectric polarization, P, responds linearly to the presence of an electric field, E, and thus we can write

where ε₀ is the electric constant, χ is the (complex) electric susceptibility, and n_r(n_i) is the real(imaginary) component of the refractive index of the medium. The effects of a parametric process will affect only n_r, whereas a nonzero value of n_i can only be caused by a non-parametric process.

Thus in linear optics a parametric process will act as a lossless dielectric with the following effects:

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