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The Interaction Between Light And Matter

Principle: Introduction to Nonlinear Wavelength Conversion

 

The interaction between light and matter


       

The propagation of light through a medium is a process of interaction between light and the medium. This process has two related sub-processes: one is the polarization (P) of the medium caused by the electric field light (e) acting on it, producing a macroscopic polarization intensity, which is the polarization process of the medium. The other is the radiation process of light. The change in electric field light (e) in the medium over time, and the resulting changing polarization intensity (P), will act as a light source to radiate light waves. In linear media, this interaction is linear, and the frequency of the light does not change, as shown in Figure 1-a; in nonlinear media, this interaction is nonlinear, as shown in Figure 1-b. The light forces these dipoles to oscillate with a nonlinear response, causing the frequency of the output light to exhibit a nonlinear effect.

 

Figure 1-a: Polarization of a linear medium
Figure 1-b: Polarization of a nonlinear medium

 

In the numbers above, χ1 is the linear polarization coefficient, while χ2 and χ3 are the second-order and third-order nonlinear harmonic polarization coefficients. The linearity coefficient χ1 determines the linear propagation of light waves, such as refraction, reflection, diffraction, and dispersion; while the higher-order harmonic polarization coefficients χ2 and χ3 determine the higher-order harmonic light generated by light waves corresponding to harmonics under a strong electric field. The second-order harmonic polarization coefficient disappears in media with inverted symmetry structures, but may be very large in some nonlinear media with non-centrosymmetric structures, such as lithium niobate (LiNbO3).