For typical intensities, optical rectification is a second-order phenomenon[1] which is based on the inverse process of the electro-optic effect.
Optical rectification can be intuitively explained in terms of the symmetry properties of the non-linear medium: in the presence of a preferred internal direction, the polarization will not reverse its sign at the same time as the driving field.
When the applied electric field is delivered by a femtosecond-pulse-width laser, the spectral bandwidth associated with such short pulses is very large.
The mixing of different frequency components produces a beating polarization, which results in the emission of electromagnetic waves in the terahertz region.
[4] Together with carrier acceleration in semiconductors and polymers, optical rectification is one of the main mechanisms for the generation of terahertz radiation using lasers.
An electron (purple) is being pushed side-to-side by a
sinusoidally
-oscillating force, i.e. the light's electric field. But because the electron is in an
anharmonic
potential
(black curve), the electron motion is
not
sinusoidal. The three arrows show the
Fourier series
of the motion: The blue arrow corresponds to ordinary (linear)
susceptibility
, the green arrow corresponds to second-harmonic generation, and the red arrow corresponds to optical rectification. (When there is no oscillating force, the electron sits at the potential minimum, but when there
is
an oscillating force, it is, on average, further to the right, by an amount shown by the red arrow.)
A schematic of an
ionic crystal
with no applied
electric field
(top), and with a sinusoidal electric field caused by a light wave (bottom). The blurryness indicates the sinusoidal oscillation of the ions. The red arrow indicates
optical rectification
: The oscillating electric field causes a shift of the ions' average positions, which in turn changes the crystal's DC
polarization
.
