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THz generation occurs via optical rectification in a <110> ZnTe. Optical rectification is a difference frequency mixing and occurs in media with large second order susceptibility, c(2). Optical rectification is actually analogous to frequency doubling. That is, a polarization is induced in the crystal that is the difference of the individual frequencies instead of their sum. This is due to the well known trigonometric relation: cos(A) * cos(B) = [cos(A+B) + cos(A-B)] / 2. Thus, light of a given frequency passing through a nonlinear medium will generate the same amount of both sum and difference frequencies, corresponding to second harmonic and dc. Another way of describing these processes is to consider the polarization induced in a medium at frequency 2w when it is driven at frequency ω:
P(2w) = c(2w; w, +w) E(w)E(w)
Frequency doubling -
P(ΩTHz) = c(ΩTHz; w, - w) E(w)E(w)
Optical Rectification -
For ultrashort laser pulses that have large bandwidth the frequency components are differenced with each other to produce bandwidth from 0 to several THz. Using either way to describe the process, the generated pulse is the envelope of the optical pulse.
Detection of the THz pulse occurs via free-space electro-optic detection in another <110> oriented ZnTe crystal. The THz pulse and the visible pulse are propagated collinearly through the ZnTe crystal. The THz pulse induces a birefringence in ZnTe crystal which is read out by a linearly polarized visible pulse. When both the visible pulse and the THz pulse are in the crystal at the same time, the visible polarization will be rotated by the THz pulse. Using a λ/4 waveplate and a beamsplitting polarizer together with a set of balanced photodiodes, we "map" the THz pulse amplitude by monitoring the visible pulse polarization rotation after the ZnTe crystal at a variety of delay times with respect to the THz pulse.
The ability to read out the full electric field, both amplitude and delay, is one of the attractive features of time-domain THz spectroscopy. Note, the visible and THz pulses are collinearly propagated through the ZnTe crystal even though in the figure they appear to be propagate at an angle.
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