For example many reports are available with thickness dependent terahertz generation. Organic single crystals exhibits higher THz efficiency at particular thickness. Could you please explain the reason for thickness dependent THz efficiency?
If you consider the physical meaning of the phase matching term within the generation process,
ITHz ~ L2Sinc2(dK L/2)
for a particular THz and optical frequency we can say that delta k will be constant but non-zero, now by increasing the length of the crystal (L) the generation efficiency grows. It continues to grow until
L = 2pi/dK
at this point the any newly generated terahertz radiation is pi out phase with any terahertz radiation generated L before it and so destructive interference means they cancel out and the efficiency decreases.
From the same equation
ITHz ~ L2Sinc2(dK L/2)
It is also possible to see now that by allowing dK to vary with THz frequency it is possible to have a frequency dependent efficiency. For instance it can occur that if the crystal is a certain thickness, the condition
L = 2pi/dK
is met for low frequencies but not met for frequencies in a higher range. In its simplest form each frequency component has its own optimal crystal thickness (given by L = 2pi/dK) and so for a given thickness each frequency component will have a slightly different efficiency.
I hope this helps.
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