Aug 5 2014
Non-linear crystals are materials in which dielectric polarization exhibits a non-linear response with respect to the electric field of the light. This nonlinearity can be observed at very high light intensities such as that of pulsed lasers.
Non-linear crystal materials give rise to a number of optical phenomena including:
- Second harmonic generation
- Third harmonic generation
- High harmonic generation
Second Harmonic Generation
Second harmonic generation, also known as frequency doubling, is a non-linear process in which photons with the same frequency interact with a non-linear material and generate new photons with twice the energy and hence twice the frequency of the initial photons. Second harmonic generation occurs only in mediums without inversion symmetry.
Third Harmonic Generation
Third harmonic generation, or frequency tripling, is a non-linear frequency conversion process in which the resulting frequency is thrice that of the input beam of laser. This can be achieved using a χ(3) non-linearity for direct third-harmonic generation despite of the phase-matching constraints.
Third harmonic generation usually starts with second harmonic generation of the input beam followed by sum frequency generation, with both processes having a χ(2) nonlinearity.
High Harmonic Generation
Strong non-linear interactions caused by focusing intense laser pulse into a gas result in the non-linear frequency conversion process, such as the formation of high odd harmonics of the pulse. This harmonic generation occurs at higher orders of optical intensities such as 1014 W/cm2 or even more. Only a small fraction of laser power can be converted into higher harmonics.
However, the output can be used for measurements in the wavelengths of hard ultraviolet or X-ray spectral region. Such high harmonics may even replace synchrotron radiation, and can be used for creating pulses having attosecond durations at extreme ultraviolet spectral region. These pulses can be used to study electronic motion in different materials.
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