We know that organic material often has a strong nonlinear optical effect due to delocalized electrons at pi-pi* orbitals. However, I still do not understand why they always have negative nonlinear refractive indices. When I iluminate CW laser on organic material I always see self-defocusing effects.
The CW laser is probably inducing a thermal lens in your sample, leading to negative nonlinear refractive index. Short pulse laser excitation with high repetition rate (MHz) may also induce thermal lens. To avoid thermal effect you should use a short pulse laser with low repetition rate (10Hz) or explorer other technic as Thermally managed Z-scan.
Dear Prof.Renato Evangelista de Araujo and Zbigniew Motyka,
I mean what do physical mechanisms result in negative nonlinear refractive index in organic?
Source of illumination is the governing factor to get electronic/thermal response of non-linearity. A CW laser is associated with non-radiative process (thermal/mechanical) of nonlinearity while with a short pulse laser excitation you will see electronic contribution for non-linear response
The CW laser is probably inducing a thermal lens in your sample, leading to negative nonlinear refractive index. Short pulse laser excitation with high repetition rate (MHz) may also induce thermal lens. To avoid thermal effect you should use a short pulse laser with low repetition rate (10Hz) or explorer other technic as Thermally managed Z-scan.
Dear Prof. Zbigniew Motyka, Prof.Renato Evangelista de Araujo, MS.Md. Khairul Amin,
I have understood somewhat the issue. As the main mechanism of the self-defocusing effect is the mechanism of heat absorption, when the temperature rises, the material expands, the density decreases, the total refractive index n = n0 + n2I decreases, then n2 must be negative.
Thank you verry much for your help !
You can see the attached publications regarding thermal response of non linearity
I think the others are right, when you use CW light and have som absorption in the material you most likely have a dominantly thermal effect. If you want to observe an electronic effect, you have to use ps or fs pulses, Even in this case where you minimize thermal intrapulse effects, you can still have inetr-pulse effects, depending on the repetition rate of the laser.
Thank you very much for your help, Prof.Renato Evangelista de Araujo, Prof.Md. Khairul Amin!
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