Presently we measured the nonlinear response of an organic compound using the Z-scan method. The magnitude of the nonlinear refractive index of the compound solution was found to be negative at a fixed laser power. What is the meaning of negative nonlinear refractive index scientifically?
dear colleague
it may mean that the compound on which you work should be classified as METAMATERIAL that could have a negative refractive index
regards
Metamaterials exhibit a negative nonlinear refractive index. Also called left-handed media. As they are left-handed according to Snell's law the incident and refracted rays are on the same side of the surface normal at an interface of positive and negative index materials.
The answer can be given from the perspective of the Kerr lensing. The intensity dependent refractive index can be written as n(I) = n0 + n2I, Where n0 is linear refractive index which is constant, the NLO refractive index is n2. For a Gaussian input intensity, due to the positive NLO n2, the sample will behave as focusing lens. That means the sample can focus light beam with its own, self-focusing. For negativevalue of the n2, the sample will act as defocusing, that means, teh light passes through the sample will defocus. This can be seen in the CA Z-scan experiments. The divergence of the output beam at detector position will be more in case of negative n2 material.
Please Note:
Although the n2 value can be negative but overall refractive index will be always positive as the contribution of n2 is very small as compare to the n0. The sample will not behave as meta material as claimed in other answers.
A negative refractive index speaks about the defocusing nature of the corresponding material. This very much depends on the response of that intensity-dependent nonlinear refractive index to adapt to the change with laser intensity. If the schematic of the phenomenon is analysed following geometrical optics (Ref. http://www.phys.unm.edu/msbahae/z-scan.htm ) clearly, the rise of a peak, followed by a valley can be understood. Similarly just opposite for the positive refractive index (valley-peak).
Usually non-structured/ amorphous materials is defocusing, negative refractive index and non-linear polarization prone.
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