The velocity of light of particular wavelength may vary as it is going deep in the nonlinear optical materials. If it is like that what is the reason for that?
Yes, nonlinear optical interactions in photonics structures happen between supported optical modes. If the thickness of the material layer is small enough (typically in the range of microns or hundreds of nanometers for visible light), then the nonlinearly interacting optical modes will "feel" several surrounding refractive indexes, because they will not be "contained" within a single layer (due to the thickness of this layer being too small). This dependance on the thickness can be used to control effective refractive index dispersion and achieve unusual phase matching regimes: https://www.researchgate.net/publication/230626656_Cascaded_third_harmonic_generation_in_lithium_niobate_nanowaveguides
Article Cascaded third harmonic generation in lithium niobate nanowaveguides
In sub micron thickness of Rb cell For example, the fluorescence and absorption response change. but I don't know of any work in refractive index. look at this paper for more information
http://journals.aps.org/pra/abstract/10.1103/PhysRevA.69.065802
theoratically index based on dipole moment . From theory we calculate the dipole moment but will not discuss about index . Based on dipole moment NLO properties will change my assumptions is similarly index also change
Yes, nonlinear optical interactions in photonics structures happen between supported optical modes. If the thickness of the material layer is small enough (typically in the range of microns or hundreds of nanometers for visible light), then the nonlinearly interacting optical modes will "feel" several surrounding refractive indexes, because they will not be "contained" within a single layer (due to the thickness of this layer being too small). This dependance on the thickness can be used to control effective refractive index dispersion and achieve unusual phase matching regimes: https://www.researchgate.net/publication/230626656_Cascaded_third_harmonic_generation_in_lithium_niobate_nanowaveguides
Article Cascaded third harmonic generation in lithium niobate nanowaveguides
The anisotropic index of refraction in NLO crystals is an instrinsic property, meaning not dependent on size. The velocity of fundamental beam (input beam) in the material is constant as it propagates through the material, however, the harmonic beam generated does have a different velocity (but constant), and therefore refracts. But the power walk from the fundamental beam to the harmonic beam IS a function of crystal length, where at maximum harmonic power, is termed the coherence length of the crystal, typically ums. NLO crystals are cut to lengths far greater the the Lc, ~ mm's.
But if the light intensity is high, due to different NLO properties, the refractive index of the material as the light go inside may change due to change of velocity of the light.
@Alexander S. Solntsev:
But the different layers of the material may have same properties, then how to consider the change in the refractive index in that material.
If all properties in different layers are the same, then how do you distinguish between different layers? It just become bulk material.
It also may be the case that the proximity of the edge introduces additional material stress and changes the optical properties, so the refractive index changes as well. This effect is typically negligible in crystals, but may be substantial in other materials.
Yes, it must be bulk material. For these materials, the change of the refractive index may be due to the interaction of light with material in different way i.e. it may continuously change the polarization of the dipoles and the polarization may effect the light velocity. these two factors changes simultaneously. I still doubt....
sure the thickness of the material plays an important role in nonlinearity especially the direction of incident light because of phase matching .
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