What materials are the film and substrate? The most obvious suggestion is to attenuate the beam until the response is weak for the film and negligible for the substrate, which will work if the substrate's nonlinear susceptibility is at least an order of magnitude weaker than the film. You should test this by doing a z-scan on a blank substrate.

The materials like carbon nanotube films and graphene on quartz, silica and lavsan subsrates. The responce from this substrates itself is not negligible even at lower pulse energies.

Please look at the attached manuscript.

Canek Fuentes-Hernandez thank you for proper reference

You need to perform two zscans one with substrate and other scanwith substrate and flim and substract one scan from the other

Thank you Fryad Henari for the answer. I am wondered how it is competent to determine the film signal just by easy substracting.

I do not think will effects the results, however if not happy with my suggestion then look for substrates which not showing the nonlinearity at the intensities used in your experiment , you may try reflection z scan but i am not sure you be able to distinguish between front and back reflection

Fryad Henari thank you, and do you know some of them that are not showing the nonlinearity?

For distinguishing between front and back reflection it will be useful to employ thing films and thick substrates. We prepared such kind of samples and here appears another question. The beam reflected from the back side of a substrate can be easy blocked by the aperture. But how we can consider a beam that is passes through the film then reflecting from the front edge of a substrate and secondly passes through the film? Can we consider this beam as a two times attenuated by a film in the course of z-scan, which is intended to get transmittance curve for further theory characterization?

Hello Sir,

Depending on whether you just want to determine z-scan features from the films or actual values you could study the effects of the substrate alone with the same beam fluence and exactly under similar conditions so that you are fully aware which of the two is responsible for specific observed attributes. It is not obvious to me whether the features of the films alone would then be a mere subtraction of the two spectra as suggested. Your scenario of the film/substrate situation may be less complex than a case I dealt with years back of metal nano-particles embedded in a non-linear substrate by ion-implantation.

There are other factors to consider as well such as the intensity of the beam that you are allowing to be incident on the films so as to prevent local heating of the samples thus varying the characteristics of the films under investigation.

Hello George Amolo,

We want to determine actual values of nonlinear refraction and nonlinear absorption of a film, and we already know that both a film and a substrate possess some nonlinearity. In the substrate this nonlinearity is present even at lower intensities which is maybe due to the thickness of the substrate (1mm),  which is much higher then the film thickness (less than 100nm).

We are also studying a case when silver nanoparticles are embedded in a glass substrate. In this case one of the way to escape the signal from a substrate is to decrease significantly its thickness.

If it is an issue of unwanted scatter, then you might consider using a FLIM microscope and only take the portion of the signal after the excitation pulse to avoid the shorter reflection signal.

Record the Z-scan curves for both (a) substrate + film and (b) film alone at similar experimental conditions and subtract the coefficients. Usually the substrate contribution will be small.

Dear Venugopal Rao Soma,

are you sure about "(b) film alone"? If we do this subtracting we will get the signal from a substrate. The question is how to escape the signal from a substrate revealing only the signal from a film.

If recording a z-scan of a film alone will be possible there will be no need to escape the signal from a substrate, therefore there will be no any problem.

Sorry. It should be substrate alone. then you will get contribution from film alone.

Venugopal Rao Soma, I am not sure that such subtracting will show the correct result, or may be you can prove it? Thank you in advance