Applicability of any test  depends on the mechanism involved. Since in nanosecond pulses (OM, 2014) you observed optical limiting, while in fs you see opposite case (bleaching) you may imagine a number of effects. E.g. Krasnoyarsk nanodiamonds are often poorly cleaned. If cleaning happens during fs irradiation? Or aggregate fracturing? Or oppositely, aggregation? Funny but when we illuminated nanodiamond suspension by 82 MHz repetition rate fs pulses we have seen formation of distorted spot in transmission. This surely was thermal effect that would be absent in ns pulses. However, z-scan seemed to be hardly applicable for our case

Of course, if you register z-scan during rather short period, long term mechanisms must not be taken into account

We have checked an aggregate fracturing as a possible reason of the transmittance  changed. We checked it by performing the measurements at different pulse energies from low energies(where the signal is observable) to higher energies (where signal is about 10%) and back to lower energies and observed no any changes in the transmittance curves. If the cleaning or breaking of the clusters at high energies was taking place the transmittance curve for low energies will not remain the same. In our case it remains the same.

I am not sure about the aggregation in this case, to reveal it we have to measure cluster sizes before and after experiments.

Vyacheslav, but what are conditions for transmission increase mentioned in the Question? If you have not seen changes in a set of expts described in the second letter, then expt in the Q conducted at different conditions. All this things are sensitive to power density, to diffusion between beam waist and the rest of solution and so on....

Do some intensity dependent studies.  Record the z-scan for 4-5 input intensities.  If the SA behavior persists then it is due to the sample. In some cases you will see switching of SA to RSA behavior with increasing peak intensities.

Dear Aleksandr Aleksandrovsky,

I mean we observed no changes in the transmittance curves at two experiments with the same low pulse energies but between them was an experiment with high energy. It means that an aggregate fracturing which you mentioned as a possible reason does not occurs.

Daer Venugopal Rao Soma,

unfortunately, we are not seeing the switching to reverse saturable absorption (RSA) at high intensities. The intensity increasing leads to continium generation in water which is employed as a host liquid.

We are not sure that the measured transmittance increase (or absorption decrease) with increasing light intensity is due to saturable absorption and asking are there any techniques to reveal it.

Change the wavelength of input laser and excite where there is no linear absorption.

Then you can find out whether it is SA or not..

1- Bistability method may be useful ?

2- Z-scan is used to measure the nonlinear refraction index: neff=n1+n2I and increases with intensity. This effect may be significant using fs laser.

3- The scattering in hybrid systems(solutions) are very crucial.

In dilute NP suspension, scattering is dominant and in dense NP concentrations the multiple scattering take place too. This causes to decrease the transmission.

You see all mechanisms are against your observation!! Scattering and nonlinear events accounts for the reduction of transmittance, contrary from your experimental result.

It is appreciated if you add the scheme of your set up for further discussion.

Dear Parviz Parvin,

would you so please give a proper reference on the method.

Dear Viatcheslav Vanyukov 

I asked two of my students add some references including our papers in this post.

Regards

Thank you Parviz Parvin

Dear P. Parvin,

The Z-scan experiment using a Gaussian laser beam in a tight-focus limiting geometry measures the transmittance of a nonlinear medium through a finite aperture placed in the far field as a function of the sample position (z) measured with respect to the focal plane. With the sample on the +z side of the focus, the negative lensing effect tends to augment diffraction, and the aperture transmittance is reduced. The approximate null at z = 0 is analogous to placing a thin lens at the focus that results in a minimal far-field pattern change. For still larger +z the irradiance is reduced and the transmittance returns to the original linear value.