I agree with Peter on light contamination.

Still, it seems like the frequency of the modulation is around 100Hz, which is strange (normally fluorescent lamps and computer screens are 50/60Hz, depending on your geographical region.) . Still, it could be you have a fancy computer screen with higher refresh rate than 60Hz, try turning it off during measurements, or at least facing the screen away fron the microscope.

If that is not the problem, than it may be mechanical vibration. Since you are performing 2D FCS, i imagine you are focusing on a 2D object, make sure the object is perfectly stable.

Yep, you are right! I am quite used to 50 Hz noise in Europe when i don't darken properly the setup, never thought too hard about it...i guess strangely pc monitors cause more trouble than lights.

 Thank you Dr.Peter and Dr. Paolo.

I do the experiments in dark room and as you have mentioned the only source of stray light is from the monitors. But if the source is due to the stray light or mechanical vibration, I should pick this up in all my experiments.

Routinely, we are measuring lipid diffusion using FCS and we never pick this oscillations. I am observing them only in experiments, where, I am studying the diffusion of quantum dots that are bound to the membrane. Also, the intensity trace of this measurements are looking different. I will pull them and send it for your opinion.

Even in 3d bulk measurements(of the same QDs in water), I get a  smooth decay. I am suspecting that this problem has to do with the 'material' rather than any external / instrumental source.

thank you.

In Brazil, many years back, I observed similar oscillations in a DLS correlogram.  They were magically cured by removing the transistor radio that sat on the flat DLS instrument. Later that year they had a jackhammer digging up their laboratory floor and wondered why results were inconsistent.  The light oscillation is a common one we see in spray laser diffraction experiments where there is an open bench - working with low or no light levels is the cure.

Hey Roobala,

If the modulation is from objects bound to a thin membrane, it could be that your objective's focus is unstable. That could be due to mechanical vibrations, or to some sort of problem with the piezoelectric focus actuator (if you have one).

For sure measurements are much more sensitive to mechanical vibrations when looking at signals confined in bidimensional surfaces. You could even verify if the periodic signal is attenuated using a lower numerical aperture objective (with matching wider pinhole), which would make a "thicker" focal volume.