I am facing a problem in the measurements of the z-scan experiment to measure the nonlinear absorption coefficient of polymer films using a cw laser due to the high temperature of the sample, and this leads to a significant increase in the intensity of radiation transmitted from the sample. Is there a way to get rid of this thermal effect
I think the only parameter available is time. You are going to have to pulse the laser. It could be as simple as using a chopper wheel
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=287
or, if you don‘t want to spend the money and you don’t care about precise timing, you can use a fan motor and a circle of cardboard with a slit cut in it. For example a 3000 RPM motor turning a disk with a slit covering 10 degrees of arc will make 0.5 ms pulses at 50 hz with 1/36 duty factor (so 1/36 the thermal load). 0.5 ms is pretty short for thermal processes. You can go further by making thinner slits on larger diameter wheels or using faster motors. You will know you’ve reduced the thermal load enough when changing the average power (and the pulse length) by, say, a factor of two no longer changes the results.
The down side is that measuring the transmitted power becomes harder. Depending on the accuracy and sensitivity of your power meter you may have to change to a different approach.
You can rid from beam local heating of your sample if would apply instead CW emission CW pulse picosecond laser though. Pulse power of a similar laser reach Megawatt level at milliwatt level of averaged power that is enough for different nonlinear effects without harmful heating.
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