If the coupling is just one-way, i.e. you take the absorbed power per volume from the optics calculation and use it as the source term in the heat transfer problem, then it's pretty straightforward. You just edit the source term in the heat transfer problem so that it's the absorbed power per volume times a function of time that represents your pulse profile (a Gaussian, for example). Make sure you scale the pulse profile properly so that the time-integrated pulse energy is correct. You'll need to set the initial time for the heat transfer simulation to well before the peak of the Gaussian to make sure you capture everything. The supplementary information and some of the references in the following paper show some examples in both 2d and 3d:

https://www.researchgate.net/publication/260525321_In_situ_imaging_of_ultra-fast_loss_of_nanostructure_in_nanoparticle_aggregates

If you're considering bidirectional coupling (for example, temperature-dependent dielectric functions or, worse yet, changes in the geometry during heating), it becomes much more complicated, because you need to re-solve the optics problem on every step (at least until the laser turns off; once the laser pulse is ended you can take the final state as the initial condition for a pure heat-flow problem).

Article In situ imaging of ultra-fast loss of nanostructure in nanop...

Thanks Bryan, So I have an equation to add as a factor of time, now I am wondering how do I implement it. I have the em frequency domain solving the heat dissipation for the plasmonic stucture , and then a time dependant solver running the heat transfer module over 1 ns, with a 4 steps. Now I cannot find where I can edit the power dependency on time. I have attached a file in case you would like to look at the setup.

I don't have access to a COMSOL license at the moment, but you edit it in the source term (I seem to recall it's called Q in COMSOL notation? Maybe lowercase q?) for the heat equation. COMSOL by default hides a lot of things from you. Have you gone into preferences and turned on the options to make advanced options visible? You should be able to find where it's solving an equation where the time derivative of temperature is accounted for by a combination of the divergence of the flux and a volumetric heating source term. You want to put time dependence on that source term.

Thanks for that Bryan, I will try and figure out how to edit the equation using the advanced function. May I also take this opportunity to ask about the power into the port. I have not been able to figure out the difference between power deposited and just the regular power input option. Since the unit is Watts, I assume I have to account for the dimensions of my model. For a pulsed laser with an average power of 3mW, each pulse roughly becomes 3MW/cm squared, I find that for my model size it is roughly 1-2mW of power being applied to the single plasmonic structure. Should I use power deposited or just the power input for the port with this value?

You need to use the power absorbed per volume. It's defined once you've done the electromagnetics part of the calculation. Go ahead and make a volumetric plot (with partial transparency) of the solution of the optics calculation and you'll see that one of the options is the deposited heat power per volume. This should be one of the pre-defined variables in the pull-down list. This will give you the variable name that needs to be fed into the source term in the heat transfer calculation. Of course, it will be zero unless you're including the imaginary part of the dielectric function in the material properties (which, depending on the material representation you've chosen to use, might also be modeled as a finite conductivity).

Thanks Bryan, I think the heat source is transferring fine, but I was asking about the EM port power input, because the temperature gradients that form in the first few ps is dependant on the EM power that I supply. I want to make sure that the laser power from the experiment is being applied correctly in this case. 

Hi , 

        I have modeled a fiber structure ( microstructured one ) in COMSOL 3.5A using FR module. I need to study how the mode profile is changing with temperature ( 30-60 degree ) in steps of 10 degree . Can anyone let me know how to use heat transfer in COMSOL for my modeled structure . 

I appreciate your help 

I have been able to use an EM model in conjunction with HT physics to evaluate heat dissipation in plasmonics. However I do not see why you need to apply HT physics in your case. The EM properties will depend on the refractive index which is dependent on the temperature. Hence if you simply provide refractive indices that are dependent on temperature which will be your variable, you can simulate the effects of temperature variation in your fiber modes.

Dear Corentin,

Afraid not, the constant EM source has provided me with enough results to explain my experimental work. Hence I have given up as I did not have time to continue on this. I would really like to know how we can have two time dependent studies such as what you have mentioned. If you do manage to pull it off, please do share it. If I do come across something similar I will put it up here. Thanks.