It depends of course on which kind of research you want to do, but I think that being a medical physics you should have already some kind of formation on laser physics, so it could be not so impossible to find a PhD position on that. Anyway the best way can be trying to apply (they usually look at your background when they make a decision) or even better try to contact some professor in the field you are interested in... look at Universities websites, find some professor whose research interests overlap with yours, consider if you could like to go to that University and then contact directly the professor... he is the best person which can indicate you if you are suitable and in case the procedure you have to follow to apply for the PhD. Also the procedure will depend on where you are =)