Thanks for contacting CareerGuide.com. I am happy to know that you are a physicist then must be having doctorate degree or btech/mtech/ms in physcial sciences or physics. You must be having any good reason to diversify the stream by accepting any complementary subject which makes your current subject more applicable in research or scientific world. Though You have not mentioned your highest obtained degree otherwise I could have given you the exact path-making approach in your academic world. Now we are living in economical era where sciences and technology gets diversified as because of commercialization in the sense of acceptability at the end users level. Keeping all these things in mind academicians, scientists, industrialists all are working together and giving the joint shape in the name of interdisciplinary and trans-disciplinary which attracts young researchers for their own reasons. I do not know whether you do understand this type of diverse and joint venture as a job machine or something else which keeps you in academic world.
As you have asked the right option for your current scientific background, I must tell you that physics is everywhere and now it is being tagged as-Biophysics, nano physics, nanotechnology, geophysics, metaphysics etc. But what you will do exactly with all these names it seems to be confusing which must be ignored with the sincere effort for your subject interest and basic knowledge. Here you are advised to follow the screening criteria by analyzing all the course structure of above given subject lines. Just see the course code basket of each and every discipline and draw the similarity matrix in terms of complementary which seems to be right combination for your studied courses. For example if you have gone through the material science very well from basic to applied then must opt nano-physics and develop the concept for nano particles. As your objective of further study is to be very confined with a certian module which will be re-difiened or re-modulated with your basic skills and concept. Therefore your skills in the same field will be flourished with the specialized research drive.



Keeping all these facts in mind you go ahead for the best institution in India or abroad and make it happens.

Good luck