physics degree, what field to go into?

Hi everyone. I'm close to receiving my bachelor of science in Physics (and minor in mathematics), and I want to apply for graduate programs in the fall, possibly a doctorate program if I can. I'm trying to consider what field I want to pursue, and hopefully you, the reader, can help me out.

Right now I actually have a sort of unpaid internship doing research with optical tweezers in a "quantum optics" lab at my university, and it is really interesting and enlightening. However I think I could definitely get more enjoyment out of other fields...

I really like theoretical physics. I enjoy the math, and that is one thing that I hope to continue doing in the field I go into. I also really just want to pursue a field where I can really help develop the technology that will change humanity for the better. Specifically, I think I would get a lot of satisfaction out of doing some sort of research that could help make nuclear power obsolete one day. I know that is probably asking a lot, but if I could help even just a little bit to make that day become possible, I would want to do that.

Another field, slightly unrelated to the previously mentioned, is theoretical research regarding black holes and quantum gravity. I've always been completely mesmerized by black holes ever since I was very young, and I still am. I'm taking an informal course on general relativity right now during my summer break, and it is really interesting.

Thanks for any ideas everyone!