I'm graduating with a B.S. in physics and I will soon be starting a Ph.D. program in nanoscale science, which is essentially an applied physics program. I chose this program because my undergraduate research was focused in this area and I've really enjoyed it. However, I'm wondering if my choice of Ph.D. program will limit my future opportunities to applied research only. There are some topics that I would love to research, such as particle physics, which are quite fundamental. Would I be able to get a post doc position in particle physics with a Ph.D. in nanoscale science, or would these positions only be offered to students that specifically researched particle physics as graduate students? At what point should you stop experimenting with different fields?