Dr. Courtney said:
My wife and I decided early on that there was no way we were going to go to separate cities for post-docs. When she landed a post-doc at the Cleveland Clinic, we decided to move to Cleveland, and there was no way I could remain in experimental atomic physics, because there were no research groups in Cleveland. But it was clearly the geographical constraint of being close to the woman I married rather than limited career opportunities. There were plenty of job opportunities in Cleveland that offered to pay me for my PhD in Physics - but most were teaching jobs or R&D jobs (using mostly my instrumentation skills).
I took an R&D job for about 7 years. After September 11, 2001, My wife and I formed a small company to do defense-related research in a field that was of great interest and need in the war on terrorism. I then took a teaching job to have more time for defense-related research. The PhD in Physics has been very important as a qualification for lots of my work, but the specific subfield mattered more as related to practical instrumentation and experimental design skills than whether or not any DoD contractor or laboratory wanted to work with me or was willing to hire our company for a project. No one has ever said (that I know of), "We're not going to use Dr. Courtney, because his PhD is in atomic physics rather than blast physics (or ballistics)." In 2009, I took a faculty position at the Air Force Academy in Mathematics. They loved the research portfolio I had compiled, and they also loved my ability to work with weaker students and to involve students in DoD-related research projects.
In the years since the reductions in DoD funding (2012-2013), the consulting work of my wife and I has shifted more to other areas, including consulting as expert witnesses on court cases. As an expert witness, I have never been subject to a Daubert challenge based on my PhD being in Atomic Physics rather than blast physics or ballistics. Being a highly published and widely cited scientist in those fields is enough for both clients and courts to accept my credentials. But I don't think either my wife or I would be in such demand or so easily accepted as courtroom experts without our PhDs. I've also managed to publish a number of peer-reviewed papers in fisheries science. Getting funding for that work with a PhD in physics is a bit trickier, but we've only run into challenges getting published a couple of times. The wildlife agencies have taken our work seriously.
In any case, my experience of earning a PhD in 1 field and then working in another is fairly common. Picking a subfield of physics is not an irrevocable career choice.