How to draft a SOP when one wants to work on a sub-domain they have never worked on before? (CS PhD USA)
A person with a bachelors degree entering into doctoral studies directly in the US has a long road ahead. First there will be (mostly or entirely) coursework in advanced topics that will give you the background necessary to pass comprehensive exams and assure you have a broad knowledge. Specialization and research only comes later in most programs.
Given that structure, there is nothing unusual in naming a research area that interests you before you have direct experience in it, provided you have the general background knowledge to begin to explore that area.
My advice, then, is just to say what your goals are and the direction you see yourself following. It will show that you have given thought to the options and have a plan. And, no one will hold you to that plan once you are accepted. Your initial advisor might be in the desired area or not, and may be more likely to be if you state the goal, but it is possible to change advisors when you get closer to choosing a research topic.
And, even stating what you now think of as a &quot;narrow&quot; area of focus, it will become much narrower once you start to choose a research question.
So, just say it and don't worry that you don't yet have background in it. That will come and is the purpose of advanced study.