subspecialize       
verb [ I ] (also sub-specialize); (UK usually subspecialise) uk/ˌsʌbˈspeʃ.əl.aɪz/ us/ˌsʌbˈspeʃ.ə.laɪz/
 专攻;专精
to study or work on a particular subject or skill more than any others within a particular area of a subject, so that you become an expert in it
  She is studying psychology and is planning to subspecialise in child psychology.
 Many psychotherapists have sub-specialized in a particular approach to therapy. 许多心理治疗师已经专攻特定的治疗方法。 
 - More examples
 - There are opportunities to specialize and even subspecialize within your field.
 - Many graduates choose to subspecialise after their initial training.
 - The only decision I had then was whether to continue in general anaesthetics or to subspecialise.