How to search subdirectories in Folder.ListFor()

My requirement is to sear all parent and sub folders for .pgp files.
I have tried parameters like below
Dictionary<string, object> parametersSearchAll = new Dictionary<string, object>(){
    { "search_all", true},
    { "search", "/parent/*.pgp" }
};

But nothing seems to be working. The only way to search sub folders is recursively. Is there a way to look up all folders in a single call?