Never use the luceneSearch operation. It is a legacy remnant and no longer receives any enhancements... Also xpathSearch is not a real XPath search, also going via the legacy Lucene search language with a very reduced XPath subset.
If you want to restrict based on the prefix of the name, you can simply add another condition to your query using AND. E.g. you can append
to query for all sub-folders / descendants with a name starting with AC (case-sensitive)