To limit the search to JUST folders use:
ext:|folder|
You can then combine that with the name search, e.g. to search for folders beginning with the text 'archive'
ext:|folder| name:archive
If you don't know the start of the name use the wildcard character, e.g.
ext:|folder| name:*archive
Note: You could perform the folder name search without the 'name' prefix but it would be slightly slower.