Several celebrities used it for their babies from 2008-2012 (
http://www.namecandy.com/celebrity-baby-names/search/baby/harper). While none of them probably *made* it popular, the more of them that used it, the more people heard it (on that page I linked, only
Shonda Rimes had a baby
Harper before it charted [not counting the one born in 1972 obviously]).
Rosalie Cassander Poet
Radiance Nocturne Silver
www.behindthename.com/pnl/59411