mongodb not using indexes

I have a collection with these indexes:

db.colaboradores.getIndexKeys()
[ { "_id" : 1 }, { "nome" : 1 }, { "sobrenome" : 1 } ]

and a query like

db.colaboradores.find({_id: ObjectId("5040e298914224dca3000006")}).explain();

thatworks fine with index

{
    "cursor" : "BtreeCursor _id_",
    "nscanned" : 0,
    "nscannedObjects" : 0,
    "n" : 0,
    "millis" : 0,
}

but when run:

db.colaboradores.find({nome: /^Administrador/}).explain()

mongodb do not use indexes any more:

{
    "cursor" : "BtreeCursor nome_1",
    "nscanned" : 10000,
    "nscannedObjects" : 10000,
    "n" : 10000,
    "millis" : 25,
}

any solutions? Thanks!