Back when my dad still worked at Aberdeen University, they had a phased array of maybe 7 or 8 transducers of about 8 cm each, that at a range of 6 inches or so, could deliver a focused beam that would penetrate a phantom / lab animal enough to cook a 2 - 3 cm sphere several cm below the skin.

The research was aimed at basically heating carcinomas above the 44 oC needed to kill the cells.

So you can focus an ultrasound beam, but it seems like a heck of a way to charge a phone.