A focused beam of light can trap a colloidal sphere, cause a specific neuron to fire, or deliver a lethal dose of energy to a cancerous cell. In biomedicine, focused light can perform nearly all the same sensing, diagnostic, and therapeutic functions as targeted x rays, without inducing harmful ionization.
Delivering light to internal tissue and organs, however, is not a straightforward task. In air or other transparent media, optical focusing is a simple matter of geometry—shape a beam with a curved lens and its rays will converge on ballistic trajectories toward a target. Scattering media such as biological tissues are not so cooperative. At penetration depths much larger than the scattering mean free path, a beam becomes distorted beyond recognition. In biological tissue, that mean free path is just 100 µm or so, roughly the width of a human hair.
Until recently it wasn’t clear that focusing light in...
