original_query: What is the Franz-Keldysh effect in optoelectronics?
query: What is the Franz-Keldysh effect?
answer: The Franz-Keldysh effect is a change in optical absorption by a semiconductor when an electric field is applied.  It's named after Walter Franz and Leonid Keldysh.  The effect allows electrons to be excited into a conduction band by absorbing a photon with energy below the band gap. It's used in electro-absorption modulators, but typically requires hundreds of volts.