  We report measurements of the infrared optical response of thin black
phosphorus under field-effect modulation. We interpret the observed spectral
changes as a combination of an ambipolar Burstein-Moss (BM) shift of the
absorption edge due to band-filling under gate control, and a quantum confined
Franz-Keldysh (QCFK) effect, phenomena which have been proposed theoretically
to occur for black phosphorus under an applied electric field. Distinct optical
responses are observed depending on the flake thickness and starting carrier
concentration. Transmission extinction modulation amplitudes of more than two
percent are observed, suggesting the potential for use of black phosphorus as
an active material in mid-infrared optoelectronic modulator applications.
