The varactor changed capacitance with changes in the bias voltage.
This one keeps the bias constant and the light varying causes change in capacitance. This could be used as a sensor for automatic gradual darkening of windows in homes, buildings and autos according to the brightness of the sun.
I wonder what kind of bandwith a “photo-varactor” diode would have as compared to a regular photo diode as a fiber optic receiver.
Or, thinking pushing LED’s, tunnel, or gunn diodes to work to transmit over coax in near the near terrahertz range, as a replacement for the microwave communication bands.