We note that this bandwidth is a conservative, lower-bound estimate, since such WSe2 are so fast that they exceed the current speed limit of our set-up as verified with a commercial photodiode In the context of energy-efficient optical communication, one may be concerned with the energy consumption of such a photodetector considering that the irradiance required for optimal speed of roughly 100 W/cm 2 is quite high. For our example and a bandwidth of (at least) 230 MHz, a laser power of 6.25 µW is sufficient. Since the detector is operated at zero bias by taking advantage of the photovoltaic voltage generated at the metal/semiconductor junction, the laser power is the only significant contribution to the overall energy consumption, which amounts to < 27 fJ/bit. This compares favorably to most optical communication components (>1000 fJ/bit) and is not much higher than the energy required to switch a simple CMOS gate (~1 fJ/bit). Exploring and expanding the ultimate speed limit for TMDC photodetectors