  The photoexcitation life-cycle from incident photon (and creation of
photoexcited electron hole pair) to ultimate extraction of electrical current
is a complex multi-physics process spanning across a range of spatio-temporal
scales of quantum materials. While often viewed through a device-technology
lens, photocurrent is a key observable of the life-cycle that is sensitive to a
myriad of physical processes across these scales. As a result, photocurrent is
emerging as a versatile probe of electronic states, Bloch band quantum
geometry, quantum kinetic processes, and device characteristics of quantum
materials. This review outlines the key multi-physics principles of
photocurrent diagnostics. In particular, we describe how the fundamental link
between light-matter interaction and quantum geometry renders photocurrent a
wavefunction-sensitive probe capable of resolving bandstructure and
characterizing topological materials. We further highlight the sensitivity of
the photoexcitation life-cycle to relaxational processes which in turn enables
photocurrent to disentangle distinct types of carrier scattering that can range
from femtosecond to nanosecond timescales. We survey the intrinsically nonlocal
character of photocurrent collection that allows new types remote sensing
protocols and photocurrent nanoscopy. These distinctive features underscore
photocurrent diagnostics as a novel multi-physics probe for a growing class of
quantum materials with properties governed by physics spanning multiple
spatio-temporal scales.
