  Ferroelectric domain wall conductivity (DWC) is an intriguing functional
property, that can be controlled through external stimuli such as electric and
mechanical fields. Optical-field control, as a non-invasive flexible handle,
has rarely been applied so far, but significantly expands the possibility for
both tuning and probing DWC. On the one hand, as known from Second-Harmonic,
Raman, and CARS micro-spectroscopy, the optical in-and-out approach delivers
parameters on the DW distribution, the DW inclination, and probes the DW
vibrational modes; on the other hand, photons might be applied also to directly
generate charge carriers within the DW, hence acting as a functional and
spectrally tunable probe to deduce the integral or local absorption properties
and bandgaps of conductive DWs. Here, we report on such an optoelectronic
approach by investigating the photo-induced DWC (PI-DWC) in DWs of the model
system lithium niobate, a material that is well known for hosting conductive
DWs. We compare three different crystals containing different numbers of domain
walls: (A) none, (B) one, and (C) many conductive DWs. All samples are
inspected for their current-voltage (I-V) behavior (i) in darkness, and (ii)
for different illumination wavelengths swept from 500 nm down to 310 nm. All
samples show their maximum PI-DWC at 310 nm, i.e., at the optical bandgap of
lithium niobate; moreover, sample (C) reaches PI-DWCs of several $\mu$A.
Interestingly, a noticeable PI-DWC is also observed for sub-bandgap
illumination, i.e., wavelengths as high as 500 nm, hinting towards the
existence and decisive role of electronic in-gap states that contribute to the
electronic transport along DWs. Finally, conductive atomic force microscopy
(c-AFM) investigations under illumination proved that the PI-DWC is confined to
the DW area, and does not originate from photo-induced bulk conductivity.
