  SnSe is a layered material that currently holds the record for bulk
thermoelectric efficiency. The primary determinant of this high efficiency is
thought to be the anomalously low thermal conductivity resulting from strong
anharmonic coupling within the phonon system. Here we show that the nature of
the carrier system in SnSe is also determined by strong coupling to phonons by
directly visualizing polaron formation in the material. We employ ultrafast
electron diffraction and diffuse scattering to track the response of phonons in
both momentum and time to the photodoping of free carriers across the bandgap,
observing the bimodal and anisotropic lattice distortions that drive carrier
localization. Relatively large (\SI{18.7}{\angstrom}), quasi-1D polarons are
formed on the \SI{300}{\femto\second} timescale with smaller
(\SI{4.2}{\angstrom}) 3D polarons taking an order of magnitude longer
(\SI{4}{\pico\second}) to form. This difference appears to be a consequence of
the profoundly anisotropic electron-phonon coupling in SnSe, with strong
Fr\"ohlich coupling only to zone center polar optical phonons. These results
demonstrate that carriers in SnSe at optimal doping levels results in a high
polaron density and that strong electron-phonon coupling is also critical to
the thermoelectric performance of this benchmark material and potentially
high-performance thermoelectrics more generally.
