  Understanding the opto-electronic properties of semiconducting polymers under
external strain is essential for their applications in flexible
opto-electronic, light-emitting and photovoltaic devices. While prior studies
have highlighted the impact of static strains applied on a macroscopic length
scale, assessing the effect of a local transient deformation before structural
relaxation occurs is challenging due to the required high spatio-temporal
resolution.
  Here, we employ scanning ultrafast electron microscopy (SUEM) to image the
dynamical effect of a photo-induced transient strain in the archetypal
semiconducting polymer poly(3-hexylthiophene) (P3HT). We observe that the
photo-induced SUEM contrast, corresponding to the local change of secondary
electron emission, exhibits a ring-shaped spatial profile with a rise time of
$\sim 300$ ps, beyond which the profile persists in the absence of a spatial
diffusion. We attribute the observation to the electronic structure modulation
of P3HT caused by a photo-induced strain field owing to its relatively low
modulus and strong electron-lattice coupling, as supported by a finite-element
analysis. Our work provides insights into tailoring opto-electronic properties
using transient mechanical deformation in semiconducting polymers, and
demonstrates the versatility of SUEM to study photo-physical processes in
diverse materials.
