  Photoactive nematic elastomers are soft rubbery solids that undergo
deformation when illuminated. They are made by incorporating photoactive
molecules like azobenzene into nematic liquid crystal elastomers. Since its
initial demonstration in 2001, it has received increasing interest with many
recent studies of periodic and buckling behavior. However, theoretical models
developed have focused on describing specific deformation modes (e.g., beam
bending and uniaxial contraction) in the absence of mechanical loads, with only
limited attention to the interplay between mechanical stress and light-induced
deformation. This paper explores photomechanical coupling in a photoactive
nematic elastomer under both light illumination and mechanical stress. We begin
with a continuum framework built on the free energy developed by Corbett and
Warner (Phys. Rev. Lett. 2006). Mechanical stress leads to nematic alignment
parallel to a uniaxial tensile stress. In the absence of mechanical stress, in
the photo-stationary state where the system reaches equilibrium, the nematic
director tends to align perpendicular to the polarization of a linearly
polarized light. However, sufficient illumination can destroy nematic order
through a first-order nematic-isotropic phase transition which is accompanied
by a snap through deformation. Combined illumination and mechanical stress can
lead to an exchange of stability accompanied by stripe domains. Finally, the
stress-intensity phase diagram shows a critical point that may be of interest
for energy conversion.
