  We study the linear and nonlinear optical properties of wurtzite GaN under
c-axial stress field, using density functional theory calculations. The fully
structural optimization at each c-axial strain was performed. The calculated
dielectric functions show that tensile c-axial strain effectively improves the
linear optical efficiency, especially for the band-edge transitions, and
significantly increase the mobility of electrons in the conduction band.
Second-order nonlinear optical susceptibilities show that the tensile c-axial
strain will enhance the zero- and low-frequency nonlinear responses of GaN. The
enhancement of the nonlinear optical property is explained by the reduction of
the polarization of wurtzite GaN under tensile c-axial strains. Based on these
findings, we propose a method for improving the electrical and optical
properties of the crystal through imposing appropriate stress on the high
symmetry crystalline directions.
