Diffraction efficiency increase by corona discharge in photoinduced surface-relief gratings on an azo polymer film

The diffraction efficiency of a surface-relief grating (SRG) was dramatically increased by corona discharge. We fabricated this SRG by applying surface modulation to azo polymer films, using laser interference fringes. The electric charge was deposited upon the SRG by a corona discharge in an oven. The first-order diffraction efficiency measured before and after corona discharge above the glass-transition temperature increased from $\sim 2\%$ to $\sim 40\%$ in a SRG written with a circularly polarized beam. The relief depth of the SRG as measured with an atomic-force microscope also increased, from $\sim 130 \mathrm{nm}$ to $\sim 450 \mathrm{nm}$. The increase in diffraction efficiency was independent of the writing polarization.