Nondestructive resolution of higher harmonics of light-induced volume gratings in PMMA with cold neutrons

Illumination of photosensitized (PMMA) with light triggers nonlinear polymerization processes. this way periodical density structures with nearly 10(4) lines/mm can be generated which are beyond the resolution limit of light-optical investigation methods. We report on the first direct experimental observation of such higher harmonics in deuterated (PMMA) by the nondestructive method of diffraction of neutrons at a wavelength of 1.2 nm. Diffraction efficiency of the second harmonic was found to be 200-500 times weaker than that of the first order. The origin of higher harmonics is attributed to the nonlinearities of the processes of chain growth and of termination.