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. In this way periodical density structures with nearly 10⁴ 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.