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 ${10}^4$ 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.