Transition of refractive index contrast in course of grating growth

Abstract

Studies on the dynamics of holographic pattern formation in photosensitive polymers, gaining deeper insight into the specific material transformations, are essential for improvements in holographic recording as well as in integrated optics. Here we investigate the kinetics of volume hologram formation in an organic cationic ring-opening polymerization system. The time evolution of the grating strength and the grating phase is presented. We found two steps of growth, separated by a depletion of the light diffraction. Capable to explore this growing behavior, a transition-theory of the refractive index contrast is established. Accordingly the growth curves appear to be ruled by the interplay of polymerization and diffusion. Hence the grating formation mechanisms can be qualified as competing effects regarding the contribution to the refractive index change. We investigate the influence of the preparation and exposure procedure on the transition and consider the usability for integrated wave guide functions.

Dynamic Bragg Angle Shift in the Course of Volume Hologram Formation

“…The act of volume hologram formation in photopolymers is of high interest for practical applications [1][2][3][4][5] and under extensive studies both in theoretical and practical terms. [6,7] A research challenge of considerable importance concerns the optical shrinkage in the course of hologram formation.…”

Volume Holography: Novel Materials, Methods and Applications

“…In this case, the growth curve reveals a transition of the refractive index contrast [29]. To ensure non-disturbing observation of the grating formation process, the wavelength for in-situ observation must be chosen outside of the absorption spectrum of the photosensitizer dye.…”

“…A position sensitive device (PSD) was implemented to detect the diffracted light. The PSD provides time-resolved information on The grating growth curve shown in Figure 6 belongs to a one-dimensional, plane-wave volume hologram of transmission type, recorded in epoxy-based polymer [29]. The characteristic two-step growth can be attributed to a transition of the refractive index contrast from positive to negative values, as a result of competing effects, taking place on overlapping time scales.…”

“…Probing only a fraction of the exposed area is primarily for the purpose of measuring precision [29,40]. However, it also enables scanning of the grating by moving the sample perpendicular to the optical axis.…”

“…Photosensitive polymers are highly interesting materials for applications in integrated optics [6,29]. This particularly applies with respect to the multi-functional applicability as well as the ability to optimize and miniaturize respective components.…”