Temperature‐Sensitive Holograms with Switchable Memory

Abstract

Herein, the dynamics and reversibility of temperature‐induced changes in volume phase holographic gratings (VPHGs) recorded in the low‐toxicity self‐processing photopolymer poly(N‐isopropyl acrylamide) (poly‐NIPA) are reported on. Transmission and reflection gratings are exposed to temperatures ranging 8–60 °C to study the effect of temperature on their properties. First, the role of the photopolymer chemical composition is investigated by comparing layers containing NIPA with those containing acrylamide (AA) and measuring the normalized diffraction efficiency (DE) at 60 °C; higher thermal sensitivity is observed for poly‐NIPA gratings (24% drop) versus low sensitivity for AA gratings (2% increase). The impact of the physical properties of the sensing layers (e.g., thickness/protective top layer) on their thermal response is studied. The reversibility of thermally induced changes in VPHGs is studied, especially the effect of exposure to the elevated temperature of 60 °C on the subsequent low‐temperature response. The results reveal the significance of water desorption/absorption for the observed changes in poly‐NIPA upon exposure to varied temperatures and indicate a thermally controlled memory effect. This effect is observed from DE values in transmission and from the spectral characteristics of the reflection gratings. A full reversibility of the DE and spectral characteristics is achieved at 8 °C.