Recording holographic gratings in commercial photochromic sunglasses

We present a cost-effective experiment designed for science and engineering students to record and read holographic gratings using a reversible material. The recording medium is a photochromic-polymer-based lens of commercially available self-tinting sunglasses, which undergoes reversible changes in its absorption coefficient upon exposure to ultraviolet and short wavelength visible light. Illumination of this medium with the interference pattern generated by two coherent blue laser beams produces a spatially periodic intensity distribution that modulates the material's optical absorption, resulting in the formation of an amplitude holographic grating. The recording mechanism is explained through a comprehensive model that accounts for a transformation between two photochromic molecular states with distinct optical absorption characteristics. This model accurately describes the dynamics of the observed diffraction efficiency across multiple diffraction orders effectively bridging the macroscopic measurements with the microscopic molecular behavior. Both the experimental setup and the holographic medium are low cost and easy to implement, making this system an ideal educational tool for developing fundamental skills in diffraction gratings and holography.