Smart holography
In a “smart” hologram, the interference pattern is stored in a material the properties of which change in response to certain environmental
conditions, thereby altering the properties of the interference pattern and thus the appearance of the hologram itself. An electron microscope
image (left) shows a hologram of a plane mirror created in a silver-halide emulsion, where the interference fringes are made up of metallic silver
particles (about 20 nm in diameter) distributed within the 5–10 μm thickness of the smart polymer. The fringes are approximately parallel to the
hologram surface, much like the pages of a closed book, and together act as a Bragg diffraction grating, which under white-light illumination
reflects a specific narrow band of wavelengths and recreates a monochromatic image of the original plane mirror (or any other object used during
hologram recording). Constructive interference between partial reflections from each fringe plane gives a characteristic spectral peak with a
wavelength approximately governed by Bragg’s law: mλ = 2ndsinθ, where m is the diffraction order, λ is the wavelength of light, n is the average
refractive index of the thin-film system, d is the spacing between the fringes and θ is the glancing angle between the propagation direction of the
incident light and the diffracting planes. Any physical, chemical or biological stimulus that changes d, n or the total number of the fringes contained
within the film thickness will generate observable changes in the wavelength (colour) or intensity (brightness) of the reflection hologram.