Abstract
Conventional volume holographic gratings (VHGs) fabricated in photosensitive emulsions such as gelatin containing silver salts enable the facile visualization of the holographic image in ambient lighting. However, for the fabrication of holographic sensors, which require more defined and chemically-functionalised polymer matrices, laser ablation has been introduced to create the VHGs and thereby broaden their applications, although the replay signal can be challenging to detect in ambient lighting. When traditional photochemical bleaching solutions used to reduce light scattering and modulate refractive index within the VHG are applied to laser ablated volume holographic gratings, these procedures decrease the holographic peak intensity. This is postulated to occur because both light and dark fringes contain a proportion of metal particles, which upon solubilisation are converted immediately to silver iodide, yielding no net refractive index modulation. This research advances a hypothesis that the reduced intensity of holographic replay signals is linked to a gradient of different sized metal particles within the emulsion, which reduces the holographic signal and may explain why traditional bleaching processes result in a reduction in intensity. In this report, a novel experimental protocol is provided, along with simulations based on an effective medium periodic 1D stack, that offers a solution to increase peak signal intensity of holographic sensors by greater than 200%. Nitric acid is used to etch the silver nanoparticles within the polymer matrix and is thought to remove the smaller particles to generate more defined metal fringes containing a soluble metal salt. Once the grating efficiency has been increased, this salt can be converted to a silver halide, to modulate the refractive index and increase the intensity of the holographic signal. This new protocol has been tested in a range of polymer chemistries; those containing functional groups that help to stabilize the metal nanoparticles within the matrix yield more intense holographic signals as the integrity of the fringe is more protected with increasing metal solubility.
Highlights
Volume holographic gratings (VHGs) fabricated in photosensitive emulsions, such as silver halide salts within a gelatin emulsion, produce intense holographic signals that enable images to be detected visually in ambient lighting [1,2]
If laser ablated VHGs are bleached to silver halide salts there will be a subsequent increase in the size of the particles present in all fringes, and little potential for refractive index modulation
When VHGs are fabricated by laser ablation and processed according to the methods outlined, the AgClO4 within the polymer matrix is first converted to AgBr and reduced to Ag0 nanoparticles with a range of sizes
Summary
Volume holographic gratings (VHGs) fabricated in photosensitive emulsions, such as silver halide salts within a gelatin emulsion, produce intense holographic signals that enable images to be detected visually in ambient lighting [1,2]. If laser ablated VHGs are bleached to silver halide salts there will be a subsequent increase in the size of the particles present in all fringes, and little potential for refractive index modulation This can be likened to rehalogenation of an unfixed VHG fabricated using photochemical means, where the exposed areas contain Ag0 nanoparticles and the unexposed regions contain silver bromide (AgBr). If nitric acid is used to dissolve the smaller particles within both fringes post ablation, it should be possible to test this hypothesis Whilst this treatment is likely to result in some loss of Ag0 in both fringes, provided the fill fraction is not substantially reduced in the dark fringes, it should be possible to use a rehalogenation bleach to modulate refractive index and thereby holographic signal intensity.
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