Abstract
Bayfol® HX200 photopolymer is a holographic recording material used in a variety of applications such as a holographic combiner for a heads-up display and augmented reality, dispersive grating for spectrometers, and notch filters for Raman spectroscopy. For these systems, the thermal properties of the holographic material are extremely important to consider since temperature can affect the diffraction efficiency of the hologram as well as its spectral bandwidth and diffraction angle. These thermal variations are a consequence of the distance and geometry change of the diffraction Bragg planes recorded inside the material. Because temperatures can vary by a large margin in industrial applications (e.g., automotive industry standards require withstanding temperature up to C), it is also essential to know at which temperature the material starts to be affected by permanent damage if the temperature is raised too high. Using thermogravimetric analysis, as well as spectral measurement on samples with and without hologram, we measured that the Bayfol® HX200 material does not suffer from any permanent thermal degradation below C. From that point, a further increase in temperature induces a decrease in transmission throughout the entire visible region of the spectrum, leading to a reduced transmission for an original 82% down to 27% (including Fresnel reflection). We measured the refractive index change over the temperature range from C to C. Linear interpolation give a slope for unexposed film, with the extrapolated refractive index at C equal to . This refractive index change decreases to when the material is fully cured with UV light, with a C refractive index equal to . Spectral properties of a reflection hologram recorded at 532 nm was measured from C to C. A consistent 10 nm spectral shift increase was observed for the diffraction peak wavelength when the temperature reaches C. From these spectral measurements, we calculated a coefficient of thermal expansion (CTE) of by using the coupled wave theory in order to determine the increase of the Bragg plane spacing with temperature.
Highlights
Over the past few decades, most imaging and nonimaging systems have been designed and built using conventional bulky glass- and metal-based optical elements
Thermogravimetric measurements show that the Bayfol® HX200 material does not suffer from any thermal degradation before 160 ◦ C
Holograms recorded in this material experience only reversible change, expressed as thermal dilatation between the Bragg planes
Summary
Over the past few decades, most imaging and nonimaging systems have been designed and built using conventional bulky glass- and metal-based optical elements. Those optical elements have been proved to perform well under many conditions, but they are heavy, expensive, and require long lead time to manufacture. Optical designers started to shift their focus on the design of thin, lightweight, and easy-to-manufacture optical elements using the method of holographic recording [1,2,3]. Holographic optical elements can be recorded on several different photosensitive materials such as dichromated gelatin, silver halide, photoresist, and photopolymer [4]. Holographic optical elements are cheap, easy to manufacture, Materials 2020, 13, 5498; doi:10.3390/ma13235498 www.mdpi.com/journal/materials
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