Thermal properties of bayfol® hx200 photopolymer

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 125^◦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 160^◦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 24^◦C to 100^◦C. Linear interpolation give a slope 4.5 × 10⁻⁴ K⁻¹ for unexposed film, with the extrapolated refractive index at 0^◦C equal to n₀ = 1.51. This refractive index change decreases to 3 × 10⁻⁴ K⁻¹ when the material is fully cured with UV light, with a 0^◦C refractive index equal to n₀ = 1.495. Spectral properties of a reflection hologram recorded at 532 nm was measured from 23^◦C to 171^◦C. A consistent 10 nm spectral shift increase was observed for the diffraction peak wavelength when the temperature reaches 171^◦C. From these spectral measurements, we calculated a coefficient of thermal expansion (CTE) of 384 × 10⁻⁶ K⁻¹ by using the coupled wave theory in order to determine the increase of the Bragg plane spacing with temperature.