| Summary: | Bayfol<sup>®</sup> 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 <inline-formula><math display="inline"><semantics><mrow><mn>125</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>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<sup>®</sup> HX200 material does not suffer from any permanent thermal degradation below <inline-formula><math display="inline"><semantics><mrow><mn>160</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>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 <inline-formula><math display="inline"><semantics><mrow><mn>24</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C to <inline-formula><math display="inline"><semantics><mrow><mn>100</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C. Linear interpolation give a slope <inline-formula><math display="inline"><semantics><mrow><mn>4.5</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup><mspace width="3.33333pt"></mspace><msup><mi>K</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></semantics></math></inline-formula> for unexposed film, with the extrapolated refractive index at <inline-formula><math display="inline"><semantics><mrow><mn>0</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C equal to <inline-formula><math display="inline"><semantics><mrow><msub><mi>n</mi><mn>0</mn></msub><mo>=</mo><mn>1.51</mn></mrow></semantics></math></inline-formula>. This refractive index change decreases to <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup><mspace width="3.33333pt"></mspace><msup><mi>K</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></semantics></math></inline-formula> when the material is fully cured with UV light, with a <inline-formula><math display="inline"><semantics><mrow><mn>0</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C refractive index equal to <inline-formula><math display="inline"><semantics><mrow><msub><mi>n</mi><mn>0</mn></msub><mo>=</mo><mn>1.495</mn></mrow></semantics></math></inline-formula>. Spectral properties of a reflection hologram recorded at 532 nm was measured from <inline-formula><math display="inline"><semantics><mrow><mn>23</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C to <inline-formula><math display="inline"><semantics><mrow><mn>171</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C. A consistent 10 nm spectral shift increase was observed for the diffraction peak wavelength when the temperature reaches <inline-formula><math display="inline"><semantics><mrow><mn>171</mn><msup><mspace width="3.33333pt"></mspace><mo>°</mo></msup></mrow></semantics></math></inline-formula>C. From these spectral measurements, we calculated a coefficient of thermal expansion (CTE) of <inline-formula><math display="inline"><semantics><mrow><mn>384</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup><mspace width="3.33333pt"></mspace><msup><mi>K</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></semantics></math></inline-formula> by using the coupled wave theory in order to determine the increase of the Bragg plane spacing with temperature.
|
|---|