In an article published in the open-access journal Photonics, researchers studied the holography recording characteristics of water-resistant, fast-curing photosensitive sol-gel layers at two distinct recording wavelengths of 532 nm and 476 nm. New techniques for enhancing the holography recording features were also investigated.
The constant enhancement in holography recording materials has made the growth of volume holographic optical elements (HOEs) increasingly feasible. Lightweight diffractive holographic optical elements can now replace conventional optical materials. The exceptional physical and environmental robustness, particularly the resilience to water and moisture and scratch resistance of a photosensitive sol-gel capable of volumetric holography, has recently attracted attention.
While water resistance is critical in weathering that several practical systems for outdoor implementation can experience, high refractive index modulation (RIM) is also crucial to enable high diffraction holography efficiently recorded in relatively thin layers. For many applications involving holographic optical elements, including solar light harvesting, optical materials display, and illumination management, a lower grating thickness provides a wide angular and wavelength range of operation.
This study achieved two-fold development in the RIM, which increased from 1.4 ´ 10-3 to 3.3 ´ 10-3 in layers 40 to 100 mm thick. The recorded increase in RIM was achieved through improved matching between layer absorption and recording wavelength, chemical variations, and thermal post-processing methods. A more than an order of magnitude boost in photosensitivity/recording speed was also reported.
Widening the Scope of Holographic Optical Elements
Holographic optical elements have recently undergone significant advancements due to developments in photopolymer holographic optical materials. As a result of their ability to be patterned through the volume of the optical materials, be laminated into a stack, and accomplish a single output beam via 100% re-direction of the incident light, volume photopolymer holographic optical elements can significantly improve surface elements.
Customization and small batch production can be made possible by the optical patterning in holographic optical elements, potentially replacing the etching and stamping procedure used in stamped diffractive optical materials. These optical materials can also achieve incredibly high diffraction and fringe-slant angles. Solar collecting, displays, and light management are the main applications for holographic optical elements.
Holography recording on thinner layers is desirable for applications requiring low angular selectivity. A good refractive index modulation is necessary to enable thinner layers and ensure that high levels of diffraction efficiency are still possible even when the path length via the grating is reduced. Therefore, optical materials will significantly enhance the functionality of any enhancements to the refractive index modulation that may be made.
Many holography recording optical materials currently in use are susceptible to environmental factors such as humidity, which is a major technical obstacle. However, with the safety of a thin plastic cover layer, this susceptibility can be advantageous for producing low-cost holography sensors.
Holographic optical elements constructed from these materials can perform well in various conventional applications. A resistant plastic can be used to seal some sensitive materials, reducing their sensitivity and preventing UV light from penetrating the hologram.
However, the variables present in the intended environment should not impact the ideal holographic optical elements. Environmental sensitivity harms solar concentration, outdoor lighting use, and outdoor displays. The requirement for resistant cover layers enhances the complexity of the manufacturing method and the cost per device of holographic optical elements.
A novel photo-polymerizable hybrid sol-gel (PHSG) that enabled the holographic recording of both reflection volume gratings and transmission was proposed. Subsequent investigations of the holographic recording processes at a wavelength of 532 nm and UV curing led to the synthesis of gratings that were approximately 90% effective in 118 mm thick layers.
This study further presented improvements to the recording sensitivity and refractive index modulation of the photosensitive sol-gel, which showed significant environmental and physical robustness and fast curing times.
Experimental Setup and Synthesis
The water-resistant, photo-polymerizable, organic-inorganic hybrid sol-gel used in this study belonged to Class II hybrid optical materials. Covalent bonding between the organic and inorganic species in the sol-gel optical materials increased their dimensional stability and durability.
The UV-Vis spectra of the layer were measured to determine the photosensitive sol-gel layer's absorption at various laser recording wavelengths. At 476 nm, the sol-gel absorption was 0.435, around 18 times greater than the absorption at 532 nm, which was 0.023.
The material absorbed significantly more effectively at 476 nm, as was evident from the difference in exposure energy needed to achieve 50% diffraction efficiency. Under these circumstances, an increase in sensitivity of about seven times was observed.
Different techniques were used to raise the average RIM value. The sol-gel sample was exposed to the recording beams until the maximum diffraction efficiency was obtained to achieve holography. Low zirconium concentrations resulted in the creation of optical materials with lower densities.
Thermally treating sol-gel samples during the dark process at 90 °C for 30 minutes produced the highest RIM samples, an improvement of a factor of three compared to unbaked samples. The heating effect on the RIM was marginally reduced when samples with three times the concentration of zirconium were heated in the dark process using this technique.
Significance of the Study
Several distinct techniques were investigated in this study using the photo-polymerizable hybrid sol-gel to enhance its holography recording properties and make it appropriate for constructing holographic optical elements for outdoor applications. By performing the measurements at a wavelength more closely aligned with the material absorption spectra, the researchers achieved a sensitivity increased by an order of magnitude.
Post-recording thermal treatment increased the RIM by a factor of 2.4. Although heat treatment involved an additional production step, it provided a shorter light exposure and a substantially higher final RIM.
The average RIM value, 1.4 ´ 10-3 ± 1 ´ 10-4 using conventional holographic exposure techniques, was increased to 3.3 ´ 10-3 ± 4 ´ 10-4 using the heat treatment approach alone. A combination of the various elements of the holographic exposure and other conventional methods, as well as a more thorough investigation of the recording dynamics and heating circumstances, might help reach the desired RIM of 4 ´ 10-2.
Rogers, B., Mikulchyk, T., Oubaha, M., Cody, D., Martin, S., Naydenova, I. (2022). Improving the Holographic Recording Characteristics of a Water-Resistant Photosensitive Sol–Gel for Use in Volume Holographic Optical Elements. Photonics, 9(9), 636. https://www.mdpi.com/2304-6732/9/9/636