Holographic recording materials can be utilised in the fabrication of microstructures for use as sensors [1,2]. In order to achieve this the holographic recording material/structure has to be functionalized by incorporation of a chemical component providing sensitivity to a specific analyte/stimuli. We introduce the different approaches to photosensitive material/photonic structure functionalization and present experimental results of fabrication of two examples of sensing devices: for selective detection of sodium and potassium ions and temperature indicator with controlled reversibility.
The first example reported here compares the performance of a surface relief grating (SRG) fabricated by holographic lithography in an acrylamide photopolymer and a volume holographic grating (VHG) recorded in a water resistant novel cellulose based photopolymer. The surface relief gratings were modified by incorporation of either dibenzo-18-crown-6 (DC) or tetraethyl 4-tert-butylcalix[4]arene (TBC) as chelating agents [3]. DC coated layers show a selective response to K+ over Na+, whereas for TBC there is a dominant response for Na+ over K+. The sensors respond to Potassium and Sodium metal ions within the physiological ranges. Normal levels of Na+ in human serum lie in the range 135-148 mmol/L and the normal K+ level is 3.5-5.0 mmol/L. The response of the SRG sensor to Sodium is compared to the response of a VHG recorded in a cellulose based novel material functionalized by incorporation of TBC. The advantages and challenges of each of these two approaches to material/structure functionalization are analysed.
The second example is a volume phase hologram, recorded in a low-toxicity thermally sensitive photopolymer Poly(N-isopropylacrylamide). Both transmission and reflection holograms were studied by exposing them to temperature ranging within 8-60 oC. It was observed that the reversibility of the hologram response to temperatures above the polymer Lower Critical Solution Temperature (32 oC) can be controlled, thus these devices can be used as elevated temperature indicators.
References
[1] A. K. Yetisen, et al, Chem. Rev. 114, 10654–10696 (2014).
[2] D. Cody, et al, Appl. Opt, 57, No. 22, pp. E173-E183 (2018).
[3] Sabad-E Gul, et al, Sensors, 19 No 5, p 1026 (2019).
Recently, a new class of holographic recording materials based on photopolymerisable glass has been developed using the sol-gel technology. Photopolymerisable sol-gel materials have demonstrated excellent capability for volume holography due to their good optical quality and high dynamic range. Other advantages include high thermal and chemical stability, good dimensional stability and robustness because of their rigid matrix. However, the preparation period of non-tacky, dry layers suitable for holographic recording depends on both the sol-gel composition and the sample thickness, and varies from 5 to 21 days. Such a long gelation time is a bottleneck for mass production and this problem needs to be addressed to move the photopolymerisable glass to the next development stage allowing creation of real-world products.
This work presents a new photopolymerisable sol-gel glass capable of producing fully dried films using considerably reduced processing time (45 min at 100 °C).The new holographic sol-gel fulfils the required standards of high dynamic range (up to 0.003), high spatial resolution (currently achieved 5600 lines/mm), low scattering, good mechanical and dimensional stability that are necessary for volume holography. Moreover, for the first time the diffraction efficiency of transmission gratings and the spectral response of reflection gratings recorded in holographic sol-gel material under exposure to water have been characterised and their stability have been observed. This provides further perspectives for utilisation of the novel photopolymerisable glass for applications where stability of the photonic structure and its non-sensitivity to ambient conditions change are crucial.
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