Development of Holographic Sensors for Monitoring Relative Humidity and Temperature

Photonic structures capable of responding to an analyte with an easily identifiable change in their optical properties have generated wide interest due to their possible application as holographic sensors. Holographic sensors are considered a low-cost, lightweight and disposable technology, and have potential for application in different areas ranging from medical diagnostics to environmental sensing including the monitoring of environmental temperature and relative humidity. In spite of the existing wide range of temperature and humidity sensors, holographic sensors are of special interest as they can provide fast, real-time, reversible or irreversible, visual colorimetric or electronic readouts. The main objective of this project was the development of holographic sensors with response to relative humidity and/or temperature. Holographic humidity sensors were fabricated by holographic recording of volume phase transmission gratings in acrylamide /diacetone acrylamide-based photopolymers containing polyvinyl alcohol as a binder. The diffraction efficiency and the Bragg angle were found to be humidity dependent in the relative humidity range 20 - 90 %. It was shown that reversibility of the response, sensitivity and operation range of the sensor can be tuned by varying the photopolymer composition. Best sensitivity up to 3%DE/%RH was observed for diacetone acrylamide-based gratings in the relative humidity range 70 - 90 %. A novel thermosensitive photopolymer containing N-isopropylacrylamide as the main monomer was developed. The diffraction efficiency up to 80 % in transmission mode and 20 % in reflection mode was achieved. It was demonstrated that the temperature switchable swelling/shrinking of the novel N-isopropylacrylamide-based photopolymer can be implemented in the development of holographic temperature sensors, temperature visual indicators and holographic optical elements with temperature controlled direction of the diffracted light and diffraction efficiency. Best sensitivity up to 2%DE/oC and 4.3nm/oC was obtained for sensors based on volume phase transmission and reflection gratings, respectively. For the first time, an optical sensor based on the surface relief structure with reversible response to relative humidity in the range 35 - 97 % was developed. The device comprises the Aztec grating as a substrate and a coating thin polymer film as a sensing medium. It was demonstrated that alteration of the coating layer composition allows tuning the operation range and sensitivity of the device

Investigation of the Sensitivity to Humidity of an Acrylamide-based Photopolymer Containing N-phenylglycine as a Photoinitiator

Sensitivity of holographic recording materials to the relative humidity (RH) of the environment often restricts their use in fabrication of holographic optical elements and other applications. It is important to develop materials with little or no sensitivity to humidity. In this paper the humidity response of transmission gratings recorded in an acrylamide-based photopolymer containing N-phenylglycine (NPG) as a photoinitiator is studied at RH = 20 – 90 %. The hologram is found to be completely insensitive to humidity at RH below 70 % and its diffraction efficiency remains constant. A decrease in diffraction efficiency is observed at RH = 80 – 90 % but this decrease is fully reversible, demonstrating quantitatively the NPG photopolymer’s excellent resistance to humidity

N-isopropylacrylamide-based Photopolymer for Holographic Recording of Thermosensitive Transmission and Reflection Grating

In recent years, functionalized photopolymer systems capable of holographic recording are in great demand due to their potential use in the development of holographic sensors. This work presents a newly developed Nisopropylacrylamide(NIPA)-based photopolymer for holographic recording in reflection and transmission modes. The optimized composition of the material is found to reach refractive index modulation of up to 5 10-3 and 1.6 10-3 after recording in transmission and reflection mode, respectively. In addition to fulfilling the requirements for holographic recording materials, the NIPA-based photopolymer is sensitive to temperature and has lower toxicity than acrylamide-based photopolymers. Possible application of the NIPA-based photopolymer in the development of a holographic temperature sensor is discussed

Humidity and Temperature Induced Changes in the Diffraction Efficiency and the Bragg Angle of Slanted Photopolymer-based Holographic Gratings

This work explores the humidity and temperature response of volume phase slanted gratings recorded in photopolymers with varied chemical composition. Acrylamide and diacetone acrylamide were used as monomers and triethanolamine and N-phenylglycine were used as photoinitiators. The study demonstrates that the response of photopolymer-based holographic gratings to relative humidity (RH) and temperature (T) can be tuned by alteration of the photopolymer composition.Humidity and temperature response of the holograms has been characterized by recording Bragg selectivity curves of transmission gratings and by monitoring the position of the maximum intensity in the spectral response of reflection gratings. Investigation of the humidity response in the range of 20–90% RH reveals that photopolymers containing triethanolamine are more responsive to moisture than photopolymers containing N-phenylglycine and display significant sensitivity to relative humidity above 40%. Full reversibility of humidity induced changes in gratings recorded in diacetone acrylamide-based photopolymer is confirmed at RH≤60%. Exposure to RH≥70% leads to irreversible changes in these gratings.The temperature response of slanted transmission gratings was investigated in the temperature range of 20–60°C. Exposure of the photopolymer layers containing triethanolamine to elevated temperature was found to cause layer shrinkage due to desorption of absorbed water. Sealed layers containing triethanolamine, however, demonstrated swelling due to the effect of thermal expansion. The photopolymer layers containing N-phenylglycine were found to be unresponsive to temperature changes below 30°C and have sensitivity to temperature above 30°C

Color-Changing Reflection Hologram for Quality Assurance of Therapeutic Ultrasound Systems

The acoustic output of clinical therapeutic ultrasound equipment requires regular quality assurance (QA) testing to ensure the safety and efficacy of the treatment and that any potentially harmful deviations from the expected output power density are detected as soon as possible. A hologram, consisting of a reflection grating fabricated in an acrylate photopolymer film, has been developed to produce an immediate, visible, and permanent change in the color of the reconstructed hologram from red to green in response to incident ultrasound energy. The influence of the therapeutic ultrasound insonation parameters (exposure time, ultrasound power density, and proximity to the point of maximum acoustic pressure) on the hologram’s response has been investigated for two types of therapeutic ultrasound systems: a sonoporation system and an ultrasound physiotherapy system. Findings show that, above a switching temperature of 45 °C, the ultrasound-induced temperature rise produces a structural change in the hologram, which manifests as a visible color change. The area of the color change region correlates with the ultrasound exposure conditions. The suitability of the hologram as a simple and quick QA test tool for therapeutic ultrasound systems has been demonstrated. A prototype ultrasound testing unit which facilitates user-friendly, reproducible testing of the holograms in a clinical setting is also reported

INVESTIGATION OF TEMPERATURE RESPONSE OF PHOTOPOLYMER MATERIAL USED FOR HOLOGRAPHIC SENSOR

Recently, functionalised photopolymer has emerged as a versatile recording material in the field of optical holography due to its novel characteristics and potential use in the development of holographic based sensors, and optical elements. This work describes the temperature response of a newly developed photopolymer, containing of a monomer Nisopropylacrylamide (NIPA) which under photopolymerisation forms a temperature sensitive polymer - Poly(Nisopropylacrylamide) (PNIPA). The photonic sensor was developed by holographic recording of volume phase transmission gratings in a self-processing NIPA-based polymer with a 532 nm laser beam. A 633 nm probe beam was used to monitor real time diffraction efficiency (DE) growth curve and the temperature dependent response of DE in the temperature range 22-50 oC. It was observed that the DE increased with increasing temperature at 10 oC min-1 and started to drop by reversing the temperature at 5 oC min-1. It was also observed that the response depends on the rate of heating/cooling and the time spent at elevated temperature. The observed response to temperature could be used to design an indicator for packaging, showing that the content of the package has been exposed to a temperature that was above a predetermined temperature limit

Improving the Holographic Recording Characteristics of a Water-Resistant Photosensitive Sol–Gel for Use in Volume Holographic Optical Elements

Continual improvements to holographic recording materials make the development of volume holographic optical elements increasingly more attainable for applications where highly efficient, lightweight diffractive optical elements can replace conventional optics. A fast-curing, water resistant photosensitive sol–gel capable of volume holographic recording has recently drawn attention for its extreme environmental and physical robustness, in particular its water/moisture and scratch resistance. However, to date, the refractive index modulation has been limited. While water-resistant properties are invaluable in the face of the weathering which many practical systems for outdoor applications will endure, high refractive index modulation is also important in order to facilitate high diffraction efficiency holograms recorded in relatively thin layers. Lower grating thickness ensures a large angular and wavelength range of operation-properties that are critical for many applications of holographic optical elements such as solar light harvesting, optical displays and illumination management. For any application where low-cost mass production is envisaged, sensitivity/writing speed is also a crucial factor. In this research, we studied the recording properties of these water resistant photosensitive sol–gel layers at two different recording wavelengths (532 and 476 nm) and investigated methods for improving these properties. We report more than two-fold improvement of the refractive index modulation from 1.4 10 3 to 3.3 10 3 in layers of thickness ranging from 40–100 m and more than an order of magnitude increase in photosensitivity/recording speed through better matching between recording wavelength and layer absorption, chemical alterations and thermal post-processing techniques

Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass

Photopolymerisable glasses are holographic recording materials which provide good recording capability, improved dimensional stability, and negligible shrinkage. Recently, a novel photopolymerisable hybrid sol-gel (PHSG) for holographic recording of volume gratings has been reported. The PHSG has significantly improved gelation time and high water resistance, both of which make it an attractive material for mass production of holographic optical elements (HOEs) with no sensitivity to ambient humidity. In order to achieve full control over the performance of the material and further improve its properties, a study of grating formation under holographic patterning is essential. This paper reports characterisation of the grating recording in PHSG. The approach is based on the analysis of grating parameters during exposure and post-recording dark processes. The obtained results suggest that photopolymerisation of the methacrylate groups is the main contributor to the creation of refractive index modulation during exposure. During the dark process, the enhancement of the refractive index modulation is observed, probably due to further polycondensation. The observations made facilitate controlled and predictable diffraction efficiency of gratings recorded on the PHSG, thereby furthering the prospect of the development of HOEs with customisable specification

Synthesis of Fast Curing, Water-Resistant and Photopolymerizable Glass for Recording of Holographic Structures by One- and Two-Photon Lithography

Advancements in hybrid sol-gel technology have provided a new class of holographic materials as photopolymerizable glasses. Recently, a number of photosensitive glass compositions with high dynamic range and high spatial resolution have been reported and their excellent capability for volume holography has been demonstrated. Nevertheless, challenges remain, particularly in relation to the processing time and environmental stability of these materials, that strongly affect the performance and durability of the fabricated holograms. State-of-the-art photopolymerizable glasses possess long curing times (few days) required to achieve thick films, thus limiting the industrial implementation of this technology and its commercial viability. This article presents a novel, fast curing, water-resistant, photopolymerizable hybrid sol-gel (PHSG) for holographic applications. Due to introducing an amine-based modifier that increases the condensation ability of the sol-gel network, this PHSG overcomes the problem of long curing time and can readily produce thick (up to a few hundred micrometers) layers without cracking and breaking. In addition, this PHSG exhibits excellent water-resistance, providing stable performance of holographic gratings for up to 400 h of immersion in water. This finding moves photopolymerizable glasses to the next development stage and renders the technology attractive for the mass production of holographic optical elements and their use across a wide number of outdoor applications

Charge Transfer Mediated Triplet Excited State Formation in Donor-Acceptor-Donor BODIPY: Application for Recording of Holographic Structures in Photopolymerizable Glass

Donor–acceptor–donor BODIPY triads bearing anthracene or pyrene as electron donating subunits were prepared through a stepwise synthesis. Photoinduced electron transfer and formation of long-lived triplet excited states via spin–orbit charge transfer intersystem crossing (SOCT-ISC) was studied by steady-state and ultrafast pump-probe spectroscopy and further supported by DFT computations. New BODIPYs were found to form triplet states and sensitize singlet oxygen in both polar and non-polar solvents which is unusual for photosensitizers operating via SOCT-ISC. BODIPY-anthracene triad (ABA) was used as a photosensitizer component in a photopolymerizable glass that was prepared by a four-step sol–gel process. ABA in combination with N-phenylglycin (NPG) showed the ability to initiate a free-radical polymerization of methacrylate monomers under 532 nm irradiation thus allowing for holographic recording of diffractive structures. High diffraction efficiency (up to 87%) obtained for ABA-NPG containing glass as compared to a reference diiodo-BODIPY (I2BDP) demonstrates for the first time that heavy-atom-free SOCT-ISC photosensitizers can efficiently operate in the solid state