Surface Holograms for Sensing Application

Abstract. Surface gratings with periodicity of 2 μm and amplitude in the range of 175 and 240 nm were fabricated in a plasticized polyvinylchloride doped with a metalloporphyrin (ZnTPP), via a single laser pulse holographic ablation process. The effect of the laser pulse energy on the profiles of the fabricated surface structure was investigated. The sensing capabilities of the fabricated diffractive structures towards amines (triethylamine, diethylamine) and pyridine vapours were then explored; the holographic structures were exposed to the analyte vapours and changes in the intensity of the diffracted light were monitored in real time at 473 nm. It was demonstrated that surface structures, fabricated in a polymer doped with a metalloporphyrin which acts as analyte receptor, have a potential in sensing applicatio

Theoretical Modelling and Design of Photonic Structures in Zeolite Nanocomposites for Gas Sensing: Part I - Surface Relief Gratings

The suitability of holographic structures fabricated in zeolite nanoparticle-polymer composite materials for gas sensing applications has been investigated. Theoretical modelling of the sensor response (i.e. change in hologram readout due to a change in refractive index modulation or thickness as a result of gas adsorption) of different sensor designs was carried out using Raman-Nath theory and Kogelnik’s Couple Wave Theory. The influence of a range of parameters on the sensor response of holographically-recorded surface and volume photonic grating structures has been studied, namely phase difference between the diffracted and probe beam introduced by the grating, grating geometry, thickness, spatial frequency, reconstruction wavelength and zeolite nanoparticle refractive index. From this, the optimum fabrication conditions for both surface and volume holographic gas sensor designs have been identified. Here in part 1, results from theoretical modelling of the influence of design on the sensor response of holographically-inscribed surface relief structures for gas sensing applications is reported

Theoretical modeling and design of photonic structures in zeolite nanocomposites for gas sensing. Part II: Volume gratings

The suitability of holographic structures fabricated in zeolite nanoparticle-polymer composite materials for gas sensing applications has been investigated. Theoretical modelling of the sensor response (i.e. change in hologram readout due to a change in refractive index modulation or thickness as a result of gas adsorption) of different sensor designs was carried out using Raman-Nath theory and Kogelnik’s Coupled Wave Theory. The influence of a range of parameters on the sensitivity of holographically-recorded surface and volume photonic structures has been studied, namely hologram geometry, hologram thickness and spatial frequency, reconstruction wavelength, and zeolite nanoparticle refractive index. From this, the optimum fabrication conditions for both surface and volume holographic gas sensor designs have been identified. Here in part 2, results from modelling of the influence of design on the sensor response of holographically-recorded volume grating structures for gas sensing applications are reported

Using Acrylamide Based Photopolymers for Fabrication of Holographic Optical Elements in Solar Energy Applications

A holographic device is under development that aims to improve light collection in solar cells. The aim is to explore the potential of using photopolymer Holographic Optical Elements (HOE) to collect light from a moving source, such as the sun, and re-direct it for concentration by a holographic lens.. A working range of 45 degrees is targeted for such a device to be useful in solar applications without tracking. A photopolymer HOE is capable of efficiently re-directing light, but the angular selectivity of a single grating is usually of the order of one degree at the thicknesses required for high efficiency. The challenge here is to increase the angular and wavelength range of the gratings so that a reasonable number may be multiplexed and/or combined to provide a device that can concentrate light incident from a large range of angles. In this paper low spatial frequency holographic recording is explored in order to increase the angular and wavelength range of an individual grating. Ultimately, a combination of gratings will be used so that a broad range of angles of incidence are accepted. A design is proposed for the combination of such elements into a holographic solar collector. The first step in achieving this is optimization of recording at low spatial frequency. This requires a photopolymer material with unique properties, such as a fast monomer diffusion rate. This paper reports results on the efficiency of holograms recorded in an acrylamide based photopolymer at low spatial frequencies (100, 200 and 300 l/mm). The diffraction efficiency and angular selectivity of recorded holograms have been studied for various photopolymer layer thicknesses and different intensities of the recording beams. A diffraction efficiency of over 80% was achieved at a spatial frequency of 200 l/mm. The optimum intensity of recording at this spatial frequency was found to be 1 mW/cm2. Individual gratings and focusing elements with high efficiency and FWHM angles of 3o are experimentally demonstrated

Study of the Effect of Magnetic Nanoparticles on the Hologram Recording Capability of Photopolymer Nanocomposite for Development of Holographic Sensor/actuator

Photopolymer nanocomposite materials utilising nanoparticles of varied refractive index are one of the most attractive materials for holography due to their tuneable properties. The preparation of the photopolymer nanocomposites can involve mixing a stable colloidal suspension in the photopolymer solution or printing of the nanoparticle colloidal suspension on top of a dried photopolymer layer. By following the first approach in this study a novel photopolymer nanocomposite material is prepared for holographic recording. It consists of N-isopropylacrylamide (NIPA)-based photopolymer as a host and iron oxide magnetic nanoparticles as nanodopant. The methodology of the photopolymer nanocomposite material preparation is explained in detail. In order to investigate the interaction of nanodopants with the host photopolymer, the photo-bleaching of the layers when exposed to a single laser beam has been studied. Initial characterisation of the holographic recording capability of the nanocomposite has been carried out. The maximum diffraction efficiency (DE) achieved in 70 μm thick layer after recording with 3.4 mW/cm2 intensity at 532 nm wavelength is 18 %

Development of Photopolymer Material with Improved Dynamic Range and Sensitivity

In this study the effect of the concentration of acrylamide and the influence of different initiators in a photopolymer composition for holographic recording of diffraction gratings is investigated. Light manipulating Holographic Optical Elements (HOEs) have a number of characteristics which can be optimised for different roles. However, at the core of these devices is the refractive index modulation that has been created in the material during recording. Typically higher refractive index modulation will enable greater diffraction efficiency. Solar concentrating HOEs can particularly benefit from material that experiences higher refractive index modulation. For a solar concentrator to have a high acceptance angle thinner photopolymer layers are preferable. Thinner layers can lead to a reduction of the device’s diffraction efficiency unless the refractive index modulation increases to compensate. This paper presents an optimisation of a photopolymer formulation for improved refractive index modulation and sensitivity of the layer. An increase in the acrylamide concentration of 66% resulted in 50% higher refractive index modulation with values reaching 5 × 10−3 in 40 micron layers. Faster recording times are an important consideration for the commercialization and mass production of photopolymer devices [1]. Higher production rates and lower costs are some of the main advantages. Altering the initiator is expected to have an effect on the material’s sensitivity and thus on recording time. Several initiators were compared, triethanolamine (TEA), methyldiethanolamine (MDEA) and dimethylethanolamine (DMEA). It was found that holograms recorded with MDEA as the initiator recorded 58% faster over TEA based photopolymer at larger layer thicknesses. The stability of the photopolymer was also tested with different protective coatings when subjected to UV light. The properties exhibited by this photopolymer composition make it a promising candidate for development of solar concentrating applications; however improvements to the durability in conditions of UV radiation will have to be made before it is suitable for solar concentration

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

Comment on C.E. Close, M.R. Gleeson and J.T. Sheridan Monomer Diffusion Rates in Photopolymer Material Part 1. Low Spatial Frequency Holographic Gratings

COMMENT: on C. E. Close, M. R. Gleeson and J. T. Sheridan “Monomer diffusion rates in photopolymer material.Part I. Low spatial frequency holographic gratings” J. Opt. Soc. Am. B / Vol. 28, No. 4 / April 2011 pp 658- 666 [1

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

Implementation of Phase-only Modulation Utilizing a Twisted Nematic Liquid Crystal Spatial Light Modulator

Twisted nematic liquid crystal spatial light modulators, (SLM) are widely used for amplitude modulation of light. The technique for measuring phase and amplitude modulation introduced by a twisted nematic cell is not new but the novelty here is that after characterisation of a Holoeye LC2002 TNLC device we were able to identify specific conditions that allow for its use as a pure 180 degree or more, phase modulator with little or no amplitude modulation at 514.5 nm laser wavelength. The phase shift introduced by the SLM was measured using a Mach-Zehnder interferometer. The experimentally measured and the theoretically predicted intensity transmission and phase shift as functions of gray levels, are compared. Finally, the effects of errors in the angular settings of the polarizers and quarter waveplates on the intensity transmission and phase shift are studied