Self-processing Photopolymer Research Articles

Self-processing photopolymer materials for versatile design and fabrication of holographic sensors and interactive holograms

The aim of this paper is to discuss the benefits as well as the limitations of utilizing photopolymer materials in the design of holograms that are responsive to changes in their environment, such as changes in the concentration of a specific substance, temperature, and pressure. Three different case studies are presented, including both surface and volume phase holograms, in order to demonstrate the flexibility in the approach of utilizing holographic photopolymers for the design of sensors and interactive optical devices. First, a functionalized surface relief hologram is demonstrated to operate as an optical sensor for the detection of metal ions in water. The sensitivity and selectivity of the sensor are investigated. The second example demonstrates a volume transmission hologram recorded in a temperature-sensitive photopolymer and the memory effects of its exposure to elevated temperature. Finally, a pressure-sensitive reflection hologram that changes color under application of pressure is characterized, and its potential application in document authentication is described.

Serialized holography for brand protection and authentication.

The problems presented by counterfeit products and documentation are discussed. Limitations of existing holograms for anti-counterfeit applications are described. We describe the advantages of full holographic serialization and the requirements in terms of materials and techniques for mass production of true serialized holograms. These requirements having been met, we report for the first time the mass production of fully serialized holograms. The novelty of the approach consists of the direct use of the product manufacturer's information as the object in a holographic recording system along with a self-processing photopolymer and modular optical system to facilitate mass production of truly serialized volume holograms. Various types of serialized holograms for overt and covert authentication are described. We discuss briefly the application of Optrace's manufacturing methods for future generation holographic devices.

LTL type nanozeolites utilized in surface photonics structures for environmental sensors

Environmental monitoring has been continuously increasing due to an emerging global emphasis on the importance of sustainable development. Improving water quality monitoring and control is a challenge that calls for innovative solutions. Current methods of water monitoring are costly and analytical techniques suitable for field use are limited. There is a need for accurate, long–term monitoring of environmental contaminants using sensors that can be operated on site. The aim of our research is to theoretically model, fabricate and characterise holographic sensors for water quality monitoring that are simple to operate, capable of sensing copper present in fresh water and have relatively low cost. The sensors are created by holographic recording of surface relief gratings (SRG) in a self-processing photopolymer material. Interrogation of these structures by light allows indirect measurements of ion concentration in real time. The SRG structures are modified by coating with porous LTL-nanoparticles (nanosized zeolites) which selectively adsorb copper ions. The suitability of the sensors for detection of copper (II) present in water at concentration levels 1–4 mM is reported. The current detection limit of the sensor is 63 ppm.

In-plane sensitive electronic speckle pattern interferometer using a diffractive holographic optical element

We describe a student project in electronic speckle pattern interferometry. The project includes holographic recording of diffraction gratings in thick self-processing photopolymer layers made from off-the-shelf chemicals. The gratings are employed in a simple electronic speckle pattern interferometer to measure in-plane rotation.

Characterisation of the humidity and temperature responses of a reflection hologram recorded in acrylamide-based photopolymer

The humidity and temperature responses of a reflection hologram recorded in a self-processing photopolymer layer have been characterised by measurement of the position of the maximum intensity in the spectral response of the hologram. It has been demonstrated that such a hologram can be used for indication of the relative humidity (RH) in a specific temperature range. The operational range and reversibility of the holograms have been characterised in a controlled humidity and temperature environment. The humidity studies were carried out in the range from 10 to 80% RH and the temperature range under study was 15–50°C.

Thermal embossing of mid-infrared diffractive optical elements by use of a self-processing photopolymer master

Surface-relief transmissive diffractive elements were fabricated by embossing. The master was made by lithography with a self-developing photopolymer. The highly cross-linked structure exhibited by the polymer has made possible the direct replication by thermal embossing of polyethylene substrates. Fabricated elements are meant to work with mid-infrared radiation. The influence of some process variables, related to the performance of the diffractive elements, is analyzed.

Fabrication of microlenses by direct photo-induced crosslinking polymerization

Growing interest in the manufacture of microlenses results from their standard use as optical components with current applications in the telecommunication industry. A number of techniques were developed by using various materials and processes. One approach involved the ability of self-processing photopolymers to generate microlens arrays. Spatially controlled illumination of a photosensitive layer induced an inhomogeneous photopolymerization involving formation of 3-D polymer network, mass-transport process of reactive species and bending of the surface resulting from a gradient of surface free energy. The imaging process that generated as a relief in the photopolymer layer, exhibited a completely self-processing character without any chemical post-treatment. The lens arrays displayed diameters ranging from less than 100μm to 1mm and focal lengths from 100μm to a few millimeters, depending on photonic, optical and physico-chemical parameters.

Improvement of holographic recording material using aerosol sealant

Self-processing photopolymer based materials offer significant practical advantages when recording holographic optical elements, HOEs. However, such materials, e.g. acrylamide based photopolymer, are seriously effected by ambient environmental conditions, in particular by fluctuations in humidity. Such sensitivity can result in a short active material shelf-life prior to recording, material variations during the recording and self-processing stages and HOE deterioration following recording. In this paper we examine the use of an aerosol sealant (varnish) to `insulate' the photopolymer holographic recording material from the environment. Application of the sealant is found to produce some deterioration in the optical quality of the resulting HOEs. However, it is also shown to produce an increased lifetime of the active material (pre-recording), an increase in the diffraction efficiency from the resulting diffraction gratings recorded and improved HOE shelf-life.

Hologram build-up in a near infrared sensitive photopolymer

A self-processing photopolymer material suitable for holographic recording in the near infrared region (at 780 nm) has been studied. It has been shown that the amount of the pre-polymerizing exposure plays a crucial role in the build-up of the holographic grating. By a proper choice of the pre-exposure it is possible to reduce the inhibition period and to optimize holographic sensitivity and maximum diffraction efficiency. Maximum diffraction efficiencies above 70% have been measured. Significant differences have been found between gratings recorded with s and p polarised beams.

Ultraviolet self-generating relief micro-optical elements through crosslinking photopolymerization of liquid resins

The use of a self-processing dry photopolymer layer capable of memorizing optical information as a local change in thickness, is proposed for the fabrication of microlenses, binary holograms and gratings. Relief micro-optical elements are generated by direct imagewise exposure through a mask. A gradient of chemical composition and a gradient of surface free energy cause the transfer of reactive species between dark and illuminated areas. Contrary to conventional lithographic techniques that require wet chemical post-treatment to remove parts of the photoresist material, the fully self-processing character of this technique makes the record available in situ and immediately after exposure.

Holography and Holographic Interferometry with a near IR Sentitive photopolymer System

0Holographic images of test targets were reconstructed from a new, self-processing photopolymer material. A laser diode operating at 780 nm was used in the experiments. Possible effects of the polarization of the construction beams on the reconstructed image were also studied. The applicability of this material to real time holographic interferometry was demonstrated.

Use of self-processing dry photopolymers for the generation of relief optical elements: a photochemical study

A new technique based on the imaging of systems in which a dry photopolymer layer is capable of memorizing optical information as a local change in thickness was developed. The generation of a relief structure takes place under the sole action of light. The latent image is revealed by the self-processing character of the material and requires no chemical treatment. The relief formation was found to depend on a spatial gradient of monomer conversion and a surface free-energy gradient. Both lead to mass transport of reactive species, that depend on photonic, optical and physicochemical parameters. This process permits the use of these optical elements in reflection after metal coating or the fabrication of masters for embossing technology. In addition, the photopolymer films appear to be suited to the recording of computer-generated focusing elements for infrared radiation.