Comparison of holographic recording options for reflection-format and transmission-format coupler-type diffractive optical elements: theoretical exploration and experimental validation.

The holographic recording of gratings that have very large diffraction angles and/or slants is challenging because recording requires beam angles that are not possible without the use of prisms. However, by using a recording wavelength that is different from (usually shorter than) the intended operating wavelength, it is possible to record with less challenging beam angles. In this paper, a recently developed model that allows systematic investigation of the potential and limits of this wavelength-shift recording technique is extended to include a reflection format. Transmission and reflection recording options are compared systematically for the first time, and it is shown that in reflection recording, some couplers can be recorded more easily when the recording wavelength is longer than the operating wavelength. This opens up new design options for previously challenging regions of the spectrum. Experimental validation of the reflection version of the model is carried out by holographic recording in reflection mode at 532 nm in Bayfol HX 200 photopolymer, demonstrating the coupling of blue light.

The Development of Optomechanical Sensors-Integrating Diffractive Optical Structures for Enhanced Sensitivity.

The term optomechanical sensors describes devices based on coupling the optical and mechanical sensing principles. The presence of a target analyte leads to a mechanical change, which, in turn, determines an alteration in the light propagation. Having higher sensitivity in comparison with the individual technologies upon which they are based, the optomechanical devices are used in biosensing, humidity, temperature, and gases detection. This perspective focuses on a particular class, namely on devices based on diffractive optical structures (DOS). Many configurations have been developed, including cantilever- and MEMS-type devices, fiber Bragg grating sensors, and cavity optomechanical sensing devices. These state-of-the-art sensors operate on the principle of a mechanical transducer coupled with a diffractive element resulting in a variation in the intensity or wavelength of the diffracted light in the presence of the target analyte. Therefore, as DOS can further enhance the sensitivity and selectivity, we present the individual mechanical and optical transducing methods and demonstrate how the DOS introduction can lead to an enhanced sensitivity and selectivity. Their (low-) cost manufacturing and their integration in new sensing platforms with great adaptability across many sensing areas are discussed, being foreseen that their implementation on wider application areas will further increase.

Improving the Angular Visibility of Photopolymer-Based Reflection Holograms for Sensing Applications.

Volume reflection hologram-based sensors are designed to visibly change colour in response to a target stressor or analyte. However, reflection holograms fabricated in thick photopolymer films are highly angularly selective, making these sensors challenging to view and interpret by non-experts. Here, the use of speckle holography to improve the visibility of reflection holograms is presented. A novel recording approach combining speckle recording techniques with Denisyuk reflection recording geometry is described. The recorded speckle reflection grating operates as a series of multiplexed reflection gratings with a range of spatial frequencies, capable of reflecting light at a wider range of angles. A comparative study of the angular and wavelength selectivity of speckle and standard reflection gratings was conducted. The FWHM of the angular selectivity curves of the speckle reflection gratings is doubled (4°) in comparison to standard 4500 lines/mm reflection gratings (2°). The wavelength selectivity FWHM is also doubled from 4.2 to 8.6 nm. The comparative ability of the speckle and standard reflection gratings to act as colour-changing compressional pressure sensors in the 0.88-5.31 MPa range is described. Finally, we present a prototype reflection hologram viewer which enables the easy observation of angularly specific reflection holograms by non-experts.

Cantilever-Based Sensor Utilizing a Diffractive Optical Element with High Sensitivity to Relative Humidity.

High-sensitivity and simple, low-cost readout are desirable features for sensors independent of the application area. Micro-cantilever sensors use the deflection induced by the analyte presence to achieve high-sensitivity but possess complex electronic readouts. Current holographic sensors probe the analyte presence by measuring changes in their optical properties, have a simpler low-cost readout, but their sensitivity can be further improved. Here, the two working principles were combined to obtain a new hybrid sensor with enhanced sensitivity. The diffractive element, a holographically patterned thin photopolymer layer, was placed on a polymer (polydimethylsiloxane) layer forming a bi-layer macro-cantilever. The different responses of the layers to analyte presence lead to cantilever deflection. The sensitivity and detection limits were evaluated by measuring the variation in cantilever deflection and diffraction efficiency with relative humidity. It was observed that the sensitivity is tunable by controlling the spatial frequency of the photopolymer gratings and the cantilever thickness. The sensor deflection was also visible to the naked eye, making it a simple, user-friendly device. The hybrid sensor diffraction efficiency response to the target analyte had an increased sensitivity (10-fold when compared with the cantilever or holographic modes operating independently), requiring a minimum upturn in the readout complexity.

Birefringent optofluidic gratings.

A set of parallel microfluidic channels behaving as a diffraction grating operating in the Raman-Nath regime has been fabricated and studied. The diffraction efficiency of such structure can be tuned by selecting a liquid with a particular refractive index and/or optical anisotropy. Alternatively the optical properties of the liquid can be characterised by measuring the diffraction efficiency and the state of polarization of the diffracted beam. In this work, the microfluidic channels under study have been filled with penicillin molecules dissolved in water. Due to the chirality of the penicillin, the liquid has been found to have circular birefringence of 2.14 × 10-7. The addition of the anisotropic liquid modifies the polarization properties of the microfluidic diffraction grating. The diffraction efficiency of the grating has been characterised for different probe beam wavelengths and states of polarization. Currently the diffraction efficiency of the device is low - 1.7%, but different approaches for its improvement have been discussed.

Modified Surface Relief Layer Created by Holographic Lithography: Application to Selective Sodium and Potassium Sensing.

Point-of-care diagnostics will rely upon the development of low-cost, noncomplex, and easily integrated systems in order to examine biological samples such as blood and urine obtained from the patient. The development of metal ion sensors is a subject of significant relevance for physiological samples. The level of different blood electrolytes, mainly H⁺, Na⁺, K⁺ and Cl- is considerably used to monitor irregular physiologies. The particular challenge in biosensing, and in fact for any other sensor, is signal differentiation between non-specifically bound material and the specific detecting of the target molecule/ion. The biosensors described in this paper are fabricated by a holographic recording of surface relief structures in a photopolymer material. The surface structures are modified by coating with either dibenzo-18-crown-6 (DC) or tetraethyl 4-tert-butylcalix[4]arene (TBC), which are embedded in a polymer matrix. Interrogation of these structures by light allows indirect measurement of the concentration of the analyte. The influence of polymer matrices with different porosities, plasticised polyvinyl chloride (PVC) and a sol-gel matrix, on the performance of the sensors for detection of K⁺ and Na⁺ is examined. Here we demonstrate a proof of concept that by using a matrix with higher porosity one can increase the sensitivity of the sensor. The results showed that the DC sensing layer provides a selective response to K⁺ over Na⁺ and the TBC modified grating is more responsive to Na⁺ over K⁺. The sensor responds to K⁺ and Na⁺ within the physiological concentration ranges.

Holographic beam-shaping diffractive diffusers fabricated by using controlled laser speckle.

A method for fabricating diffractive holographic optical diffusers is reported, allowing a high degree of control of the resulting diffuser characteristics. The method consists of recording a laser speckle pattern using a single carrier beam, with controlled speckle size and shape, in an acrylamide-based volume photopolymer. The multiple interferences that create the speckle pattern form the hologram. Results are presented verifying the diffusers are volume holographic in nature and the speckle pattern is recorded accurately in the photopolymer. Diffusers recorded by this method are analysed to characterise the optical performance of the diffusers and to illustrate their beam-shaping capabilities, particularly in producing asymmetric beam outputs.

Development of a photopolymer holographic lens for collimation of light from a green light-emitting diode.

A holographic optical element (HOE) was developed to collimate a monochromatic light-emitting diode (LED). The fabrication was achieved by the interference of collimated and diverging beams from a 532 nm laser to produce the required photonic structure in a self-developing photopolymer material. The experimental values of diffraction efficiency and spatial period across the HOE were compared with their expected theoretical values. Good agreement was found for the spatial period; however, the diffraction efficiency varied significantly across the lens. In this paper, two approaches have been taken to address this variation: (1) modification of recording geometry and (2) optimization of recording intensity and exposure time at constant energy. The performance of an optimized HOE (cylindrical holographic lens) was then demonstrated using a 532 nm laser beam. The optimized conditions for the cylindrical holographic lens were used to fabricate a spherical holographic lens of the same numerical aperture for LED applications. This type of lens has the potential to be used in combination with LED sources.

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.

Application of phase shifting electronic speckle pattern interferometry in studies of photoinduced shrinkage of photopolymer layers.

Photoinduced shrinkage occurring in photopolymer layers during holographic recording was determined by phase shifting electronic speckle pattern interferometry. Phase maps were calculated from the changes in intensity at each pixel due to the phase differences introduced between object and reference beams. Shrinkage was then obtained from the changes in phase as recording proceeded. The technique allows for whole field measurement of the dimensional changes in photopolymers during holographic recording.

N-isopropylacrylamide-based photopolymer for holographic recording of thermosensitive transmission and reflection gratings.

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 N-isopropylacrylamide (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.

Theoretical modeling of the effect of polymer chain immobilization rates on holographic recording in photopolymers.

This paper introduces an improved mathematical model for holographic grating formation in an acrylamide-based photopolymer, which consists of partial differential equations derived from physical laws. The model is based on the two-way diffusion theory of [Appl. Opt.43, 2900 (2004)10.1364/AO.43.002900APOPAI1559-128X], which assumes short polymer chains are free to diffuse, and generalizes a similar model presented in [J. Opt. Soc. Am. B27, 197 (2010)10.1364/JOSAB.27.000197JOBPDE0740-3224] by introducing an immobilization rate governed by chain growth and cross-linking. Numerical simulations were carried out in order to investigate the behavior of the photopolymer system for short and long exposures, with particular emphasis on the effect of recording parameters (such as illumination frequency and intensity), as well as material permeability, on refractive index modulation, refractive index profile, and grating distortion. The model reproduces many well-known experimental observations, such as the decrease of refractive index modulation at high spatial frequencies and appearance of higher harmonics in the refractive index profile when the diffusion rate is much slower than the polymerization rate. These properties are supported by a theoretical investigation which uses perturbation techniques to approximate the solution over various time scales.

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 (HOEs) to collect light from a moving source, such as the sun, and redirect it for concentration by a holographic lens. A working range of 45° is targeted for such a device to be useful in solar applications without tracking. A photopolymer HOE is capable of efficiently redirecting 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 create a device that can concentrate light incident from a large range of angles. In this paper, low spatial frequency holographic recording is explored 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 3° are experimentally demonstrated.

Optical Response of PDMS Surface Diffraction Gratings under Exposure to Volatile Organic Compounds.

Monitoring volatile organic compounds (VOCs) in indoor air is significantly gaining importance due to their adverse effects on human health. Among the diverse detection methods is optical sensing, which employs materials sensitive to the presence of gases in the environment. In this work, we investigate polydimethylsiloxane (PDMS), one of the materials utilized for gas sensing, in a novel transducer: a surface relief diffraction grating. Upon adsorption of the volatile analyte, the PDMS grating swells, and its refractive index changes; both effects lead to increased diffraction efficiency in the first diffraction order. Hence, the possibility of VOC detection emerges from the measurement of the optical power transmitted or diffracted by the grating. Here, we investigated responses of PDMS gratings with varying surface profile properties upon exposure to VOCs with different polarities, i.e., ethanol, n-butanol, toluene, chloroform, and m-xylene, and compared their response in the context of the Hansen theory of solubility. We also studied the response of the grating with a 530 nm deep surface profile to different concentrations of m-xylene, showing a sensitivity and limit of detection of 0.017 µW/ppm and 186 ppm, respectively. Structures in the PDMS were obtained as copies of sinusoidal surface gratings fabricated holographically in acrylamide photopolymer and revealed good sensing repeatability, reversibility, and a fast response time. The proposed sensing technique can be directly adopted as a simple method for VOC detection or can be further improved by implementing a functional coating to significantly enhance the sensitivity and selectivity of the device.

Effect of glycerol on a diacetone acrylamide-based holographic photopolymer material.

The composition of the low-toxicity, environmentally compatible diacetone acrylamide (DA) photopolymer has been modified with the inclusion of different additives. The addition of glycerol to the photopolymer composition is described. Results show that the incorporation of glycerol results in a uniform maximum refractive-index modulation for recording intensities in the range of 1-20 mW/cm(2). This may be attributed to glycerol's nature as a plasticizer, which allows for faster diffusion of an unreacted monomer within the grating during holographic recording. An optimum recording intensity of 0.5 mW/cm(2) is observed for exposure energies of 20-60 mW/cm(2). The modified photopolymer achieves a refractive-index modulation of 2.2×10(-3), with diffraction efficiencies up to 90% in 100 µm layers. Glycerol has also shown to reduce the rate of photobleaching of the DA photopolymer. This is possibly due to more prevalent inhibition effects caused by increased oxygenation of the photopolymer layers. The stability of the photopolymer samples is also improved with the addition of glycerol.

Shrinkage during holographic recording in photopolymer films determined by holographic interferometry.

Shrinkage of photopolymer materials is an important factor for their use in holographic data storage and for fabrication of holographic optical elements. Dimensional change in the holographic element leads to a requirement for compensation in the reading angle and/or wavelength. Normally, shrinkage is studied at the end of the polymerization process and no information about the dynamics is obtained. The aim of this study was to use holographic interferometry to measure the shrinkage that occurs during holographic recording of transmission diffraction gratings in acrylamide photopolymer layers. Shrinkage in photopolymer layers can be measured over the whole recorded area by real-time capture of holographic interferograms at regular intervals during holographic recording using a complimentary metal-oxide-semiconductor camera. The optical path length change, and hence the shrinkage, are determined from the captured fringe patterns. Through analysis of the real-time shrinkage curves, it is possible to distinguish two processes that determine the value of shrinkage in the photopolymer layer. These processes are ascribed to monomer polymerization and crosslinking of polymer chains. The dependence of shrinkage of the layers on the conditions of recording such as recording intensity, single or double beam exposure, and the physical properties of the layers, such as thickness, were studied. Higher shrinkage was observed with recordings at lower intensities and in thinner layers. Increased shrinkage was also observed in the case of single beam polymerization in comparison to the case of double beam holographic exposure.

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.

Holographic Recording of Unslanted Volume Transmission Gratings in Acrylamide/Propargyl Acrylate Hydrogel Layers: Towards Nucleic Acids Biosensing.

The role of volume hydrogel holographic gratings as optical transducers in sensor devices for point-of-care applications is increasing due to their ability to be functionalized for achieving enhanced selectivity. The first step in the development of these transducers is the optimization of the holographic recording process. The optimization aims at achieving gratings with reproducible diffraction efficiency, which remains stable after reiterative washings, typically required when working with analytes of a biological nature or several step tests. The recording process of volume phase transmission gratings within Acrylamide/Propargyl Acrylate hydrogel layers reported in this work was successfully performed, and the obtained diffraction gratings were optically characterized. Unslanted volume transmission gratings were recorded in the hydrogel layers diffraction efficiencies; up to 80% were achieved. Additionally, the recorded gratings demonstrated stability in water after multiple washing steps. The hydrogels, after functionalization with oligonucleotide probes, yields a specific hybridization response, recognizing the complementary strand as demonstrated by fluorescence. Analyte-sensitive hydrogel layers with holographic structures are a promising candidate for the next generation of in vitro diagnostic tests.

Development and Evaluation of a Multifrequency Ultrafast Doppler Spectral Analysis (MFUDSA) Algorithm for Wall Shear Stress Measurement: A Simulation and In Vitro Study.

Cardiovascular pathology is the leading cause of death and disability in the Western world, and current diagnostic testing usually evaluates the anatomy of the vessel to determine if the vessel contains blockages and plaques. However, there is a growing school of thought that other measures, such as wall shear stress, provide more useful information for earlier diagnosis and prediction of atherosclerotic related disease compared to pulsed-wave Doppler ultrasound, magnetic resonance angiography, or computed tomography angiography. A novel algorithm for quantifying wall shear stress (WSS) in atherosclerotic plaque using diagnostic ultrasound imaging, called Multifrequency ultrafast Doppler spectral analysis (MFUDSA), is presented. The development of this algorithm is presented, in addition to its optimisation using simulation studies and in-vitro experiments with flow phantoms approximating the early stages of cardiovascular disease. The presented algorithm is compared with commonly used WSS assessment methods, such as standard PW Doppler, Ultrafast Doppler, and Parabolic Doppler, as well as plane-wave Doppler. Compared to an equivalent processing architecture with one-dimensional Fourier analysis, the MFUDSA algorithm provided an increase in signal-to-noise ratio (SNR) by a factor of 4-8 and an increase in velocity resolution by a factor of 1.10-1.35. The results indicated that MFUDSA outperformed the others, with significant differences detected between the typical WSS values of moderate disease progression (p = 0.003) and severe disease progression (p = 0.001). The algorithm demonstrated an improved performance for the assessment of WSS and has potential to provide an earlier diagnosis of cardiovascular disease than current techniques allow.