Search

A method to capture three-dimensional (3D) objects image data under extremely low light level conditions, also known as Photon Counting Imaging (PCI), was reported. It is demonstrated that by combining a PCI system with computational integral imaging algorithms, a 3D scene reconstruction and recognition is possible. The resulting reconstructed 3D images often look degraded (due to the limited number of photons detected in a scene) and they, therefore, require the application of superior image restoration techniques to improve object recognition. Recently, Deep Learning (DL) frameworks have been shown to perform well when used for denoising processes. In this paper, for the first time, a fully unsupervised network (i.e., U-Net) is proposed to denoise the photon counted 3D sectional images. In conjunction with classical U-Net architecture, a skip block is used to extract meaningful patterns from the photons counted 3D images. The encoder and decoder blocks in the U-Net are connected with skip blocks in a symmetric manner. It is demonstrated that the proposed DL network performs better, in terms of peak signal-to-noise ratio, in comparison with the classical TV denoising algorithm.

To advancement in security applications, for the first time, a bio-chaotic key generated from the emerging biometric trait “finger knuckle print (FKP)” has been proposed for data security and authentication. In the system, the feasibility to generate multiple keys from the single FKP image has been exploited to secure multiple information, which results in significant enhancement of the security strength. The encryption and decryption processes keep the user-specificity and perform better in reducing space for key storage and transmission. The research contribution is to facilitate a simplified optical implementation. Simulation results show the validity and effectiveness of the proposed scheme.

The holographic principle, discovered by Gabor, and realized by Leith & Upatnieks and Denisyuk is discussed. The intrinsic value of this idea and its continuing ability to motivate and inspire engineers and applied physicists is reviewed. Science Foundation Ireland, The Irish Research Council for Science Engineering and Technology, Enterprise Ireland, and the European Commission. “Generalitat Valenciana” of Spain (projects PROMETEO/2021/006 and IDIFEDER/2021/014, cofunded by European Union through the FEDER Programme) and by the “Ministerio de Ciencia e Innovación” of Spain (projects FIS2017-82919-R and PID2019-106601RB-I00).

A simplified (i.e., single shot) method is demonstrated to generate a Fourier hologram from multiple two-dimensional (2D) perspective images (PIs) under low light level imaging conditions. It was shown that the orthographic projection images (OPIs) can be synthesized using PIs and then, following incorporation of corresponding phase values, a digital hologram can be generated. In this work, a fast dictionary learning (DL) technique, known as Sequential Generalised K-means (SGK) algorithm, is used to perform Integral Fourier hologram reconstruction from fewer samples. The SGK method transforms the generated Fourier hologram into its sparse form, which represented it with a linear combination of some basis functions, also known as atoms. These atoms are arranged in the form of a matrix called a dictionary. In this work, the dictionary is updated using an arithmetic average method while the Orthogonal Matching Pursuit algorithm is opted to update the sparse coefficients. It is shown that the proposed DL method provides good hologram quality, (in terms of peak signal-to-noise ratio) even for cases of ~ 90% sparsity.

A simplified (i.e., single shot) method is demonstrated to generate a Fourier hologram from multiple two-dimensional (2D) perspective images (PIs) under low light level imaging conditions. It was shown that the orthographic projection images (OPIs) can be synthesized using PIs and then, following incorporation of corresponding phase values, a digital hologram can be generated. In this work, a fast dictionary learning (DL) technique, known as Sequential Generalised K-means (SGK) algorithm, is used to perform Integral Fourier hologram reconstruction from fewer samples. The SGK method transforms the generated Fourier hologram into its sparse form, which represented it with a linear combination of some basis functions, also known as atoms. These atoms are arranged in the form of a matrix called a dictionary. In this work, the dictionary is updated using an arithmetic average method while the Orthogonal Matching Pursuit algorithm is opted to update the sparse coefficients. It is shown that the proposed DL method provides good hologram quality, (in terms of peak signal-to-noise ratio) even for cases of ~ 90% sparsity.

Estimation of the delay between two signals has physical significance in synchronisation problems in, e.g. telecommunications, measurement of motion and vibration, and image registration. Low complexity algorithms can be performed extremely quickly even on limited hardware, and have improved energy consumption over more complex algorithms. We present a partial Fourier analysis of a previously reported algorithm to estimate the magnitude of the delay; the algorithm is the sum of absolute differences. The analysis offers insight into why the algorithm requires the absolute value operation. The algorithm is insensitive to direction of the delay, but the same analysis demonstrates that new approaches are possible to find the signed magnitude of the delay. Arising from that analysis, we propose one such algorithm, and demonstrate its efficacy in simulation, along with its robustness to additive and quantization noise. Our algorithm could be useful in a very wide range of applications, including image stitching, measurement of vibrations in buildings, and synchronisation problems in telecommunications.

Airway epithelium structure/function can be altered by local inflammatory/immune signals, and this process is called epithelial remodeling. The mechanism by which this innate response is regulated, which causes mucin/mucus overproduction, is largely unknown. Exosomes are nanovesicles that can be secreted and internalized by cells to transport cellular cargo, such as proteins, lipids, and miRNA. The objective of this study was to understand the role exosomes play in airway remodeling through cell–cell communication. We used two different human airway cell cultures: primary human tracheobronchial (HTBE) cells, and a cultured airway epithelial cell line (Calu-3). After intercellular exosomal transfer, comprehensive proteomic and genomic characterization of cell secretions and exosomes was performed. Quantitative proteomics and exosomal miRNA analysis profiles indicated that the two cell types are fundamentally distinct. HTBE cell secretions were typically dominated by fundamental innate/protective proteins, including mucin MUC5B, and Calu-3 cell secretions were dominated by pathology-associated proteins, including mucin MUC5AC. After exosomal transfer/intake, approximately 20% of proteins, including MUC5AC and MUC5B, were significantly altered in HTBE secretions. After exosome transfer, approximately 90 miRNAs (z4%) were upregulated in HTBE exosomes, whereas Calu-3 exosomes exhibited a preserved miRNA profile. Together, our data suggest that the transfer of exosomal cargo between airway epithelial cells significantly alters the qualitative and quantitative profiles of airway secretions, including mucin hypersecretion, and the miRNA cargo of exosomes in target cells. This finding indicates that cellular information can be carried between airway epithelial cells via exosomes, which may play an important role in airway biology and epithelial remodeling.

Many image processing applications require a quantitative estimation of underlying correspondence (of relative position in space) between two or more images. Here, we propose a novel, less numerically intensive method, which we refer to as the Subtraction Method, to measure in-plane object motion with sub-pixel accuracy. This method can be usefully employed in conjunction with correlation to provide fine motion information very quickly (100 times faster). In this paper we demonstrate, explain, and examine the proposed method using simulated digital image data. Subpixel motion of discrete Gaussian function, four different digital images and speckle image are determined. Performance is compared to that of correlation.

We proposed a novel, fast, sub-pixel accuracy digital image motion estimation method, in Opt. Lett, 45(24), 6611- 6614 (2021). In this paper, we experimentally examine the method’s performance in more detail. Images are captured using both optical and THz imaging systems. A high-resolution megapixel camera is used in the optical system and a low-resolution camera with 16×16 pixel is used to perform the THz measurements. Both 1D x and y direction motion measurement data are examined and analysed. In all cases sub-pixel resolution is achieved. The results demonstrate that the method can be flexibly used in different areas.

A novel (to the best of our knowledge), fast method to measure in-plane object motion in 1D with sub-pixel accuracy which complements the correlation technique is proposed. The method is verified experimentally using both visible and terahertz images. The absolute sum of grey level accumulated change is used to quantify object motion. The method requires calibration for each target, but only addition and subtraction operations. This results in a decrease of two orders of magnitude in the computation time.

In this paper, we assess the quality of three speckle image processing techniques - namely Laser Speckle Contrast Imaging (LSCI), Fuji method and Generalised Differences (GD) - in the detection of veins in various plant leaves. We develop a simple NIR laser speckle imaging set-up and find that the Fuji algorithm is best suited for producing highly contrasted and spatially resolved veins in leaf tissue, with the LSCI algorithm producing lower resolution results and GD failing to locate fine vascular structure. Determining the practicality of various speckle image processing techniques for vein detection aides future research for possible subcutaneous blood ow detection in non-invasive biomedical applications.

We explore and discuss the process of PVA/AA fabrication and the doping of the material with a photosensitive dye creating a photosensitive polymer. The dye used is Erythrosine B (EB), which is sensitive to wavelength of λ = 532nm. We expose the photopolymer with the donor wavelength and initiate the self-writing waveguide (SWW) process and polymerization, creating a single homogenous SWW. We investigate the progress of the SWW as it propagates within the photopolymer and map its change in refractive index relative to its surroundings. When the SWW has been completed we then use a multi-mode Y-Coupler to combine two wavelengths λ = 532nm and λ = 625nm. These two light-waves are injected into the SWW via a Y-Coupler test bed. When the combined light-waves beam exits the photo-sensitive polymer it is directed into another MM fiber. Here it is then measured by an optical analyser that investigates the combined optical wavelengths for interference, coherence, wavelength differential group delay (WDRG), non-linear effects and optical loss. We examine the light-waves for modal interference and birefringence and calculate the combined optical power. The optical insertion testing is now revered and a single wavelength is used. The light-wave enters the MM fiber and then the SWW. It then exists and enters the Y-Coupler. Here the wavelength is captured and the coherence testing is undertaken and the optical properties of the wavelength are analysed and a comparison is carried out to investigate the coherence of the light-wave under test.

Photo-polymerization is the reaction between monomers to form polymer chains, and these polymer chains have a higher refractive index than the monomer in the photopolymer, and so it is higher in the polymerized areas. There are four main processes that occur during photo-polymerization in the photopolymer, which are Initiation, Propagation, Termination, and Inhibition. Self-written waveguides (SWWs) in photopolymers, and the process of self-focusing and self-trapping, has been seen that certain important areas which require further research. In this article, the effect a SWW has on the transmission of light through the photopolymer is further tested using a PVP as a media with Rhodamine 6G (R6G) as a dye. Simulations were focused on investigating the SWW mechanism, testing different quantities of beams entering the material. The SWW process was seen to be of most interest for the connection and splitting of multiple optical waveguides.

Traditional imaging systems including microscopes depend on the generation of real images to be recorded by the sensor element, which is only sensitive to the intensity and not the phase. The object distance, which is crucial for the spatial resolution of the system, therefore is restricted to be larger than the focal length of the objective lens. This leads to a limitation of the achievable lateral, diffraction-limited spatial resolution. In order to reach a resolution enhancement with the same system components, we explore – in the sub-THz frequency regime – heterodyne detection of the scene’s complex-valued spatial Fourier spectrum in the image-sided focal plane of the optical system. The existence of the Fourier spectrum is independent of the object distance. The measured complex-valued field distribution enables a numerical back-propagation to any place in the object-sided free space from which radiation has reached the detector. Heterodyne Fourier imaging hence enables 3D imaging and – the relevant theme here – it lifts the restriction of the imaging distance. This enables object distances smaller than the focal length of the objective lens and, with it, an enhanced diffraction-limited spatial resolution. In the experiments presented here, the heterodyne data acquisition of the 0.3-THz continuous wave radiation employs Si CMOS TeraFET detectors, i.e., THz sensors based on antenna-coupled field-effect transistors which have been developed in our laboratory. First imaging results show an enhancement of the maximal resolution by a factor of 1.4 for the specific measurement conditions of our experiments, with considerable room for improvement.

THz imaging is being increasingly applied to produce images of the interiors of objects and to identify variations. THz detectors are steadily improving, but lag behind visible light systems in terms of their resolution, i.e. due in part to the limited pixel number and the size of the array detector area. Synthetic aperture methods seek to increase the numerical aperture (and so image resolution). For example, the position of the detector is altered, and multiple images are captured in order to obtain the entirety of the image plane information. High precision registration and image fusion algorithms are then required to stitch together the individual images captured. Here, we propose a novel subpixel estimation method for THz imaging, which enables real-time operation with high temporal and spatial resolution. A THz imaging system is implemented using a continuous-wave THz source emitting at 300 GHz and a THz camera with 16×16 pixels. The subpixel estimation method is applied to the resulting THz images. We demonstrate that this method can be used to calculate and calibrate image position in a conventional THz imaging system, with significantly reduced computational expense.

In this paper, we propose a new asymmetric method for double image encryption using the two-dimensional non-separable linear canonical transform (2D NS-LCT) and an iterative phase retrieval algorithm (PRA). First an encryption security key is generated using the PRA. Then two images (2D intensities) are combined to form a complex image. The 2D NS-LCT of this complex valued image is then obtained. A nonlinear phase truncation operation is applied to the transformed image and the amplitude is retained and used as the private key. The phase term is multiplied by the security PRA generated key and the inverse 2D NS-LCT is applied to the result giving the output encrypted image. The robustness of the proposed technique is tested against noise, occlusion and chosen-plaintext attacks. Numerical results are presented demonstrating the security of the proposed technique.

The asymptotic property of the maximum likelihood estimator (MLE) has been utilized to reconstruct three-dimensional (3D) sectional images in the photon counting imaging (PCI) regime. At first, multiple 2D intensity images, known as Elemental images (EI), are captured. Then the geometric ray-tracing method is employed to reconstruct the 3D sectional images at various depth cues. We note that a 3D sectional image consists of both focused and defocused regions, depending on the reconstructed depth position. The defocused portion is redundant and should be removed in order to facilitate image analysis e.g., 3D object tracking, recognition, classification and navigation. In this paper, we present a subpixel level three-step based technique (i.e. involving adaptive thresholding, boundary detection and entropy based segmentation) to discard the defocused sparse-samples from the reconstructed photon-limited 3D sectional images. Simulation results are presented demonstrating the feasibility and efficiency of the proposed method.

Semiconductor materials are widely used in integrated circuit and are important material platform in the fabrication and development of different technologies. Their material properties, e.g. complex permittivity and permeability, must be accurately known when designing high frequency microwave and millimeter wave devices. The applications of terahertz (THz) waves is receiving considerable attention because of the many potential applications. In this paper we provide results of an initial study to characterize low resistance silicon. First a free-space measurement setup is used to measure the characteristics of four commercial silicon wafers. These differently doped silicon wafers have different resistivity, i.e. 10, 20, 30 & 40 Ω cm. The complex relative dielectric constant and the loss tangent of the four types of silicon are measured. Second, the absorption properties of the same samples are measured using a compact terahertz time domain spectrometer (THz-TDS). In both measurements the results show that the sample doped using Phosphorus has a higher loss (absorption) and lower permittivity in the THz range compared the three wafers doped with Boron. In the case of the three Boron doped samples the loss tangent was found to depend on the resistivity of the material.

We present a simple and customizable microneedle mold fabrication technique using a low-cost desktop SLA 3D printer. As opposed to conventional microneedle fabrication methods, this technique neither requires complex and expensive manufacturing facilities nor expertise in microfabrication. While most low-cost 3D-printed microneedles to date display low aspect ratios and poor tip sharpness, we show that by introducing a two-step “Print & Fill” mold fabrication method, it is possible to obtain high-aspect ratio sharp needles that are capable of penetrating tissue. Studying first the effect of varying design input parameters and print settings, it is shown that printed needles are always shorter than specified. With decreasing input height, needles also begin displaying an increasingly greater than specified needle base diameter. Both factors contribute to low aspect ratio needles when attempting to print sub-millimeter height needles. By setting input height tall enough, it is possible to print needles with high-aspect ratios and tip radii of 20–40 µm. This tip sharpness is smaller than the specified printer resolution. Consequently, high-aspect ratio sharp needle arrays are printed in basins which are backfilled and cured in a second step, leaving sub-millimeter microneedles exposed resulting microneedle arrays which can be used as male masters. Silicone female master molds are then formed from the fabricated microneedle arrays. Using the molds, both carboxymethyl cellulose loaded with rhodamine B as well as polylactic acid microneedle arrays are produced and their quality examined. A skin insertion study is performed to demonstrate the functional capabilities of arrays made from the fabricated molds. This method can be easily adopted by the microneedle research community for in-house master mold fabrication and parametric optimization of microneedle arrays., Microfabrication: Low-Cost 3D Printing Method for Producing Microneedle Molds A low-cost, simple, customizable method has been developed for fabricating microneedle molds by means of a commercially available stereolithography 3D printer. Microneedle arrays (MNAs) are devices composed of micron-sized needles that allow the transfer of compounds, such as drugs, across the outer layer of a tissue, usually the skin. Conventional methods to fabricate the microneedles for such MNAs demand complex, expensive manufacturing facilities and expertise in microfabrication. However, a team headed by Kevin Krieger in Dr. Eoin O’Cearbhaill’s Medical Device Design Group at University College Dublin, Ireland was able to develop a simple fabrication method using a commonly available, affordable 3D printer that allows rapid production of microneedle master molds with easily customizable needle parameters and demands no microfabrication expertise. The authors believe that their method can be readily adopted by researchers for master mold fabrication and parametric optimization of MNAs.

In this paper, first the Self-Written Waveguide (SWW) process in wet photopolymer media (liquid solutions), are examined for three examples: single-, counter-, and co-fibers exposure. Then the SWWs formed inside solid material are examined including the effects of manipulating the alignment of the fibers. In all cases high precision measurements are used to position the fiber optic cables (FOCs) before exposure using a microscope. The self-writing process is indirectly monitored by observing (imaging) the light emerging from the side of the material sample during SWW formation. In this way the optical waveguide trajectories formed in an Acrylamide/Polyvinyl Alcohol (AA/PVA) a photopolymer material (sensitized at 532 nm) are examined. First the transmission of light by this material is characterized. Then the bending and merging of the waveguides which occur are investigated. The predictions of our model are shown to qualitatively agree with the observed trajectories. The largest index changes taking place at any time during the exposure, i.e. during SWW formation, are shown to take place at the positions where the largest exposure light intensity is present. Typically, such maxima exist close to the input face and the first maximum is referred to as the location of the Primary Eye. Other local maxima also appear further along the SWW and are referred to as Secondary Eyes, i.e. deeper within the material.

We present a THz detector based on AlGaN/GaN HEMT technology implemented for a multi-pixel array and tested in a confocal THz imaging system. The multi-pixel array shows good homogeneity where the ratio between the most and the least sensitive pixels is less than two times. The detector has been tested in a confocal system at 300 GHz and the pictures show a good resolution.

A novel continuous-wave THz imaging system based on the coherent recording of the Fourier space spectrum (FSS) with heterodyne detection is described. The system consists of two 300-GHz electrical multiplier-chain sources with a slight frequency offset, and a single narrow-band TeraFET detector working in raster-scanning mode. The complex-valued FSS is recorded in either the focal-plane or in the free space of the optical system in focusing and non-focusing mode, respectively. Two- and three-dimensional (2D and 3D) images are reconstructed numerically with an inverse Fourier transform algorithm. With a 80 × 80-mm2 detection area and 1-mm2 sampling steps, 1-mm resolution and 76.2×76.2-mm2 image area are achieved.

Recently the numerical and experimental results of optical self-written waveguides (SWWs) has been demonstrated intensively in the photopolymer media. In order to further understand the mechanism of self-written net-waveguide in photosensitive polymers, light-induced material response is analyzed. Optical netwaveguide trajectories formed using solid bulk of acrylamide/polyvinyl alcohol (AA/PVA) photopolymer material. As part of this work presents a studying of non-linear optics in photopolymer systems to form a net-waveguides. Which deals with the nonlinearity behaviors of transmitted light in photopolymer media, during refractive index changed throughout the optical self-propagating process. The self-interactions of crossing beams inside photopolymer material during SWWs process are studied. It is shown that there is good agreement between the numerical simulation results and experimental observations. These are confirmed the validity of the numerical model that was used to simulate these experiments.

Here we study self-assembled of 9-diethylamino-5-benzo[α]phenoxazinone 9 . AFM, SEM, Raman imaging, studies demonstrate that such materials form micron-sized tub-like structures with rectangular shape. These microstructures show both active and passive wave guiding modes. Applying wavelengths in resonance 532 nm, and in non-resonance (785nm) with the molecular electronic absorption bands was undertaken for active and passive wave guiding studies. PL spectra capping point, the body and emerging light points were investigated. Meanwhile, Raman scattered photons were also acquired which maintain the polarized light propagation direction(s) and its interaction with tube molecules. The hollow features and tubes defect were identified using Raman imaging. These studies showing that excellent wave guiding features are present using these self-assembled micron-tube structures.

The two-dimensional non-separable linear canonical transform (2D-NS-LCT) can model a wide range of paraxial optical systems. Digital algorithms to calculate the 2D-NS-LCTs are of great interested in both light propagation modeling and digital signal processing. We have previously reported that the transform of a 2D image with rectangular sampling grid generally results in a parallelogram output sampling grid, thus complicating further calculations. One possible solution is to use interpolation techniques. However, it usually leads to poor calculation speed and reduced accuracy. To alleviate this problem, we previously proposed a unitary algorithm by choosing an advantageous sampling rate related to the system parameters. In this paper, a fast algorithm is further proposed based on a novel matrix decomposition, which can significantly improve the efficiency of the numerical approximations.

The creation of self-trapping waveguides within a photopolymer material and the resultant permanent self-written waveguides (SWW) has been investigated for their propagation properties. We look at two light beams as they converge on the photopolymer and interact with the material to produce two separate waveguides fabricated within the material. Here we study these self-written waveguides and investigate their coupling as they propagate within the PVA/AA and combine to form a single SWW within the material. The angle of insertion is interrogated to identify the best angle to produce the SWW coupling effect. The single coupled SWW that is produced, is measured for attenuation so as to characterize the coupled waveguide.

An experimental and theoretical investigation of the preparation and exposure of multilayer photosensitive materials is presented. It is shown how the recorded change in the refractive index in each layer depends on the dye (photosensitizer) concentrations in each layer. It is also shown how the photosensitive material properties in each layer can be controlled to optimize some recording characteristics for particular applications. To do so, a set of equations, predicting the amplitude of higher harmonics refractive index amplitudes induced in the material layers with depth during exposure, is derived. This results in a technique for varying the dye concentration in each layer of a multilayer, so as to optimize volume diffraction grating performance. In part I of this paper, the 3D nonlocal photopolymerization-driven diffusion (NPDD) model is applied to calculate the resulting combined multilayer absorption and polymerization processes. The NPDD describes the time-varying behaviors taking place during exposure in such photopolymer materials. Simulations are performed for an acrylamide/polyvinyl alcohol-based photopolymer containing erythrosine-B dye. It is predicted that, in general, non-uniform gratings are formed, with the resulting refractive index being distorted both from the ideal sinusoidal cross-sectional spatial distribution and also with depth. This agrees with previous results indicating that increasing the thickness of a single photopolymer layer does not in practice lead to ever-increasing angular selectivity. In part II of this paper, it is confirmed experimentally that a suitably modified multilayer can be used to increase grating angular selectivity, i.e., reduce the width of the off-Bragg replay curve.

The application of artificial light to carry information, that is, to transmit signals over longer distances, dates from the use of fire to send messages. In the 1880s, Bell demonstrated his system for transmitting speech using a modulated beam of light. This invention paved the way for more complex light communications over distances of several hundred meters. However, such techniques only became widespread with the invention and implementation of the optical laser. The availability of coherent (monochromatic) light sources has led to the development of modern light-based, dense data transmission. Since then, many methods for transmitting information optically have been proposed and investigated. For example, initially, laser beams were shone through free space (i.e., the atmosphere). This approach has some drawbacks, for instance, system unreliability due to natural atmospheric effects (rain, snow, fog, and turbulence). These difficulties indicate why the propagation of light waves within shielded environments, such as through gas-filled pipes and tubes containing sequential arrangements of optical lenses, have been investigated. Of such approaches, optical fibers provide the best method of channeling light over long distances. For instance, a 550,000-mile optic fiber cable is hidden under the ocean transmitting vast quantities of information at high speed across the globe. However, in the vacuum of outer space, wave guidance is necessary, and in 2013, as part of the first demonstration of free-space laser communication, NASA scientists beamed an image of the Mona Lisa to the Lunar Reconnaissance Orbiter, a satellite orbiting the moon, from Earth.

From its inception holography has proven an extremely productive and attractive area of research. While specific technical applications give rise to 'hot topics', and three-dimensional (3D) visualisation comes in and out of fashion, the core principals involved continue to lead to exciting innovations in a wide range of areas. We humbly submit that it is impossible, in any journal document of this type, to fully reflect current and potential activity; however, our valiant contributors have produced a series of documents that go no small way to neatly capture progress across a wide range of core activities. As editors we have attempted to spread our net wide in order to illustrate the breadth of international activity. In relation to this we believe we have been at least partially successful. This Roadmap is organized under three headings: Materials, Applications and Concepts . We hope this structure is not misleading as in some cases the contributions do not fit neatly under any single heading and in several cases, there is significant overlap between articles listed in all the three sections. Significantly, some articles discuss work in areas not traditionally addressed side by side with others. We sincerely hope our approach will lead to an increase in awareness and a cross-fertilization in this wonderful field. Materials development and characterisation have always been of critically importance for the development of holography. The Roadmap begins with Wang et al (section 1) who briefly discuss novel materials for use in dynamic holographic displays. Tomita (section 2) then describes the development of nanocomposite photopolymers. Neipp and Frances (section 3) discuss the recording of holographic waveguides, while Gallego and Pascual (section 4) discuss shrinkage effects arising in photopolymers. Marovina et al (section 5) describe recent photorefractive materials and devices. Bruder et al (section 6) then discuss industrial-scale material production and volume holographic optical elements (vHOEs) fabrication. Under the heading of Applications, Kostuk (section 7) leads with the use of non-imaging HOES for solar energy conversion. Odinokov (section 8) discussed holographic data storage (HDS) and security issues. Matoba et al (section 9) reviews some advances in Multimodal 3D data acquisition using digital holography (DH). Wachulak (section 10) describes the performance of extreme ultraviolet (EUV) and soft x-ray (SXR) holography and tomography employing compact short wavelength sources. Gorelaya et al (section 11) then discuss holographic wavefront sensors (WFS) and next Chmelik (section 12), describes advance in incoherent holographic microscopy (HM). Ferrara and Coppola (section 13) describe the applications of digital polarized holography in the sciences. Márquez and Beléndez (section 14) discuss the use of liquid crystal on silicon (LCoS) microdisplays for use in HDS systems. Yang and Yuste (section 15) discuss the two-photon holographic imaging and manipulation of neural activity in vivo. Finally, in this section, Bianco and Zanutta (section 16) describe the use of holography in astronomical spectrographs. Under the heading Concepts, we begin with the description by Falldorf (section 17) of DH employing the spatial coherence function. Healy et al (section 18) describe the performance of autofocus in holographic imaging. Zhurminsky et al (section 19) discuss interference lithography for the fabrication of nanostructures. Situ and Wang (section 20) examine the applications of deep learning (DL) in DH. Abdurashitov and Tuchin (section 21) examine digital focusing in laser speckle contrast imaging (LSCI). Petrov (section 22) introduces ultrafast digital techniques and spatio-temporal metrology and Nomura (section 23) describes the evolution from the concepts of conventional DH to wide-sense DH. To conclude Morim and Saravanamuttu (section 24) describe the fabrication of functional 3D waveguide microstructures with nonlinear waves of light.

The development of high-resolution THz imaging systems is currently of great practical significance, (e.g. in the areas of security, agri-food, biomedicine etc.). Implementing and calibrating such systems is essential, as is the verification and appropriate use of models. Confocal microscopes are well known to improve resolution and provide 3D images. In this paper a novel terahertz (THz) confocal transmission scanning microscopy is implemented using a commercial 300 GHz (~1 mm) electronic THz source and sensor array. In order to calibrate the system, two objects, a high contrast amplitude metallic object and a low contrast surface relief phase pattern, are produced. These simple samples are imaged to quantify the polarization and sectioning properties of the imaging system. A scalar model is shown to be appropriate and is shown to predict the measured lateral and axial resolutions. Experimental results using two more complex objects are then presented, confirming confocal operation. This paper details the implementation and calibration of a 3D THz confocal imaging system.

This study is novel for several reasons: We used a thin drop cast layer of dry photosensitive materials to study the behaviors of wet photopolymer media using microscopic distances during the Self-Written Waveguide (SWW) process, then, we examined the self-trajectories formed inside the solid material. The results provide a framework for theoretical and experimental examinations by handling the effects of manipulating the alignment of fibers. The other main advantage of these techniques is their lightweight, easy to process, highly flexible, and ultimately low-cost nature. First, the SWW process in wet photopolymer media (liquid solutions) was examined under three cases: single-, counter-, and co-fiber exposure. Then, the SWWs formed inside the solid material were examined along with the effects of manipulating the alignment of the fibers. In all cases, high precision measurements were used to position the fiber optic cables (FOCs) before exposure using a microscope. The self-writing process was indirectly monitored by observing (imaging) the light emerging from the side of the material sample during SWW formation. In this way, we examined the optical waveguide trajectories formed in Acrylamide/Polyvinyl Alcohol (AA/PVA), a photopolymer material (sensitized at 532 nm). First, the transmission of light by this material is characterized. Then, the bending and merging of the waveguides that occur are investigated. The predictions of our model are shown to qualitatively agree with the observed trajectories. The largest index changes taking place at any time during exposure, i.e., during SWW formation, are shown to take place at the positions where the largest exposure light intensity is present. Typically, such maxima exist close to the input face. The first maximum is referred to as the location of the Primary Eye. Other local maxima also appear further along the SWW and are referred to as Secondary Eyes, i.e., eyes deeper within the material.

The creation of self-written waveguides within photopolymer material and their permanent nature are optically characterized and investigated for the purpose of propagating near infrared wavelengths for use within optoelectronics systems and communication networks.

A high-resolution, high-precision interferometer system for measuring terahertz spectrum is proposed and experimentally demonstrated using a 300 GHz source and an array THz camera.

A novel continuous-wave terahertz holography imaging system based on heterodyne detection is presented. It employs a field-effect transistor as a fast coherent detector for digital data recording.

THz in-line holography, is a simple and effective way of three-dimensional (3D) imaging. In this work, a THz in-line holography system is presented to investigate the imaging performance. An electrical multiplier-chain emitter working at 300 GHz was utilized as illuminating source. A highly sensitive broad-band CMOS TeraFET detector mounted on a 2D mechanical translation stage for raster-scanning acted as a virtual camera to record the hologram. The data capture area was 60x60 mm2 with a pixel size of 0.25 0.25 mm2. The test objects were polyvinyl chloride (PVC) boards, one taped with 2-mm-wide straight aluminum stripes separated by 2-mm intervals, another one taped with 1.5-mm-wide aluminum stripes forming the word ’GUF’ (size of each letter: 6x8 mm2), and the other two with the same patterns, but now realized as grooves in the material. The imaging results show that a resolution of at least 2 mm is achieved, with a large dynamic range of 60 dB due to the high sensitivity of the TeraFET detector. A property of in-line holography is its self-homodyning capability with the consequence that, in addition to the intensity, also the phase information is encoded in the hologram. The phase information of the transparent part of the object, and of the scattered and diffracted waves are retrieved with the same spatial resolution as the intensity, revealing the 3D imaging capability of the system. The quality and fidelity of the image results can be substantially improved in the future by enlarging the recording area and using an iterative phase recovery method. Furthermore, with a camera substituting the single-pixel scanning technique, the system will have the capability of real-time imaging. Combined with the 3D imaging ability, it will then have a wide application range for various topical areas.

In general, the holographic grating refractive index profiles in photopolymer materials are not identical to the exposing pattern. During exposure, high harmonics of the fundamental refractive index period are generated within the layer volume. A set of equations to calculate the amplitudes of the higher harmonics of refractive index induced in the grating is introduced. Then, an algorithm involving the use of the 3D nonlocal photopolymerization-driven diffusion model is presented and applied to calculate the resulting grating diffraction efficiencies. The experimental observation that the grating diffraction efficiency cannot reach the theoretical maximum value (ηmax=100%) and that, in the case of over-modulation, the minimum value (ηmin=0%) is also never achieved, are explained theoretically. The predictions of the simulations are also fit to experimental data for an acrylamide/polyvinyl alcohol photopolymer material with good agreement being achieved.