Preface

The book "Holograms –Recording Materials and Applications" comprises five sections.

The first section has eight chapters on holographic recording materials including ionic liquids in photopolymerisable materials (Chapter 1), Norland optical adhesive 65® as holographic material (Chapter 2), porous glass and polymer nanocomposite (Chapter 3), amorphous chalcogenide films (Chaper 4), azo-dye containing materials (Chapter 5 and 6) and photochromic materials (Chapter 7 and 8). The remaining four sections are dedicated to a variety of holographic applications. Section two has two chapters on further insights to holographic data storage – in depth analysis of collinear holographic storage (Chapter 9) and theoretical analysis of polychromatic reconstruction for volume holographic memory (Chapter 10). Section three is dedicated to holographic devices for application in wavelength division multiplexers in optical fiber systems (Chapter 11), optical circulators based on polarisation – selective substrate-mode volume holograms (Chapter 12) and holograms for beam shaping and generation of diffraction free beams (Chapter 13). Section four has two chapters on security applications – hologram based optical security systems for practical applications in verification of authenticity are described in Chapter 14 and nanophotonic hierarchical holograms are discussed in Chapter 15. The final section is dedicated to reconfigurable phase holograms for microwave signal processing (Chapter 16).

Many of the chapters describe the historical developments leading to the specific topic under discussion and will provide the reader with interesting and useful background information.

The following paragraphs give a brief summary of contents.

*Ionic Liquids in Photopolymerizable Holographic Materials* identifies ionic liquids suitable for use as additives in photopolymerisable materials. The authors demonstrate the application of these materials in the fabrication of symmetric and asymmetric optical diffusers with directional properties.

*Norland Optical Adhesive 65® as Holographic Material* presents a photosensitive material comprising Norland Optical Adhesive 65*®* mixed with crystal violet dye with a high potential for recording holographic elements in real time. The results from recording of Fourier holograms of binary objects are presented.

#### XII Preface

*Light-Sensitive Media - Composites for Recording Volume Holograms Based on Porous Glass and Polymer* describes a novel holographic material AgHal-PG. The list of the most important parameters of silver-halide media is supplemented by AgHal-PG-media with new possibilities: obtaining samples with thickness of several millimetres, low shrinkage and limitation of the maximum particle size in the light-sensitive material in pre and post processed form. A high-efficiency hologram is recorded in latent form and, after post-treatment, a distortionless interference structure in a wide dynamic range is obtained.

Preface XI

*Theory of Polychromatic Reconstruction for Volume Holographic Memory* develops the theory of holographic reconstruction with polychromatic light. In particular, focusing on its application to holographic memory, the required spectral width, distortion of the reconstructed image, diffraction efficiency, intra- and inter-page crosstalk, and

*Application of Holograms in WDM Components for Optical Fiber Systems* discusses the design of a singular device for use both in Coarse and Dense Wavelength Division Multiplexing CWDM/DWDM systems. Applications such as tunable optical filters, demultiplexers and wavelength routers, using holographic SLM technology are

*Polarization-Selective Substrate-Mode Volume Holograms and Its Application to Optical Circulators* introduces polarization-selective substrate-mode volume holograms which are applied in several novel designs of optical circulator. The described optical circulators have a number of advantages such as polarization-independence, compactness, high isolation, low polarization mode dispersion, easy fabrication, and low cost. In addition, the port number of the proposed multi-port device can be

*Holographic Synthesis of Diffraction Free Beams and Dark Hollow Beams* describes a simple method using holographic techniques in order to generate a variety of diffraction free beams and dark hollow beams. A fundamental part of the study consists in the generation of the boundary condition for the optical field characterized by a transmittance function. This is obtained by interfering two zero

*Optimization of Hologram for Security Applications* studies optical security systems for practical applications in authentication, such as in a card system. The advantages of the optical method in a security are the fast decoding of an encrypted image and the identification of it. Firstly, the authors study a common method of joint transform correlation for optical security systems and the optimization of binary holograms, and prove that the optimization of the hologram can be a powerful tool in the enhancement of system performance. An alternative method employs a phase-coding technique which enables easier realization of practical applications in optical security

*Nanophotonic Hierarchical Holograms: Demonstration of Hierarchical Applications Based on Nanophotonics* describes the basic concept of the nanophotonic hierarchical hologram with embedded nanophotonic codes and the fabrication of a sample device. One of the most notable characteristics of the proposed approach is embedding a nanophotonic code within the patterns of a hologram composed of one-dimensional grid structures. Because embedding and retrieval of a nanophotonic code require highly advanced technical know-how, this approach can also improve the strength of anti-

storage capacity are investigated.

reviewed.

expanded easily.

order Bessel beams.

counterfeiting measures.

systems.

*Digital Holographic Recording in Amorphous Chalcogenide Films* discusses the possibilities of hologram recording in As-S-Se chalcogenide films. The authors demonstrate that the As-S-Se chalcogenide films can be successfully used in applied dot-matrix and image-matrix holography as an excellent alternative to organic photoresists for producing high-quality security holograms with high diffraction efficiency.

*Azobenzene-Containing Materials for Hologram* describes block copolymers with welldefined structures that can eliminate the scattering of visible light by microphase separation and prohibit photoinduced surface deformation when azobenzene blocks form the minority phase. Thick films (> 200 microns) with good optical transparency can be prepared with random copolymers or blended block copolymers, for recording volume holograms.

*Holography Based on the Weigert's Effect* outlines the advantages of holography based on Weigert's effect and some new approaches in the investigation of the photo processes that occur during holographic recording in photoanisotropic materials.

*Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film* studies the holographic storage applications of 3-indoly-benzylfulgimide/PMMA film including ordinary and polarization holography, which are based respectively on the photochromic and photoinduced anisotropy properties of the material.

*Three-Dimensional Vector Holograms in Photoreactive Anisotropic Media* describes the principle of vector holography and investigates the optical characteristics of vector holograms recorded in a photoreactive anisotropic medium. Diffraction properties of the holograms recorded in a model medium, are characterised and the results are analyzed with the use of the finite-difference time-domain (FDTD) method. By comparing the experimental and calculated results, the authors elucidate the formation mechanism of the vector holograms.

*Diffraction Property of Collinear Holographic Storage System* describes the collinear holographic storage system proposed by Optware. The basic theories, including two existing models for collinear systems and Volume Hologram as an Integrator of the Lights Emitted from Elementary Light Sources (VOHIL) model, are introduced. Based on Fresnel transform and the VOHIL model, paraxial solutions to describe the diffraction characteristic of the collinear holographic system are obtained.

*Theory of Polychromatic Reconstruction for Volume Holographic Memory* develops the theory of holographic reconstruction with polychromatic light. In particular, focusing on its application to holographic memory, the required spectral width, distortion of the reconstructed image, diffraction efficiency, intra- and inter-page crosstalk, and storage capacity are investigated.

X Preface

range is obtained.

efficiency.

volume holograms.

*Light-Sensitive Media - Composites for Recording Volume Holograms Based on Porous Glass and Polymer* describes a novel holographic material AgHal-PG. The list of the most important parameters of silver-halide media is supplemented by AgHal-PG-media with new possibilities: obtaining samples with thickness of several millimetres, low shrinkage and limitation of the maximum particle size in the light-sensitive material in pre and post processed form. A high-efficiency hologram is recorded in latent form and, after post-treatment, a distortionless interference structure in a wide dynamic

*Digital Holographic Recording in Amorphous Chalcogenide Films* discusses the possibilities of hologram recording in As-S-Se chalcogenide films. The authors demonstrate that the As-S-Se chalcogenide films can be successfully used in applied dot-matrix and image-matrix holography as an excellent alternative to organic photoresists for producing high-quality security holograms with high diffraction

*Azobenzene-Containing Materials for Hologram* describes block copolymers with welldefined structures that can eliminate the scattering of visible light by microphase separation and prohibit photoinduced surface deformation when azobenzene blocks form the minority phase. Thick films (> 200 microns) with good optical transparency can be prepared with random copolymers or blended block copolymers, for recording

*Holography Based on the Weigert's Effect* outlines the advantages of holography based on Weigert's effect and some new approaches in the investigation of the photo processes

*Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film* studies the holographic storage applications of 3-indoly-benzylfulgimide/PMMA film including ordinary and polarization holography, which are based respectively on the

*Three-Dimensional Vector Holograms in Photoreactive Anisotropic Media* describes the principle of vector holography and investigates the optical characteristics of vector holograms recorded in a photoreactive anisotropic medium. Diffraction properties of the holograms recorded in a model medium, are characterised and the results are analyzed with the use of the finite-difference time-domain (FDTD) method. By comparing the experimental and calculated results, the authors elucidate the

*Diffraction Property of Collinear Holographic Storage System* describes the collinear holographic storage system proposed by Optware. The basic theories, including two existing models for collinear systems and Volume Hologram as an Integrator of the Lights Emitted from Elementary Light Sources (VOHIL) model, are introduced. Based on Fresnel transform and the VOHIL model, paraxial solutions to describe the

diffraction characteristic of the collinear holographic system are obtained.

that occur during holographic recording in photoanisotropic materials.

photochromic and photoinduced anisotropy properties of the material.

formation mechanism of the vector holograms.

*Application of Holograms in WDM Components for Optical Fiber Systems* discusses the design of a singular device for use both in Coarse and Dense Wavelength Division Multiplexing CWDM/DWDM systems. Applications such as tunable optical filters, demultiplexers and wavelength routers, using holographic SLM technology are reviewed.

*Polarization-Selective Substrate-Mode Volume Holograms and Its Application to Optical Circulators* introduces polarization-selective substrate-mode volume holograms which are applied in several novel designs of optical circulator. The described optical circulators have a number of advantages such as polarization-independence, compactness, high isolation, low polarization mode dispersion, easy fabrication, and low cost. In addition, the port number of the proposed multi-port device can be expanded easily.

*Holographic Synthesis of Diffraction Free Beams and Dark Hollow Beams* describes a simple method using holographic techniques in order to generate a variety of diffraction free beams and dark hollow beams. A fundamental part of the study consists in the generation of the boundary condition for the optical field characterized by a transmittance function. This is obtained by interfering two zero order Bessel beams.

*Optimization of Hologram for Security Applications* studies optical security systems for practical applications in authentication, such as in a card system. The advantages of the optical method in a security are the fast decoding of an encrypted image and the identification of it. Firstly, the authors study a common method of joint transform correlation for optical security systems and the optimization of binary holograms, and prove that the optimization of the hologram can be a powerful tool in the enhancement of system performance. An alternative method employs a phase-coding technique which enables easier realization of practical applications in optical security systems.

*Nanophotonic Hierarchical Holograms: Demonstration of Hierarchical Applications Based on Nanophotonics* describes the basic concept of the nanophotonic hierarchical hologram with embedded nanophotonic codes and the fabrication of a sample device. One of the most notable characteristics of the proposed approach is embedding a nanophotonic code within the patterns of a hologram composed of one-dimensional grid structures. Because embedding and retrieval of a nanophotonic code require highly advanced technical know-how, this approach can also improve the strength of anticounterfeiting measures.

#### XIV Preface

*Photonic Microwave Signal Processing Based on Opto-VLSI Technology* discusses reconfigurable phase holograms for the realization of microwave and RF filters, truetime delays, and beamformers in order to overcome the disadvantages of poor tunability, inflexibility, and low resolution of conventional photonic microwave signal processors. The authors demonstrate that holography is a promising technology for flexible processing of microwave and wideband RF signals with high resolution.

> **Dr. Izabela Naydenova**  Dublin Institute of Technology Ireland

**Part 1** 

**Holographic Recording Materials** 

**1** 

*Germany* 

**Ionic Liquids in Photopolymerizable** 

*INM – Leibniz Institute for New Materails, Campus D 2 2, Saarbruecken,* 

A variety of materials have been used to record hologram, such as silver halide emulsions, hardened dichromated gelatin, ferroelectric crystals, photochromics, photoresist, photodichroics and photopolymerizable materials [1-3]. Photopolymerizable holographic materials due to their low cost and dry processing have attracted great interest in academics and industry. They have broad applications in holographic memories, recording media, LCD displays, helmet-mounted display, optical interconnects, waveguide couples, holographic diffusers, laser eye protection devices, automotive lighting, and security holograms. The photopolymerizable holographic composite contains mainly a matrix binder, a photopolymerizable monomer, an initiator system, a plasticizer and additives [4- 17]. Due to the inter diffusion of the unpolymerized monomers in a holographic film, areas with high and low refractive index are formed during the irradiation with an interference pattern. Many photopolymer systems have been developed including binary photopolymer composites, organic-inorganic nanocomposites, a hybrid organic-inorganic host consisting of porous glass, and a system using monomers capable of cationic ring-opening

The addition of a plasticizer or an additive can increase the refractive index modulation and the final diffraction efficiency. Monroe et al. reported that tri(2-ethylhexyl)phosphate, glyceryl tributyrate, polyethylene glycol or functional polyethylene glycol etc. as plasticizers may increase the refractive index modulation [18]. Frank recommended photopolymerizable compositions with triglycerides as additives, which provide a stable holographic material with high refractive index modulation [19]. Tucker et al. used trithiocarbonate as additive to increase the diffraction efficiency, uniformity and reproducibility in the formation of electrically switchable holographic gratings [20]. Finally, one publication reports about an additive to improve the sensitivity of

Ionic liquids are organic salts that are liquid at ambient temperatures, preferably at room temperature. They are nonvolatile, thermally and chemically stable, highly polar liquids, high ionic conductivity, large electrochemical window and ease of solubilization of a large organic molecules and transition metal complexes [22-25]. Applications of ionic liquids include their use in synthesis, catalysis, separation, electrochemistry, electrolytes, lubrication, biomass processing, drug delivery and others. The cations of ionic liquids are

**1. Introduction** 

polymerization.

photopolymerizable hologram material [21].

**Holographic Materials** 

Hechun Lin and Peter W. de Oliveira
