Chemical routes to materials rely on the availability of appropriate precursors. The high lability of the metal alkoxide linkage opens up large possibilities of tailoring of solubility, hydrolysis rates, introduction of special functionalities such as polymerizable ones into the coordination sphere and thus into the final network. The coordination modes of common chemical modifiers are discussed. Differences between silicon and electrophilic metals are stressed.
Simple ways to build up mixed-metal molecules which can act as potential single-source precursors to multimetallic oxides are reviewed. Emphasis is given to Lewis acid-base reactions between metal alkoxides M(OR)n, and between metal alkoxides and more accessible oxide precursors, namely carboxylates M(O2CR)n and β- diketonates M(β-dik)n. The reactions proceed toward the formation of aggregates in which the different metals display their usual coordinations numbers, often six for transition metals, as shown single crystal X-ray diffraction. Strategies for fixing the stoichiometry between the metals are developed. The reactivity of the MM'species (dissociation, effects of chemical modifiers, of other metallic species, hydrolytic or nonhydrolytic condensation,...) will be indicated.
In this paper two examples of application of the sol-gel method to the preparation of nonlinear optical materials are described. The first example deals with glasses or glassy-like materials doped with semiconductor quantum dots. The properties of the doped materials depend on the size and size distribution of the nanoclusters embedded in the matrix, which depends on the preparation procedure used. The sol-gel preparation procedures proposed in the literature are critically reviewed. The results obtained by the author in the preparation of semiconductor doped planar optical waveguides are described in more detail. In the second example the preparation of optical limiting materials obtained by incorporation of C60 derivatives are described.
Inorganic/organic gels are prepared by hydrolyzing silica-containing alkoxides. Organics are introduced either by functionalizing the silica chemical hybrids) or by dissolving organic polymer in the solvent (physical hybrids). When the organic is a linear polymer, such as poly(vinyl acetate) or poly(ethylene oxide), the interactions between the organic and inorganic components are largely through surface hydroxyl groups. These interactions are responsible for the degree of visible light transmission.
A review of developments to date in the field of optical chemical sensors based on solgel- derived thin films is presented. The focus throughout is on those publications which have made a significant contribution to the area. The advantages and versatility of the solgel process are highlighted, with particular emphasis on the immobilisation of analytesensitive reagents. Possible sensing configurations are evaluated with respect to practicality and performance. Those problems, such as microstructural instability and leaching, which may restrict sol-gel sensors to short-term or disposable use are discussed and possible solutions are identified.
New organic materials have played an important role in optical data storage devices. In this paper, the film preparation process, optical and spectroscopic properties, optical storage performances of dye-doped polymer and ormosil materials used as write-once and rewritable optical recording media (photochromic media and others) are reviewed. The existing problems, solutions to these problems and the future development are discussed also.
The possible use of sol-gel films and monoliths for optical guided wave components is receiving increasing attention. This paper describes the requirements for such components in fibre communication systems, and reviews the status of the principal integrated optical technologies which compete for these applications. The development of waveguides based on sol-gel, and on sol-gel materials relevant to integrated optics, is reviewed. The issues relating to the future success of sol-gel in this field, i.e. cost, performance and functionality, are discussed, and some areas of current and future work described. Key challenges to competitive success identified.
The current potential of organic-inorganic hybrid materials, based on a sol-gel derived siloxane matrix, for photonic applications is reviewed. Materials development for bulk and waveguide applications is considered. The hybrid materials are classified and approaches to synthesis and shaping are discussed. Structure is reviewed especially the implications for optical performance and material robustness. Linear optical loss mechanisms are discussed and potential photonic applications are briefly reviewed.
There is a strong need for the development of cheap component technologies for optical functions such as switching, demultiplexing and amplification. Silica-on-silicon integrated optics using sol-gel processing is probably the best technology for such low cost applications. This review focuses on the sol-gel based thin film fabrication technologies for integrated optics (IO) lasers and amplifiers, using Nd3+ and Er3+ as the active species. Special emphasis is given to the work performed under the European Union sponsored projects NODES (ESPRIT) and CAPITAL (ACTS), in particular to the processing and characterization of Nd3+ and Er3+-doped silica-titania planar waveguides for IO lasers and amplifiers.
Sol-gel technologies represent a powerful tool for making almost all kinds of transparent materials with interesting optical or photonic properties. During the last two decades, a large number of materials, mainly in form of coatings, have been developed. It is to say that the materials properties would be suitable for many applications. Draw-backs in many cases are processing, chemical engineering for quality-assured material production, availablilty of materials, skills for combining chemistry, engineering and device making in academia as well as in many industries, especially SMEs. These definitions in combination with the lack of knowledge lead to a situation that despite of the interesting prospects the industrial breakthrough has still not yet taken place.
This paper examines sol-gel materials for photonics in terms of partnerships with other material contenders for processing optical devices. The discussion in four sections identifies semiconductors, amorphous and crystalline inorganic dielectrics, and amorphous and crystalline organic dielectrics as strategic agents in the rapidly evolving area of materials and devices for data communications and telecommunications. With Zyss, we trace the hierarchical lineage that connects molecular hybridization (chemical functionality), through supramolecular hybridization (collective properties and responses), to functional hybridization (device and system level constructs). These three concepts thread their way through discussions of the roles sol-gel glasses might be anticipated to assume in a photonics marketplace. We assign a special place to glass integrated optics and show how high temperature consolidated sol-gel derived glasses fit into competitive glass fabrication technologies. Low temperature hybrid sol-gel glasses that combine attractive features of organic polymers and inorganic glasses are considered by drawing on examples of our own new processes for fabricating couplers, power splitters, waveguides and gratings by combining chemical synthesis and sol-gel processing with simple photomask techniques.
The sol-gel process is an interesting alternative to make glass integrated optics components. Basically, reasonable performances can be reached with low cost fabrication. The sol-gel process is coupled to an one-step method to imprint channel waveguides as basic parts of optical integrated circuits. The basic one layer structure is enriched with a protective layer and a buffer layer and we show here the advantages of these additions in the quality of the propagation. The fabrication of gratings is also described since they can be also easily imprinted with two methods. Passive devices illustrate the possibility of this technology.
Electro-optic (EO) polymer films for use in optical modulators and switches continue to show promise for lowering costs, increasing bandwidth, and allowing high levels of monolithic integration. Now more emphasis must be placed on designing EO polymer films with low optical loss while maintaining adequate EO coefficient and thermal stability. Recently demonstrated, highly rigid and polarizable polymers have quite high thermal stability, but their optical clarity has suffered. The higher optical loss in films of these highly rigid polymers is an unsolved problem at this point. Furthermore, electricfield- poled films of these rigid polymers often have lower EO coefficients, which may be due to higher electrical conductivity at poling temperature that prevents the build-up of an adequate electric field across the film. Crosslinking of moderately rigid, fluorinecontaining films may provide the best compromise of thermal stability, optical clarity and EO coefficient.
Until recently, the product of chromophore dipole moment, β, and molecular first hyperpolarizability, p, divided by chromophore molecular weight was considered to be an appropriate chromophore figure of merit. Substantial progress has been made designing and synthesizing chromophores characterized by large μβ values. If such high pp chromophores could be translated to hardened acentric polymer lattices with the same efficiency achieved for disperse red (azobenzene) chromophores then optical nonlinearities in excess of 50 pm/V could be expected. Although high μβ chromophores have been available for several years, such macroscopic optical nonlinearities have only beat recently realized. We demonstrated that the problem of translating microscopic to macroscopic optical nonlinearity can be traced to the attenuation of electric field polinginduced order by chromophore-chromophore electrostatic interactions. Such interactions are frequently treated within the approximations of London theory. We extend theoretical analysis to take into account the size and shapes of chromophores; such theory permits essentially quantitative prediction of variation of electro-optic coefficient with chromophore loading. Theory also suggests structural modification of chromophores to improve the maximum realizable optical nonlinearity as a function of chromophore loading and theoretical predictions have been experimentally realized in a number of cases leading to doubling and tripling of previously realized maximum electro-optic coefficient values. Chromophore-chromophore electrostatic interactions also contribute to aggregation and phase-separation which result in unacceptably high values of optical loss. Such interactions can also inhibit lattice hardening (e.g., thermosetting) reactions. A systematic analysis of such effects is presented.
Photorefractive polymer are emerging as promising photonic materials for a variety of applications including holographic storage, real-time optical processing, imaging, nondestructive testing, and phase conjugation. These materials combine the low cost and ease of processing of polymers with the highly sensitive optical encoding mechanism of photorefractive materials in general. This paper will introduce the basic concepts of photorefractivity, organic nonlinear optical materials, and amorphous organic photoconductors. Then we will review the physics and the chemistiy of photorefractive polymers and give a survey of different classes of materials that have been proposed during the relatively short history of organic photorefractive materials. Finally, we will discuss some of the potential and future challenges of these materials in the context of photonic applications.
We review our recent research in the field of photochromic polymeric structures for optical data storage and nonlinear optics. We unify some of the sub-themes of azopolymer structures in the light of photo-induced movement of azobenzene molecules. In particular, we discuss photo-induced effects in supramolecular assemblies containing azobenzene molecules (e.g. Langmuir-Blodgett-Kuhn structures and ultrathin silane layers). Reorientation of azobenzenes in these structures will be compared to that observed in spin-cast films. Photoisomerization and photo-induced orientation of azobenzene molecules is also studied at the molecular level by means of azosilane molecules chemisorbed on Silicon Oxide substrates. We establish a correlation between polymer architecture and sub-glass transition temperature (Tg) light-induced molecular movement in high Tg nonlinear optical azo-polyimide polymers. We show that the isomerization reaction itself depends on the polymer molecular structure, and we present evidence of light-induced molecular movement 325 °C below Tg of a rigid NLO azopolyimide polymer containing no flexible connector or tether.
We have investigated crosslinkable polyimides for both passive and active electro-optic devices. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination yielding propagation losses as low as 0.4 dB/cm in the near infrared. Channel waveguides formed by a simple wet etch process are observed to have no excess loss over slab structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent.
Polymer optical fibers may suitable for a wide variety of applications because they are easy to process into a wide variety of structures. For example, graded index fibers are widely believed to be ready for high bandwidth local area network applications such as fiber to the home. The polymer graded index fiber is inexpensive, easy to splice, and insensitive to small misalignment at a connector. Single mode polymer optical fibers, on the other hand, are promising candidates for all-optical devices because of the high light intensitities they can support, ability to tailor materials to meet a given application, and ease of fiber fabrication. In this paper, we review methods to make polymer fibers, discuss the fabrication process that is used to make graded-index fibers, single-mode polymer fibers and more complex fiber structures such as dual-core fibers. We also describe linear and nonlinear characterization techniques and review fiber properties. In particular, we discuss refractive index profile measurements in both graded index and step index fiber preforms, waveguiding studies in dualcore optical fibers, nonlinearity, and loss. We also introduce a new class of devices that are based on the photomechanical effect and discuss characterization studies and demonstration devices.
Poled polymers are an exciting class of materials for second-order and third-order-like (cascading) nonlinear optical applications at the telecom wavelengths around 1.3 and 1.55μm, as material parameters can be tailored for each specific application. In this survey, the most recent developments in the field of second-order nonlinear optical polymers for frequency conversion and cascading are briefly reviewed. The material parameters required for efficient frequency conversion are critically discussed together with the waveguide preparation, electrical poling and in-situ characterization of the achieved nonlinearity for various phase-matched device schemes. The state of the art in achieved second-harmonic generation conversion efficiencies is documented and predictions on the possible further development of the field are made.
A versatile polymeric waveguide technology is proposed for low-cost high-performance photonic devices that address the needs of both the telecom and the datacom industries. We have developed advanced organic polymeric materials that can be readily made into both multimode and single-mode optical waveguide structures of controlled numerical aperture (NA) and geometry. These materials are formed from highly-crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, loss, and stability with temperature and humidity. These monomers are intermiscible, providing for precise adjustment of the refractive index from 1.3 to 1.6. Waveguides are formed photolithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser direct-writing. A wide range of rigid and flexible substrates can be used, including glass, quartz, oxidized silicon, glassfilled epoxy printed circuit board substrate, and flexible polyimide film. We discuss the use of these materials on chips, on multi-chip modules (MCM’s), on boards, and on backplanes. Light coupling from and to chips is achieved by cutting 45° mirrors using Excimer laser ablation. Fabrication of the planar polymeric structures directly on the modules provides for stability, ruggedness, and hermeticity in packaging.
We present the fabrication of polyimide-based H-tree waveguides for a multi-GBit/sec optical clock signal distribution in a Si CMOS process compatible environment. Such a clock distribution system is to replace the existing electronic counterpart associated with high-performance computers. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the l-to-48 waveguide fanout device, l-to-2 splitting loss and bending loss were measured to be 0.25 dB and higher. The planarization requirement of the optical interconnection layer among many electrical interconnection layers makes the employment of tilted grating a choice of desire. Theoretical calculation predicts the 1-to-l free-space to waveguide coupling with an efficiency as high as 95%. Currently, a coupling efficiency of 35% was experimentally confirmed due to the limited index difference between guiding and cladding layers. Further experiments aimed at structuring a larger guiding/cladding layer index differences are under investigation. To effectively couple an optical signal into the waveguide through the tided grating coupler, the accuracy of the wavelength employed is pivotal. This makes the usage of the vertical cavity surface-emitting lasers (VCSELs) and VCSEL arrays the best choice when compared with edge-emitting lasers. Modulation bandwidth as high as 6 GBit/sec was demonstrated at 850 nm. Such a wavelength is compatible with Si-based photodetectors. Temperature dependence of the threshold current up to 155 °C was measured which will determine the power dissipation issue of the optoelectronic packaging. Finally, the first fully monolithic Si-MOSFET integrated receiver was made as the optical clock signal detector. To further enhance the bandwidth of such a detector, a resonant cavity structure with Si/Si02 as the bottom mirror was employed. The measured demodulation bandwidth is over 10 GHz. A fully integrated guided-wave optical clock signal distribution system having planarized grating couplers, H-tree Si- CMOS process compatible waveguides, VCSELs and Si-based photo-receivers will be demonstrated in the near future.