This review article describes recent developments in the field of second-order nonlinear optical (NLO) polymers with specific focus on their characterization methods, materials synthesis, chromophore orientation techniques, and device applications. For the characterization techniques of NLO properties of organics and polymeric materials, electric field-induced second harmonic generation, hyper-Raleigh scattering, electro-optic coefficient measurement, etc. are discussed. The significant progress obtained from the authors’ investigations, resulted in various types of polymeric materials including dendrimers and organic-inorganic hybrids with specific structures of academic and technological significance which are presented here. To produce highly efficient macroscopic nonlinearity in NLO polymeric systems, several chromophore orientation techniques such as static field poling, photoassisted poling, all optical poling, contact and corona poling are also demonstrated. In addition, the prospects for practical applications of the NLO polymers in information technology are reviewed.

Polymeric Materials and their Orientation Techniques for Second-Order Nonlinear Optics

Transparent optical nonlinear crystallized glasses with the composition of 25La2O3 25B2O3 50GeO2, stoichiometric to ferroelectric stillwellite-type LaBGeO5 crystalline phase, have been prepared by a two-step heat-treatment (first heat treatment: T1=670 °C, t1=10 h, second heat-treatment: T2, t2), and their second harmonic (SH) intensities have been examined using the Maker fringe method. The samples obtained by heat treatments at T2=720∼725 °C for t2=3 h show only surface crystallization and exhibit clear and fine (narrow) fringe patterns. The samples heat treated at T2=740 and 750 °C exhibit relatively strong SH intensities, but the fringe patterns in such samples are broad. It is proposed that SH waves generated from surface LaBGeO5 crystalline layers scatter at LaBGeO5 crystals formed in the interior of glass, causing the disappearance of fine fringe patterns.

Second harmonic generation in transparent surface crystallized glasses with stillwellite-type LaBGeO5

A method of calculating the characteristics of noncollinear phase matching in both uniaxial and biaxial crystals is presented. Al- though significant work has been done to characterize collinear phase matching and to present many of its applications, noncollinear phase matching also has unique characteristics, leading to several useful ap- plications. The method presented enables calculations of both the collin- ear and noncollinear cases, and enables a far larger set of nonlinear crystals and configurations to be studied. © 2000 Society of Photo-Optical In- strumentation Engineers. (S0091-3286(00)02304-7)

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Calculating characteristics of noncollinear phase matching in uniaxial and biaxial crystals

Recent developments in the application of poled optical fibers to second harmonic generation and electrooptic light modulation are reviewed.

Glass fiber poling and applications

Applying a dc electric field across a fused silica sample at elevated temperatures followed by cooling the sample with the field applied (thermal poling) leads to a second-order nonlinearity that has been linked to the formation of a space-charge region in bulk glass. The first microscopic information on the extent of the space-charge region and its behavior with poling time is reported using secondary ion mass spectrometry to monitor the distribution of charged impurities. Lithium and sodium ions are observed to form depletion regions. Potassium and sodium ions as well as a hydrogenated species appear to be injected from the surface. The extent of the space-charge region evolves approximately logarithmically with poling time well after the nonlinearity as measured by second-harmonic generation has been established. The evolution of the space charge region can be qualitatively understood by an ion-exchange model that allows interaction of two ionic carriers with vastly different mobilities.

Secondary ion mass spectrometry study of space-charge formation in thermally poled fused silica

Second order nonlinearity via corona poling at room temperature in a Corning 7059 glass film on a Pyrex glass substrate was studied. It was clearly pointed out that the origin of the induced nonlinearity in this waveguide was located at the interface region between a top layer glass film and a glass substrate. This region is formed to be a depletion layer in the consequence of a drift of positive ions towards a negative electrode during corona poling. X‐ray photoelectron spectroscopy measurement on the corona‐poled Pyrex glass substrate gave the evidence that the main mobile ions were sodium ions.

Optical quadratic nonlinearity in multilayer corona‐poled glass films

Second harmonic generation (SHG) from silica glasses doped with sodium or water was investigated to clarify the effects of the impurities on the second-order nonlinearity. Sodium ions were introduced into pure silica glasses from a polymer coating containing a trace amount of sodium with electrical poling. The doping of sodium drastically increased the SHG intensity from the poled silica glasses. The nonlinearity localized in the anode surface region is suggested to be ascribed to a frozen electric field formed by the drift of sodium ions. Introduction of water molecules into silica glasses was performed by hydrothermal treatment in purified water using an autoclave. The SHG intensity from the electrically poled glasses also increased with the increasing content of water molecules. In this case, the SHG was emitted from the entire region of the sample plate. The origin of the nonlinearity is tentatively assumed to be due to a frozen electric field created by charge separation with protonic conduction.

Effects of introduction of sodium and water on second-order nonlinearity in poled synthetic silica glass

Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method has been used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable for nonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations.

Finite-element modal analysis of large-core multimode optical fibers.

Quadratic optical effects in corona-poled silicate glass films on glass substrates were investigated using second harmonic generation. A time-dependent decay of the quadratic non-linear coefficients observed at room temperature was found to be composed of two components which have different relaxation times. The slow decay component increased with an increase of BaO-B 2 O 3 or BaO-Al 2 O 3 in the glass composition and with a decrease in the film thickness. The quadratic non-linearity showing the slow decay is assumed to be ascribed to a microscopic change in a local structure of the glass films. On the other hand, the fast decay component is suggested to be caused by a static-electric field in the entire film which formed by the induced surface charge and the space charge at the interface between the film and the substrate.

Time-dependent decay of quadratic non-linearity in corona-poled silicate glass films

Nonlinear-optical properties of an organic crystal, 2-furyl methacrylic anhydride, are characterized. The crystal, whose cutoff wavelength is 380 nm, belongs to the 4mm point group and is optically uniaxial. Dispersions of refractive indices and the quadratic nonlinear coefficients d33 (24 pm/V) and d31 (16 pm/V) are measured. Collinear phase-matched second-harmonic generation at 532.1 nm with a polished crystal is demonstrated, and patterns of noncollinear phase matching are discussed and observed experimentally. The crystal is phase matchable for fundamental wavelengths as low as 908 nm. The second-harmonic wavelength is tuned from 451.9 to 455.4 nm by temperature-controlled 90° phase matching.

S. Horinouchi

Papers

Optical quadratic nonlinearity in multilayer corona‐poled glass films

Effects of introduction of sodium and water on second-order nonlinearity in poled synthetic silica glass

Finite-element modal analysis of large-core multimode optical fibers.

Time-dependent decay of quadratic non-linearity in corona-poled silicate glass films

Nonlinear-optical properties of a novel organic crystal: 2-furyl methacrylic anhydride