Showing papers in "Pure and Applied Optics: Journal of The European Optical Society Part A in 1996"

Chemical sensing by surface plasmon resonance in a multimode optical fibre

Abstract: A new fibre optic sensor for the detection of refractive index variations is presented. The principle of the sensor is based on the generation of surface plasma waves at the interface between a thin metallic layer deposited on the fibre core and a surrounding dielectric medium. Specific injection of monochromatic light into the fibre allows the detection of a refractive index variation as low as for dielectric media lying in the 1.33 - 1.40 refractive index range. Modelling of sensing signal and experiments are performed. Potential applications are aqueous solution analyses and detection of gases and solvents with a specific polymeric layer.

Optical properties of isotropic chiral media

Abstract: A review is given of the optical properties of isotropic chiral media, based on the symmetrized constitutive relations of Condon. The review includes discussion of wave propagation in chiral media, and derivation of the reflection and transmission amplitudes of an isotropic optically active medium, and of a layer resting on a substrate. Boundary conditions and energy conservation relations are derived. For the chiral layer, simple formulae are given for the reflection and transmission coefficients at normal incidence, in the weak chirality limit, near the critical angles, and for a thin layer. Analytic expressions are given for all the reflection and transmission amplitudes in the general case. An ellipsometric method of measuring the chirality of very small sample volumes is suggested.

Nonlinear optical properties of nanocomposite materials

Abstract: This paper reviews some of the authors' recent research aimed at obtaining an understanding of the physical processes that determine the linear and nonlinear optical properties of nanocomposite materials. One result of this research is the prediction and experimental verification that under proper conditions two materials can be combined in such a manner that the nonlinear susceptibility of the composite exceeds those of the constituent materials. This paper also presents a survey of the various geometrical structures of composite materials. A common approach to the development of nonlinear optical materials entails searching for materials that possess, at the molecular level, desirable nonlinear optical properties. An alternative approach, which will be explored in this paper, entails combining known materials into a composite material. Under proper conditions, this composite material might combine the more desirable properties of the starting materials, or ideally, might possess properties superior to those of the starting materials. Some of the commonly encountered structures of composite materials are shown in figure 1. The Maxwell Garnett (1) geometry consists of small inclusion particles embedded in a host material. The Bruggeman (2) geometry consists of two intermixed components. These two model geometries are the structures most often encountered in theoretical discussions of composite materials. Two additional structures are that of porous silicon and that of layered materials. Recent research (3) has shown that an electrochemical etching procedure can be used to turn silicon into a porous structure. The resulting structure then contains 'worm holes' which can be modelled as cylindrical columns in which the silicon has been eaten away from the host material. When still more material has been eaten away, the resulting structure can be modelled as cylindrical columns of silicon surrounded by voids. In either case, the voids can be filled with a second material to form a composite structure. These composite materials can be thought of as a two-dimensional version of the Maxwell Garnett structure. Research on porous silicon is still quite new and will not be discussed further in this paper. The final structure illustrated in figure 1 is the layered geometry, consisting of alternating layers of two materials with different linear and nonlinear optical properties. In all of the structures shown in figure 1, we assume that the two materials are intermixed on a distance scale much smaller than an optical wavelength. Under these conditions, the propagation of light can be described by effective values of the optical constants that are obtained by performing a suitable volume average of the local optical response of the material. In fact, performing such an average can be rather subtle for situations involving the nonlinear optical response, because it is the nonlinear polarization that must be averaged, and the nonlinear polarization depends on the spatially inhomogeneous electric field amplitude

Quantitative evaluation of digital shearing interferogram using the spatial carrier method

Abstract: In this paper we describe a digital shearing interferometer system. The set-up is composed of a Mach - Zehnder interferometer which allows us to split the image of one object and to recombine the two images shifted and tilted. The tilt between the two images is chosen in order to produce a carrier frequency. The spatial carrier phase-shifting method is used for the quantitative evaluation. Two separate shearograms of an object under test are recorded using a CCD-sensor and stored in a frame grabber. The phases of the two shearograms are calculated from the complex amplitudes and the gradient is obtained from the phase difference. Experimental results are presented.

Phase function simulation in tissue phantoms: a fractal approach

Abstract: Artificial media are needed for the calibration of optical diagnostic methods in order to work on reproducible, stable and well known samples. Since the scattering and absorption coefficients can easily be adjusted by using appropriate concentrations of scattering and absorbing components, the most difficult part in the design of a good tissue phantom is to obtain an actual phase function. The most common way to create phantoms is to use scattering microspheres of equal size, but the Mie phase function of such a phantom does not match the tissue's real phase function. Moreover, we show in this paper that the similarity relations often used for the analysis of the results obtained with this type of phantom may sometimes be very inaccurate. The use of a mixture of different sized scattering particles is then considered, in order to imitate the whole phase function. However, as the determination of adequate sizes and concentrations is a difficult mathematical task, we describe a simple method to solve this problem. We first demonstrate that the extreme optical complexity of real biological samples could be simulated by a mixture of spheres with a fractal diameter distribution. Then, we present a few simple rules based on the knowledge of this fractal distribution, which can be used to obtain a realistic phase function with a limited number of sphere diameters.

Coupling gratings as waveguide functional elements

Abstract: Waveguide grating couplers allow the essential function of waveguide access to be integrated together with optical processing functions on a monolithic planar substrate. Grating coupling technology is topologically and, to a large extent, technologically compatible with the planar processes which define the optical processing waveguide circuit. Poor coupling efficiency, highly dispersive character, absence of user-friendly modelling tools and fabrication difficulties have long prevented this technology from being implemented into practical sensors and microsystems. This paper makes a review of the points where decisive technological progress has been made, illustrates some useful features of waveguide coupling gratings and underlines some of the difficulties which the designer may encounter.

Dye-doped liquid crystal composite for real-time holography

Abstract: In this paper we report the results of dynamic phase grating formation in an anthraquinone derivative dichroic dye-doped nematic liquid crystal in which an external DC electric field was used to change the planar alignment of liquid crystal molecules into a near homeotropic one. This field was also needed to produce an efficient grating via photoconductivity-induced molecular reorientation in a degenerate two-wave mixing experiment with He - Ne (632.8 nm) laser light serving as a low power excitation source. The characteristic of the measured effect shows that the mechanism of grating formation is evidently a non-photorefractive one. A diffraction efficiency of up to 20% has been measured in the system. Fast hologram recording and erasing times (1 ms) make this system attractive for image processing applications.

Lead phthalocyanine reverse saturable absorption optical limiters

Abstract: Systematic studies of the nonlinear optical properties of organic materials have led to the development of promising new materials for optical limiting. Several heavy metal substituted phthalocyanines are effective materials for optical limiters in the visible and near IR. Nonlinear absorption is the principal mechanism near the limiting threshold. In fast optical systems, refractive contributions also contribute at higher energies. These refractive contributions are predominantly thermal. The spectral window over which the limiter operates can be engineered by altering both the main ring and peripheral substitution of the molecules.

Some topics in extending the C method to multilayer gratings of different profiles

Abstract: In a recent Letter to the Editor (1995 Pure Appl. Opt. 4 1 - 5), the differential formalism of Chandezon et al (the C method) was extended to treat layered gratings in which the profiles of the medium interfaces within the same grating are different from each other. Numerical experiments have shown that a computer program based on the recipe given in the Letter gives excellent numerical results for diffraction efficiencies. However, a crucial assumption was implicitly used without justification. In this paper, we examine this assumption and comment on its validity. In addition, we suggest two alternative ways to extend the C method, and show that one of them is as effective as the recipe given in the Letter.

Salt-based approach for frequency conversion materials

Abstract: A salt-based approach for the development of frequency conversion materials is presented. Salts are generally rugged materials due to their strong ionic bonding. A high degree of chemical substitution (both of anion and cation) is possible, including highly nonlinear organic ions. This flexibility is consistent with the use of empirical survey techniques to identify promising candidate materials. We describe in detail our experimental approach and present examples of nonlinear optical crystals we have identified in our materials surveys.

Transfer of radiance by twisted Gaussian Schell-model beams in paraxial systems

Abstract: The radiometric brightness theorem for a generalized radiance function is used to study the propagation of twisted Gaussian Schell-model beams in paraxial optical systems. First it is shown that a certain unique correspondence exists between the Gaussian radiance and cross-spectral density functions associated with these beams. The method is then applied to the passage of the twisted beam field through a thin lens, and the analysis yields algebraic expressions for the quantities characterizing the transmitted beam. The phenomenon of focal shift occurring in imaging by wavefields of this kind is examined, and the effects of partial spatial coherence and beam twist are elucidated.

Gaussian quadrature approach to the calculation of the optical constants in the vicinity of inhomogeneously broadened absorption lines

Abstract: A fast numerical procedure to calculate the convolution of Gaussian and Lorentzian functions is introduced as well as its application to the approximation of experimental spectra. It is shown that these spectra can be well approximated with the method, especially in the case when the inhomogeneous and homogeneous linewidth contributions as parameters of a Voigt lineshape are of the same order of magnitude. Particularly, the method is applicable to refractive index modelling in the vicinity of inhomogeneously broadened absorption lines.

Focusing of electric-dipole waves

Abstract: Diffraction of electromagnetic waves through an aperture in a plane screen is discussed with particular emphasis on the focusing properties of a converging electric-dipole wave. Explicit results for the field in the focal region are given both within the Debye and the Kirchhoff approximations. Within the Debye approximation, the results for a focused electric-dipole wave are compared analytically and numerically with those obtained earlier for the focusing of a linearly polarized plane wave by a rotationally symmetric, aplanatic system. It is shown that at high angular apertures, the converging electric-dipole wave has better energy concentration and less cross-polarization, both in the focal plane and along the optical axis.

Use of self-processing dry photopolymers for the generation of relief optical elements: a photochemical study

Abstract: A new technique based on the imaging of systems in which a dry photopolymer layer is capable of memorizing optical information as a local change in thickness was developed. The generation of a relief structure takes place under the sole action of light. The latent image is revealed by the self-processing character of the material and requires no chemical treatment. The relief formation was found to depend on a spatial gradient of monomer conversion and a surface free-energy gradient. Both lead to mass transport of reactive species, that depend on photonic, optical and physicochemical parameters. This process permits the use of these optical elements in reflection after metal coating or the fabrication of masters for embossing technology. In addition, the photopolymer films appear to be suited to the recording of computer-generated focusing elements for infrared radiation.

Hybrid organic - inorganic materials with second-order optical nonlinearities synthesized via sol - gel chemistry

Abstract: The sol - gel process provides a method to blend, down to the molecular scale, organic and inorganic components which can be mixed in virtually any ratio to obtain the desired properties. These hybrid nanocomposites are extremely versatile in their composition, processing and optical and mechanical properties. Hybrid organic - inorganic materials offer a wide range of interesting applications especially in optics. A short review of the most striking examples of hybrid organic - inorganic material used to produce materials with second-order optical nonlinearities is presented.

Resonances in a single thin dielectric layer: enhancement of the Goos - Hänchen shift

Abstract: We report experimental results on a new type of resonance induced in a single dielectric film. In our system, a single dielectric layer, evaporated onto a prism base, acts as a Fabry - Perot-type cavity, with narrow resonances around the cut-off frequency. Near a resonance, this device, which is simpler than those based on a multilayered structure, has been used for amplifying the Goos - Hanchen shift.

Diffraction of a time Gaussian-shaped pulsed plane wave from a slit

Abstract: The spectral shifts of a Gaussian-shaped pulsed source in diffraction from a slit are discussed.

Exciton dynamics and trapping in J-aggregates of carbocyanine dyes

Abstract: Exciton relaxation processes in J-aggregates of two thiacarbocyanine dyes (TDC and THIATS) in water/ethylene glycol low temperature glass have been studied by means of steady-state, site-selective and time-resolved spectroscopy. The results of different experiments can be rationalized within the framework of exciton self-trapping (ST) in a 1D molecular chain. The measured Stokes shift is found to be proportional to the exciton - phonon coupling strength squared that is in accordance with the model of large radius ST excitons. A negative fluorescence anisotropy was observed for J-aggregates of both investigated dyes upon excitation outside the J-band. The observed monotonic dependence of the fluorescence anisotropy on the excitation wavelength is interpreted quantitatively within the framework of intraband absorption and Davydov splitting.

From one- to two-dimensional complexes for quadratic nonlinear optics: the influence of ligand and complexing metal atoms

Abstract: Metal complexes display interesting properties in terms of molecular optimization of quadratic nonlinear properties, combined with a good thermal stability and a large flexibility of geometrical and electronic properties. The role of dimensionality and intramolecular charge transfer of the pure ligand is investigated here, in particular for Schiff base derivatives. Also, the influence of metal electronic configurations on the quadratic hyperpolarizability is investigated in the series of Ni II, Co II and Cu II atoms, showing the interest of open-shell structures as compared to closed-shell complexes in the case of a weakly nonlinear ligand. Finally, strongly octupolar two-dimensional systems, subphthalocyanines, are shown to present very large second-order nonlinear polarizabilities, with static values as high as measured on a trinitro-substituted SubPc by the harmonic light scattering technique.

Polarimetric fibre-optic strain gauge using two-mode highly birefringent fibres

Abstract: Possible strategies of practical fibre-optic strain gauges using polarimetric and polarization sensitive intermodal interferometric responses in different types of two-mode highly birefringent (Hi-Bi) fibres are discussed. Using general Mueller - Stokes polarization analysis practical expressions for the single-mode polarimetric and two-mode interferometric responses are briefly derived and the corresponding visibilities are obtained. A special attention is paid to the influence of temperature in both regimes of operation, including the cross-sensitivity effect. While a temperature compensated solution by means of a splice is used in the single-mode polarimeter, a simultaneous strain-and-temperature measurement is suggested in the bimodal case. The appropriate conditions for such a two-parameter measurement are considered in detail.

A reflective grating interferometer for measuring the refractive index of liquids

Abstract: A simple and compact interferometric system, which makes use of a reflective-type diffraction grating, has been developed for measuring the refractive index of liquids. This paper describes the details of the system, which is easy to align and compact, and a series of measurements performed on liquids. An analysis of the accuracy of the method is presented, which is proven to be within one part in .

On-axis focal shift effects in focused truncated Bessel beams

Abstract: The focusing properties of truncated J0 Bessel beams are studied by means of the Huygens-Fresnel diffraction integral. Numerical calculations are presented for different Fresnel numbers, showing the effects of the finite aperture on the on-axis irradiance distribution.

Laser-induced excitation and dispersed fluorescence spectra of the ethoxy radical

Abstract: Extensive laser excitation and dispersed fluorescence spectra of the ethoxy radical have been recorded in a supersonic jet expansion. Neon transitions have been used to calibrate both the wavelength of the excitation dye laser and the optical multichannel analyser system used to record the wavelength-resolved emission spectra. Both the excitation and dispersed spectra are characterized by prominent progressions involving the C - O stretch vibrational mode. Seven vibrational frequencies for the excited electronic state and eight for the ground state have been assigned. To the best of our knowledge, 10 of these 15 assigned frequencies are reported here for the first time. Vibrational and anharmonic constants for the C - O stretch mode have been determined via least-squares fits for the X state and the B state .

The concept of guiding light with light and negative third-order optical nonlinearities of organics

Abstract: Dark spatial solitons can be used for generating light-induced waveguides and guiding light with light. A negative (self-defocusing) third-order nonlinearity is necessary to generate dark solitons. We show that the negative electronic nonlinearity is possible at 800 nm in several classes of organics including squaraine dyes, -conjugated polymers of poly(p-phenylenevinylene) type and polyaniline.

Determination of the refractive indices of highly biaxial anisotropic coatings using guided modes

Abstract: Precise determination of the main refractive indices of biaxial anisotropic coatings is essential for the calculation of their optical performance. Earlier measurements based on wave guiding methods have proved to yield excellent results with obliquely deposited and hence anisotropic layers, deposited under an angle of or less. But with higher deposition angles, new problems arise due to the less defined microstructure and the increasing influence of the accuracy of the column inclination angle on the resulting indices. In a theoretical study, these effects are shown to limit the precision of the method. Experimental results at on coatings, deposited at angles of , , and , are given. All three main refractive indices were found to decrease with an increasing deposition angle, while anisotropy increases almost linearly.

Demodulation scheme for fibre Bragg sensors based on source spectral characteristics

Abstract: A passive all-fibre technique for Bragg wavelength shift detection is presented which can be considered as the minimum configuration for fibre grating sensing demodulation. The scheme involves a light source with a particular spectral profile that allows the direct tracking of Bragg wavelength shifts. Referencing to source power fluctuations is also provided. This simple technique shows satisfactory resolution over a large dynamic range. Experimental results are presented for temperature and strain measurements.

An optical fibre delay line with a 318 mm stroke

Abstract: We report the implementation of an optical fibre delay line demonstrating the capability of silica fibre to induce a large variable group delay. It is the first time, to our knowledge, where a stretching process has been used to achieve a large delay line. A stroke equivalent to an air path of 318 mm is reached by stretching a 20 m waveguide. The low distortions of polarization and dispersion properties allow us to observe well contrasted fringes over the stroke. The induced losses are low.

Refractive index profile and geometry measurements in multicore fibres

Abstract: The refracted near-field technique (RNF) has been applied to the measurement of the refractive index profiles in multicore fibres (MCF). The fibre core diameter and the intercore distances have been determined.

Ultrashort pulse second harmonic generation in quasi-phase-matched structures

Abstract: Second harmonic generation of ultrashort pulses in first- and higher-order quasi-phase-matched nonlinear structures is investigated theoretically It is shown that using such structures high conversion efficiency can be achieved while preserving the ultrashort pulse duration Using periodically poled bulk lithium niobate as an example, it is found that nearly complete conversion of a 900 nm wavelength 100 fs pulse to the second harmonic may be possible in a first-order structure without any broadening of the pulse width

Sol - gel glasses for nonlinear optics

Abstract: In recent years a large amount of activity has been carried out searching for novel or advanced materials which exhibit large nonlinear third-order susceptibility . Emphasis has been put on materials suitable for integrated optical circuits, since the guided-wave format can provide additional routes to the conception and realization of photonic devices. The preparation of nanoparticle-doped glass by the sol - gel technique represents one of the most promising approaches: here we give a short overview of alternative processes to control the size of semiconductor clusters and we present experimental results from CdS- and PbS-doped silica - titania sol - gel films.