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COST, European Cooperation in the Field of Scientific and Technical Research, was established in 1971 as a framework for scientific and technical cooperation, allowing the coordination of national research on a European level. COST Actions consist of basic and precompetitive research. The goal of COST is to ensure that Europe holds a strong position in the field of scientific and technical research for peaceful purposes, by increasing European cooperation and interaction in this field. COST has developed into one of the largest frameworks for research cooperation in Europe and is a valuable mechanism coordinating national research activities at European level. Today its nearly 200 Actions involve some 40000 participating scientists from 32 European member countries and nearly 50 more participating institutions from an additional 14 countries. These networks represent a value of more than 2 billion.

The Technical Committee of Physics is the most recently formed of the COST Technical Committees, having been approved in 1997, and currently consists of six Actions. The COST Action P2, Applications of Nonlinear Optical Phenomena, has been running since 1997 and has to date been signed by 16 member countries. In the frame of this Action a general workshop is held every year: the second general workshop was held in Amalfi (Italy) from 6-9 October 1999, with the title `Applications of Nonlinear Optical Phenomena and Related Industrial Perspectives'.

Our aim was to provide an industrial and academic forum for presentations and panel discussions on the state-of-the-art nonlinear optical phenomena applications in Europe. Further, we wanted to allow for open discussions on current industrial and academic achievements, as well as future perspectives in this area. For this purpose the Workshop was also open to scientists not belonging to the COST Action P2 and it was promoted all over Europe.

The Workshop was held in Amalfi at the Hotel Cappuccini Convento. This is a prestigious Hotel in a historical monastery dated from the 11th century, which provided a most enjoyable and suitable location for a very exciting scientific meeting. By unanimous consent this event was a complete success in every aspect, but primarily in the scientific one. During the three and a half days of the Workshop, we had 77 registered participants, 4 keynote speakers, 6 invited chairpersons and speakers, 21 contributed speakers, 18 poster presentations and 2 general discussions, plus the Management Committee meeting of the Action P2. The whole organization of the Workshop was our attempt to fulfil the spirit of COST. We hope to have at least partially reached this objective, and we wish that new fruitful collaborations will establish themselves and that old ones may become stronger. The Management Committee of the COST Action P2 can confirm these results since the next general workshop is in Twente, Netherlands, in October 2001.

The workshop was not intended to be a small topical conference but instead, as said before, a forum for discussions; nevertheless the scientific content of the presentations was on average very high. Thus, in order to keep a permanent record of the presentations given and of the most important scientific achievements discussed in Amalfi, and with the support of the Management Committee of the COST Action P2, two important initiatives were taken.

The first was the realization of a CD-ROM containing the relevant information about the Workshop, namely the programme, the list of participants, the list of abstracts and all the available presentations in electronic format.

The second, in agreement with Institute of Physics Publishing and the Editors of the journal, was the publication of a selected number of original papers presented in Amalfi in this special issue of the Journal of Optics A: Pure and Applied Optics. The papers published were selected and submitted to the standard peer review procedure of the journal. The reviewing was performed strictly following the rules and indications of the journal by a number of experts in the field: they deserve our gratitude for their time and contribution to ensuring the high scientific quality. Many aspects of nonlinear optical applications are covered, ranging from telecommunications to optical data storage and pattern recognition, as well as hot topics such as photonic band gaps. Some very interesting and important papers deal with molecular engineering, which is a crucial point in the challenge amongst NLO materials. The issue opens with a review article by Toskikuni Kaino, which gives a highly valuable overview of recent developments of polymeric materials for NLO. Showing the possibility to overcome many of the drawbacks of older polymeric optical devices, it gives a possible explanation of the re-growing interest of the NLO community in these kind of materials.

For the financial support that permitted not only the organization of the workshop but also the realization of this special issue, we are indebted to the Department of Physical Sciences of the University of Naples Federico II, to the Istituto Nazionale per la Fisica della Materia INFM and to the COST Secretariat.

As guest editors, we thank all those who submitted their contribution for inclusion in this special issue. Special thanks are also due to the editorial staff of the journal, who supported and helped us in our work.

Among many types of nonlinear optical (NLO) polymers developed, this paper has focused on azo-dye-functionalized polymers and discussed their channel waveguide fabrication for photonic applications. After examining the critical issues of the polymer waveguide fabrication, serially-grafted quasi-phase-matching waveguide for frequency conversion, 3rd-order NLO polymer waveguide for all optical switch and trench-filled waveguide grating for wavelength filter were discussed. The serially-grafted waveguide structure described in this paper will be a key area for future photonic signal processing systems.

Calculations of the electronic and vibrational second hyperpolarizabilities of various quadrupolar π-conjugated molecules performed at the ab initio Hartree-Fock level of approximation show that the vibrational contribution is large for most nonlinear optical phenomena. As for unsubstituted π-conjugated compounds, the Raman term which contributes to the dc-Kerr and degenerate four-wave mixing phenomena is of the same order of magnitude as the static electronic counterpart. The term related to the infrared and hyper-Raman intensities is substantial and, in the case of a squaraine dye, dictates the sign of the global response. This infrared-hyper-Raman term which is often negligible for unsubstituted compounds contributes substantially to the vibrational dc-Kerr and electric field-induced second harmonic generation responses.

The purpose of this paper is twofold. First, we demonstrate optical fixing through two-step gated recording in a crystal of La₃Ga₅SiO₁₄ doped with praseodymium. Second, based on these experiments we suggest an excitation-recombination model for the photorefractive effect in the crystal and comment on the nature of the intermediate level involved in the gated recording process.

We describe the utilization of a cascade of two second-order processes for the efficient generation of a wavelength-shifted field. The overall process is equivalent to a nearly degenerate, four-wave mixing process. The interesting aspect is that the corresponding effective third-order nonlinearity is fast and nonabsorbative, and is much larger than that available in third-order materials. Experiments have been performed with a periodically poled lithium niobate (LN) crystal and a single-mode LN waveguide, by using 20 ps pulses in the near infrared. The obtained results represent a first step towards the realization of a nonlinear device for application to optical communications systems.

We report on nonlinear absorption measurements of tetraphenyl-diamine dyes developed for use as transparent materials for optical limiting applications in the visible range. All the excited state properties which are relevant to the process are investigated experimentally using three different and complementary nonlinear spectroscopy tools. Through a modification of the substitution of the peripheral benzene rings of the original dye, we could significantly improve its optical limiting activity, especially in the red region where it had a rather poor efficiency.

Azobenzene chromophores are promising as molecularly engineered materials for reversible optical data storage based on molecular reorientation. In this paper, the optical properties of several different azobenzene chromophores are studied using molecular quantum calculations. Special emphasis is put on molecular anisotropy since a high degree of anisotropy is essential for the storage performance. The trans isomers are all found to be practically one-dimensional whereas the anisotropy of the cis isomers is highly dependent on substituents. Molecular reorientation of chromophores in liquid-crystalline polymers is simulated in order to study the influence of lacking cis anisotropy. It is demonstrated that photoinduced birefringence is significantly reduced in materials characterized by a low degree of cis anisotropy.

We report on measurements of laser emission from poly-methylmethacrylate and poly-vinyl carbazole polymer films doped with rhodamine-6G, DCM and coumarin laser dyes in an optically pumped distributed feedback scheme. We obtain tunability on a broad spectral range for all samples. We show the impact of waveguiding in the polymer film on reducing the laser threshold. We also show that the number of laser modes increases with the polymer film thickness, following the guided mode dispersion.

A molecular engineering strategy is developed around bis(di-n-methylamino)-E-stilbene 1 in order to design efficient two-photon absorbers in the visible range. This strategy is based on the role of the charge transfer conjugated system, and of donor substituents on theoretical linear and two-photon absorption (TPA). The best trade-off TPA-linear absorption is fulfilled by two tetraphephenyl-diamine derivatives (biphenyl and fluorene), which could allow, if mixed, efficient coverage of the visible range.

Spectra of high-order harmonics, generated by 30 fs and sub-10 fs laser pulses in neon and krypton, have been obtained. The main characteristics of the measured spectra are analysed and discussed. For the 30 fs regime experimental results are compared with spectra calculated by means of a 1D propagation code which partially accounts for 3D focusing effects.

Optical reorientation in nematic liquid crystals turns out to depend not only on the intensity and polarization of the pumping laser beam, but also on the shape of the transverse cross-section. A longitudinal extra torque acting on the molecular director appears whenever the beam cross-section shape breaks the system cylindrical symmetry.

The theory of parametric generation and amplification in artificially gyrotropic (magneto-optic) media is presented. Complete solutions for signal and idler are given in terms of Stokes' parameters, and polarization state dependences of applied static magnetic field strength are discussed in terms of trajectories of the reduced Stokes vector on the Poincaré sphere.

A self-regulating method for the control of spontaneous instabilities of the transversal beam profile in nonlinear optical systems is experimentally realized. The control method is the high-dimensional analogue of classical negative feedback regulators. An all-optical implementation with its capabilities of parallel processing is essential for the experimental feasibility. Unstable system-inherent transversal patterns are stabilized, including also the stationary homogeneous state. Even spatio-temporal disorder is removed in favour of the highly symmetric, stationary patterns and nearly without losses of output power. The manipulation of the output state is demonstrated to be noninvasive. This allows investigations of the otherwise inaccessible unstable patterns, for which an example is given.

We report on saturated-absorption spectroscopy of H₂¹⁸O at 1.39 µm by means of a low-power distributed feedback diode laser. In the transit-time regime, we observed a 2.5 MHz wide line. This is the first narrow resonance in the spectral window around 1.4 µm. Possible applications in metrology are discussed. In addition, we propose some experimental schemes to measure the absolute frequency of H₂¹⁸O lines in this spectral region.

We present both a theoretical and experimental analysis of nonlinear polarization rotation in an optical fibre. Starting from the coupled nonlinear Schrödinger equations an analytical solution for the evolution of the state of polarization, valid for fibres with large linear birefringence and quasi cw input light with arbitrary polarization, is given. It allows us to model straightforwardly go-and-return paths as in interferometers with standard or Faraday mirrors. In the experiment all the fluctuations in the linear birefringence, including temperature- and pressure-induced ones, are successfully removed in a passive way by using a double pass of the fibre under test with a Faraday mirror at the end of the fibre. This allows us to use long fibres and relatively low input powers. The match between the experimental data and our model is excellent, except at higher intensities where deviations due to modulation instability start to appear.

This paper reports on an investigation of optical crosstalk caused by nonlinear interactions in short- and long-haul wavelength division multiplexion transmission systems called inter-domain interfaces. These investigations are based on numerical simulations of multichannel optical transmission systems, using specialist software: Gigabit Optical Line Designer v.2.0 from Virtual Photonics. The nonlinear phenomena in fibres that have to be taken into account include four-wave mixing, self-phase modulation, cross-phase modulation, stimulated Raman scattering, stimulated Brillouin scattering and nonlinear interactions in erbium-doped fibre amplifiers.

Band edge effects such as increased density of modes, large field enhancement, and low group velocity will provide highly efficient parametric interaction if the proper phase matching conditions are established in one-dimensional photonic band gap (PBG) structures. The analysis of the dispersion properties of one-dimensional PBG structure allows us to define the suitable phase matching conditions for parametric interaction when quadratic nonlinear materials constitute the layered geometry.

It has been possible to excite a spatial soliton in a dye doped liquid crystal confined in a capillary tube. The phenomenon is explained by a pure thermal nonlinear effect due to dye absorption.

We investigate the evolution of Gaussian input conditions in fibre links with dispersion management. Gaussian pulses, which do not exactly match any system eigenmode, suffer from both initial transient evolutions and asymptotic slow oscillations. By means of chirped Hermite-Gauss bases, we analytically characterize and discuss these pulse dynamics in terms of interplay among the different Hermite-Gauss modes.

We study a mechanism of light-by-light control, based on changing the refractive index of the narrow-band layers in a semiconductor photonic band gap structure (PBGS). This change is caused by the contribution of the non-equilibrium charge carriers generated by the controlling radiation. We theoretically show that this mechanism can be efficient if an optimal controlling light wavelength is taken near the proper absorption edge of the narrow-band semiconductor layers, since small changes of the layer refractive indices at the band gap edge of a PBGS cause essential change of the transmission characteristics. A modulation depth of the controlled light of more than 90% is achieved for a PBGS based on GaAs at the wavelength 1.54×10^-6 m under the controlling light power equal to 5×10⁷ W m^-2. The inertia of the mechanism considered is determined by the lifetime of the non-equilibrium charge carriers in semiconductor layers and is equal to 10^-7 s in our case.