Nanowalled polymer microtubes fabricated by using strained semiconductor templates.

Herein we describe the realization of nanowalled polymeric microtubes through a novel and versatile approach combining the layer-by-layer (LbL) deposition technique, the self-rolling of hybrid polymer/semiconductor microtubes and the subsequent removal of the semiconductor template. The realized channels were characterized in detail using scanning electron and atomic force microscopes. Additionally, we report on the incorporation of a dye molecule within the nanowalls of such microtubes, demonstrating a distribution of the fluorescence signal throughout the whole channel volume. This approach offers the possibility to tailor the properties of micro/nanotubes in terms of size, wall thickness and composition, thus enabling their employment for several applications.

Optical polarization based logic functions (XOR or XNOR) with nonlinear Gallium nitride nanoslab.

We present a scheme of XOR/XNOR logic gate, based on non phase-matched noncollinear second harmonic generation from a medium of suitable crystalline symmetry, Gallium nitride. The polarization of the noncollinear generated beam is a function of the polarization of both pump beams, thus we experimentally investigated all possible polarization combinations, evidencing that only some of them are allowed and that the nonlinear interaction of optical signals behaves as a polarization based XOR. The experimental results show the peculiarity of the nonlinear optical response associated with noncollinear excitation, and are explained using the expression for the effective second order optical nonlinearity in noncollinear scheme.

Mapping the nonlinear optical susceptibility by noncollinear second-harmonic generation.

We present a method, based on noncollinear second-harmonic generation, to evaluate the nonzero elements of the nonlinear optical susceptibility. At a fixed incidence angle, the generated signal is investigated by varying the polarization state of both fundamental beams. The resulting polarization charts allows us to verify if Kleinman's symmetry rules can be applied to a given material or to retrieve the absolute value of the nonlinear optical tensor terms, from a reference measurement. Experimental measurements obtained from gallium nitride layers are reported. The proposed method does not require an angular scan and thus is useful when the generated signal is strongly affected by sample rotation.

The effects of the focus ion beam milling process on the optical properties of semiconductor nanostructures.

In this work, the effects of the focus ion beam (FIB) milling process on the optical properties of semiconductor nanostructures were investigated. With this aim, a sensitive materials system based on InGaAs/GaAs quantum dots with well known and excellent optical properties was selected for the FIB treatment. The FIB technique was used to locally remove a metallic mask deposited on top of the quantum dot sample. The photoluminescence (PL) signal, collected from the circular openings, was used to infer the possible damage effects of the ion beam on the properties of the dots.

Novel grating design approach by radiation modes coupling in nonlinear optical waveguides.

In integrated optics the radiation modes represent a negative aspect regarding the propagation of guided modes. They characterize the losses of the substrate region but can contribute to enhance the guided modes by considering the coupling through properly designed gratings arranged at the core/substrate interface. By tailored gratings, the radiation modes become propagating modes and increase the guided power inside the waveguide guiding region. This enhancement is useful especially in low intensity processes such as second harmonic chi(2) conversion process. For this purpose, we analyze accurately the radiation modes contribution in a chi(2) GaAs/AlGaAs nonlinear waveguide where second harmonic signal is characterized by a low power intensity. This analysis considers a new design approach of multiple grating which enhances a fundamental guided mode at lambda(FU) =1.55 microm and a codirectional second harmonic guided mode at lambda(SH) =0.775 microm. In particular we analyze the second harmonic conversion efficiency by studying the coupling effect of three gratings. The combined effects of the gratings provide an efficient second harmonic field conversion. Design considerations, based on the coupled mode equations analysis, are theoretically discussed. A good agreement between analytical and numerical results is observed.

Scalar time domain modeling and coupling of second harmonic generation process in GaAs discontinuous optical waveguide.

We present in this work the scalar potential formulation of second harmonic generation process in chi((2)) nonlinear analysis. This approach is intrinsically well suited to the applications of the concept of circuit analysis and synthesis to nonlinear optical problems, and represents a novel alternative method in the analysis of nonlinear optical waveguide, by providing a good convergent numerical solution. The time domain modeling is applied to nonlinear GaAs asymmetrical waveguide with dielectric discontinuities in the hypothesis of quasi phase matching condition in order to evaluate the efficiency conversion of the second harmonic signal. The accuracy of the modeling is validated by the good agreement with the published experimental results. The effective dielectric constant method allows to extend the analysis also to 3D optical waveguides.

Linear increase of the modal gain in 1.3 µm InAs/GaAs quantum dot lasers containing up to seven-stacked QD layers.

The authors have recently demonstrated the enhancement of the quantum dot laser modal gain, linearly scaling with the number of stacked QD layers. These results allowed the achievement of multi-quantum dot (MQD) lasers, the zero-dimensional counterpart of MQW lasers, with a modal gain as high as 42 cm(-1), in a seven-layer structure. A detailed investigation of the structural and optical properties was performed on laser structures with three, five and seven QD layers. Such an investigation clearly shows that the high uniformity of QD layer features is responsible for the linear increase of the modal gain and its high value.

Design and modeling of tapered waveguide for photonic crystal slab coupling by using time-domain Hertzian potentials formulation.

This work introduces a new simulation approach to the evaluation of the time-domain electromagnetic (EM) field useful in the modeling of tapered waveguide for the Photonic Crystal Slab (PCS) coupling. Only solutions of two scalar Helmholtz-equations are used in the evaluation of electric and magnetic Hertzian-potentials that yields the EM field and the frequency response of the tapered waveguide. By considering simultaneously an analytical and a numerical approximation it is possible to reduce the computational burden. In order to compare the computational time we analyze the 2D structure by also using the Finite Difference Time Domain (FDTD) method and by the 3D Finite Element Method (FEM). The method is applied by starting from design criteria of the tapered structures in order to set the correct geometrical and physical parameters, and considers the field-perturbation effect in proximity of the dielectric discontinuities by generators modeling.

Correlation between shape and electronic states in nanostructures

We show how the electronic states of quantum wires and quantum dots can be evaluated exactly starting from the profile of the nanostructure observed by transmission electron microscopy, scanning tunneling microscopy and atomic force microscopy. The calculated quantization energies reproduce the energy position of the luminescence resonances in the optical spectra of different samples, without fitting parameters.