Optical second harmonic generation in a low-bandgap polymer

Recently, much research has been performed on developing low-bandgap polymers for e.g. harvesting solar energy. In the quest to improve these properties, little attention has been paid to their nonlinear optical properties, despite their interesting linear optical spectra and structural similarities to certain nonlinear optically active compounds. We characterized the optical second harmonic generation of corona poled films of poly(cyclopenta[2,1-b;3,4-b′] dithiophen-4-ylidenedioctylmalonate). The unexpectedly large nonlinear optical susceptibilities and the thermal and temporal stability of the material compare favorably to other novel nonlinear optical materials despite the lack of a donor-acceptor dye. Additionally, the polymer displays a very low absorption in the relevant wavelength region. These results demonstrate the promise of these materials for nonlinear optical devices.     

Point Group Symmetry Determination via Observables Revealed by Polarized Second-Harmonic Generation Microscopy: (1) Theory

We present a methodology based on polarization-controlled second-harmonic generation microscopy that allows one to determine the point group symmetry of noncentrosymmetric structures in situ and in vivo in complex systems regardless of the occurrence of periodicity. Small, randomly oriented structures suffice for the analysis, which is based on simple recognition of observables in four tests. These can be performed in any standard SHG-microscope that allows polarization control of the incident and detected light. The method is resilient to birefringence and light dispersion.     

Subsurface atomic force microscopy: towards a quantitative understanding

Recent experiments in the field of subsurface atomic force microscopy have demonstrated that it is possible to nondestructively image micro- and even nanoparticles that are embedded significantly deep within the bulk of a sample. In order to get insights into the contrast formation mechanism, we performed a finite element analysis and an analytical study, in which we calculated the amplitude and phase variation on the surface of an ultrasound wave that has traveled through the sample. Our calculations were performed as closely as possible to the situation in the experiments to enable a (future) comparison based on our predictions. We show that Rayleigh scattering of acoustic waves accounts for the measured contrast and we verify the characteristic Rayleigh dependences. The numerical results show that the contrast is independent of the depth at which a particle is buried, whereas the analytical study reveals a 1/depth dependence. In addition, we find a large deviation in the width of the particle in the contrast at the surface when applying the numerical or the analytical calculation respectively. These results indicate the importance of both the reflections of sound waves at the sample interfaces and bulk damping, as both are treated differently in our two models.     

Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals,Recent Progress,and Outlook

Strong chiroptical effects recently reported result from the interaction of light with chiral plasmonic nanostructures. Such nanostructures can be used to enhance the chiroptical response of chiral molecules and could also significantly increase the enantiomeric excess of direct asymmetric synthesis and catalysis. Moreover, in optical metamaterials, chirality leads to negative refractive index and all the promising applications thereof. In this Progress Report, we highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures. These strategies consecutively highlight the importance of chirality in the nanostructures (for linear and nonlinear chiroptical effects), in the experimental setup and in the light itself. Because, in the future, manipulating chirality will play an important role, we present two examples of chiral switches. Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light.     

On the use of evolutionary feature selection for improving fuzzy rough set based prototype selection

The k-nearest neighbors classifier is a widely used classification method that has proven to be very effective in supervised learning tasks. In this paper, a fuzzy rough set method for prototype selection, focused on optimizing the behavior of this classifier, is presented. The hybridization with an evolutionary feature selection method is considered to further improve its performance, obtaining a competent data reduction algorithm for the 1-nearest neighbors classifier. This hybridization is performed in the training phase, by using the solution of each preprocessing technique as the starting condition of the other one, within a cycle. The results of the experimental study, which have been contrasted through nonparametric statistical tests, show that the new hybrid approach obtains very promising results with respect to classification accuracy and reduction of the size of the training set.     

Drift detection and removal for sequential structure from motion algorithms

In sequential Structure from Motion algorithms for extended image or video sequences, error build up caused by drift poses a problem as feature tracks that normally represent a single scene point will have distinct 3D reconstructions. For the final bundle adjustment to remove this drift, it must be told about these 3D-3D correspondences through a change in the cost function. However, as a bundle adjustment is a nonlinear optimization technique, the drift needs to be removed from the supplied initial solution to allow for convergence of the bundle adjustment to the real global optimum. Before drift can be removed, it has to be detected. This is accomplished through understanding of the long term behavior of drift which leaves 3D reconstructions from short sequences intact. Drift detection boils down to identifying reconstructions of the same scene part that only differ up to a projective transformation. After detection, the drift can be removed from future processed images and an Adapted Bundle Adjustment using correspondences supplied by the drift detection can remove the drift from previous images. Several experiments on real video sequences demonstrate the merit of drift detection and removal.     

Visual Modeling with a Hand-Held Camera

In this paper a complete system to build visual models from camera images is presented. The system can deal with uncalibrated image sequences acquired with a hand-held camera. Based on tracked or matched features the relations between multiple views are computed. From this both the structure of the scene and the motion of the camera are retrieved. The ambiguity on the reconstruction is restricted from projective to metric through self-calibration. A flexible multi-view stereo matching scheme is used to obtain a dense estimation of the surface geometry. From the computed data different types of visual models are constructed. Besides the traditional geometry- and image-based approaches, a combined approach with view-dependent geometry and texture is presented. As an application fusion of real and virtual scenes is also shown.