RESUMO
The second order nonlinear optical response of gold nanoprisms arrays is investigated by means of second harmonic generation (SHG) experiments and simulations. The polarization dependence of the nonlinear response exhibits a 6-fold symmetry, attributed to the local field enhancement through the excitation of the surface plasmon resonances in bow-tie nanoantennas forming the arrays. Experiments show that for polarization of the input light producing excitation of the plasmonic resonances in the bow-tie nanoantennas, the SHG signal is enhanced; this despite the fact that the linear absorption spectrum is not dependent on polarization. The results are confirmed by electrodynamic simulations which demonstrate that SHG is also determined by the local field distribution in the nanoarrays. Moreover, the maximum of SHG intensity is observed at slightly off-resonance excitation, as implemented in the experiments, showing a close relation between the polarization dependence and the structure of the material, additionally revealing the importance of the presence of non-normal electric field components as under focused beam and oblique illumination.
RESUMO
An experimental method for quantifying disorder within the anulus fibrosus is described based on polarization-modulated second harmonic generation imaging (PM-SHG-I). This method is demonstrated by imaging the anular lamellar architecture of a mouse model of compressive loading. Results were consistent with those obtained in an earlier study where organization was quantified directed secants image analysis on photomicrographs. In this study the orientation within individual lamellia is quantified by average orientation of the collagen molecules within a defined volume of a single lamellar as measured by the PM-SHG-I. Lamellar boundaries can be identified through the SHG intensity images, and confirmed through co-registration with photomicrographs of the same region. The orientation within the lamellar is quantified by the polarization angle of the maximum second harmonic intensity. PM-SHG-I offers several advantages as compared with the method of directed secants: first, it is nondestructive, allowing repeated measurements of the same tissue; second, images are captured on the order of seconds and capable of obtaining information up to a depth of 200-300 microns, thus allowing for real-time assessment of load damage; third, organization is measured at a much higher resolution, as it is based on disorder within the molecular arrays of a single lamella.
RESUMO
An analytical solution is presented to the vectorial coupled wave equations for the steady state amplitudes of two waves interacting in cubic photorefractive crystals. The solution accounts for pump depletion as well as an arbitrary phase shift between the interference pattern and photorefractive grating. It is shown that bidirectional vectorial amplification and polarization orthogonalization of the interacting beams take place in photorefractive crystal with diffusion, drift or mixed mechanism of space-charge formation.
RESUMO
We present a new method of light amplification based on vectorial two-wave mixing in cubic crystal via an unshifted photorefractive grating. The energy transfer between interacting waves is achieved in an experimental configuration that provides the amplitudes of modulation of refractive indices of opposite signs, Dn(1)=-Dn(2), for two orthogonal polarization modes of the interacting waves. The configuration demonstrates bidirectional amplification; i.e., the weak wave is always amplified, independent of its direction with respect to the strong pump beam. We present the results of numerical analysis along with the experimental results for a Bi(12)TiO(20) crystal with a dc applied electric field.