Showing papers by "Sophie Brasselet published in 2007"

Defocused imaging of second harmonic generation from a single nanocrystal.

Abstract: We demonstrate the direct imaging of the second harmonic generation radiation from a single nonlinear nanocrystal using defocused nonlinear microscopy. This technique allows the retrieval of complete information on the 3D orientation of a nanocrystal as well as possible deviations from its purely crystalline nature, in a simple experimental implementation. The obtained images are modeled by calculation of the radiation diagram from a nonlinear dipole that accounts for the excitation beam, the crystal symmetry and the particle size. Experimental demonstrations are performed on Potassium Titanyl Phosphase (KTP) nanocrystals. The shape and structure of the radiation images show a strong dependence on both crystal orientation and field polarization state, as expected by the specific nonlinear coherent coupling between the induced dipole and the excitation field polarization state.

Role of spatial distortions on the quadratic nonlinear optical properties of octupolar organic and metallo-organic molecules

Abstract: Following on the recent experimental demonstration of a discrepancy between the nonlinear optical (NLO) behavior of several π-conjugated chromophores and their assumed octupolar symmetry, the authors investigate how geometrical distortions influence the NLO response of multipolar push-pull molecules. Their analytical model is set on a basis of valence-bond and charge-transfer states to estimate the hyperpolarizability of organic and metallo-organic chromophores using the lowest possible number of variables. Since symmetry breakdown changes the definition of the molecular Cartesian framework, tensorial spherical coordinates are implemented. The evolution of the nonlinear molecular anisotropy with possible rotational deviations is then evaluated for two recently studied chromophores. Zero-frequency calculations show that, outside optical resonance, weak geometrical distortions lead to strong anisotropy variations in agreement with experimental data. Their goal is to underscore which molecular engineering st...

Octupolar Films with Significant Second-Harmonic Generation

Abstract: Considerable efforts are being made to develop octupolar nonlinear optical materials for possible application in optical and electro-optical devices. [1–3] An advantage of such molecules in comparison to the more conventional dipolar molecules is the existence of a broader range of tensorial coefficients, providing optimum nonlinear efficiency with a polarization-independent second-harmonic response with respect to the incident light. Great progress has been made by optimization at the molecular level, as exemplified by the established structure–property relationship of two-dimensional octupoles and the development of highly efficient molecules. [3–5] Recently, we reported that 1,3,5-tricyano-2,4,6tris(p-diethylaminostyryl)benzene (TTB) produces non-centrosymmetric crystals that show very large second-harmonic generation (SHG). [1] To integrate such materials in optical devices, it is essential to prepare thin films showing large SHG and high thermal stability. As electric poling is not possible because of the lack of a ground-state dipole moment, it is not readily obvious how to align octupolar molecules non-centrosymmetrically in a thin film. Although optical poling in polymers has been proposed, it requires photoisomerizable molecular functions and resonant excitation of the molecules by intense laser pulses, which may be a limitation for industrial application. An ideal solution to this problem would be to prepare self-oriented films without applying external fields. For this purpose, we have studied thin films of TTB in a polymethylmetacrylate (PMMA) matrix prepared by using free

Photostable Single KTiOPO 4 Nanocrystals for Second-Harmonic Generation Microscopy

Abstract: The finding of nonlinear nanometric-sized probes is of key importance for the development of nonlinear microscopy in nanosciences and biology. We isolate nonlinear KTiOPO4 nanocrystals with remarkable photostability in second-harmonic generation under femtosecond infrared laser light excitation. Their size distribution is determined using dynamic light scattering and atomic force microscopy. With both polarization analysis and defocused imaging of the emitted second-harmonic field, we also extract the Euler angles of the crystalline axes of a single nanocrystal. These sub-wavelength particles can find application as near-field vectorial probes.

Investigation of the second-harmonic light emission by KTiOPO 4 nanometric-sized crystals as an in situ nonlinear nanosource

Abstract: In this paper, we propose the inorganic nonlinear KTiOPO4 (KTP) crystals of nanometric size with a strong second-harmonic coherent emission when illuminated by femtosecond infrared pulses. The non-resonant character of the nonlinear interaction prevents from any photobleaching or blinking. The signal therefore exhibits a perfect photostability. We show that analysis of coherently emitted non-linear field allows us to characterize isolated nanocrystal orientation.