Dominique Chauvat

Photostable Second-Harmonic Generation from a Single KTiOPO4 Nanocrystal for Nonlinear Microscopy†

Nonlinear second-harmonic generation (SHG) microscopy has become a commonly used technique for investigating interfacial phenomena and imaging biological samples. Different non-centrosymmetric nanometric light sources have been recently studied in this context, for example, organic nanocrystals. For those systems, resonant optical interaction leads to an enhancement of the nonlinear response but also to a parasitic effect that is detrimental for practical applications, namely photobleaching due to two-photon residual absorption. Conversely, inorganic non-centrosymmetric materials with far-off resonance interaction avoid this limitation. Recent achievements have been obtained using KNbO3 nanowires as a tunable source for subwavelength optical microscopy and Fe(IO3)3 nanocrystallites as promising new SHG-active particles with potential applications in biology. However, either the dimensions of the used crystals are still of the order of the micrometer along one axis or the corresponding bulk material is not easily grown, so that the crystal characteristics are not directly available. A complementary approach consists of considering a well-known SHG-active bulk material and investigating its properties in nanoparticle form. Potassium titanyl phosphate (KTiOPO4, KTP) is a widely used nonlinear crystal. [11] Studies of this material have focused on the optimized growth of largesize single crystals, which have found numerous applications in

Second‐Harmonic Generation from a Single Core/Shell Quantum Dot

Nanoparticles emitting two-photon luminescence are broadly used as photostable emitters for nonlinear microscopy. Second-harmonic generation (SHG) as another two-photon mechanism offers complementary optical properties but the reported sizes of nanoparticles are still large, of a few tens of nanometers. Herein, coherent SHG from single core/shell CdTe/CdS nanocrystals with a diameter of 10 to 15 nm is reported. The nanocrystal excitation spectrum reveals resonances in the nonlinear efficiency with an overall maximum at about 970 nm. Polarization analysis of the second-harmonic emission confirms the expected zinc blende symmetry, and allows extraction of the three-dimensional nanocrystal orientation. The small size of these nonlinearly active quantum dots, together with the intrinsic coherence and orientation sensitivity of the SHG process, are well adapted for ultrafast probing of optical near-fields with high resolution as well as for orientation tracking for bioimaging applications.

Second-harmonic generation from coupled plasmon modes in a single dimer of gold nanospheres

We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy.

Defocused imaging of second harmonic generation from a single nanocrystal

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.

Measurement of positive and negative Goos–Hänchen effects for metallic gratings near Wood anomalies

Large Goos-Hänchen effects are isolated for reflection on a metallic grating. These shifts occur in the vicinity of Wood anomalies. Depending on the nature of the anomaly, these displacements are found to be either positive or, contrary to the usual GH effect, clearly negative. Those shifts, associated with forward and backward leaky surface waves, are as large as plus or minus tens of wavelengths for a classic metallic grating.

Balanced homodyne detection of second-harmonic generation from isolated subwavelength emitters

We demonstrate the association of two-photon nonlinear microscopy with balanced homodyne detection for investigating second harmonic radiation properties at nanoscale dimensions. Variation of the relative phase between second-harmonic and fundamental beams is retrieved, as a function of the absolute orientation of the nonlinear emitters. Sensitivity down to approximately 3.2 photon/s in the spatio-temporal mode of the local oscillator is obtained. This value is high enough to efficiently detect the coherent second-harmonic emission from a single KTiOPO4 crystal of sub-wavelength size.The authors demonstrate the association of two-photon nonlinear microscopy with balanced homodyne detection for investigating second-harmonic radiation properties at nanoscale dimensions. Variation of the relative phase between second-harmonic and fundamental beams is retrieved, as a function of the absolute orientation of the nonlinear emitters. Sensitivity down to a few photons per second in the spatiotemporal mode of the local oscillator is obtained. This value is high enough to efficiently detect the coherent second-harmonic emission from a single KTiOPO4 crystal of subwavelength size.

Coherent nonlinear emission from a single KTP nanoparticle with broadband femtosecond pulses

We demonstrate that the intensity of the second harmonic (SH) generated in KTiOPO(4) nanoparticles excited with femtosecond laser pulses increases with decreasing duration of the infrared pump pulses. The SH intensity scales, approximately, as the inverse of the laser pulse duration ranging between 13 fs and 200 fs. The SH intensity enhancement requires careful compensation of the high-order spectral phase, being achieved with a genetic algorithm. Using ultrashort laser pulses improves the signal-to-noise ratio and will allow the detection of 10-nm size particles. Finally, we demonstrate that the spectrum of broadband (100 nm) pulses can be shaped to generate non-degenerate sum-frequency mixing. This opens up access to the polarization degrees of freedom of this second-order nonlinear process at the nanoscale.

Polarization-sensitive two-photon microscopy study of the organization of liquid-crystalline DNA.

Highly concentrated DNA solutions exhibit self-ordering properties such as the generation of liquid-crystalline phases. Such organized domains may play an important role in the global chromatin topology but can also be used as a simple model for the study of more complex 3D DNA structures. In this work, using polarized two-photon fluorescence microscopy, we report on the orientation of DNA molecules in liquid-crystalline phases. For this purpose, we analyze the signal emitted by fluorophores that are noncovalently bound to DNA strands. In nonlinear processes, excitation occurs exclusively in the focal volume, which offers advantages such as the reduction of photobleaching of out-of-focus molecules and intrinsic 3D sectioning capability. Propidium iodide and Hoechst, two fluorophores with different DNA binding modes, have been considered. Polarimetric measurements show that the dyes follow the alignment with respect to the DNA strands and allow the determination of the angles between the emission dipoles and the longitudinal axis of the DNA double strand. These results provide a useful starting point toward the application of two-photon polarimetry techniques to determine the local orientation of condensed DNA in physiological conditions.

Direct measurement of the central fringe velocity in Young-type experiments

Abstract We show that in the Youngs two-slit experiment, the velocity of fringes on the axis of propagation is superluminal. This velocity is directly measured with a slightly modified Young set-up. The differential measurement method is generalized to the case of the Arago–Fresnel–Poisson spot in optics.

Liquid crystal phases of DNA: Evaluation of DNA organization by two-photon fluorescence microscopy and polarization analysis†

We report on the investigation of the structure of DNA liquid crystal (LC) phases by means of polarization sensitive two-photon microscopy (PSTPM). DNA was stained with fluorescent dyes, an intercalator propidium iodide, or a groove binder Hoechst 3342, and the angular dependence of the intensity of two-photon excited fluorescence emitted by the dye was collected. The local orientation of DNA molecules in cholesteric and columnar LC phases was established on the basis of the relative angle between the transition dipole of the dye and the long axis of DNA helix. Three-dimensional images of the cholesteric phase were obtained making use of the intrinsic 3D resolving ability of two-photon microscopy. We also discuss the influence of dyes on the parameters of DNA LC phases and comment on advantages and limitations of the PSTPM technique in comparison with other LC characterization techniques.