Marcin Zielinski

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.

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.

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.

Nanoengineering the second order susceptibility in semiconductor quantum dot heterostructures

We study second-harmonic generation from single CdTe/CdS core/shell rod-on-dot nanocrystals with different geometrical parameters, which allow to fine tune the nonlinear properties of the nanostructure. These hybrid semiconductor-semiconductor nanoparticles exhibit extremely strong and stable second-harmonic emission, although the size of CdTe core is still within the strong quantum confinement regime. The orientation sensitive polarization response is analyzed by means of a pointwise additive model of the third-order tensors associated to the nanoparticle components. These findings prove that engineering of semiconducting complex heterostructures at the single nanoparticle scale can lead to extremely bright nanometric nonlinear light sources.

Second-harmonic generation from single core-shell CdTe/CdS quantum dots

We observed second harmonic generation from semiconducting CdTe(CdS) nanocrystals with a diameter below 15 nm. Their submicron size, high nonlinearity and orientation sensitive SHG response are adapted for ultrafast, high-resolution probing of optical near-fields.

Second-harmonic generation from single core-shell CdTe(CdS) quantum dots

We observed second harmonic generation from semiconducting CdTe(CdS) nanocrystals with a diameter below 15 nm. Their submicron size, high nonlinearity and orientation sensitive SHG response are adapted for ultrafast, high-resolution probing of optical near-fields.

Nonlinear polarimetric analysis of DNA liquid crystalline domains

In nuclei of eukaryotic cells, DNA molecules are tightly packed up to a concentration level of several hundreds of mg/ml. At high concentration, in order to decrease the intermolecular volume the DNA strands self-organize and form domains containing molecules oriented along preferential directions. These self-organizing properties give rise to various liquid crystalline phases, similar to the process of physiologic DNA condensation [1]. Liquid crystalline phases were observed in concentrated DNA solutions, but also in nucleosome-like particles or plasmids solutions [2–4]. Besides their biologic implication, these self-organized structures may serve as a simple model for the structural study of DNA higher order organization schemes and as a first approach mimicking the complex chromatin topology.