Digital phase conjugation of second harmonic radiation emitted by nanoparticles in turbid media

We demonstrate focusing coherent light on a nanoparticle through turbid media based on digital optical phase conjugation of second harmonic generation (SHG) field from the nanoparticle. A SHG active nanoparticle inside a turbid medium was excited at the fundamental frequency and emitted SHG field as a point source. The SHG emission was scattered by the turbid medium, and the scattered field was recorded by off-axis digital holography. A phase-conjugated beam was then generated by using a phase-only spatial light modulator and sent back through the turbid medium, which formed a nearly ideal focus on the nanoparticle

Second harmonic generation from nanocrystals under linearly and circularly polarized excitations

We study second harmonic generation (SHG) from non-centrosymmetric nanocrystals under linearly polarized (LP) and circularly polarized (CP) excitations. Theoretical models are developed for SHG from nanocrystals under both plane-wave and focused excitations. We find that the focused excitation reduces the polarization dependency of the SHG signal. We show that the SHG response under CP excitation is generally inferior to the average of LP excitations over all orientations. We verify the theory by measuring the SHG polar responses from BaTiO_3 nanocrystals with a scanning confocal microscope. The experimental data agrees well with the theory

Nonlinear Optical Properties of Core-Shell Nanocavities for Enhanced Second-Harmonic Generation

A nonlinear optical plasmonic core-shell nanocavity is demonstrated as an efficient, subwavelength coherent light source through second-harmonic generation. The nonlinear optical plasmonic nanocavity incorporates a noncentrosymmetric medium, which utilizes the entire mode volume for even-order nonlinear optical processes. In previous plasmonic nanocavities, enhancement of such processes was only possible at the interface but symmetry prohibited in the body. We measured an enhancement of over 500 times in the second-harmonic radiation power. Calculations show that an enhancement of over 3500 times is achievable

Imaging through turbid layers by scanning the phase conjugated second harmonic radiation from a nanoparticle

We demonstrate imaging through a turbid layer by using digital phase conjugation of the second harmonic field radiated from a beacon nanoparticle. We show that the phase-conjugated focus can be displaced from its initial position by illuminating the same region of the turbid layer with an angular offset. An image is obtained by scanning the phase-conjugated focus through the turbid layer in a region around the nanoparticle. We obtain a clear image of the target by measuring the light transmitted through it when scanning the focused beam

Barium titanate nanoparticles used as second harmonic radiation imaging probes for cell imaging

Luminescent markers play a key role in imaging techniques for life science since they provide a contrast mechanism between signal and background. We describe a new type of marker using second harmonic generation (SHG) from noncentrosymmetric BaTiO_3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. In this paper, we report the use of nanoparticles for cell imaging. We first stabilized the BaTiO_3 nanoparticles in suspension and characterized the optical properties. We also demonstrated the functionalization of BaTiO_3 nanoparticles by conjugating IgG antibody on the surface of the nanoparticles. These functionalized nanocrystals are capable of specific labeling the antigens of interest

Characterization of the cytotoxicity and imaging properties of second-harmonic nanoparticles

We develop second-harmonic nanoparticles as the contrast agents for cell imaging. Second-harmonic nanoparticles show promise as cell imaging probes due to their non-bleaching, non-blinking, and coherent signal. Nanoparticles of noncentrosymmetric crystal structures have high second-harmonic generation (SHG) efficiency and provide high contrast in a generally non-structured cell environment. Here, we use barium titanate (BaTiO_3) nanoparticles with tetragonal crystal structure as imaging probes. Cytotoxicity tests performed on BaTiO_3 nanoparticles with mammalian cells did not result in toxic effects. Specifically, we observed no change in the cell metabolism after 24 hours incubation of the cells with high concentration of BaTiO_3 nanoparticles. We demonstrate two methods of cell labeling with BaTiO_3 nanoparticles for imaging. One is non-specific labeling via endocytosis of the cells, which results in a great number of the nanoparticles randomly distributed inside the cells. The other is specific labeling via surface functionalization of the nanoparticles with antibodies, which enables us to label specific cell membrane proteins with the nanoparticles. SHG imaging is compatible to two-photon microscopy and the SHG signal from nanoparticles can be easily detected with a standard two-photon confocal microscope. Our work provides the opportunity for long-term, three-dimensional cell tracking with secondharmonic nanoparticles

Tracking single particles on supported lipid membranes: multi-mobility diffusion and nanoscopic confinement

Supported lipid bilayers have been studied intensively over the past two decades. In this work, we study the diffusion of single gold nanoparticles (GNPs) with diameter of 20 nm attached to GM1 ganglioside or DOPE lipids at different concentrations in supported DOPC bilayers. The indefinite photostability of GNPs combined with the high sensitivity of interferometric scattering microscopy (iSCAT) allows us to achieve 1.9 nm spatial precision at 1 ms temporal resolution, while maintaining long recording times. Our trajectories visualize strong transient confinements within domains as small as 20 nm, and the statistical analysis of the data reveals multiple mobilities and deviations from normal diffusion. We present a detailed analysis of our findings and provide interpretations regarding the effect of the supporting substrate and GM1 clustering. We also comment on the use of high-speed iSCAT for investigating diffusion of lipids, proteins or viruses in lipid membranes with unprecedented spatial and temporal resolution

Optofluidic applications with lithium niobate nanowires

We report the hydrothermal synthesis of free-standing lithium niobate nanowires. We show that the versatile properties of bulk lithium niobate such as nonlinear optical effects can be exploited at the nanoscale. We describe the fabrication of polydimethylsiloxane (PDMS) microfluidics as well as indium tin oxide (ITO) electrodes with different design for dedicated applications. The control of microfluidic channel dimensions and the corresponding particle concentration is explored. Finally, the selection of fluidic conductivity for optimal dielectrophoretic trapping conditions is discussed

Harmonic holographic microscopy with circularly polarized excitation

We have developed a luminescent marker using the second harmonic generation (SHG) from noncentrosymmetric BaTiO_3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. In this paper, we report the SHG response of BaTiO_3 nanocrystals under a circularly polarized excitation. We observed a more uniform SHG signal intensity from nanocrystals of different crystal orientations under a circularly polarized excitation. The three-dimensional (3D) SHG fields generated from the SHRIMPs were recorded by a harmonic holographic microscope. Submicron resolution in both lateral and axial directions has been achieved. We show that the circularly polarized excitation is useful for harmonic holographic microscopy

Second harmonic nanoparticles in imaging applications

Nanocrystals with second harmonic response is a new class of nonlinear optical nanoprobes with dramatically different properties from fluorescent agents. Compared with two-photon fluorescence, second harmonic generation is an ultrafast, lossless, and coherent process. In particular, the absence of photobleaching and emission intermittency in the optical response of the second harmonic nanoparticles is likely to complement the fluorescent agents widely used today in many imaging applications. Furthermore, the coherent emission from the second harmonic generation process provides unique opportunities for the application of coherence domain techniques that are not available with fluorescent agents. We review the application of the second harmonic nanocrystals in imaging applications, especially those pertaining to biomedicine