Room temperature synthesis of upconversion fluorescent nanocrystals.

Synthesis of nanocrystals that exhibit strong upconversion (UC) luminescence upon infrared excitation has been challenging due to the stringent control needed over experimental variables. Herein, we report a method to synthesize nanocrystals demonstrating high UC at room temperature in aqueous solution on graphene.

Deep-tissue access with confocal fluorescence microendoscopy through hypodermic needles.

We report on the design and implementation of a gradient-index microendoscope suitable for accessing tissues deep within the body using confocal fluorescence imaging. The 350-µm diameter microendoscope has a length of 27 mm, which enables it to be inserted through a 22-gauge hypodermic needle. A prototype imaging system is demonstrated to obtain images of tissue samples at depths of ~15 mm with a lateral resolution of ~700 nm. To the best of our knowledge, this is the highest resolution and imaging depth reported for a confocal probe of these dimensions. We employ a scanning arrangement using a lensed fiber that can conveniently control the input beam parameters without causing off-axis aberrations typically present in the optical relay lenses used in galvanometer-mirror scanning systems.

Dispersion-based pulse shaping for multiplexed two-photon fluorescence microscopy.

We demonstrate selective two-photon excited fluorescence microscopy with shaped pulses produced with a simple yet efficient scheme based on dispersive optical components. The pulse train from a broadband oscillator is split into two subtrains that are sent through different amounts of glass. Beam recombination results in pulse-shape switching at a rate of 150MHz. Time-resolved photon counting detection then provides two simultaneous images resulting from selective two-photon excitation, as demonstrated in a live embryo. Although less versatile than programmable pulse-shaping devices, this novel arrangement significantly improves the performance of selective microscopy using broadband shaped pulses while simplifying the experimental setup.

Multiplexed two-photon microscopy of dynamic biological samples with shaped broadband pulses.

Coherent control can be used to selectively enhance or cancel concurrent multiphoton processes, and has been suggested as a means to achieve nonlinear microscopy of multiple signals. Here we report multiplexed two-photon imaging in vivo with fast pixel rates and micrometer resolution. We control broadband laser pulses with a shaping scheme combining diffraction on an optically-addressed spatial light modulator and a scanning mirror allowing to switch between programmable shapes at kiloHertz rates. Using coherent control of the two-photon excited fluorescence, it was possible to perform selective microscopy of GFP and endogenous fluorescence in developing Drosophila embryos. This study establishes that broadband pulse shaping is a viable means for achieving multiplexed nonlinear imaging of biological tissues.

Analysis of the influence of spherical aberration from focusing through a dielectric slab in quantitative nonlinear optical susceptibility measurements using third-harmonic generation.

The third-order nonlinear susceptibility (chi(3)) can be measured quantitatively using third-harmonic generation (THG) from two different interfaces. For the first time it is demonstrated both in experiments and theory that the magnitude of the THG signals from the two interfaces is not only determined by material properties (refractive index and chi(3)), but also by optical aberrations. It is found that this method of chi(3) determination can be applied without additional correction factors only for focusing conditions with a numerical aperture (NA)

Imaging colloidal particle induced topological defects in a nematic liquid crystal using third harmonic generation microscopy.

The nature of the third-harmonic generation (THG) process in a nematic liquid crystal is investigated for the case of tightly focused, low intensity, laser beams. Colloidal particle induced topological defects in a liquid crystal are visualized in three-dimensions using the dependence of the THG signal on both changes in non-linear susceptibility and the orientation of the liquid crystal director relative to the incident laser polarization state. We have found that the interpretation of THG images in a liquid crystal is complicated not only by the change in polarisation of the electric field as it propagates through the medium but also by anisotropic refractive index mismatch induced aberrations.