Spatiotemporal beam self-cleaning for high-resolution nonlinear fluorescence imaging with multimode fiber.

Beam self-cleaning (BSC) in graded-index (GRIN) multimode fibers (MMFs) has been recently reported by different research groups. Driven by the interplay between Kerr effect and beam self-imaging, BSC counteracts random mode coupling, and forces laser beams to recover a quasi-single mode profile at the output of GRIN fibers. Here we show that the associated self-induced spatiotemporal reshaping allows for improving the performances of nonlinear fluorescence (NF) microscopy and endoscopy using multimode optical fibers. We experimentally demonstrate that the beam brightness increase, induced by self-cleaning, enables two and three-photon imaging of biological samples with high spatial resolution. Temporal pulse shortening accompanying spatial beam clean-up enhances the output peak power, hence the efficiency of nonlinear imaging. We also show that spatiotemporal supercontinuum (SC) generation is well-suited for large-band NF imaging in visible and infrared domains. We substantiated our findings by multiphoton fluorescence imaging in both microscopy and endoscopy configurations.

A readily usable two-photon fluorescence lifetime microendoscope.

A two-photon fluorescence lifetime (2P-FLIM) microendoscope, capable of energetic metabolism imaging through the intracellular nicotinamide adenine dinucleotide (NADH) autofluorescence, at sub-cellular resolution, is demonstrated. It exhibits readily usable characteristics such as convenient endoscope probe diameter (≈2 mm), fiber length (>5 m) and data accumulation rate (16 frames per second (fps)), leading to a FLIM refreshing rate of ≈0.1 to 1 fps depending on the sample. The spiral scanning image formation does not influence the instrument response function (IRF) characteristics of the system. Near table-top microscope performances are achieved through a comprehensive system including a home-designed spectro-temporal pulse shaper and a custom air-silica double-clad photonic crystal fiber, which enables to reach up to 40 mW of ≈100 fs pulses @ 760 nm with a 80 MHz repetition rate. A GRadient INdex (GRIN) lens provides a lateral resolution of 0.67 µm at the focus of the fiber probe. Intracellular NADH fluorescence lifetime data are finally acquired on cultured cells at 16 fps.

Particle-based photodynamic therapy based on indocyanine green modified plasmonic nanostructures for inactivation of a Crohn's disease-associated Escherichia coli strain.

Particle-based photodynamic therapy (PPDT) holds great promise in theranostic applications. Herein, we demonstrate that PPDT based on gold nanorods coated with an indocyanine green (ICG)-loaded silica shell allows for the inactivation of the Crohn's disease-associated adherent-invasive Escherichia coli strain LF82 (E. coli LF82) under pulsed laser light irradiation at 810 nm. Fine-tuning of the plasmonic structures together with maximizing the photosensitizer loading onto the nanostructures allowed optimizing the singlet oxygen generation capability and the PPDT efficiency. Using a nanoparticle concentration low enough to suppress photothermal heating effects, 6 log10 reduction in E. coli LF82 viability could be achieved using gold nanostructures displaying a plasmonic band at 900 nm. An additional modality of nanoparticle-based photoinactivation of E. coli is partly observed, with 3 log10 reduction of bacterial viability using Au NRs@SiO2 without ICG, due to the two-photon induced formation of reactive oxygen species. Interaction of the particles with the bacterial surface, responsible for the disruption of the bacterial integrity, together with the generation of moderate quantities of singlet oxygen could account for this behavior.

Highly effective photodynamic inactivation of E. coli using gold nanorods/SiO2 core-shell nanostructures with embedded verteporfin.

The potential of gold nanorods post-coated with a 20 nm silica shell loaded with verteporfin (Au NRs@SiO2-VP) as efficient near-infrared nanostructures for photodynamic therapy under continuous wave and pulsed-mode excitation to eradicate a virulent strain of E. coli associated with urinary tract infection is described.

Label-free microscopy and stress responses reveal the functional organization of Pseudodiaptomus marinus copepod myofibrils.

Pseudodiaptomus marinus copepods are small crustaceans living in estuarine areas endowed with exceptional swimming and adaptative performances. Since the external cuticle acts as an impermeable barrier for most dyes and molecular tools for labeling copepod proteins with fluorescent tags are not available, imaging cellular organelles in these organisms requires label free microscopy. Complementary nonlinear microscopy techniques have been used to investigate the structure and the response of their myofibrils to abrupt changes of temperature or/and salinity. In contrast with previous observations in vertebrates and invertebrates, the flavin autofluorescence which is a signature of mitochondria activity and the Coherent Anti-Stokes Raman Scattering (CARS) pattern assigned to T-tubules overlapped along myofibrils with the second harmonic generation (SHG) striated pattern generated by myosin tails in sarcomeric A bands. Temperature jumps from 18 to 4 °C or salinity jumps from 30 to 15 psu mostly affected flavin autofluorescence. Severe salinity jumps from 30 to 0 psu dismantled myofibril organization with major changes both in the SHG and CARS patterns. After a double stress (from 18 °C/30 psu to 4° C/0 psu) condensed and distended regions appeared within single myofibrils, with flavin autofluorescence bands located between sarcomeric A bands. These results shed light on the interactions between the different functional compartments which provide fast acting excitation-contraction coupling and adequate power supply in copepods muscles.

Plasmonic photothermal destruction of uropathogenic E. coli with reduced graphene oxide and core/shell nanocomposites of gold nanorods/reduced graphene oxide.

The development of non-antibiotic based treatments against bacterial infections by Gram-negative uropathogenic E. coli is a complex task. New strategies to treat such infections are thus urgently needed. This report illustrates the development of pegylated reduced graphene oxide nanoparticles (rGO-PEG) and gold nanorods (Au NRs) coated with rGO-PEG (rGO-PEG-Au NRs) for the selective killing of uropathogenic E. coli UTI89. We took advantage of the excellent light absorption properties of rGO-PEG and Au NR particles in the near-infrared (NIR) region to photothermally kill Gram-negative pathogens up to 99% in 10 min by illumination of solutions containing the bacteria. The rGO-PEG-Au NRs demonstrated better photothermal efficiency towards E. coli than rGO-PEG. Targeted killing of E. coli UTI89 could be achieved with rGO-PEG-Au NRs functionalized with multimeric heptyl α-d-mannoside probes. This currently offers a unique biocompatible method for the ablation of pathogens with the opening of probably a new possibility for clinical treatments of patients with urinary infections.

Application of a high power Yb fiber-based laser compatible with commercial optical parametric oscillator for coherent anti-Stokes Raman scattering microscopy.

Coherent anti-Stokes Raman scattering (CARS) microscopy is a powerful tool for chemical analysis at a subcellular level, frequently used for imaging lipid dynamics in living cells. We report a high-power picosecond fiber-based laser and its application for optical parametric oscillator (OPO) pumping and CARS microscopy. This fiber-based laser has been carefully characterized. It produces 5 ps pulses with 0.8 nm spectral width at a 1,030 nm wavelength with more than 10 W of average power at 80 MHz repetition rate; these spectral and temporal properties can be slightly modified. We then study the influence of these modifications on the spectral and temporal properties of the OPO. We find that the OPO system generates a weakly spectrally chirped signal beam constituted of 3 ps pulses with 0.4 nm spectral width tunable from 790 to 930 nm optimal for CARS imaging. The frequency doubling unconverted part is composed of 7-8 ps pulses with 0.75 nm spectral width compatible with CARS imaging. We also study the influence of the fiber laser properties on the CARS signal generated by distilled water. In agreement with theory, we find that shorter temporal pulses allow higher peak powers and thus higher CARS signal, if the spectral widths are less than 10 cm(-1) . We demonstrate that this source is suitable for performing CARS imaging of living cells during several hours without photodamages. We finally demonstrate CARS imaging on more complex aquatic organisms called copepods (micro-crustaceans), on which we distinguish morphological details and lipid reserves.

Fiber-based device for the detection of low-intensity fluctuations of ultrashort pulses.

We describe a fiber-based device that can significantly enhance the low-intensity fluctuations of an ultrashort pulse train to detect them more easily than with usual direct detection systems. Taking advantage of the Raman intrapulse effect that progressively shifts the central frequency of a femtosecond pulse propagating in an anomalous dispersion fiber, a subsequent spectral filtering can efficiently increase the level of fluctuations by more than 1 order of magnitude. We show that attention has to be paid to maintain the shape of the statistical distribution unaffected by the nonlinear process.