Stimulated Raman Scattering (SRS) microscopy is a nonlinear microscopy technique based on Raman vibrational resonances determined by the frequency difference between Pump and Stokes laser pulses. Modulation of one laser beam transfers the modulation to the other, as either a gain in Stokes (SRG) or a loss in Pump power (SRL). SRS microscopy does not exhibit the four-wave mixing nonresonant background characteristic of CARS microscopy. However, other background signals due to two-photon absorption, thermal lensing or cross-phase modulation (XPM) do reduce the detection sensitivity and can distort the hyperspectral scans. Phase sensitive lock-in detection can reduce contributions from two-photon absorption, which is out-of-phase for the SRG case. However, the background signal due to XPM, which can be in-phase with SRS, can reduce the detection sensitivity.
We present a novel polarization modulation (PM) scheme in SRS microscopy which greatly reduces the nonresonant XPM background, demonstrated here for the SRL case. Since many Raman vibrational transitions are parallel polarized, the SRS signal is maximum (minimum) when the polarizations of the pump and the Stokes beams are parallel (perpendicular). However, in both parallel and perpendicular Pump-Stokes geometries, XPM is non-zero in many media. Therefore, PM can remove the XPM background without significantly reducing the SRS signal. Our results show that the PM-SRS successfully removes the nonresonant signal due to XPM. High imaging contrast is observed, concomitant with high sensitivity at very low analyte concentrations and undistorted Raman spectra.
Energetic electrons generation by longitudinal field acceleration from a laser pulse was demonstrated. The longitudinal field was generated by focusing a radially polarised TM01 ultrashort laser pulse (1,8 microns, 550 uJ, 15 fs) with a high numerical aperture parabola. The created longitudinal field was intense enough to ionised and accelerated electrons with a few tens of keV from a low density oxygen gaz. The energy, spectrum, number of charges per shot and divergence of the generated electron bunches have been measured and will be presented. Electron bunch pulse duration, space charge effects and energy tunability will also be discussed.
We present a coupled study of laser-induced damage and ablation of fused silica in the femtosecond regime. Both
thresholds are essentially different and investigations under a wide excursion of pulse duration (< 10 fs to 300 fs) and
applied fluence (Fth < F < 10 Fth) provide quantitative knowledge on i) the strength of the so-called "deterministic"
character of femtosecond laser damaging, linked to ionization mechanisms ; ii) the physical characteristics of surface
ablation craters demonstrating that high selectivity and nanometric resolution is achievable.
Multiphoton microscopy is a powerful technique for high spatial resolution thick tissue imaging. In its simple version, it
uses a high repetition rate femtosecond oscillator laser source focussed and scanned across biological sample that contains fluorophores. However, not every biological structure is inherently fluorescent or can be stained without causing biochemical changes. To circumvent these limitations, other non-invasive nonlinear optical imaging approaches are currently being developed and investigated with regard to different applications. These techniques are: (1) second harmonic generation (SHG), (2) third harmonic generation (THG), and (3) coherent anti-Stokes Raman scattering
(CARS) microscopy. The main advantage of the above mentioned techniques is that they derive their imaging contrast
from optical nonlinearities that do not involve fluorescence process. As a particular application example we investigated
collagen arrays. We show that combining SHG-THG-CARS onto a single imaging platform provides complementary information about the sub-micron architecture of the tissue. SHG microscopy reveals the fibrillar architecture of collagen arrays and confirm a rather high degree of heterogeneity of χ<sup>(2)</sup> within the focal volume, THG highlights the boundaries between the collagen sheets, and CH<sub>2</sub> spectroscopic contrast with CARS.
CARS microscopy has emerged as a powerful tool in imaging of biological matter. In addition to a high-3D spatial resolution, the technique delivers an attractive set of properties such as chemical specificity, high sensitivity, and fast data acquisition rates thus making it very suitable for biomedical applications. However, these advantages come at a cost of complex tunable laser sources, beam guiding and delivery optics. In particular, two high intensity laser pulses, whose carrier frequencies ωp (pump) and ωs (Stokes) are separated by the corresponding Raman shift value, are required to interact with the imaged media. In this paper, we present experimental results corresponding to our first step towards an integrated CARS-microscope. We demonstrate optical fiber delivery of two color picosecond pulses before they interact in the focus of microscope objective in order to produce CARS image. Certain aspects related to the effect of the pump and Stokes pulse parameters on image quality (e.g. contrast, sensitivity) after the pulses' propagation in the fiber will be addressed. The incorporation of fiber delivery feature significantly improves the microscope performance and ease its operation. In addition, we are exploring certain approaches in further development of CARS-microscope as a biomedical tool towards fully functional endoscope for in vivo chemically sensitive imaging.