We have developed and tested a wide-field coherent anti-Stokes Raman scattering (CARS) microscopy technique, which provides the simultaneous imaging of an extended illuminated area without scanning. This method is based on the non-phase-matching illumination of a sample and imaging of a CARS signal with a CCD camera using conventional microscope optics. We have identified a set of conditions on the illumination and imaging optics, as well as on sample preparation. Imaging of test objects proved high spatial resolution and chemical selectivity of this technique.
New system for a wide-field CARS microscopy is demonstrated, including two schemes of non-phase-matching illumination. Several advantages including high Stokes pulse energy, pulse-to-pulse stability and inherent synchronization between pump and Stokes pulses were brought by use of methane-filled Raman converter. Spatial resolution of the system with axially symmetric illumination, 0.5 microm, was found to correspond to diffraction limit of the imaging objective. Selective sensitivity to lipid-rich myelin sheaths in the nerve tissue has been demonstrated and confirmed by comparison with histological samples stained with myelin-specific dye. Single-shot imaging capability of the system has been demonstrated with a speckling-free illumination on a monolayer of 3 microm polystyrene beads.
We demonstrate a highly elongated (aspect ratio over 500:1) optical breakdown in water produced by a single pulse of a picosecond laser focused with a combination of an axicon and a lens. Locations of the proximal and distal ends of the breakdown region can be adjusted by modifying radial intensity distribution of the incident beam with an amplitude mask. Using Fresnel diffraction theory we derive a transmission profile of the amplitude mask to create a uniform axial intensity distribution in the breakdown zone. Experimentally observed dynamics of the bubbles obtained with the designed mask is in agreement with the theoretical model. A system producing an adjustable cylindrical breakdown can be applied to fast linear or planar dissection of transparent materials. It might be useful for ophthalmic surgical applications including cataract surgery and crystalline lens softening.
Transparent biological tissues can be precisely dissected with ultrafast lasers using optical breakdown in the tight focal zone. Typically, tissues are cut by sequential application of pulses, each of which produces a single cavitation bubble. We investigate the hydrodynamic interactions between simultaneous cavitation bubbles originating from multiple laser foci. Simultaneous expansion and collapse of cavitation bubbles can enhance the cutting efficiency, by increasing the resulting deformations in tissue, and the associated rupture zone. An analytical model of the flow induced by the bubbles is presented and experimentally verified. The threshold strain of the material rupture is measured in a model tissue. Using the computational model and the experimental value of the threshold strain one can compute the shape of the rupture zone in tissue resulting from application of multiple bubbles. With the threshold strain of 0.7 two simultaneous bubbles produce a continuous cut when applied at the distance 1.35 times greater than that required in sequential approach. Simultaneous focusing of the laser in multiple spots along the line of intended cut can extend this ratio to 1.7. Counterpropagating jets forming during collapse of two bubbles in materials with low viscosity can further extend the cutting zone-up to approximately a factor of 1.5.
We report a wide-field Coherent Anti-Stokes Raman Scattering (CARS) microscopy technique based on non-phasematching illumination and imaging systems. This technique is based on a non-collinear sample illumination by broad laser beams and recording image of sample at anti-Stokes wavelength using full-frame image detector. An amplified Ti:Sapphire laser and an Optical Parametric Amplifier (OPA) provided picosecond pump and Stokes beams with energies sufficient for CARS generation in an area of 100 µm in diameter. The whole field of view of the microscope was illuminated simultaneously by the pump and Stokes beams, and CARS signal was recorded onto a cooled CCD, with resolution determined by the microscope objective. Several illumination schemes and several types of thin sample preparations have been explored. We demonstrated that CARS image of a 100x100 µm sample can be recorded with submicrometer spatial resolution using just a few laser pulses of microJoule energies.
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