The recent development of Ultra Short Pulse lasers has widely broadened the range of possibilities of laser material processing. Associated with a proper beam splitting it enables adding to the surface new properties by texturing it.
We present here a fully reflective three by three beam splitter compatible with high power up to 300W with 500fs pulses lasers. The process results are presented including the repeatability of the pattern, and the achievable ablation rate. The pattern is 15µm waist gaussian beams with 300µm pitch.
Compatibility with scanning system and F-theta lenses, enabling micro-processing throughput improvement, is described.
After travelling through a disordered medium an ultrashort pulse of light gets completely scrambled both temporally and spatially. Multiple scattering tends to elongate the pulse duration and to distort it spatially, as each spectral component of the pulse generates a different speckle pattern. This mixing results in a complex spatio-temporal speckle pattern. By determining the Multi Spectral Transmission Matrix (MSTM) of the medium, one can achieve full control of transmitted light both in time and space only by exploiting spatial degrees of freedom of a single SLM. This operator is a stack of monochromatic Transmission Matrices (TM), measured for all the spectral components of the pulse. Although this technique has proved its efficiency, its technical complexity precludes its broad dissemination. Primarily, it requires a pulsed laser capable of tunable CW operation to measure all monochromatic TMs. Additionally, the scattering medium needs to be stable during the whole experiment, which is a major limitation for thick media with large number of independent TMs.
Here, we report a new technique to parallelize the full MSTM measurement of a highly scattering medium. It speeds up acquisition an order of magnitude, and does not require a tunable source. To this end, a micro-lens array and a diffraction grating are used to encode both spatial and spectral information of the output speckle on a single CMOS camera. We experimentally demonstrate the advantage of the technique by measuring MTSM in very strong scattering regime (N_λ>30) where the conventional method would be impractical.
Raman spectroscopy is a valuable tool for non-invasive and label-free identification of sample chemical composition. Recently a few miniaturized optical probes emerged driven by the need to address areas of difficult access, such as in endoscopy. However, imaging modality is still out of reach for most of them. Separately, recent advances in wavefront shaping enabled different microscopies to be applied in various complex media including multimode fibers. Here we present the first and thinnest to date Raman fiber imaging probe based on wavefront shaping through a single multimode fiber without use of any additional optics. We image agglomerates of bacteria and pharmaceuticals to demonstrate the capability of our method. This work paves the way towards compact and flexible Raman endoscopy.
Second harmonic generation (SHG) is a powerful technique to observe fibrillar collagen without any staining and with a good contrast. More information about the molecular structure of collagen fibrils in tissues and their 3D distribution can be gained with polarization-resolved SHG imaging. Nevertheless, strong focusing is required for effective imaging and light propagation in tissues is complex and not thoroughly understood yet, preventing accurate and reproducible measurements. Theoretical analysis, vectorial numerical simulations and experiments were implemented to understand how the SHG signal builds up and how geometrical parameters affect polarization-resolved measurements in homogeneous collagen-rich tissues.
Type I collagen is a major structural protein in mammals that shows highly structured macromolecular organizations specific to each tissue. This biopolymer is synthesized as triple helices, which self-assemble into fibrils (Ø =10-300 nm) and further form various 3D organization. In recent years, Second Harmonic Generation (SHG) microscopy has emerged as a powerful technique to probe in situ the fibrillar collagenous network within tissues. However, this optical technique cannot resolve most of the fibrils and is a coherent process, which has impeded quantitative measurements of the fibril diameter so far.
In this study, we correlated SHG microscopy with Transmission Electron Microscopy to determine the sensitivity of SHG microscopy and to calibrate SHG signals as a function of the fibril diameter in reconstructed collagen gels. To that end, we synthetized isolated fibrils with various diameters and successfully imaged the very same fibrils with both techniques, down to 30 nm diameter. We observed that SHG signals scaled as the fourth power of the fibril diameter, as expected from analytical and numerical calculations. This calibration was then applied to diabetic rat cornea in which we successfully recovered the diameter of hyperglycemia-induced fibrils in the Descemet’s membrane without having to resolve them. Finally we derived the first hyperpolarizability from a single collagen triple helix which validates the bottom-up approach used to calculate the non-linear response at the fibrillar scale and denotes a parallel alignment of triple helices within the fibrils. These results represent a major step towards quantitative SHG imaging of nm-sized collagen fibrils.
Precise control of particle positioning is desirable in many optical propulsion and sorting applications. Here, we develop an integrated platform for particle manipulation consisting of a combined optical nanofiber and optical tweezers system. Individual silica microspheres were introduced to the nanofiber at arbitrary points using the optical tweezers, thereby producing pronounced dips in the fiber transmission. We show that such consistent and reversible transmission modulations depend on both particle and fiber diameter, and may be used as a reference point for in-situ nanofiber or particle size measurement. Therefore we combine SEM size measurements with nanofiber transmission data to provide calibration for particle-based fiber assessment. We also demonstrate how the optical tweezers can be used to create a ‘particle jet’ to feed a supply of microspheres to the nanofiber surface, forming a particle conveyor belt. This integrated optical platform provides a method for selective evanescent field manipulation of micron-sized particles and facilitates studies of optical binding and light-particle interaction dynamics.
Double nanohole apertures in metal films have proven to be efficient plasmonic devices for trapping nanoparticles as small as single proteins.1 To date, this technique has relied on weak transmission far beyond the wavelength cutoff, ignoring the prospect of plasmon field enhancement. In this work we present details on the design and fabrication of arrays of nanoring apertures on gold films. These devices feature efficient light localization in small gaps similar to double nanohole apertures, but additionally benefit from surface plasmon resonances in the near infrared spectrum. We perform polarization-resolved spectrometry on the arrays and discuss their potential for nanoparticle trapping.
We combined polarization-resolved SHG microscopy with mechanical assays in rat-tail-tendon and measured collagen
remodeling upon controlled stretching. This approach aimed to analyze the relationship between macroscopic response
and sub-micrometer scale organization of collagen fibrils. We observed a straightening of the crimps followed by a
sliding of the fibrils with increasing stretching of the tendon fascicles. Polarization resolution of the SHG images
provided complementary information about the orientation dispersion of collagen fibrils within the focal volume and
enabled monitoring of collagen remodeling at the sub-micrometer scale. Our approach can be readily generalized to
other tissues and should bring new valuable information about biomechanics of microstructured tissues.
Multiphoton microscopy provides specific and contrasted images of unstained collagenous tissues such as tendons or
corneas. Polarization-resolved second harmonic generation (SHG) measurements have been implemented in a laserscanning
multiphoton microscope. Distortion of the polarimetric response due to birefringence and diattenuation during
propagation of the laser excitation has been shown in rat-tail tendons. A model has been developed to account for these
effects and correct polarization-resolved SHG images in thick tissues. This new modality is then used in unstained
human corneas to access two quantitative parameters: the fibrils orientation within the collagen lamellae and the ratio of
the main second-order nonlinear tensorial components. Orientation maps obtained from polarization resolution of the
trans-detected SHG images are in good agreement with the striated features observed in the raw images. Most
importantly, polarization analysis of the epi-detected SHG images also enables to map the fibrils orientation within the
collagen lamellae while epi-detected SHG images of corneal stroma are spatially homogenous and do not enable direct
visualization of the fibrils orientation. Depth profiles of the polarimetric SHG response are also measured and compared
to models accounting for orientation changes of the collagen lamellae within the focal volume. Finally, in vivo
polarization-resolved SHG is performed in rat corneas and structural organization of corneal stroma is determined using
epi-detected signals.
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