The second-order nonlinear polarization properties of fibrillar collagen in various rat tissues (vertebrae, tibia, tail tendon, dermis, and cornea) are investigated with polarization-dependent second-harmonic generation (P-SHG) microscopy. Three parameters are extracted: the second-order susceptibility ratio, R = [Formula: see text] ; a measure of the fibril distribution asymmetry, |A|; and the weighted-average fibril orientation, <δ>. A hierarchical organizational model of fibrillar collagen is developed to interpret the second-harmonic generation polarization properties. Highlights of the model include: collagen type (e.g., type-I, type-II), fibril internal structure (e.g., straight, constant-tilt), and fibril architecture (e.g., parallel fibers, intertwined, lamellae). Quantifiable differences in internal structure and architecture of the fibrils are observed. Occurrence histograms of R and |A| distinguished parallel from nonparallel fibril distributions. Parallel distributions possessed low parameter values and variability, whereas nonparallel distributions displayed an increase in values and variability. From the P-SHG parameters of vertebrae tissue, a three-dimensional reconstruction of lamellae of intervertebral disk is presented.

Collagen (type I) fibers are readily visualized with second harmonic generation (SHG) microscopy though the molecular origin of the signal has not yet been elucidated. In this study, the molecular origin of SHG from type I collagen is investigated using the time-dependent coupled perturbed Hartree-Fock calculations of the hyperpolarizibilities of glycine, proline, and hydroxyproline. Two effective nonlinear dipoles are found to orient in-the-plane of the amino acids, with one of the dipoles aligning close to the pitch orientation in the triple-helix, which provides the dominant contribution to the SHG polarization properties. The calculated hyperpolarizability tensor element ratios for the collagen triple-helix models: [(Gly3)n]3, [(Gly-Pro2)n]3, and [(Gly-Pro-Hyp)n]3, are used to predict the second-order nonlinear susceptibility ratios, χ(zzz)(2)/χ(iiz)(2) and χ(zii)(2)/χ(iiz)(2) of collagen fibers. From SHG microscopy polarization in, polarization out (PIPO) measurements of type I collagen in human lung tissue, a theoretical method is used to extract the triple-helix orientation angle with respect to the collagen fiber. The study shows the dominant role of amino acid orientation in the triple-helix for determining the polarization properties of SHG and provides a method for determining the triple-helix orientation angle in the collagen fibers.

A full numerical description of second-and third-harmonic generation (SHG and THG) at the focus of a nonlinear microscope is presented. The numerical implementation takes into account reflections and refraction by an arbitrary number of interfaces perpendicular to the optical axis in the focal region. The calculation of the second-and third-harmonic far-field radiation pattern is based on a Green function approach and is presented for any collection direction. The calculations are sped up by using the chirp-z transform for the focusing fields as well as for the far-field radiation calculation. Numerical evidence is presented for deviations in the measurement of the secondorder nonlinear susceptibility ratio ρ ≡ χ 2 yyy ∕χ 2 yxx of collagen fibers in SHG microscopy at high excitation numerical aperture. When interface reflections are taken into account, significant direct backward THG is demonstrated from interfaces and multilayer structures.

Abstract:We report on the development of a high pulse energy femtosecond Yb:KGd(WO4)2 laser pumped by a fibercoupled diode module. Passive mode locking was initiated using a semiconductor saturable absorber mirror. The laser produced 420 fs pulses with up to 145 nJ of energy at a repetition rate of 14.5 MHz. The output spectrum was centered at 1030 nm. At the lower pulse energy of 100 nJ the laser produced 300 fs long pulses.Spectral intensity, a.u.

We present a new laser system and nonlinear microscope, designed for differential nonlinear microscopy. The microscope features time-correlated single photon counting of multiphoton fluorescence generated by an alternating pulse-train of orthogonally polarized pulses. The generated nonlinear signal is separated using home-built electronics. Results are presented on fluorescence-detected nonlinear absorption linear anisotropy (FDNALA) of chloroplasts in Asparagus Sprengerii Regel and of Congo Red-stained cellulose.