We study the nonlinear optical propagation of two different classes of light beams with space-varying polarization-radially symmetric vector beams and Poincaré beams with lemon and star topologies-in a rubidium vapor cell. Unlike Laguerre-Gauss and other types of beams that quickly experience instabilities, we observe that their propagation is not marked by beam breakup while still exhibiting traits such as nonlinear confinement and self-focusing. Our results suggest that, by tailoring the spatial structure of the polarization, the effects of nonlinear propagation can be effectively controlled. These findings provide a novel approach to transport high-power light beams in nonlinear media with controllable distortions to their spatial structure and polarization properties.

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SummaryWe report a method for characterizing the focussing laser beam exiting the objective in a laser scanning microscope. This method provides the size of the optical focus, the divergence of the beam, the ellipticity and the astigmatism. We use a microscopic‐scale knife edge in the form of a simple transmission electron microscopy grid attached to a glass microscope slide, and a light‐collecting optical fibre and photodiode underneath the specimen. By scanning the laser spot from a reflective to a transmitting part of the grid, a beam profile in the form of an error function can be obtained and by repeating this with the knife edge at different axial positions relative to the beam waist, the divergence and astigmatism of the postobjective laser beam can be obtained. The measured divergence can be used to quantify how much of the full numerical aperture of the lens is used in practice. We present data of the beam radius, beam divergence, ellipticity and astigmatism obtained with low (0.15, 0.7) and high (1.3) numerical aperture lenses and lasers commonly used in confocal and multiphoton laser scanning microscopy. Our knife‐edge method has several advantages over alternative knife‐edge methods used in microscopy including that the knife edge is easy to prepare, that the beam can be characterized also directly under a cover slip, as necessary to reduce spherical aberrations for objectives designed to be used with a cover slip, and it is suitable for use with commercial laser scanning microscopes where access to the laser beam can be limited.

Self-focusing of intense laser beams and pulses of light in real nonlinear media is in general accompanied by material losses that require corrections to the conservative Nonlinear Schrödinger equations describing their propagation. Here we examine loss mechanisms that exist even in lossless media and are caused by shedding of energy away from the self-trapping beam making it to relax to an exact solution of lower energy. Using the conservative NLS equations with absorbing boundary conditions we show that energy shedding not only occurs during the initial reshaping process but also during oscillatory propagation induced by saturation of the nonlinear effect. For pulsed input we also show that, depending on the sign and magnitude of dispersion, pulse splitting, energy shedding, collapse or stable self-focusing may result.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXXWe have developed a simple wavelength tunable optical parametric generator (OPG), emitting broad band ultrashort pulses with peak wavelengths at 1530-1790 nm, for nonlinear label-free microscopy. The OPG consists of a periodically poled lithium niobate crystal, pumped at 1064 nm by a ultrafast Yb:fiber laser with high pulse energy. We demonstrate that this OPG can be used for label-free imaging, by third harmonic generation, of nuclei of brain cells and blood vessels in a >150 m thick brain tissue section, with very little decay of intensity with imaging depth and no visible damage to the tissue at an incident average power of 15 mW . © 2015 Optical Society of America OCIS Codes: (180.4315) Nonlinear microscopy, (190.2620) Harmonic generation and mixing, (190.4970) Parametric oscillators and amplifiers.Label-free imaging allows for the study of biological samples without chemical modifications and the associated sample preparation that could modify the structure or mechanism under study. In label-free third harmonic generation (THG) imaging, the image contrast arises from refractive index differences within the specimen, as well as from differences in 3 rd order susceptibility  (3) [1], with the THG signal being generated across the interfaces [2]. This allows for label-free visualization of, for example, blood vessels and red blood cells [3,4], lipid structures [1,5], cell boundaries and membranes [5,6] and brain tissue [3,7-9]. Since THG imaging is a nonlinear imaging method, it provides inherent optical sectioning as well as larger imaging depth than simpler label-free imaging methods such as differential interference contrast (DIC). Furthermore, for imaging deep into brain tissue, it has been shown that three-photon imaging has potential for retaining the resolution and therefore support imaging deeper into thick grey matter than two-photon imaging [7,10]. Since no optical energy is deposited in the tissue, in contrast to in fluorescence imaging, the sample damage is potentially low, despite the high laser peak powers needed. However, the use of THG in biological imaging has been limited by the available laser sources. As THG is a third order nonlinear process, ultra-short laser pulses are required to generate sufficiently high peak powers for efficient signal generation. Furthermore, these pulses should preferably be at longer wavelengths than those available with a Ti:Sapphire laser (>1080nm) because the detected wavelength, which is a third of the excitation wavelength, is otherwise in the UV wavelength range, where standard microscope optics, as well as the tissue itself, have low transparency. Longer excitation wavelengths also give larger penetration depth in tissue because of reduced scattering [7], as well as less specimen damage [11]. The laser should also be tunable so that the wavelength can be chosen to suit the scattering properties of the sample. The tunable f...