Generation of an optical needle beam with a laser inscribed Pancharatnam-Berry phase element under imperfect conditions.

Beams exhibiting long focal lines and small focal spot sizes are desired in a variety of applications and are called optical needles, with Bessel beams being a common example. Conical prisms are regularly used to generate Bessel beams, however, this method is usually plagued by an appearance of on-axis oscillations. In this work, we consider an optical element based on the space-domain Pancharatnam-Berry phase (PBP) to generate a high-power optical needle with a smooth and constant on-axis intensity profile. The phase in PBP elements is not introduced through optical path differences but results from the geometric phase that accompanies space-variant polarization manipulation. Our implementation is based on a type 2 modification of bulk transparent glass material, resulting in the formation of nanogratings with slow axes aligned perpendicular to the grating corrugation. We investigate both numerically and experimentally the stability of an optical needle generation under imperfect conditions. Influences of misalignments in the optical schema are investigated numerically and experimentally.

Depolarization compensation with a spatially variable wave plate in a 116 W, 441 fs, 1 MHz Yb:YAG double-pass laser amplifier.

A subpicosecond laser system featuring a fiber chirped pulse amplification-based seed laser and a double-pass end-pumped Yb:YAG crystal power amplifier was investigated. The key novelty of the system was the application of depolarization compensation using a specially designed spatially variable wave plate. To the best of our knowledge, this method was applied for the first time. The presented laser system produced pulses of 441 fs duration, 116 µJ pulse energy at 116 W average power with a beam quality of M2∼2.1, featured optical-to-optical efficiency of 32% at room temperature (T=20∘C), and had residual depolarization level of 2.7%.

Femtosecond laser micro-machined polyimide films for cell scaffold applications.

Engineering of sophisticated synthetic 3D scaffolds that allow controlling behaviour and location of the cells requires advanced micro/nano-fabrication techniques. Ultrafast laser micro-machining employing a 1030-nm wavelength Yb:KGW femtosecond laser and a micro-fabrication workstation for micro-machining of commercially available 12.7 and 25.4 µm thickness polyimide (PI) film was applied. Mechanical properties of the fabricated scaffolds, i.e. arrays of differently spaced holes, were examined via custom-built uniaxial micro-tensile testing and finite element method simulations. We demonstrate that experimental micro-tensile testing results could be numerically simulated and explained by two-material model, assuming that 2-6 µm width rings around the holes possessed up to five times higher Young's modulus and yield stress compared with the rest of the laser intacted PI film areas of 'dog-bone'-shaped specimens. That was attributed to material modification around the micro-machined holes in the vicinity of the position of the focused laser beam track during trepanning drilling. We demonstrate that virgin PI films provide a suitable environment for the mobility, proliferation and intercellular communication of human bone marrow mesenchymal stem cells, and discuss how cell behaviour varies on the micro-machined PI films with holes of different diameters (3.1, 8.4 and 16.7 µm) and hole spacing (30, 35, 40 and 45 µm). We conclude that the holes of 3.1 µm diameter were sufficient for metabolic and genetic communication through membranous tunneling tubes between cells residing on the opposite sides of PI film, but prevented the trans-migration of cells through the holes. Copyright © 2016 John Wiley & Sons, Ltd.

Micromachining and validation of the scanning acoustic microscope spatial resolution and sensitivity calibration block for 20-230 MHz frequency range.

Scanning acoustic microscopy (SAM) is used as a routine non-destructive test tool for different diagnostic examinations: detection of defects such as microcracks, delamination, disbonding, inclusions, subsurface features in materials such as pores and cracks. SAM can be operated in a wide frequency range from Megahertz to Gigahertz. SAM measurement spatial resolution is diffraction limited by the wavelength of the acoustic wave in particular medium and also depends on individual transducers geometry. Actual SAM spatial resolution can be determined by measuring calibrated lithographically formed microstructures in high acoustic impedance materials. Numerical acoustic signal simulation method, based on the diffraction approach, was employed for the selection of the calibration block pattern geometry and linear dimensions of the elements. Universal calibration block for SAM operating in a 20-230 MHz frequency range was micromachined in high acoustic impedance ceramic substrates. Differently spaced (from 18 to 185 µm) lines of the same width and different widths (from 17 to 113 µm) but similar spacing lines were imposed in alumina ceramics employing one step lithography process, i.e. femtosecond laser ablation. Proposed SAM calibration pattern linear dimensions were characterized employing optical and scanning electron microscopy. Finally the samples were measured with SAM employing different frequency transducers and results were compared with the numerical simulations. It was obtained that resolution of SAM operating with 230 MHz transducer is not worse than 40 µm.