Frequency-Doubling of Femtosecond Pulses in “Thick” Nonlinear Crystals With Different Temporal and Spatial Walk-Off Parameters

We present a comparative study on frequency-doubling characteristics of femtosecond laser pulses in thick nonlinear crystals with different temporal and spatial walk-off parameters. Using single-pass second harmonic generation (SHG) of 260 fs pulses at 1064 nm from a high-average-power femtosecond Yb-fiber laser in 5-mm-long crystals of β-BaB2O4 (BBO) and BiB3O6 (BIBO), we find that for comparable values of temporal and spatial walk-off parameters in each crystal, the optimum focusing condition for SHG is more strongly influenced by spatial walk-off than temporal walk-off. It is also observed that under such conditions, the Boyd and Kleinman theory commonly used to define the optimum focusing condition for frequency-doubling of cw and long-pulse lasers is also valid for SHG of ultrafast lasers. We also investigate the effect of focusing on the spectral, temporal, and spatial characteristics of the second harmonic (SH) radiation, as well as angular acceptance bandwidth for the SHG process, under different temporal and spatial walk-off conditions in the two crystalsPeer ReviewedPostprint (author's final draft

Aligning Carbon Fibers in Micro-Extruded Composite Ink

Direct write processes include a wide range of additive manufacturing techniques with the ability to fabricate structures directly onto planar and non-planar surfaces. Most additive manufacturing techniques use unreinforced polymers to produce parts. By adding carbon fiber as a reinforcing material, properties such as mechanical strength, electrical conductivity, and thermal conductivity can be enhanced. Carbon fibers can be long and continuous, or short and discontinuous. The strength of carbon fiber composite parts is greatly increased when the fibers are preferentially aligned. This research focuses on increasing the strength of additively manufactured parts reinforced using discontinuous carbon fibers that have been aligned during the micro extrusion process. A design of experiments (DOE) approach was used to identify significant process parameters affecting fiber alignment. Factors such as the length of carbon fibers, nozzle diameter, fiber loading fraction, air pressure, translational speed and standoff distance were considered. A two dimensional Fast Fourier Transform (2D FFT) was used to quantify the degree of fiber alignment in the extruded composite inks. ImageJ software supported by an oval profile plugin was used with micrographs of printed samples to obtain the carbon fiber alignment values. The optimal value for the factors was derived by identifying the significant main and interaction effects. Based on the results of the DOE, tensile test samples were printed with fibers aligned parallel and perpendicular to the tensile axis. A standard test method for tensile properties of plastic revealed that the extruded parts with fibers aligned along the tensile axis were better in tensile strength and modulus

Three Dimensional Digital Alloying with Reactive Metal Inks

3D printing of multifunctional components using two or more materials is a rapidly growing area of research. Metallic alloy inks have been used with various 3D printing techniques to create functional components such as antennas, inductors, resistors, and biocompatible implants. Most of these printing techniques use premixed metallic alloy inks or nanoalloy particles with a fixed composition to fabricate the functional part. Since the properties of alloys vary with changes in the elemental composition, a printing process which could digitally dispense alloy inks having specific desired compositions would enable different functionalities and be highly desirable. Using the binary copper-nickel system as an example, the formation of alloy with metal precursor inks is presented. Since copper and nickel both have a face centered cubic (FCC) structure and show complete miscibility in each other, formation of their nanoalloy is, in theory, relatively easy. By printing metal precursor inks rather than nanoparticle suspensions, problems associated with the nanoparticle inks such as ink stability and nozzle clogging can be avoided. Copper and nickel precursor inks were formulated having rheological properties suitable for inkjet printing. Reduction of metal inks was studied under various conditions. The sintered metal and alloy structures were characterized using thermal analysis, infrared spectroscopy, energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction. Nickel, a ferromagnetic metal, showed novel microstructures such as aligned nanowires and nanowire grids when reduced in the presence of a magnetic field. These microstructures had enhanced anisotropic electrical and magnetic properties along the direction of the nanowire. The reduction of combined ink solutions (copper and nickel) showed formation of a two phase with copper as one phase and a nickel rich alloy as other. These structures demonstrated no change in electrical resistivity when exposed to an oxidation rich environment. To achieve a homogeneous alloy formation, the copper phase and the nickel rich phase were diffused together at high temperatures. Copper nickel alloy inks with ratios Cu30Ni70, Cu50Ni50, and Cu70Ni30 were formulated and reduced at 230 °C and later high temperature diffusion was achieved at 800 °C. The lattice parameter of the alloy phase for the inks with ratio Cu30Ni70 was 3.5533Å, Cu50Ni50 was 3.5658 Å, and Cu70Ni30 was 3.5921 Å. Using Vegard’s law, the composition of the alloy phases for the three samples were estimated to be Cu32Ni68, Cu46Ni54, and Cu75Ni25. This formation of the desired alloy composition can open the door to numerous applications in biomedical and electronics sectors, among other

A Structural Model for the HIV-1 Rev–RRE Complex Deduced from Altered-Specificity Rev Variants Isolated by a Rapid Genetic Strategy

AbstractA broadly applicable genetic strategy was developed for investigating RNA–protein interactions and applied to the HIV-1 Rev protein. By rapidly screening thousands of Rev–RNA interactions in Escherichia coli, we isolated Rev suppressor mutations that alleviated the deleterious effect of mutations in RRE stem–loop IIB, the high affinity RNA-binding site for Rev. All of these suppressor mutations map to a single arginine-deficient face of a Rev α-helix, and some alter the binding specificity of the protein, providing genetic evidence for direct contacts between specific Rev amino acids and RNA nucleotides in the RNA complex of Rev. The spatial constraints suggested by these data have enabled us to model the structure of this complex

Optical parametric generator based on orientation-patterned gallium phosphide

We report the first pulsed optical parametric generator based on Orientation-patterned Gallium Phosphide. The output is tunable from 1721-1850 nm (signal) and 2504-2787 nm (idler), providing a total output power of 18 mW.Peer ReviewedPostprint (author's final draft

Critically phase-matched Ti:sapphire-laserpumped deep-infrared femtosecond optical parametric oscillator based on CdSiP2

We report a high-repetition-rate femtosecond optical parametric oscillator (OPO) for the deep-infrared (deep-IR) based on type-I critical phase-matching in CdSiP2 (CSP), pumped directly by a Ti:sapphire laser. Using angle-tuning in the CSP crystal, the OPO can be continuously tuned across 7306–8329 nm (1201–1369  cm−1) in the deep-IR. It delivers up to 18 mW of idler average power at 7306 nm and >7  mW beyond 8000 nm at 80.5 MHz repetition rate, with the spectra exhibiting bandwidths of >150  nm across the tuning range. Moreover, the signal is tunable across 1128–1150 nm in the near-infrared, providing up to 35 mW of average power in ∼266  fs pulses at 1150 nm. Both beams exhibit single-peak Gaussian distribution in TEM00 spatial profile. With an equivalent spectral brightness of ∼5.6×1020photons s−1 mm−2 sr−10.1% BW−1, this OPO represents a viable alternative to synchrotron and supercontinuum sources for deep-IR applications in spectroscopy, metrology, and medical diagnostics.Peer ReviewedPostprint (author's final draft

Femtosecond deep-infrared optical parametric oscillator pumped directly by a Ti:sapphire laser

We report a high-repetition-rate femtosecond optical parametric oscillator (OPO) for the deep-infrared (deep-IR) based on the nonlinear optical crystal, CdSiP2 (CSP), pumped directly by a Ti:sapphire laser, for the first time. By pumping CSP at <1 μm, we have achieved practical output powers at the longest wavelengths generated by any Ti:sapphire-pumped OPO. Using a combination of pump wavelength tuning, type-I critical phase-matching, and cavity delay tuning, we have generated continuously tunable radiation across 6654−8373 nm (1194−1503 cm-1) at 80.5 MHz repetition rate, providing up to 20 mW of average power at 7314 nm and <7 mW beyond 8000 nm, with idler spectra exhibiting bandwidths of 140−180 nm across the tuning range. Moreover, the near-IR signal is tunable across 1127−1192 nm, providing up to 37 mW of average power at 1150 nm. Signal pulses, characterised using intensity autocorrelation, have durations of ∼260–320 fs, with corresponding time-bandwidth product of ∆υ∆τ∼1. The idler and signal output exhibit a TEM00 spatial profile with single-peak Gaussian distribution. With an equivalent spectral brightness of ∼6.68×1020 photons s-1 mm-2 sr-1 0.1% BW-1, this OPO represents a viable table-top alternative to synchrotron and supercontinuum sources for deep-IR applications in spectroscopy, metrology and medical diagnostics.Peer ReviewedPostprint (author's final draft