Chirped pulse amplification in single mode Tm:fiber using a chirped Bragg grating

We report femtosecond pulse generation and chirped pulse amplification in Tm:fiber. A mode-locked oscillator operating in the soliton regime produced 800 fs pulses with 5 nm spectral bandwidth, at 40 pJ pulse energy. This oscillator seeded a pre-amplifier that utilizes a Raman soliton self-frequency shift to produce wavelength tunable pulses with 3 nJ energy, reduced pulse duration of 150 fs, and increased bandwidth of 30 nm. For further amplification, the pulses were stretched up to 160 ps using a chirped Bragg grating (CBG). Stretched pulses were amplified to 85 nJ after compression in single-mode Tm:fiber and recompressed with the CBG as short as 400 fs. Compressed pulses were coupled into a highly nonlinear tellurite fiber to investigate the potential of this ultrashort pulse 2-μm fiber source as a pump for mid-IR supercontinuum generation.     

Sulfur adsorption on Fe(1 1 0): a DFT study

The adsorption of atomic S on the Fe(1 1 0) surface is examined using density functional theory (DFT). Three different adsorption sites are considered, including the atop, hollow and bridge sites and the S is adsorbed at a quarter monolayer coverage in a p(2 × 2) arrangement. The hollow site is found to be the most stable, followed by the bridge and atop sites. At all three sites, S adsorption results in relatively minor surface reconstruction, with the most significant being that for the hollow site, with lateral displacements of 0.09 Å. Comparisons between S-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the S. At the hollow site, the presence of S causes an increase in the surface Fe d-orbital density of states between 4 and 5 eV. However, S adsorption has no significant effect on the structure and magnetic properties of the lower substrate layers.     

A moving grid finite element method applied to a model biological pattern generator

Many problems in biology involve growth. In numerical simulations it can therefore be very convenient to employ a moving computational grid on a continuously deforming domain. In this paper we present a novel application of the moving grid finite element method to compute solutions of reaction–diffusion systems in two-dimensional continuously deforming Euclidean domains. A numerical software package has been developed as a result of this research that is capable of solving generalised Turing models for morphogenesis.     

Molecular contrast in optical coherence tomography by use of a pump-probe technique

We describe a novel technique for contrast enhancement in optical coherence tomography (OCT) that makes possible molecular-specific imaging for what is believed to be the first time. A pump-probe technique is employed in which a pulsed pump laser is tuned to ground-state absorption in a molecule of interest. The location of the target molecule population is derived from the resulting transient absorption of OCT sample-arm light acting as probe light. A signal processing technique for three-dimensional localization of the transient absorption signal is described, and preliminary results exhibiting OCT contrast from methylene blue dye in multilayer and scattering phantoms are presented.     

A comparison of the fragmentation thresholds and inertial cavitation doses of different ultrasound contrast agents

Contrast bubble destruction is important in several new diagnostic and therapeutic applications. The pressure threshold of destruction is determined by the shell material, while the propensity for of the bubbles to undergo inertial cavitation (IC) depends both on the gas and shell properties of the ultrasound contrast agent (UCA). The ultrasonic fragmentation thresholds of three specific UCAs (Optison, Sonazoid, and biSpheres), each with different shell and gas properties, were determined under various acoustic conditions. The acoustic emissions generated by the agents, or their derivatives, characteristic of IC after fragmentation, was also compared, using cumulated broadband-noise emissions (IC "dose"). Albumin-shelled Optison and surfactant-shelled Sonazoid had low fragmentation thresholds (mean = 0.13 and 0.15 MPa at 1.1 MHz, 0.48 and 0.58 MPa at 3.5 MHz, respectively), while polymer-shelled biSpheres had a significant higher threshold (mean = 0.19 and 0.23 MPa at 1.1 MHz, 0.73 and 0.96 MPa for thin- and thick-shell biSpheres at 3.5 MHz, respectively, p<0.01). At comparable initial concentrations, surfactant-shelled Sonazoid produced a much larger IC dose after shell destruction than did either biSpheres or Optison (p<0.01). Thick-shelled biSpheres had the highest fragmentation threshold and produced the lowest IC dose. More than two and five acoustic cycles, respectively, were necessary for the thin- and thick-shell biSpheres to reach a steady-state fragmentation threshold.     

Emission spectrum of hot HDO below 4000 cm

Fourier transform emission spectra were recorded using a mixture of H₂O and D₂O at a temperature of 1500 °C. The spectra were recorded in three overlapping sections and cover the wavenumber range 1800-3932 cm⁻¹. This spectrum is analyzed together with a previously reported one spanning the 380-2190 cm⁻¹ range [Parekunnel et al., J. Mol. Spectrosc. 2001 (28) 101]. This analysis leads to 4409 newly assigned HDO emission lines. This work particularly extends data on the (200) and (120) states of HDO for which newly determined energy levels are presented.     

Wet-etch optimization of free-standing terahertz frequency-selective structures

Free-standing frequency-selective surfaces consisting of approximately 10-microm-thick copper films with cross-aperture arrays are found to be tunable toward lower frequencies by means of wet chemical etching. Center frequencies were tuned from 1.57 to 1.53 THz while maintaining high transmittance. Wet etching also adjusts bandwidth, peak transmittance, and sidelobe transmittance. The advantage of the wet-etch technique is demonstrated by employment of these devices as bandpass filters for difluoromethane-based terahertz lasers. Adjustment in aperture dimensions because of etching results in suppression of a competing laser line (133.93 microm) by 15 dB while maintaining high transmittance at the operating wavelength of 192.06 microm.     

Experimental observations of a left-handed material that obeys Snell's law

We measure two-dimensional profiles of collimated microwave beams transmitted through composite wire and split-ring resonator prisms. Prior experiments suggest these structures have a negative index of refraction, though these claims have been questioned. Our 2D measurements demonstrate that transmission obeys Snell's law with a negative index, confirming the refractive nature of this signal and refuting alternatives posed in the criticisms. In addition, we present preliminary evidence that a flat rectangular slab of this material can focus power from a point source.     

Growth model for atomic ordering: the case for quadruple-period ordering in GaAsSb alloys

Quadruple-period ordering in GaAsSb alloys is studied both theoretically and experimentally. A growth model is proposed to account for the observed three-dimensional (3D) ordered structure. The model is qualitatively different from the widely accepted surface reconstruction and dimerization-induced ordering models that strictly speaking explain only the in-plane 2D patterns. Here, we show that the already ordered substrate will affect the reconstruction of the growth front with respect to the substrate to ensure a correct stacking of the individual 2D ordered layers into the observed 3D lattice.     

New interpretations of XPS spectra of nickel metal and oxides

A current interpretation of XPS spectra of Ni metal assumes that the main 6 eV satellite is due to a two hole c3d⁹4s² (c is a core hole) final state effect. We report REELS observation in AES at low voltages of losses (plasmons and inter-band transitions) corresponding to the satellite structures in Ni metal 2p spectra. The satellite near 6 eV is attributed to a predominant surface plasmon loss. A current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignments of the main peak at 854.6 eV and the broad satellite centred at around 861 eV to the cd⁹L and the unscreened cd⁸ final-state configurations, respectively (L is a ligand hole). Multiplet splittings have been shown to be necessary for assignment of Fe 2p and Cr 2p spectral profiles and chemical states. The assignments of Ni 2p states are re-examined with intra-atomic multiplet envelopes applied to Ni(OH)₂, NiOOH and NiO spectra. It is shown that the free ion multiplet envelopes for Ni²⁺ and Ni³⁺ simulate the main line and satellite structures for Ni(OH)₂ and NiOOH. Fitting the NiO Ni 2p spectral profile is not as straightforward as the hydroxide and oxyhydroxide. It may involve contributions from inter-atomic, non-local electronic coupling and screening effects with multiplet structures significantly different from the free ions as found for MnO. A scheme for fitting these spectra using multiplet envelopes is proposed.