Double-pump technique – one step closer towards efficient liquid-based THz sources

By irradiating a water jet with double pulses, we demonstrate 4-fold higher THz wave generation than for a single pump pulse. The dependence of the enhanced THz signal on the temporal delay between two collinear pulses reveals the optimal time for launching signal pulse is near 2-4 ps, which corresponds to the time needed to create the complete pre-ionization state when sufficient electron density is already induced, and there is no plasma reflection of the pump pulse radiation. The increase in THz waves generation efficiency corresponds to the case of water jet excitation by the pulses with an optimal duration for a certain jet thickness, which is determined by the spatial pulse size. Using a theoretical model of the interaction of a high-intensity sub-picosecond pulse with an isotropic medium, we held a numerical simulation, which well describes the experimental results when using 3 ps value of population relaxation time. Thus, in this work, double pump method allows not only to increase the energy of the generated THz waves, but also to determine the characteristic excited state lifetime of liquid water. The optical-to-terahertz conversion efficiency in case of double pulse excitation of water column is of the order of 0.5·10−3, which exceeds the typical values for THz waves generation during two-color filamentation in air and comparable with the achievable values due to the optical rectification in some crystals

Improving the hydrophobicity of polymers through surface texturing

Improving the hydrophobic property of polymers is essential considering the wide usage of polymers in various applications, including biomedical applications, and their tendency to degrade with long-term exposure to moisture environments. Several surface modification techniques have been developed over the years to improve the hydrophobic property of polymers. However, their influence on enhancing the hydrophobicity and long-term mechanical performance is yet to be fully understood. Although surface modification such as texturing may improve hydrophobicity, these surfaces tend to have low wear and abrasion resistance, which can deteriorate the long-term mechanical performance of polymers used in various prosthesis applications. This research employs surface texturing as the surface modification technique to study the effect of surface modification on hydrophobic property of polymers. Theoretical study is conducted to investigate the feasibility of introducing surface textures on the polymers. Square and cylindrical surface textures, both protrusion and cavity, with variation in texture dimension are introduced on the surfaces of two model polymers, Ultrahigh Molecular Weight Polyethylene (UHMWPE) and High Density Polyethylene (HDPE); using a combination of laser engraving and hot pressing techniques. The surface textures are analyzed using a digital microscope and the hydrophobicity of polymer surfaces is measured using a custom-built water contact angle measurement setup. The results show that introduction of surface textures significantly improve the hydrophobicity of UHMWPE and HDPE. However, the relationship between texture type and dimension, and improvement in hydrophobicity is not straightforward. The study provides useful guidelines on improving the hydrophobic property of polymers

Characteristics of laser produced plasmas of hafnium and tantalum in the 1–7 nm region

Soft X-ray (SXR) spectra from hafnium and tantalum laser produced plasmas were recorded in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 170 ps and 10 ns, respectively, operating at a range of power densities. The maximum focused peak power density was 2.  3 × 1014 W cm-2 for 170 ps pulses and 1.  8 × 1012 W cm-2 for 10 ns pulses, respectively. Two intense quasicontinuous intensity bands resulting from n = 4 - n = 4 and n = 4 - n = 5 unresolved transition arrays (UTAs) dominate both sets of experimental spectra. Comparison with calculations performed with the Cowan suite of atomic structure codes as well as consideration of previous experimental and theoretical results aided identification of the most prominent features in the spectra. For the 10 ns spectrum, the highest ion stage that could be identified from the n = 4 - n = 5 arrays were lower than silver-like Hf25+ and Ta26+ (which has a 4d104f ground configuration) indicating that the plasma temperature attained was too low to produce ions with an outermost 4d subshell, while for the 170 ps plasmas the presence of significantly higher stages was deduced and lines due to 4d–5p transitions were clearly evident. Furthermore, we show an enhancement of emission from tantalum using dual laser irradiation, and the effect of pre-pulse durations and delay times between two pulses are demonstrated