D.M. Bubb

Saturable and reverse saturable absorption in silver nanodots at 532 nm using picosecond laser pulses

Saturable absorption (SA) and reverse saturable absorption (RSA) at 532 nm were observed in Ag nanodots prepared by pulsed laser deposition. The real [Re χ(3)] and imaginary [Im χ(3)] parts of the third order nonlinearity of these films were measured using the Z-scan technique. At low input irradiances, the decrease in absorption near the focal point results from a negative Im χ(3) and yields SA. At higher input irradiance, RSA becomes dominant. The transition from SA to RSA suggests that there is a threshold irradiance at which additional nonlinear process(es) is (are) involved and become dominant. The recovery time of these nonlinear processes was measured by a degenerate pump-probe experiment. The effects are explained in terms of the electron dynamics in the excited states.

Liquid crystal composites with a high percentage of gold nanoparticles

We have reported in our previous work that doping low concentrations (up to 10% by weight) of gold nanoparticles (GNP) in a polar nematic 4’-hexyl-4-biphenylcarbonitrile (HBPCN) increases the dielectric anisotropy, while the switching voltage and times, and the nematic–isotropic liquid (IL) transition point of the mixtures are not affected by doped nanoparticles. In the current work we extend our study of the behaviour of HBPCN doped with higher than 10% GNP. We show that at certain gold concentrations – 35% and 45% – the nematic–IL phase transition point increases by 15°C in comparison with the pure nematic value. At the same concentrations the dielectric anisotropy increases from its value for the pure nematic by about 2.2 times for 35% and twice for 45%. Also, the threshold voltage increases by 0.2 V for 35% and decreases by 0.15 V for 45%. However, the switching-off times decrease for both concentrations: 7 ms for 35% and 12 ms for 45%. We propose that the described effects of doped GNP on the properties of the nematic are due to the formation of different kinds of aggregations between two components of the mixtures.

Effects of the absorption coefficient on resonant infrared laser ablation of poly(ethylene glycol)

We describe experiments on resonant infrared laser ablation of poly(ethylene glycol) (PEG) at two different resonant excitation wavelengths and for different molecular weights of PEG. The two resonant wavelengths correspond to different stretching vibrations of the polymer and have absorption coefficients that differ by roughly an order of magnitude. Ablation via excitation of the O–H terminal group stretching mode at 2.94 μm, the weaker of the two absorptions, is delayed in time by several microseconds with respect to ablation at 3.47 μm, the more strongly absorbing C–H stretching mode of the polymer. Time-resolved plume shadowgraphs along with ablation rate measurements for the two modes reveal that the absorption coefficient strongly affects the physical characteristics of the ejecta and plume, as well as the time scale for material removal. Temperature-rise calculations demonstrate that phase explosion is likely the operative mechanism in ablation at the C–H mode, while normal boiling may play a role ...

Laser vaporization of trace explosives for enhanced non-contact detection

Trace explosives contamination is found primarily in the form of solid particulates on surfaces, due to the low vapor pressure of most explosives materials. Today, the standard sampling procedure involves physical removal of particulate matter from surfaces of interest. A variety of collection methods have been used including air-jetting or swabbing surfaces of interest. The sampled particles are typically heated to generate vapor for analysis in hand held, bench top, or portal detection systems. These sampling methods are time-consuming (and hence costly), require a skilled technician for optimal performance, and are inherently non-selective, allowing non-explosives particles to be co-sampled and analyzed. This can adversely affect the sensitivity and selectivity of detectors, especially those with a limited dynamic range. We present a new approach to sampling solid particles on a solid surface that is targeted, non-contact, and which selectively enhances trace explosive signatures thus improving the selectivity and sensitivity of existing detectors. Our method involves the illumination of a surface of interest with infrared laser light with a wavelength that matches a distinctive vibrational mode of an explosive. The resonant coupling of laser energy results in rapid heating of explosive particles and rapid release of a vapor plume. Neighboring particles unrelated to explosives are generally not directly heated as their vibrational modes are not resonant with the laser. As a result, the generated vapor plume includes a higher concentration of explosives than if the particles were heated with a non-selective light source (e.g. heat lamp). We present results with both benchtop infrared lasers as well as miniature quantum cascade lasers.

On the mechanism of resonant infrared polymer ablation: the case of polystyrene

We investigate the fundamental mechanisms of resonant-infrared laser ablation of polymers using polystyrene as a model material. Time-resolved plume shadowgraphy coupled with laser-induced temperature-rise calculations indicate that spinodal decomposition of a superheated surface layer is the primary mechanism for the initial stages of material removal. The majority of the ablated material is then released by way of recoil-induced ejection of liquid which proceeds for some tens of microseconds following a ~μs laser pulse excitation. The recoil-induced ejection of liquid material as the dominant ablation mechanism helps to explain previous observations of laser deposition of intact polymeric material.

Mechanism of resonant infrared laser vaporization of intact polymers

Experiments on pulsed laser vaporization of many different kinds of polymers have demonstrated that it is possible to eject intact polymers into the ambient, whether air or vacuum, by resonant pulsed laser excitation, using both neat and matrix targets. Two recent studies of resonant infrared ablation - one on polystyrene, the other on poly(amic acid), the precursor for the thermoset polyimide - show moreover that the ablation process is both wavelength selective and surprisingly non-energetic, especially compared to ultraviolet laser ablation. We propose a wavelength-selective photothermal mechanism involving breaking of intermolecular hydrogen bonds that is consistent with these observations.

ANTIMICROBIAL TESTING, MORPHOLOGICAL CHARACTERIZATION, AND SURFACE POTENTIAL MAPPING OF SILVER-POLY-(METHYL METHACRYLATE) NANOCOMPOSITE FILMS MADE THROUGH MATRIX-ASSISTED PULSED LASER DEPOSITION AGAINST S. AUREUS

Silver-poly(methyl methacrylate) nanocomposite thin films were deposited on silicon substrates by means of matrix-assisted pulsed laser deposition. Atomic force microscopy revealed that the surface morphologies of the silver-poly(methyl methacrylate) nanocomposite thin films were dependent on the matrix-assisted pulsed laser evaporation processing parameters. Films produced using 4% (by weight of solvent) silver nanoparticles exhibited the highest antimicrobial activity. Antimicrobial activity of the thin films was dependent on the silver concentration, the microenvironment created by the polymer–silver interaction, and the surface potential.

Mechanism of resonant infrared laser ablation of polystyrene

Although resonant infrared (RIR) pulsed laser deposition (PLD) was discovered in 2001 [1] and has since been applied to produce technologically interesting thin films and rudimentary opto-electronic devices [2], the mechanism governing RIR-PLD has been primarily a matter of conjecture. Understanding these mechanisms at the microscopic level has significant implications for process control in thin-film deposition, by linking laser parameters to film characteristics such as surface roughness. In this paper, we describe new experiments and calculations on the model material polystyrene that facilitate quantitative conclusions about these issues.

Laser Processing of Polymers and Biomaterials with Matrix Assisted Methods and Resonant Interactions

Recent and new work with laser processing of polymers and biomaterials is reviewed. Matrix assisted pulsed laser deposition has been used in order to deposit thin films and characterized the films with a variety of techniques.

Resonant infrared pulsed laser deposition of polymers using a picosecond tunable free-electron laser

Summary from only given. Resonant picosecond-pulse, mid-infrared laser irradiation has been shown to ablate glassy and crystalline solids with high efficiency and low collateral damage. We have extended this concept to show that resonant infrared (IR) pulsed-laser deposition (PLD) is an effective method for depositing polymer films with physical and chemical structure as well as optical properties virtually identical to those of the bulk starting material. This contrasts sharply with PLD at ultraviolet (UV) wavelengths, where deposited polymer material is significantly degraded.