Swelling and modification of silicone surface by F2 laser irradiation

The surface of silicone rubber swelled and was modified by 157-nm F₂ laser irradiation at a laser fluence less than the ablation threshold. The irradiated surface swelled to a height of approximately 3 m. Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy showed that the irradiated surface was modified to SiO₂. 193-nm ArF laser irradiation of the silicone rubber induced the surface to swell, but not to modify to SiO₂. The IR peaks of end groups of silicone were observed in the FT-IR spectra of the surface. From these results, it is concluded that the main chains (Si-O) of silicone were photodissociated and generation of low molecular weight silicones caused the swelling. In addition, it was observed that methane and carbon dioxide were released from silicone rubber when each laser beam irradiated it. These gases were generated by photodissociation of the side chains (Si-CH₃) of silicone. An F₂ laser beam can photodissociate the Si-CH₃ bonds and the Si-O bonds of silicone and O₂ effectively even at a laser fluence less than the ablation threshold, resulting in the modification to SiO₂ and the swelling. PACS 61.80.Ba; 61.82.Pv; 82.50.Hp     

Pulsed laser deposition of c * axis oriented pentacene films

Pulsed laser desorption and film deposition behaviors have been investigated on pentacene as an organic molecule primarily due to its applications in electronics. The laser desorption time-of-flight (LDTOF) mass spectrum exhibited a single parent peak when a pressed pentacene pellet was ablated by an N₂ laser beam of its fluence lower than 100 mJ/cm², indicating that pentacene could be evaporated without an appreciable photodecomposition by the pulsed laser beam. Nd:YAG pulsed laser deposition of pentacene films was performed using such optimization parameters as laser fluences and wavelength (second, third and forth harmonic generations (SHG, THG, FHG)). The analyses with AFM, XRD and UV-Vis spectroscopy revealed the fabrication of c^* axis oriented pentacene films on quartz, silicon, and CaF₂ substrates by the SHG. The SHG films have a surface morphology superior to those of films deposited by THG and FHG. PACS 81.15.Fg; 81.05.Lg; 82.80.Rt     

Morphology and composition on Al surface irradiated by femtosecond laser pulses

We study the surface chemicals and structures of aluminum plates irradiated by scanning femtosecond laser pulses in air for a wide range of laser fluence from 0.38 to 33.6 J/cm². X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate clearly that crystalline anorthic Al(OH)₃ is formed under femtosecond laser pulse irradiation. Besides aluminum hydroxide, crystalline Al₂O₃ is also found in the samples irradiated at high laser fluence. Field emission scanning electron microscopy demonstrates that the surfaces of the samples irradiated with low laser fluence are colloidal-like and that nanoparticles with a few nanometers in size are embedded in glue-like substances. For high laser fluence irradiated samples, the surfaces are highly porous and covered by nanoparticles with uniform size of less than 20 nm.     

Experimental study of laser induced decomposition based processing of a brittle plastic,CR-39 (allyl diglycol carbonate)

A cw CO₂ laser, coupled with an astigmatism free beam focussing mirrors arrangement is used for processing a brittle plastic, CR-39 without producing cracks, vents or chips. The processing is based on the formation of volatile products of laser-induced decomposition in the plastic. Threshold fluence for the decomposition (found to be independent of the power density and beam residence time) in CR-39 at=10.6m is determined to be 25 J cm^–2 and the decomposition threshold power density for cw irradiation 2.1±0.5 W cm^–2. The depth and width of the tapered laser processed region are observed to increase with power density and beam residence time. The widths attain a steady state value of 1 mm at beam residence time above 65 ms, for a fixed power density (2.5×10⁴ W cm^–2) and sheet thickness (250 m). Taper angle of the edges decreases with increasing power density and/or beam residence time. The heat affected zone (measured in crossed polarisers) around the processed region is found to extend with increasing beam residence time but remains unaffected on changing power density. The results are discussed in terms of the optical and thermophysical properties of CR-39 and the parameters of the interacting laser beam.     

Excimer-laser-induced permanent electrical conductivity in solid C60 films

After being irradiated in air by a XeCI (308 nm) excimer laser, the electrical conductivity of solid thin-film C₆₀ has been improved by more than six orders of magnitudes. The products resulting from laser irradiation of C₆₀ films have been investigated by Raman scattering and the onset of conductivity can be attributed to laser-induced oxygenation and disintegration of the fullerene. Irradiated by 40 ns laser pulses with different fluence, products with different microstructure were observed. At lower fluence, the Raman features of microcrystalline graphite and fullerene polymer were observed. At a fluence just below the ablation threshold (36 mJ/cm²), the fullerene molecules in the film were disintegrated completely and transformed to amorphous graphite.     

Dopant-induced excimer laser ablation of poly(tetrafluoroethylene)

Clean ablation of poly(tetrafluoroethylene) (PTFE) at etch rates in excess of 7µm/pulse has been achieved with an excimer laser using 308nm radiation and a 25 ns pulse width. This was accomplished by doping the ultraviolet-transparent PTFE polymer with polyimide. Ablation rates were investigated as a function of fluence in the range from 1 to 12J/cm² and dopant levels up to 15% (wt/wt). Results show that at a given fluence there exists an optimum absorption coefficient _max, for which maximum ablation rates are achieved. The value of _max was found to decrease with increasing fluence. The relationship between _max and fluence was determined from existing ablation rate models and found to compare favorably with empirical results.     

Pulsed CO2 laser induced multiphoton decomposition of cyclobutanone: A comparison of predictions for three models of decomposition probability vs. fluence

The multiphoton decomposition (MPD) probability (f) for photolysis of cyclobutanone (pressure: 0.333 kPa) with a pulsed (pulse width: 40 ns) CO₂ laser tuned to the 9P(20) line, was studied,in a collimated beam geometry, as a function of fluence (=0.75 to 11.7 J/cm²). The values off () were fitted to three suggested 1- or 2-parameter models. Models with two parameters correlated the experimental values equally well, whereas a 1-parameter model was less accurate for low fluences. The MPD yield for two focussed beam geometries was measured and compared to predicted yields obtained by integrating the three models forf () over the focussed beam geometries. All three models predicted yields which agreed well with the measured values.Issued as AECL Contribution Number 8943AECL National Program Summer (1983)     

Laser-induced control of (multichannel) intracluster reactions

An experimental and theoretical study on the excited Ba FCH₃(A) photodissociation yield as a function of the excitation laser fluence is reported. Experimentally, it was found that the two-photodissociation channel yields, i.e. the reactive BaF and non-reactive Ba^* products, increased exhibiting a similar behaviour, as the laser fluence changed from 0.2 up to ca. 4 mJ/cm². Beyond this value the BaF yield levels off and the Ba^* decreases over the 4-7 mJ/cm² range. The theoretical simulation of the excited state electron-ion dynamics within the time-dependent density functional theory revealed that the reactive channel dominated the photofragmentation dynamics as it occurs within a femtosecond time scale and became accelerated as the photodissociation laser fluence increased. By contrast, the non-reactive channel only manifested for low laser fluences at the nano/picosecond time regime resulted inactive as the laser fluence increased. A simple scheme to control the dynamics of the intracluster multichannel reaction is suggested in which the slowest the channel the easiest to close it as the excitation laser power increases.Received: 3 July 2003PACS: 36.40.Jn Reactivity of clusters - 36.40.Qv Stability and fragmentation of clusters - 34.50.Rk Laser-modified scattering and reactions     

The characteristics of multiple-photon absorption of SF6 molecules cooled in free jet expansion from a pulsed supersonic nozzle

The multiple-photon absorption of pulsed TEA CO₂ laser radiation by SF₆ molecules cooled toT _R40K andT _v160K in the free jet expansion from a pulsed supersonic nozzle has been investigated at energy fluences of 0.1 to 3.0 J·cm^–2.For practically all laser lines which coincide with the linear absorption spectrum of thev ₃ vibrational mode of SF₆ [P(12)...P(28), 10,6 m], the dependence of the absorbed energyE _ab on the exciting energy fluence was found to be steeper than linearE _abⁿ, wheren=(1.1 to 1.8). Considerable increases of the absorption cross sections with increasing energy fluence were observed.The fraction of the molecules interacting with the laser radiation is estimated.     

Growth of polycrystalline La0.5Sr0.5CoO3 films by femtosecond pulsed laser deposition

In this paper we present the growth of La_0.5Sr_0.5CoO₃ (LSCO) films on MgO, quartz, and silicon substrates by pulsed laser deposition (PLD) using a Ti:sapphire laser (50 fs, 800 nm wavelength). The morphology and the structure of the films were studied by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The films were polycrystalline and exhibit a good adherence to the Si substrate. Different deposition parameters such as substrate temperature, oxygen pressure, and laser fluence were varied to achieve good surface quality and low resistivity crystalline films. We also defined the optimum conditions in which the deposited film surface is particulate free. The best films (droplets free) were grown at 625 °C, in an ambient oxygen pressure of 6 mbar, with an incident laser fluence of 0.19 J/cm². This is a mandatory step in the complex work of fabricating La_0.5Sr_0.5CoO₃/BaTiO₃/La_0.5Sr_0.5CoO₃ heterostructures for the development of thin film capacitors for non-volatile ferroelectric access memory devices. PACS 81.15 Fg; 42.62-b; 68.65.Ac     

Selective ir laser chemistry of CDF3 in natural fluoroform

Selective decomposition of CDF₃ at natural abundance level (150ppm) in fluoroform has been achieved by infra-red multiple-photon excitation at moderate substrate pressure using 100 ns FWHM CO₂ laser pulses. Effects of energy fluence, number of laser pulses, buffer gas pressure and substrate pressure on decomposition yield and bulk selectivity are reported and discussed.     

Dual excimer and CO2 laser etching studies of polyethylene terephthalate

Polyethylene terephthalate (PET) films preheated with a pulsed CO₂ laser have been ablatively etched with an XeCl laser. The observed reduction in ablation threshold, from 170 to 140 mJ cm^–2, is consistent with a thermal mechanism for XeCl laser ablation of PET. Transient changes in the UV absorption coefficient of PET caused by heating with pulsed CO₂ laser radiation have also been studied and a significant increase in absorption observed at 308 nm. Permanent changes in the ultraviolet absorption of PET following exposure to low fluence XeCl laser radiation are also reported.     

Observations on the spectral dependence andT/D isotope selectivity in the CO2 laser multiple-photon dissociation of trifluoromethane

Pulsed CO₂ laser multiple-photon dissociation of CTF₃ (v ₂ mode) bathed in argon and CDF₃, CHF₃ or CCl₄ is examined as a function of laser wavelength (9.2–9.6 m) and fluence. The dependence of the dissociation profile on wavelength is analyzed and comparisons are made to prior work. The single-stepT/D enrichment factor for infrared photolysis of trifluoromethane is measured; potential practical isotope separation is discussed. Pulsed infrared laser photolysis of CTF₃ (v ₅ mode) using a 12 m NH₃ laser is also attempted.     

UV laser micromachining of piezoelectric ceramic using a pulsed Nd:YAG laser

UV laser (=355 nm) ablation of piezoelectric lead zirconate titanate (PZT) ceramics in air has been investigated under different laser parameters. It has been found that there is a critical pulse number (N=750). When the pulse number is smaller than the critical value, the ablation rate decreases with increasing pulse number. Beyond the critical value, the ablation rate becomes constant. The ablation rate and concentrations of O, Zr and Ti on the ablated surface increase with the laser fluence, while the Pb concentration decreases due to the selective evaporation of PbO. The loss of the Pb results in the formation of a metastable pyrochlore phase. ZrO₂ was detected by XPS in the ablated zone. Also, the concentrations of the pyrochlore phase and ZrO₂ increase with increasing laser fluence. These results clearly indicate that the chemical composition and phase structure in the ablated zone strongly depend on the laser fluence. The piezoelectric properties of the cut PZT ceramic samples completely disappear due to the loss of the Pb and the existence of the pyrochlore phase. After these samples were annealed at 1150 °C for 1 h in a PbO-controlled atmosphere, their phase structure and piezoelectric properties were recovered again. Finally, 1–3 and concentric-ring 2–2 PZT/epoxy composites were fabricated by UV laser micromachining and their thickness modes were measured by impedance spectrum analysis and a d₃₃ meter. Both composites show high piezoelectric properties. PACS 81.40.Gh; 77.84.-s     

Efficient ablation of organic polymers polyether sulphone and polyether ether ketone by a TEA CO2 laser with high perforation ability

Organic polymer (PES: PolyEther Sulphone and PEEK: PolyEther Ether Ketone) ablation with oscillation-line selected TEA CO₂ lasers is successfully demonstrated. With different irradiation conditions the ablative etch-rate slopes were varied, which means that the ablation process is dependent on the ablation conditions such as incident laser intensity and ambient gas. In perforation processing of the PEEK film, the TEA CO₂ laser had a higher etch rate of 42 m/pulse at a fluence of 70 J/cm² in vacuum than the XeCl laser.     

Comparison of the transmission behavior of a triazeno-polymer with a theoretical model

The threshold fluence,F _Th, of ablation of a triazeno-polymer was measured in the low fluence range for thin films using conventional UV-spectroscopy. It was found that there is a clearly definedF _Th for 308 nm irradiation between 20 and 25 mJ cm^–2. In the case of 248 nm irradiation, a threshold fluence range between 16 and 32 MJ cm^–2 was found. The ablation rate for both irradiation wavelengths depends on film-thickness. For the XeCl excimer-laser, the point at which the rate becomes independent of thickness was observed to lie at a value which did not correspond to the calculated laser penetration depth, whereas for the KrF laser the independence was not reached within the applied thickness range (up to 0.35 m). Additional transmission measurements have been performed showing that the target transmission at 248 nm increases only slightly, whereas for 308 nm the transmission increases by a factor of approximately 4. This result shows that dynamic target absorption properties are very important for describing the ablation process. The results derived from the transmission studies and etch rates were analyzed theoretically with a two-level model of chromophore absorption. For 248 nm irradiation this model can describe the transmission behavior and the ablation rate. In the case of 308 nm irradiation, it was only possible to match one data set. A good agreement with the experimental transmission ratio does not match the ablation rate and vice versa.     

Generation of more than 40 laser emission lines from the ultraviolet to the visible regions by two-color stimulated raman effect

An intense and monochromatic laser beam consisting of more than 40 vibrational and rotational lines is generated by the stimulated Raman effect, when two-color laser beams separated by 590 cm^–1 are tightly focused into pressurized hydrogen. This phenomenon is reasonably explained by four-wave mixing; one beam is used for two-step excitation (₁+₁) to an imaginary level and another beam (₂) acts as a seed beam, generating a fourth beam (2₁–₂). Through cascade processes, so many laser emissions appear with a frequency difference of (₁–₂) in entire ultraviolet and visible regions.     

High-contrast all-optical switching with Pt:ethynyl complex

We have theoretically analyzed all-optical switching in Pt:ethynyl complex based on nonlinear excited-state absorption. A detailed analysis for Pt:ethynyl complex has been presented based on rate equation approach. It is shown that a pulsed pump laser beam at 355 nm switches the transmission of a cw probe laser beam at 633 nm through a Pt:ethynyl sample. The effect of various parameters, such as pump pulse width, peak pumping intensity, normalized parameter , transition times of S₁→S₀ and S₁→T₁ states and lifetime of triplet state, on switching characteristics has been analyzed in detail. It has been shown that the probe beam can be completely switched off (i.e. 100% modulation) by a pulsed pump laser beam at 50 kW/cm². These results have been used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses.     

Growth enhancement of UF5 nano-particles assisted byα-ray ionization

It was demonstrated experimentally for the first time that particle growth of photoproduced UFS was enhanced by ionizing radiation from an -emitting²⁴¹Am source. A gas mixture of UF₆, CH₄, and Ar in a reaction cell was irradiated with a single shot of a KrF laser at 248 nm to form UF5 particles. Transmission electron microscope images of the samples indicated that the average size of UF₅ particles produced without the -ray source was independent of the laser energy. UFS particles produced with the -ray source were found to be much larger in size than those produced without the a-ray source, and the enhancement of particle growth was marked when the laser energy was low. An explanation was attempted for the anomalous behaviors of particle size distribution from the viewpoint of competition between the second-order coagulation rate and the rate of diffusion from the reaction zone.     

Light source and geometrical requirements for the optimization of optical anemometry signals

The criteria for the design of optical arrangements for laser anemometry are formulated for reference-beam, two-beam and single-beam modes of operation. The dependence of useful light intensity upon optical path-length difference and number of axial laser modes is calculated. Laser power requirements are evaluated and the dependence upon band-pass filtering is quantitatively assessed. A new two-channel integrated-optical unit, with light-path compensation, and embodying the proposed design criteria, is described.Nomenclature c velocity of light - D _1/e diameter of laser beam at 1/e-point - d half distance separating the beams leaving the lens - d _m effective diameter of measuring control volume - d _ph diameter of aperture in front of photo-multiplier - d _r waist diameter of focused reference beam - d _s waist diameter of scattering light beam - d _l waist diameter of fucused light beam - F f-number of lens - scattering function introduced in Mie's theory - f signal frequency - f bandwidth of filter - f _D Doppler line width of laser radiation - f _G effective bandwidth of gain envelope of a laser - f _M frequency difference between two adjacent axial modes - h Planck constant (6.6256×10^–34J sec) - K 2/ wave number - L cavity length of laser - l _m length of measuring control volume - m total number of axial modes of laser - M magnification (a/b) - N _fr number of fringes in crossing region of the two light beams - N _ph number of fringes seen by photomultiplier - N _s number of photons scattered per particle passage - N _e number of electrons leaving photo cathode per unit time - P _l total light power emitted by laser - P _s total light power scattered by particle - Q _scat scattering coefficient - R distance from particle centre to point on plane of observation - r _p radius of scattering particle - u velocity component perpendicular to fringe pattern - _fr distance between fringes inside measuring control volume - _c efficiency of light collecting system - _q overall quantum efficiency of photodetector - g _q coordinate measuring angle from the optical axis - half angle between wave fronts - wave length of light - frequency of incident light wave - _e standard deviation of electron flux - transit time of scattering particle - solid angle - d solid angle element This paper was presented at the Conference on Electro-optic Systems in Flow Measurement, Southampton, September 1972.