Time-resolved Photography Research Articles

Laser micromachining of indium tin oxide films on polymer substrates by laser-induced delamination

A Q-switched neodymium : yttrium–aluminium–garnet (Nd : YAG) laser was used to ablate indium tin oxide (ITO) thin films from polyethylene terephthalate substrates. Film damage and partial removal with no evidence of a melt zone was observed above 1.7 J cm−2. Above the film removal threshold (3.3 J cm−2) the entire film thickness was removed without substrate damage, suggesting that ablation was a result of delamination of the film in the solid phase. Measurements of ablated fragment velocities near the ablation threshold were consistent with calculations of velocities caused by stress-induced delamination of the ITO film, except for a high velocity component at higher fluences. Nanosecond time-resolved shadowgraph photography revealed that the high velocity component was a shock wave induced by the rapid compression of ambient air when the film delaminated.

Plasma plume photography and spectroscopy of Fe—Oxide materials

Time-resolved photography was employed to study plasma dynamics and particle ejection of laser-irradiated iron oxide materials. Nano-particle powder, pressed powder pellets and sintered ceramics were ablated in air and Ar gas background by means of short laser pulses (Nd:YAG laser wavelength λ = 1064 nm and pulse duration τ L ≈ 6 ns; KrF laser λ = 248 nm and τ L ≈ 20 ns). Plasma plume dynamics significantly depended on sample morphology. The ejection of non-luminous particles up to several hundreds of microseconds after the laser pulse was observed for powder and pressed powder target materials. Laser-induced breakdown spectroscopy (LIBS) was employed for element analysis of iron oxide powders, pressed pellets and sintered ceramics. LIBS spectra of the different targets were comparable to each other and qualitatively independent of target morphology.

Mechanisms of Laser-Induced Dissection and Transport of Histologic Specimens

Rapid contact- and contamination-free procurement of histologic material for proteomic and genomic analysis can be achieved by laser microdissection of the sample of interest followed by laser-induced transport (laser pressure catapulting). The dynamics of laser microdissection and laser pressure catapulting of histologic samples of 80 μm diameter was investigated by means of time-resolved photography. The working mechanism of microdissection was found to be plasma-mediated ablation initiated by linear absorption. Catapulting was driven by plasma formation when tightly focused pulses were used, and by photothermal ablation at the bottom of the sample when defocused pulses producing laser spot diameters larger than 35 μm were used. With focused pulses, driving pressures of several hundred MPa accelerated the specimen to initial velocities of 100–300 m/s before they were rapidly slowed down by air friction. When the laser spot was increased to a size comparable to or larger than the sample diameter, both driving pressure and flight velocity decreased considerably. Based on a characterization of the thermal and optical properties of the histologic specimens and supporting materials used, we calculated the evolution of the heat distribution in the sample. Selected catapulted samples were examined by scanning electron microscopy or analyzed by real-time reverse-transcriptase polymerase chain reaction. We found that catapulting of dissected samples results in little collateral damage when the laser pulses are either tightly focused or when the laser spot size is comparable to the specimen size. By contrast, moderate defocusing with spot sizes up to one-third of the specimen diameter may involve significant heat and ultraviolet exposure. Potential side effects are maximal when samples are catapulted directly from a glass slide without a supporting polymer foil.

Principles of laser-induced separation and transport of living cells

Separation and transport of defined populations of living cells grown on a thin membrane can be achieved by laser microdissection (LMD) of the sample of interest, followed by a laser-induced forward transport process [laser pressure "catapulting" (LPC)] of the dissected cell cluster. We investigate the dynamics of LMD and LPC with focused and defocused UV-A laser pulses by means of time-resolved photography. Catapulting is driven by plasma formation when tightly focused pulses are used, and by confined thermal ablation at the bottom of the sample for defocused catapulting. With both modalities, the initial specimen velocity amounts to about 50 to 60 ms. Time-resolved photography of live cell catapulting reveals that in defocused catapulting, strong shear forces arise when the sample is accelerated out of the culture medium covering the cells. By contrast, pulses focused at the periphery of the specimen cause a fast rotational movement that minimizes the flow of culture medium parallel to the sample surface, and thus the resulting shear stresses. Therefore, the recultivation rate of catapulted cells is much higher when focused pulses are used. Compared to collateral damage by mechanical forces, side effects by heat and UV exposure of the cells play only a minor role.

Dynamics of a pulsed laser generated tin plasma expanding in an oxygen atmosphere

Semiconducting tin oxide can be successfully deposited by means of the laser ablation technique. In particular by ablating metallic tin in a controlled oxygen atmosphere, thin films of SnO x have been deposited. The partial oxygen pressure at which the films are deposited strongly influences both the stoichiometry and the structural properties of the films. In this work, we present a study of the expansion dynamics of the plasma generated by ablating a tin target by means of a pulsed laser using time and space resolved optical emission spectroscopy and fast photography imaging of the expanding plasma. Both Sn I and Sn II optical emission lines have been observed from the time-integrated spectroscopy. Time resolved-measurements revealed the dynamics of the expanding plasma in the ambient oxygen atmosphere. Stoichiometry of the films has been determined by means of X-ray photoelectron spectroscopy and correlated to the expansion dynamics of the plasma.

Ultrafast time-resolved photography of femtosecond laser induced modifications in BK7 glass and fused silica

The dynamics of absorption after excitation of fused silica and BK7 glass with femtosecond laser radiation are visualized by transient absorption spectroscopy. Focusing laser radiation with pulse durations in the picosecond time regime in BK7 glass generates free electrons with relaxation by emission of radiation or by formation of defects. The temporal and spatial emission characteristics are observed by high-speed photography in the streak mode. The beam waist moves within the pulse duration towards the incoming laser radiation by self-focusing and with the laser radiation absorbed by multi-photon processes. The dynamics of the long lasting stress formation is visualized by time-resolved Nomarski-Photography. The modification of the glass is investigated during and after irradiation with ultra-short pulsed laser radiation (100 fs<tp<3 ps) at the wavelength λ=810 nm. The formation of a sound wave in fused silica and BK7 glass is observed and the mechanical stress, depending on the excitation pulse duration, is measured.

Studies of pulsed high-current arcs used to prepare carbon films

Electric arcs have been widely used for the preparation of carbon films. In particular, the filtered cathode arc technique is known to be one of the best methods for the production of diamond-like carbon with high concentrations of sp 3-bonded carbon. In 1999 the possibility of producing diamond micro-crystallites [Nature 402 (1999) 162] using a high-current arc pulse, >1000 A with a 60-ms duration, between two 0.5-mm-diameter graphite rods. For such high-density plasma systems, the relative density of the deposit precursors, and their spatial variation, energy, degree of ionisation, etc. are very important, since these determine the characteristics of the deposits produced. In this paper we report optical studies involving time-resolved emission spectroscopy (identification of the emitted species at different delay times), time-resolved photography (temporal and spatial evolution of the arc) and shadowgraphy (temporal and spatial evolution of the plasma and particle formation) of a pulsed arc similar to that used in the microcrystalline diamond work. In addition, we report preliminary results of the characteristics of carbon films deposited on both a close substrate (as in the report in Nature) and on a variably biased substrate placed approximately 10 cm from the arc.

The thickness of detonation waves visualised by slight obstacles

The reflection of detonation waves from slight obstacles, which hardly disturb the wave propagation, is observed by time-resolved schlieren photography. The following stoichiometric mixtures are used: pure and argon-diluted hydrogen-oxygen, hydrogen-air, acetylene-oxygen, and acetylene-air. Initial pressures are varied such that cell widths range from 1.4 up to 108 mm, which is twice the side length of the square cross-section of the tube. The trajectories of the incident and the reflected waves in the x,t-plane are used to determine lower limit values for the wave thickness. The considerable influences of the obstacle shape and of the evaluation method on the results are discussed in detail, and error sources are analyzed. The method has been improved since a previous publication by the authors. The ratio of the lower limit values to the cell width spreads from 0.4 to 0.8 in the medium cell size range. It decreases with increasing marginality and seems to increase at small scale. A unique correlation between the lower limit value and the tube diameter, both referred to the cell size, that was proposed earlier in the literature has to be refused. The velocities of the reflected waves are presented as additional information on the post-detonation wave state. The sonic transition is discussed theoretically, enhancing the stream tube model, and practically, based on detailed observations for marginal detonations.

Recent Trends and Developments in Excimer Laser Ablation for Medical Applications.

The ArF excimer laser ablates the cornea and other tissue with micrometer-size precision and minimal residual thermal damage. However, there is still no clear picture regarding the mechanisms for the excimer laser ablation (ELA) giving rise to this favorable and characteristic removal of hydrated tissue. In order to gain obvious insight into the role of water in the ELA mechanism of the cornea, we have macroscopically investigated the ablation behavior of gels in the swelled states. The ELA of the hydrated collagen and polyacrylamide gel using ArF laser were scrutinized using time-resolved photography, atomic force microscopy (AFM), and FT-IR attenuated total reflectance spectroscopy (FTIR-ATR). The observed results led us to the conclusion that the control of the water content in and/or on the gel matrix during ELA would reduce any thermal and mechanical damage to the soft tissue.

Characterization of materials ejected by excimer laser ablation of hydrated collagen gel

The materials ejected from the hydrated collagen gel by ArF laser (193 nm) ablation were investigated using time-resolved photography, atomic force microscopy (AFM), and FTIR attenuated total reflectance spectroscopy (FTIR-ATR). The time-resolved photography suggested that the characteristics of the ejected materials were affected by the laser fluence and the surface condition, and the driving force of the material ejection would be the cavitation bubble collapse. The AFM and FTIR-ATR measurements demonstrated that measurable-sized materials were ejected by the laser ablation and the structure of the collagen in the ejected materials was partially changed from the original. Some of the ejected materials were found to consist of low molecular weight polypeptides.

Characterization of water contribution to excimer laser ablation of collagen

Abstract In order to gain an obvious insight into the role of water in the mechanism of the excimer laser ablation of the cornea, we have macroscopically investigated the ablation behavior of collagen gel in the swelled state by direct photoetching using an ArF excimer laser with time-resolved photography, and furthermore, the thermal effects on the microscopic structures of the collagen molecules by FTIR–ATR spectroscopy. The hydrated collagen film (HF) has a smaller threshold fluence than the dried collagen film (DF). From the time-resolved photographs, the ejected materials were detected only for HF. It was predicted that the effect of bubble formation for HF contributes to the etching. The FTIR–ATR spectroscopic results revealed that the existence of the water suppressed the denaturation of the collagen to gelatin on the surface in the irradiated region. Overall, it was inferred that during the ablation process for HF, the laser energy would be mostly consumed as the latent heat of evaporation of water, that is, the water in the gel matrix would contribute to the suppression of the increment in the temperature in the irradiated region.

Acute and chronic effects of transmyocardial laser revascularization in the nonischemic pig myocardium by using three laser systems.

Transmyocardial laser revascularization (TMLR) improves symptoms in patients with coronary heart disease. It is based on the hypothesis of direct perfusion of ischemic myocardium by means of laser-created channels. Three different lasers were used to study alternative effects on myocardium. The present study was conducted to evaluate comprehensively and compare the short and long-term tissue effects and the basic interaction mechanisms of CO2, Ho:YAG, and Er:YAG laser radiation with myocardium. The dynamics of laser-induced impacts in gel used as tissue phantom was visualized by time-resolved flash photography. Pressure measurements performed during perforation of myocardium in vitro revealed the explosive character of the ablation process. Channels made into the left ventricle of normal pig hearts were examined immediately and 6 weeks after creation. Regardless of laser source, all channels became occluded within 6 weeks by scar. Minimal acute thermal damage by Er:YAG laser corresponded to smaller scars. Pulsed Ho:YAG caused stronger tissue tearing than continuous wave CO2 irradiation. An increased volume density of intramyocardial vessels was found about the scars 6 weeks after treatment with all lasers. The laser sources permitted to study outcome of pressure effects and thermal damage in vivo. There were only minor differences between the three laser systems used. Rapid channel occlusion suggests that rather than revascularization, subsidiary physiologic tissue effects elicited by the thermal, oxidative, or mechanical action of the laser impact may contribute to the beneficial clinical effects of TMLR.

Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Highly oriented pyrolytic graphite (HOPG) was ablated by a 193-nm ArF excimer laser in air. The fluence was varied in the range 1–25 J/cm2. Every laser shot hit a pristine graphite surface. The emerging shock wave was recorded by a nanosecond-resolution photographic arrangement. The velocity of the shock wave as a function of time and laser fluence was measured. The amount of energy that generates the shock wave was determined and found to be about 5–7% of the incident laser energy. The shock wave is already present 10–15 ns after the maximum of the incident laser pulse. These facts imply that, even if high-energy (10–100 eV) ions, atoms, or clusters leave the surface, a layer several 10 nm thick has to be removed during this short period. The temperature of the shock front is ∼2500–4000 K, as derived from the measured velocities. Measuring the ablation depth by atomic force microscopy as a function of fluence revealed that the single-shot ablation threshold is 1.4±0.2 J/cm2, and the effective absorption coefficient is ∼1.5×105 cm-1.

Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Characterization and modeling of the ablation plumes formed by intense-pulsed ion beam impact on solid targets

An investigation of the properties of the ablation products from intense-pulsed ion beam impact on solid targets is described. Measurements and calculations of the properties of the ablation plume are presented and correlated with incident beam parameters. Experimental techniques include Thomson parabola particle spectroscopy to measure the incident ion beam atomic composition and the energy spectrum of each beam component, thermal imaging to measure the incident-beam energy density, time-resolved photography to measure the plume expansion time history and geometry, and time-resolved energy-density measurements of the plume. The results of a thermal transport model of the beam-target interaction are presented, and a detailed comparison with measurements is made.

Photographic studies of laser-induced bubble formation in absorbing liquids and on submerged targets: implications for drug delivery with microsecond laser pulses

Pulsed laser ablation of blood clots in a fluid-filled blood ves- sel is accompanied by an explosive evaporation process. The resulting vapor bubble rapidly expands and collapses to disrupt the thrombus (blood clot). The hydrodynamic pressures following the bubble expan- sion and collapse can also be used as a driving force to deliver clot- dissolving agents into thrombus for enhancement of laser thrombolysis. Thus, the laser-induced bubble formation plays an important role in the thrombus removal process. We investigate the effects of boundary con- figurations and materials on bubble formation with time-resolved flash photography and high-speed photography. Potential applications in drug delivery using microsecond laser pulses are also discussed. © 1998 So- ciety of Photo-Optical Instrumentation Engineers. (S0091-3286(98)00108-1)

Pressure impulses during microsecond laser ablation

The collapse of laser-induced cavitation bubbles creates acoustic transients within the surrounding medium and also pressure impulses to the ablation target and light-delivery fiber during microsecond laser ablation. The impulses are investigated here with time-resolved flash photography, and they are found to occur whether or not the light-delivery fiber is in contact with the target. We demonstrate that the impulses depend primarily on the energy stored in the cavitation bubble. They are not directly dependent on the mode of light delivery (contact versus noncontact), and they are also not directly correlated to the other acoustic transients. The pressure impulses do seem to be associated with the bubble-driven jet formation caused by the bubble collapse.

Power enhancement by increasing the initial array radius and wire number of tungstenZpinches

Tungsten wire array implosions on the 7- to 8-MA Saturn generator have been optimized using wire number and array diameter variations to produce $75\ifmmode\pm\else\textpm\fi{}10\mathrm{TW}$ of x rays with total energy outputs of $450\ifmmode\pm\else\textpm\fi{}50\mathrm{kJ}.$ By increasing the number of wires in a 12.5-mm-diam array from 24 to 70 and simultaneously decreasing the individual wire diameter from 13 to 7.5 \ensuremath{\mu}m, the total radiated power increased from $20\ifmmode\pm\else\textpm\fi{}3$ to $40\ifmmode\pm\else\textpm\fi{}6\mathrm{TW}$ and the x-ray pulse width decreased from 18 to 8.5 ns. In addition, a diameter scan at an implosion time of $50\ifmmode\pm\else\textpm\fi{}5\mathrm{ns}$ showed that the pulse width has a strong dependence on collapse velocity and wire thickness. For the largest diameter load of 17.5 mm with 120 5-\ensuremath{\mu}m-diam wires, a 4-ns pulse width with a peak power of $75\ifmmode\pm\else\textpm\fi{}10\mathrm{TW}$ was achieved: four times power gain over the 20-TW electrical power generated by the pulsed power system. Time-resolved pinhole photography confirms that the power enhancement with increased wire number is associated with the plasma achieving a tighter compression and better axial uniformity. For the higher-velocity implosions, we infer from two-dimensional radiation-magnetohydrodynamic calculations that the plasma becomes hotter and hence radiates at a higher brightness temperature. Zero- and two-dimensional load models coupled with a detailed circuit model have shown expected radial kinetic energies in the range of 100--200 kJ. The total radiated energy of $&gt;400\mathrm{kJ}$ in a 4--20-ns FWHM pulse exceeds the total kinetic energy by more than a factor of 2. Two-dimensional, three-temperature simulations reproduce the observed trends in powers and pulse widths by using a variable initial random density perturbation. These calculations also indicate that the radiated energy is accounted for by the total work done on the plasma by the magnetic field.

Laser–Tissue Interaction During Transmyocardial Laser Revascularization

Background. The clinical procedure known as transmyocardial revascularization has recently seen its renaissance. Despite the promising preliminary clinical results, the associated mechanisms are subject to much discussion. This study is an attempt to unravel the basics of the interaction between 800-W CO 2 laser radiation and biological tissue. Methods. Time-resolved flash photography was used to visualize the laser-induced channel formation in water and in vitro porcine myocardium. In addition, laser-induced pressures were measured. Light microscopy and birefringence microscopy were used to assess the histologic characteristics of laser-induced thermal damage. Results. The channel depth increased logarithmically with time (ie, with pulse duration) in water and porcine myocardium. Pressure measurements showed the occurrence of numerous small transients during the laser pulse, which corresponded with channel formation, as well as local and partial channel collapse during the laser pulse. Twenty millimeters of myocardium was perforated in 25 ms. Increasing the pulse duration had a small effect on the maximum transversable thickness, but histologic analysis showed that thermal damage around the crater increased with increasing pulse duration. Conclusions. Several basic aspects of the interaction of high-power CO 2 laser radiation with myocardial tissue and tissue phantoms were studied in vitro. Although the goal of this study was not to unravel the mechanisms responsible for the beneficial effects of transmyocardial revascularization, it provided important information on the process of channel formation and collapse and tissue damage.

Bifunctional fusion proteins consisting of a single-chain antibody and an engineered lanthanide-binding protein.

The combination of an antibody fragment with a lanthanide chelating protein has desirable characteristics for fluorescence-based immunoassays and tumor radioimmunotherapy. As a model for this design, a fusion protein consisting of a single-chain antibody linked to an engineered version of oncomodulin, a protein with two Ca(2+)-binding motifs (the CD and EF loops), was produced by secretion from Escherichia coli in good yield. The single-chain antibody was specific for a Salmonella O-polysaccharide. The CD loop of oncomodulin had been redesigned to bind lanthanide ions with high affinity. The fusion protein was shown to have antigen-binding activity that was comparable to that of the unfused single-chain antibody, to bind Tb3+ with very high affinity and to give strong, sensitized Tb3+ luminescence via excitation of the tryptophan residue in the CD loop. A second fusion protein containing a 30-residue helix-loop-helix motif as the lanthanide-binding component was also prepared, but showed considerably lower solubility. Competition for Tb3+ binding by a series of metal chelators indicated that the affinities of the oncomodulin and 30 residue fusions for Tb3+ were approximately 10(11) M-1 and 10(7) M-1, respectively. Time-resolved lanthanide luminescence photography of electrophoresis gels demonstrated that the helix-loop-helix Ca(2+)-binding could be used to specifically visualize the scFv fragment.