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The dynamics of liquid-vapor phase-change in the nanosecond time-scale induced by pulsed-laser heating of a liquid on a solid sample is studied by means of optical reflectance and scattering measurements, and the piezoelectric detection technique. The Iiquids studied include water, ethanol, merhanol, IsoproPropyl Alcohol (IPA), and mixtures of water and IPA. The threshold fluence for nucleation is determined with high accuracy using the optical and acoustic signals. Heat diffusion calculations performed for the threshold fluences indicate that the liquids are sufficiently superheated before nucleation sets on. The transient optical reflectance signal is analyzed by an effective-medium theory to provide bubble-growth kinetics, so that the bubble-growth velocity for the test liquids could be estimated. In addition, it is observed that, following the thermally induced nucleation, repetitive acoustic cavitation at the surface of the solid sample occurs, with a time interval related to the speed ofsound in the liquid.

The enhancement of acoustic caviation at a liquid-solid interface following laser-induced bubble formation is studied. The experiment results indicate that metastable ultramicroscopic bubbles formed on the solid surface cause a long-term memory effect on acoustic cavitation. By performing a double-pulse experiment using two excimer lasers, the temporal decay of this memory effect is determined for two different liquids on a chromium surface. An explanation of the observed decay mode by a diffusion model is presented.

The explosion or rapid vaporization of a liquid film on an opaque surface by a pulsed laser is studied experimentally. Using a probe‐beam deflection sensing (PDS) scheme, together with a previously developed transmission monitor, the distortion of the PDS signal due to the generation of shock waves by the exploding liquid is investigated. Various liquids, including alcohols and pure water in contact with substrates such as polyimide, amorphous carbon, and silicon, are studied for a wide range of excimer laser fluences. It is concluded that the present PDS technique is highly sensitive to the explosion threshold.

Optical reflectance and light scattering measurements are used to study bubble nucleation of various liquids and subsequent explosive vaporization at the surface of a thin chromium film heated by a 248-nm KrF excimer laser. Liquids studied include water, ethanol, methanol, isopropanolm, and different mixtures of water and isopropanol. The reflectance and scattering signals show distinct transients when the excimer laser fluence exceeds a certain liquid-dependent threshold. The reflectance signals are analyzed to provide bubble growth kinetics and explosive vaporization thresholds of the liquids.

The dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser is studied experimentally on top of an amorphous silicon (a‐Si) film deposited on fused quartz. In particular, the transient temperature as well as the nucleation of the superheated liquid at the interface for laser fluences below and above the spontaneous nucleation threshold are monitored using an optical transmission technique. Results for different pure and mixed liquids are presented for cases of different thicknesses of the a‐Si film. Details for the calibration of the transmission signal to obtain the interface temperature are provided through the application of the one‐dimensional diffusion model and the characteristic matrix theory using data obtained from previous steady‐state experiments. The calibrated results are in reasonable agreement with the known data for superheated liquids in the literature as well as theoretical calculations. In addition, the delay times for spontaneous nucleation of the liquids are estimated to be ≲1 μs, generally comparable with those observed in droplet explosion experiments.

The temperature dependence of the optical properties for amorphous silicon is studied at wavelengths of 632.8 and 752 nm. Both the refractive index and extinction coefficient increase linearly with temperature for 752 nm, while the refractive index decreases and the extinction coefficient increases for 632.8 nm. The rate of increase of the extinction coefficient at 632.8 nm is twice as much as that for 752 nm.

CO/sub 2/-laser produced micro-bumps on glass substrates are not only of interest in tribological applications, e.g. zone texturing of substrates as discussed in this paper, but for other applications such as micro-optics fabrication. These bumps are positive protruding domes with net volume gain. Mass conservation implies a local density decrease inside the bump. We have studied this density variation using nanoindentation to probe local hardness. Our findings indicate a clear and significant decrease in bump hardness (up to 30%) relative to the background glass-consistent with this local change in density. SIMS and electron microprobe analysis indicate no chemical changes between the bump surface, and background regions. Regardless, the glass texture zone displays excellent bump durability and contact start/stop performance. Additionally, the role played by the chemical surface strengthening and its effect on bump height has been investigated. For chemically engineered surface stresses ranging from 100 to 500 MPa, the bump height increases with increasing surface stress at a rate of 0.064 nm/MPa. Extrapolation of the plot to zero stress confirms a non-zero thermal expansion contribution to the bump height-consistent with our theory for bump generation. Reflectivity measurements have shown that the optical constants vary little with changing surface stress, implying that the laser coupling to the surface is essentially unaltered.

Multiple pulse exposure provides a means for making bumps in the 1-100 nm height range on Ni-P substrates. Although the bump height is generally a nonlinear function of the number of pulses, below the fluence threshold for chemicapillary effects, bump height increases linearly with the number of pulses. At low fluence levels, when the peak melt temperature is below 1400/spl deg/K, the bump surface becomes rough. Avoiding the low fluence regime, this technique can be reliably used for making smooth bumps with flexible shapes and sizes which are useful for tribology studies.

Important issues of laser zone texturing in manufacturing are discussed. We report practical strategies in terms of tooling and process control. We found that good bump height uniformity is achieved by operating in the most stable region where the effects of laser pulse energy fluctuation, disk thickness variation, and disk runout are minimized. By selecting optimum process parameters, a bump height standard deviation of less than 1.5% can be achieved.

Infrared laser pulses of microsecond duration derived from a stabilized carbon dioxide laser are used to produce a textured zone on a glass disk substrate. Each laser pulse can produce a microscopically smooth dome-shaped bump that protrudes from the initial disk surface. Typical bump heights of interest are 20-30 nm with bump diameters /spl sim/10 /spl mu/m. The laser-textured zone, composed of /spl sim/10/sup 5/ bumps, provides a dedicated region for contact start/stop. The zone exhibits excellent tribological properties (stiction and durability).

The variable capillary force arising between the surfaces of a slider and a textured magnetic disk is explored. Disk texture is modeled with an ensemble of bumps having geometries characteristic of laser-texture. Lubricant redistribution is modeled dynamically while elastic deformation of bumps is described by Hertzian contact models. The results demonstrate a significant impact of geometry on capillary spreading and on the subsequent net capillary force.

We provide here extended experimental data on the bump shape formed on Ni-P disk substrate under various laser and substrate surface conditions, and describe a model for the Sombrero-shape bump formation based on competition between thermocapillary and chemicapillary flows. This model rationalizes a large body of experimental data and provides insight into how one might tailor bump shapes to specific requirements. For comparison, laser bump formation on non-Ni-P substrates has also been studied.

The transient temperature and pressure field development in the excimer laser-induced vaporization of liquids in contact with a solid surface is studied. A thin silicon film, which has temperature-dependent optical properties, is embedded between an absorbing chromium film and a transparent fused quartz substrate. Static reflectivity measurement is performed to determine the thin film optical properties at elevated temperatures. The transient backward reflectance responses from the silicon layer are compared with heat transfer modeling results. The backward reflectance probe is not affected by the creation of bubbles and is successfully employed for the first time to measure non-intrusively the temperature development during the rapid vaporization process. The optical reflectance probes are applied from the front-side and back-side of the sample simultaneously to monitor the dynamic bubble nucleation behavior and transient temperature development, respectively, at various ambient pressures using a high- pressure cell. The investigation on the effect of ambient pressure on the bubble nucleation threshold combined with the surface temperature measurement determines the thermodynamic state of the superheated metastable liquid at the interface and subsequently the explosion pressure.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The formation of bubbles at a liquid-solid interface due to acoustic cavitation depends particularly on the preconditions of the interface. Here, it is shown that following laser- induced bubble formation at the interface the acoustic cavitation efficiency is strongly enhanced. Optical reflectance measurements reveal that this observed enhancement of acoustic cavitation due to preceding laser-induced bubble formation, which could be termed as a memory effect, decays in a few hundred microseconds. By performing a double-pulse experiment using two excimer lasers the influence of process parameters, such as liquid temperature and salt concentration, on the temporal decay of the memory effect has been studied. An analysis of the experimental results by a diffusion model is presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The threshold for explosive vaporization of a liquid layer on an opaque solid surface heated by an ultraviolet excimer pulsed laser is studied by a photoacoustic probe-beam deflection method. The probe beam traverses the liquid in the vicinity of the laser-heated liquid-solid interface. Below the explosion threshold, photoacoustic generation in the solid occurs only through a thermoelastic mechanism, which results mainly in shear waves that do not couple well into the liquid. Above the explosion threshold, photoacoustic pulses in the solid are also produced by explosive recoil, hence producing longitudinal pulses in the solid that couple well into the liquid after reflections. By setting the probe-beam refraction to detect longitudinal pulse echoes coupled back into the liquid, a sensitive detection of the explosive threshold can be established. There is much recent interest in the study of the nucleation dynamics and explosive vaporization of a liquid film on a solid surface that is flash heated by a laser pulse.'- 3 This kind of liquid-film explosion is observed to lead to large forces exerted on microscopic particles on the surface, which leads to their rocketed ejection.' Previously, we have used a piezoelectric detector 7 as well as optical transmission" 3 techniques to probe such nucleation dynamics. However, the piezoelectric probe is limited to a far distance (-1 cm) from the irradiated source owing to its relatively large size;, also, accurate detection of the transient photoacoustic (PA) pulse shape is difficult because of transducer ringing and the lack of bandwidth. The optical transmission probe is applicable only to certain thin samples with significant temperature-dependent transmissions. Here, we apply the PA probe-beam refraction techniques 8 ' 9 to monitor for the first time to our knowledge multiple PA echoes generated in the solid sample both below and above the explosion threshold of the liquid at the interface. We show that a transition from a mainly shear-wave nature to a strongly longitudinalwave nature for the PA wave in the solid signifies the onset, of liquid explosion. This new technique is applicable to any opaque solids rather than only certain thin solids. Also, the probe beam can be set at a small distance (s 1 mm) from the PA source so that excessive damping of the high-frequency signals can be avoided. Figure 1 shows our experimental setup. We have built a compact probe-beam refraction sensing unit that contains a sample holder in the middle of a rigid structure. The probe He-Ne laser (633 nm) is mounted on one side of the structure, and a bicell position sensor is mounted on the opposite side. The rise time of the bicell is -10 ns. The He-Ne beam is expanded by a fivefold beam expander to -2.5 mm in diameter and is focused by a lens of focal length 5 cm. The theoretical beam waist (w) at the focal spot for a Gaussian beam is estimated to be -16 ,m. A direct measurement of w using a knife edge, a chopper, and the bicell gives a value for w of -43 ,Am in air. The probe beam is -0.4 mm from the surface of the sample. The KrF (248 nm, -16 ns) UV excimer laser spot size is measured to be -1 cm 2 . A slit is used so that the width of the illuminated area of the sample is limited to 3 mm. This will optimize the probe refraction signal with the given beamwaist geometry.'" By having the probe refraction measurement done in one compact and rigid unit, noise that is due to vibrations is eliminated to a large extent. In this experiment, we report only the PA refraction signal 9 and leave out the photothermal signal, 8 which could be recorded at a delay time of a few microseconds after the UV pulse. In addition, in the case of thin-film samples such as amorphous silicon (a-Si) deposited onto quartz, an additional probe based on optical transmission is measured simultaneously by using a cw diode

The drive in data storage technology towards utilizing magnetic films with lower magnetostriction (to reduce the magnetoelastic energy term) and reduced thickness has resulted in the requirement for more sensitive, reliable, and easy-to-use tools to monitor magnetostriction. A measurement tool based on an in-plane rotating and saturating magnetic field and laser-beam-deflection technique, which is able to meet these requirements, is described. The tool developed offers high accuracy, large dynamic range, long-term stability, simple sample insertion, and a fast, easy measurement procedure. With this tool, the measurement of small magnetostriction coefficients of thin soft-magnetic films can become a simple, fast, and reliable procedure, thus helping the development of magnetic thin-film production processes and routine composition control. >

We review the recent advances in the use of laser sources in the field of electrophotography. New experimental investigations of the photogeneration and transport processes utilized in electrophotography are now possible because of the short pulse duration, high peak power, high collimation, monochromaticity, and/or coherence of a laser source. Also, the increasing availability of reliable and inexpensive lasers has resulted in many new technological uses of lasers in electrophotographic applications. In addition to the continued use of HeNe and HeCd lasers, new products have been configured with GaAs lasers.

The optogalvanic effects of the He 2.058-?m line (21P-21S) from He lamps illuminating a weak dc He discharge (current density ?10 ?A/cm2, field/pressure ~25 V/cm - torr) are reported. For illumination at the positive column, we have made quantitative measurements of the decreases in the discharge current, electron density, and metastable densities, as weli as the increase in the electric field in the positive column, as the intensity of illumination increases. We have also observed that for sufficiently strong illumination (using He lamps only), the optogalvanic effect is catastrophic, i.e., the discharge is switched off; this clearly shows that a sufficiently large metastable density (which is reduced by the illumination) is necessary to maintain a weak He discharge. For illumination at the cathode regions, the optogalvanic effects are "anomalous": the discharge current is strongly suppressed by illumination at the cathode dark space next to the cathode, but is enhanced by iumination at the adjacent negtive glow region.

Photothermal (PT) effects are produced by the heating a sample and its surroundings due to the absorption of electromagnetic radiation, and is a type of energy conversion. Using a laser beam for PT generation, many PT effects can be measured, including refractive-index gradients, acoustic emission, surface deformation, reflectivity changes, desorption, and “grey-body” infrared emission, providing useful techniques for materials characterization. These PT material probing or characterization techniques generally rely on the use of high-sensitivity detection methods, involving the use of “probe” laser beams, transducers, or infrared detectors to monitor the effects caused by PT heating of a sample. Many of these PT effects occur simultaneously, e.g., PT heating of a sample in air will produce temperature rise, photoacoustic waves and refractive-index changes in the sample and in the adjacent air, infrared thermal radiation increase, etc., all at the same time. Thus, the choice of a suitable PT effect for detection will depend on the nature of the sample and its environment, the light source used, and the measurement desired. We have previously reviewed [1,2,3] the various experimental arrangements and detection schemes for the different PT effects, and given a summary of possible applications. We have shown that it is possible to obtain high PT generation efficiencies using a short-pulsed laser beam; various sensitive detection schemes have also been developed to monitor the resultant temperature or pressure transients in the sample or its vicinity.

In this paper, we demonstrate the applicability of PA pulse generation for the monitoring of ultrasonic properties of powders, for example, uncured carbon-loaded epoxy resin powders, whose average particle diameter is ~15 μm. The acoustic pulse is generated by a short laser pulse (10-nsec or 1-μsec duration) of 1–10-mJ energy and is detected by a piezoelectric detector (polyvinylidene difluoride foil or commercial lead zirconate titanate transducer). This allows ultrasonic measurements over a broad range of porosity and pressure conditions, from loose unconsolidated powers to heat- and pressure-sintered pellets. For loose powders at atmospheric pressures (with a porosity of ~50%), the acoustic velocity is remarkably small (typically 3 × 103 cm/sec), and attenuation and dispersion are large. This appears to be the first demonstration of the use of a PA technique to measure ultrasonic properties in powders.

A wide variety of acoustic pulses (different magnitudes and temporal/frequency profiles) can be produced in a sample by optical absorption of various light beams, i.e., photoacoustic (PA) pulse generation. Various generation mechanisms are possible: thermal expansion, gas evolution, chemical changes, dielectric breakdown, electrostriction, etc. PA pulse generation has found several important applications, including ultrasensitive spectroscopic detections of linear and nonlinear optical absorptions, noncontact ultrasonic measurements in hostile environments, PA imaging techniques, PA operation of mechanical devices, and so on. This paper presents some basic understanding of a PA generation process, and examples of important recent applications.