Explosion of a liquid film in contact with a pulse-heated solid surface detected by the probe-beam deflection method.

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