Experimental observations of the stress experienced by a solid surface when a laser-created bubble oscillates in its vicinity.

In an attempt to shed more light on the complex process by which cavities interact destructively with rigid boundaries we consider experimentally the interaction of a laser-generated bubble with a nearby solid boundary. To determine the stresses the surface of the boundary experiences, particularly during the final stages of the first collapse phase, we present a series of pressure transducer traces observed when the laser-created cavity is produced close to the solid boundary. Through careful control of experimental parameters we record pressure increases for cavities created between 0.56 and 1.5 times the maximum bubble radius from the solid boundary. Previous studies have shown this to be a complex process which is particularly sensitive to the working parameters of the problem, which is borne out by our observations.

The generation of negative pressure waves for cavitation studies.

A technique for producing strong focused negative pressure waves in water is described. The method is based on the phase inversion of the planar shock wave from an electromagnetic transducer. This is achieved by reflection and focusing at a pressure release boundary. The acoustic concentrator has a phase inverting central element and a phase maintaining annular mirror. Focal pressures of approximately -16 MPa from an initial +5 MPa EMAT source have been measured and the focal volume was found to be approximately the same as that obtained with a conventional ultrasonic lens. The electromagnetic transducer and the parabolic concentrator is an excellent source of cavitation bubbles in water. The focused negative wave has been observed using high speed laser-lit Schlieren photography.

A schlieren study of the interaction between a lithotripter shock wave and a simulated kidney stone.

The interaction between a lithotripter shock wave and a simulated kidney stone has been imaged using laser illuminated schlieren photography. The images clearly show the incident, transmitted and reflected components of the shock wave and the associated cavitation.

Optical distortion in the field of a lithotripter shock wave.

The schlieren observation of cavitation phenomena produced in the tail of a lithotripter shock wave has indicated the presence of some interesting features. The images produced appear to indicate that cavitation transients in the field of a shock wave propagate nonsymmetrically; this is not the case. The apparent lack of symmetry exhibited by the primary cavitation transients is due to a complex optical lensing effect, which is brought about by the change in refractive index associated with the pressure profile of the shock wave. Objects seen through or immersed in the shock-wave field of an electromagnetic acoustic transducer, such as cavitation, appear highly distorted because of the strong positive and negative lensing effects of the compression and rarefaction cycles of the shock wave. A modification of the schlieren technique called the scale method has been used to model the distortion introduced by the shock wave and consequently explain the cavitation distortion. The technique has also been used to quantitatively analyze and partially reconstruct the lithotripter shock wave. The combination of schlieren and scale imaging gives more information about the refractive index field and therefore the shock-wave structure itself.

Discrete reconstruction of real phase objects: a comparison with computer-simulated phase objects.

The Gerchberg-Saxton algorithm for iterative phase retrieval is applied in reconstructing an optical wave arriving from a coherently illuminated phase object. Computer simulations show that the algorithm converges to the correct solution when magnitude information for both image and Fourier domains is provided. Phase retrieval from measured intensities in an experimental optical system is compared with the simulations. Charge-coupled-device sensors, in which the output voltage is a nonlinear function of the light intensity, are used to perform measurements of the quasi magnitude information. Because of the accuracy of the measured data, phase retrieval for real simple objects is possible, and schlieren images can be generated on the computer screen.

Optical imaging of shock waves produced by a high-energy electromagnetic transducer.

An electromagnetic acoustic transducer (EMAT) and acoustic lens have been constructed to provide reproducible and well characterized acoustic shock waves in water. The peak pressure amplitude variation for the unfocused and focused shock waves was found to be within +/- 5%. The focal region has been characterized both optically and by conventional piezoelectric pressure measurements. Optical investigations centre around ruby laser illuminated Schlieren and focused shadow imaging. The temporal resolution of the optical technique was determined by the ruby laser pulse length of 20 ns, giving a spatial resolution of 30 microns for a shock front in water.

Pulse-length scaling of laser damage at 249 nm in oxide and fluoride multilayer coatings.

We have measured laser damage thresholds of a variety of fluoride-based multilayer coatings at 249 nm by using pulse lengths between 450 fs and 25 ns. Rare-earth fluoride materials yield coatings with damage resistance approximately three times greater than conventional oxide multilayers. In the scaling law, where the damage threshold is proportional to the nth power of the pulse length, the value of n changed between 1 and 0.25 over the range of pulse lengths employed.

Differential phase quadrature surface profiling interferometer.

This paper describes an optical surface profiling system based on phase quadrature differential interferometry. The optical path difference between two adjacent optical probe beams is measured. Interference phase calculation and sample scanning is controlled by a PC computer. Height sensitivity is of the order of 1 nm and lateral resolution is ~10 microm. Results are given which demonstrate the reproducibility and stability of the system.

High resolution imaging studies into the formation of laser-induced periodic surface structures.

We report the results of an investigation into the formation mechanism of laser-induced ripple structures based on obtaining direct images of a surface while the transient heating induced by a KrF excimer laser is still present. These images reveal transient but well-defined periodic heating patterns which, if enough subsequent excimer pulses are incident on the surface, become permanently induced ripple structures. It is evident from these transient images that the surface heating is confined to the induced structures, thus strongly supporting the idea that at low fluences the ripples are formed by localizing surface melting.