Todd W. Murray

Detection of ultrasound-modulated photons in diffuse media using the photorefractive effect.

Ultrasound-modulated optical tomography is a dual-wave sensing technique in which diffusive light in a turbid medium interacts with an imposed acoustic field. A phase-modulated photon field emanates from the interaction region and carries with it information about the optomechanical properties of the medium. We present a technique for detection of ultrasound-induced optical phase modulation using an adaptive, photorefractive-crystal-based interferometry system. Experimental results are presented demonstrating detection of ultrasound-modulated signals in highly scattering media by use of pulsed ultrasound insonation.

Nucleating cavitation from laser-illuminated nano-particles

Vapor bubble generation from laser-illuminated gold nano-particles has been investigated as a means of providing nucleation sites for cavitation induced by high-intensity focused ultrasound (HIFU). Pulses from a 532-nm Nd:Yag laser were synchronized with a pulsed 1.1-MHz HIFU source in an acrylamide phantom seeded with 82-nm-diameter gold particles. Emissions from bubble collapses were detected by a 15-MHz focused transducer at a laser pulse energy and HIFU focal pressure of 0.10 mJ and 0.92 MPa, respectively. In comparison, a HIFU peak focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination.

Laser-based ultrasonic generation and detection of zero-group velocity Lamb waves in thin plates

A novel laser-based ultrasonic technique for the inspection of thin plates and membranes is presented, in which a modulated continuous-wave laser source is used to excite narrow bandwidth Lamb waves. The dominant feature in the acoustic spectrum is a sharp resonance peak that occurs at the minimum frequency of the first-order symmetric Lamb mode, where the group velocity of the Lamb wave goes to zero while the phase velocity remains finite. Experimental results with the laser source and receiver on epicenter demonstrate that the zero-group velocity resonance generated with a low-power modulated excitation source can be detected using a Michelson interferometer coupled to a lock-in amplifier. This resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable, for example, for the measurement and mapping of nanoscale thickness variations.

Experimental evaluation of enhanced generation of ultrasonic waves using an array of laser sources

Abstract An array of ten pulsed Nd: YAG lasers was constructed in order to study the effects of generating ultrasound with an array of laser sources. The laser system permitted the spatial and temporal control of the firing of the individual lasers in the array necessary for the production of both narrow-band ultrasonic signals and phased array single pulses. The increase in sensitivity of a laser ultrasonic system associated with the generation of narrow-band and phased array acoustic waves is discussed theoretically and verified experimentally for surface and bulk wave generation. The ultrasonic signals were generated in aluminium samples of various thicknesses and with source laser power densities consistent with generation in the thermoelastic regime, thus causing no damage to the surface of the specimens. The signals were detected using a path stabilized Michelson interferometer. In the narrow-band case, the waveforms were digitally filtered in order to take advantage of the reduced spectral range of the generated acoustic energy. A significant increase in the sensitivity of the laser ultrasonic system, consistent with theoretical predictions, was observed in both the narrow-band and phased array cases.

Laser generation of ultrasound in films and coatings

A model for the pulsed laser generation of ultrasound in an isotropic film on a semi-infinite substrate is presented. The model gives the time domain displacement of the system as a function of the density and mechanical properties of the film and substrate and the thermal properties of the film. The model has been verified experimentally using a 1 ns Nd:YAG laser source for acoustic wave generation and a stabilized Michelson interferometer for detection. Experimental and theoretical signals agree well for both the case of a fast layer on a slow substrate (zirconium nitride/steel) and a slow layer on a fast substrate (titanium/aluminum).

Photoacoustic characterization of the mechanical properties of thin films

Narrow band photoacoustics (laser ultrasonics) are used to characterize the properties of free-standing nanometer-sized thin films. Photoacoustic generation is achieved by use of a microchip laser which deposits pulsed laser energy in the form of a spatially periodic source on the structure. The resulting narrow band ultrasonic modes are monitored using a Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of acoustic wave numbers is probed. Results are presented for two-layer thin film aluminum/silicon-nitride (Al/Si3N4) membranes. For such thin films, only the two lowest order guided modes are generated and these in turn can be related to sheet and flexural modes in plates. The mechanical properties and residual stress in the thin films are evaluated from measured acoustic dispersion curves for these two lowest order modes.

Building embedded microchannels using a single layered SU-8, and determining Young's modulus using a laser acoustic technique

In this paper, an innovative method to create embedded microchannels is presented. The presented technology is based on a direct-write technique using a scanning laser system to pattern a single layered SU-8. The enormous flexibility of the scanning laser system can be seen in two key features: the laser pulsing can be controlled spot-by-spot with variable exposure doses, and the laser intensity penetrating into samples can be adjusted by varying the laser focus level. The UV laser direct-write method greatly simplifies the fabrication processes. Moreover, it can be set up in a conventional manufacturing environment without the need for clean room facilities. The second part of this paper describes the underlying theory and method to determine Youngs modulus of exposed SU-8 by using a laser acoustic microscopy system. The laser-based ultrasonic technique offers a non-contact, non-destructive means of evaluation and material characterization. This paper will determine Youngs modulus of UV exposed SU-8 generated with different exposure doses. Measurements show that Youngs modulus is highly dependent on exposure dose. Youngs modulus ranges from 3.8 to 5.4 GPa when the thickness of a fully cross-linked SU-8 microbeam varies from 100 to 205 µm with a gradually increased UV exposure dose.

Gold nanoparticle targeted photoacoustic cavitation for potential deep tissue imaging and therapy

The laser generation of vapor bubbles around plasmonic nanoparticles can be enhanced through the application of an ultrasound field; a technique referred to as photoacoustic cavitation. The combination of light and ultrasound allows for bubble formation at lower laser fluence and peak negative ultrasound pressure than can be achieved using either modality alone. The growth and collapse of these bubbles leads to local mechanical disruption and acoustic emission, and can potentially be used to induce and monitor tissue therapy. Photoacoustic cavitation is investigated for a broad range of ultrasound pressures and nanoparticle concentrations for gold nanorods and nanospheres. The cavitation threshold fluences for both nanoparticle types are found to drastically reduce in the presence of an ultrasound field. The results indicate that photoacoustic cavitation can potentially be produced at depth in biological tissue without exceeding the safety limits for ultrasound or laser radiation at the tissue surface.

Laser generation of acoustic waves in the ablative regime

A practical model of acoustic wave generation by a pulsed laser source in the ablative regime is presented. The pressure exerted on the surface during Q-switched laser heating is calculated through a finite difference solution of the vaporization problem. The epicentral displacement is found through summation of the displacement field induced by the vaporization process with that caused by thermoelastic expansion. The model is restricted to the weakly ablative regime in the absence of a backing gas. The results are compared to the epicentral displacements generated in aluminum samples under rough vacuum conditions at generating wavelengths of 532 and 1064 nm. The waveforms compare well over a limited irradiance range. The effects of rough vacuum conditions on the generated acoustic signals are also examined and compared to signals generated in the presence of a backing gas. The divergence in the shape and amplitude of these signals observed under highly ablative conditions is discussed.

Simulation of laser-generated ultrasonic waves in layered plates

A model is presented for the pulsed laser generation of ultrasound in isotropic layered plates. The stresses and displacements of the plate have been formulated in the Hankel and Laplace transform domains using the Thompson transfer matrix approach. The time domain response has been obtained by numerically inverting the transforms. Several numerical results are presented showing the normal surface displacement in the following configurations: single-layer film on a semi-infinite substrate, two layers on a semi-infinite substrate, and three-layer plates. The model provides a useful tool for the determination of which modes are generated by a laser source in a layered system. It can also be used to determine how sensitive the modes are to small changes in density, thickness, or elastic properties of the layers and to help in the selection of experimental parameters (laser spot size, pulse length, and source to receiver distance) for optimal sensitivity.