Session T46: Invited Session: Keithley Award Session: Photoacoustic and Photothermal Measurement Science

Joseph F. Keithley Award For Advances in Measurement Science Lecture: Thermophotonic and Photoacoustic Radar Imaging Methods for Biomedical and Dental Imaging

In the first part of this presentation I will introduce thermophotonic radar imaging principles and techniques using chirped or binary-phase-coded modulation, methods which can break through the maximum detection depth/depth resolution limitations of conventional photothermal waves. Using matched-filter principles, a methodology enabling parabolic diffusion-wave energy fields to exhibit energy localization akin to propagating hyperbolic wave-fields has been developed. It allows for deconvolution of individual responses of superposed axially discrete sources, opening a new field: depth-resolved thermal coherence tomography. Several examples from dental enamel caries diagnostic imaging to metal subsurface defect thermographic imaging will be discussed. The second part will introduce the field of photoacoustic radar (or sonar) biomedical imaging. I will report the development of a novel biomedical imaging system that utilizes a continuous-wave laser source with a custom intensity modulation pattern, ultrasonic phased array for signal detection and processing coupled with a beamforming algorithm for reconstruction of photoacoustic correlation images. Utilization of specific chirped modulation waveforms (``waveform engineering'') achieves dramatic signal-to-noise-ratio increase and improved axial resolution over pulsed laser photoacoustics. The talk will conclude with aspects of instrumental sensitivity of the PA Radar to optical contrast using cancerous breast tissue-mimicking phantoms, super paramagnetic iron oxide nanoparticles as contrast enhancement agents and in-vivo tissue samples.   

Photoacoustics and Photothermal instrumentation in the study of thermal properties of liquids and semisolids

The fundamentals of the measurements of the thermal properties of solids and semisolids using photoacoustics and photothermal techniques are presented. It is shown that photoacoustics is a high stability technique which allows the monitoring of complex process in which the physical properties of the liquid can evolve. Additionally, it is shown that the methodology known as thermal wave cavity can be used successfully in high accuracy measurements for a wide variety of materials. In particular the case of magnetic fluids in which the viscosity can be varied, using an external magnetic fluid, is presented. It is also shown that the thermal wave cavity permits the study of magnetic fluids in which high aspect ratio particles are introduced in the fluid matrix. The effect of the orientation of non-magnetic particles inside the magnetic fluid generated by external magnetic field is also investigated. The possible applications and consequences in the development of windows of controlled thermal conductivity are discussed.   

Photoacoustic and Photothermal Effects in Periodic Structures and Acoustic Resonators

Laser excited photoacoustic and photothermal waves can be generated in one-dimensional structures whose acoustic or thermal properties vary sinusoidally in space. The wave equations describing the pressure or the temperature in such structures can be shown to reduce to inhomogeneous Mathieu equations. Solutions of the Mathieu equation are obtained based on both the method of variation of parameters and expansion of the pressure or temperature in a summation over eigenfunctions. The solutions for the photoacoustic effect show the space equivalent of subharmonic generation where resonances occur at one half of the period of the structure. The positions of the band gaps and the dispersion relations for any modulation depth of the acoustic properties of the structure can be found directly from the Mathieu characteristic exponent. Since the photoacoustic effect is governed by an inhomogeneous differential equation, excitation within forbidden gaps is possible. For excitation within a finite region of the structure, the Mathieu equation equivalent of Hankel functions are defined. From these functions the properties of the photoacoustic waves excited within or outside of the band gaps are found. For thermal waves, the character of the waves and the dispersion relation can be found as well, however no band gaps result from the periodicity of the thermal properties of the structure. The generation of sound by continuous, unmodulated irradiation of an absorbing gas in a resonant cavity is discussed. A longitudinal resonance of the cavity is predicted to be excited since any pressure increase from optical absorption is accompanied by a density increase, the latter resulting in additional energy deposition by the laser beam. Thus, on each return of the pressure pulse to the window of the resonator where laser beam enters the acoustic signal is amplified. Calculations show that for a strongly absorbing gas, the acoustic modes of the resonator become mode locked.   

Linear and Nonlinear Laser-Based Guided Acoustic Waves Propagating at Surfaces (2D) and Edges (1D)

In recent years photoacoustics opened the door to many new applications of 2D linear surface acoustic waves (SAWs), e.g., nondestructive evaluation (NDE) of surface-breaking cracks. Real partially-closed cracks of micrometer size have been analyzed. More recently also pulsed laser excitation of solitary elastic surface pulses and their detection with a continuouswave probe laser has been achieved, by generating dispersion with a thin film coating that introduces a length scale. In addition, such laser-based pump-probe experiments allow the excitation of short nonlinear SAW pulses developing steep shock fronts that fracture brittle materials such as silica or silicon. With this method it is possible to measure the fracture strength of materials and compare the critical failure stress with ab initio calculations of the ideal strength of the corresponding perfect single crystal. The excitation and detection of 1D edge or wedge waves propagating along a wedge formed by two planar surfaces that meet at the apex of the wedge or wedge tip has been performed by laser irradiation. The characteristic features of the non-dispersive linear wedge waves such as their small phase velocity below the Rayleigh velocity, the very high degree of localization of the displacement field at the wedge tip, and their existence for certain geometries in anisotropic media such as silicon could be verified by photoacoustic experiments. Despite the strong nonlinearity of certain edge-localized modes, as expected from theoretical considerations, 1D solitary waves and nonlinear wedge waves with steep pulse profiles could not be detected up to now. The latest progress will be discussed.   

Understanding climate: the role of photoacoustic spectroscopy at NIST

Sophisticated climate models predict that soot aerosols have a significant impact on Earth's energy budget; however, the uncertainty of these predictions is large, in part, because soot in the atmosphere and in the laboratory is poorly characterized. In the atmosphere, soot's optical and physical properties change as it combines with water vapor and sulfuric acid. We will describe a novel photoacoustic spectrometer system that measures the optical absorption cross section of various soots as they age in diverse environments. We also measure the albedo (optical scattering) of aerosols ranging from black-carbon-like to brown-carbon-like using simultaneous photoacoustic spectroscopy and cavity ring-down spectroscopy. Lastly, we developed a photoacoustic spectrometer system that measures the concentration of carbon dioxide in atmospheric air with sub-ppm uncertainty. We will report results of field tests of this spectrometer.