Abstract
A novel form of acoustic modulation of light from an incoherent source has been detected in water as well as in turbid media. We demonstrate that patterns of modulated light intensity appear to propagate as the optical shadow of the density variations caused by ultrasound within an illuminated ultrasonic focal zone. This pattern differs from previous reports of acousto-optical interactions that produce diffraction effects that rely on phase shifts and changes in light directions caused by the acoustic modulation. Moreover, previous studies of acousto-optic interactions have mainly reported the effects of sound on coherent light sources via photon tagging, and/or the production of diffraction phenomena from phase effects that give rise to discrete sidebands. We aimed to assess whether the effects of ultrasound modulation of the intensity of light from an incoherent light source could be detected directly, and how the acoustically modulated (AOM) light signal depended on experimental parameters. Our observations suggest that ultrasound at moderate intensities can induce sufficiently large density variations within a uniform medium to cause measurable modulation of the intensity of an incoherent light source by absorption. Light passing through a region of high intensity ultrasound then produces a pattern that is the projection of the density variations within the region of their interaction. The patterns exhibit distinct maxima and minima that are observed at locations much different from those predicted by Raman-Nath, Bragg, or other diffraction theory. The observed patterns scaled appropriately with the geometrical magnification and sound wavelength. We conclude that these observed patterns are simple projections of the ultrasound induced density changes which cause spatial and temporal variations of the optical absorption within the illuminated sound field. These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.
Highlights
We report novel experimental observations of the modulation of an incoherent light beam by an ultrasonic field that are distinct from previous reported interactions of sound and light, and which demonstrate the ability to directly observe sound pressure patterns as changes in light absorption
This work was originally motivated by our interest in developing and evaluating a novel type of hybrid imaging system for molecular imaging in biological samples, and in the interpretation of previous reports of acousto-optical imaging using fluorescent light sources
Optoacoustic/Photoacoustic imaging, in which sound is produced and detected after light interacts with a target, has been successfully developed as a hybrid imaging method that combines the molecular sensitivity of optical methods with the spatial resolution and depth penetration of ultrasound, and is in practical use in clinical and pre-clinical applications [1,2,3]
Summary
We report novel experimental observations of the modulation of an incoherent light beam by an ultrasonic field that are distinct from previous reported interactions of sound and light, and which demonstrate the ability to directly observe sound pressure patterns as changes in light absorption. This work was originally motivated by our interest in developing and evaluating a novel type of hybrid imaging system for molecular imaging in biological samples, and in the interpretation of previous reports of acousto-optical imaging using fluorescent light sources. Acousto-optical imaging is an alternative technology to photoacoustic imaging, in which sound is used to modulate a light source, but it potentially shares some of the advantages of photoacoustic imaging, and our studies were motivated by the need to better understand the nature of acousto-optic interactions. There have been no previous reports of the direct effects of sound causing changes in the absorption of ballistic photons, as reported here
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