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Detection of a novel mechanism of acousto-optic modulation of incoherent light.

Jarrett CW, Caskey CF, Gore JC - PLoS ONE (2014)

Bottom Line: 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.These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

View Article: PubMed Central - PubMed

Affiliation: Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, United States of America; Program in Chemical and Physical Biology, Vanderbilt University, Nashville, Tennessee, United States of America.

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.

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Related in: MedlinePlus

Measured peak separations vs those predicted from simple theory, for different sound frequency and geometry.A linear fit of the observed data agrees with the theory: Measured peak spacing (mm) = 1.03×Predicted spacing+0.6.
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pone-0104268-g008: Measured peak separations vs those predicted from simple theory, for different sound frequency and geometry.A linear fit of the observed data agrees with the theory: Measured peak spacing (mm) = 1.03×Predicted spacing+0.6.

Mentions: It was observed that the average distance between peaks scaled precisely with the expected geometrical magnification of a simple optical projection of the focal zone when changing the distances from the LED to the focal zone, from the focal zone to the measurement plane, and the wavelength. For simplicity, assume the LED acts as a point source of light. If the distance from the LED to the focal zone axis is d and the distance from the LED to the measurement plane is D then D/d is a geometrical magnification factor m. Distances along the axis of the sound beam become magnified by m at the measurement distance. If the regions of alternating absorption along the axis of the focal zone of the sound beam have a width of a half wavelength (where Λ is the sound wavelength, f the frequency, and c the speed of sound, taken here to be ≈1500 m.sec−1) these become in extent at the light detector. Note that, because we measure the temporally modulated light signal (rather than the mean ambient light level), regions of increased density along the axis show the same modulation as regions of decreased density or rarefaction, so the peaks in modulated light occur every half wavelength rather than every wavelength. As shown in figure 8, the regions of compression and rarefaction also have finite thickness t in the direction of light propagation, and a sinusoidal variation along the sound field axis, so their projections are expected to have unsharp edges and may extend over a distance . Thus we predict that alternating peaks of average width up to separated by , which may reduce the peak to trough modulation. Figure 9 shows the composite data from all the above experiments (varying m and f), where we have plotted the measured peaks separations versus those predicted by simple theory. The measured separations of the peaks are accurately predicted, and linear regression gives the following relationship.


Detection of a novel mechanism of acousto-optic modulation of incoherent light.

Jarrett CW, Caskey CF, Gore JC - PLoS ONE (2014)

Measured peak separations vs those predicted from simple theory, for different sound frequency and geometry.A linear fit of the observed data agrees with the theory: Measured peak spacing (mm) = 1.03×Predicted spacing+0.6.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126715&req=5

pone-0104268-g008: Measured peak separations vs those predicted from simple theory, for different sound frequency and geometry.A linear fit of the observed data agrees with the theory: Measured peak spacing (mm) = 1.03×Predicted spacing+0.6.
Mentions: It was observed that the average distance between peaks scaled precisely with the expected geometrical magnification of a simple optical projection of the focal zone when changing the distances from the LED to the focal zone, from the focal zone to the measurement plane, and the wavelength. For simplicity, assume the LED acts as a point source of light. If the distance from the LED to the focal zone axis is d and the distance from the LED to the measurement plane is D then D/d is a geometrical magnification factor m. Distances along the axis of the sound beam become magnified by m at the measurement distance. If the regions of alternating absorption along the axis of the focal zone of the sound beam have a width of a half wavelength (where Λ is the sound wavelength, f the frequency, and c the speed of sound, taken here to be ≈1500 m.sec−1) these become in extent at the light detector. Note that, because we measure the temporally modulated light signal (rather than the mean ambient light level), regions of increased density along the axis show the same modulation as regions of decreased density or rarefaction, so the peaks in modulated light occur every half wavelength rather than every wavelength. As shown in figure 8, the regions of compression and rarefaction also have finite thickness t in the direction of light propagation, and a sinusoidal variation along the sound field axis, so their projections are expected to have unsharp edges and may extend over a distance . Thus we predict that alternating peaks of average width up to separated by , which may reduce the peak to trough modulation. Figure 9 shows the composite data from all the above experiments (varying m and f), where we have plotted the measured peaks separations versus those predicted by simple theory. The measured separations of the peaks are accurately predicted, and linear regression gives the following relationship.

Bottom Line: 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.These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

View Article: PubMed Central - PubMed

Affiliation: Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, United States of America; Program in Chemical and Physical Biology, Vanderbilt University, Nashville, Tennessee, United States of America.

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.

Show MeSH
Related in: MedlinePlus