Manipulating Sound with Lasers and Optics

Lasers are used for a wide range of industrial, medical, spectroscopic, and military applications. Daniel Kazal, a graduate research assistant in the Department of Chemistry and Biochemistry at the University of Maryland Baltimore County, has developed a novel technique for channeling sound using a tube-shaped laser beam that forms a thermal gradient. Based on his work with this approach, he received the 2017 FACSS Innovation Award. We recently spoke with him about this research. This interview is part of a series of interviews with the winners of awards presented at SciX.

Your work involves the manipulation of the direction and magnitude of sound waves traveling in air using laser light to generate photothermal barriers (1). Can you briefly describe the basic elements of the approach for both suppression and channeling of sound?

A species present in the air-in our case ethanol vapor-absorbs the laser light, which creates a localized thermal gradient that causes the local air density to change. This air density change results in an abrupt change in compressibility at the edges of the beam, which provides an effective barrier for sound reflection. If you create a tube-shaped laser beam you can generate an acoustic waveguide capable of channeling sound.

How is your approach an improvement over (or departure from) earlier techniques for photothermal manipulation of sound waves?

This is really the first time it has been demonstrated that it is possible to optically manipulate sound in such a manner. Before this study, the closest demonstration and observation of interacting thermally with sound waves was by John Tyndall during the 1880s, where he showed that acoustic waves can be suppressed when propagated over multiple heat barriers from a slot burner.

What were your instrumental setups or arrangements for your studies of suppression and channeling?

We employed a carbon dioxide laser, modulated with an optical chopper to create both photothermal barriers and channels inside a plexiglass chamber (used to contain the ethanol vapor). The acoustic channel was formed by using a beam expander and masking the center of the expanded laser beam with an earbud speaker, resulting in a tube-shaped channel.

What is the dependence of the suppression efficiency on wavelength and distance?

The wavelength of laser light affects the efficiency of suppression only as a function of its absorptivity by the air (or in our case ethanol vapor). The greater the population of excited molecules generated, the more efficient the change in compressibility will be and thus the greater the acoustic reflection will be.

How does laser power affect suppression?

Because this phenomenon relies on an initial optical absorption event, greater laser powers result in a larger number of molecules being excited and a greater thermal depletion zone. These conditions provide greater acoustic reflection efficiencies over larger distances.

What decibel range was studied? Were you able to achieve complete suppression of acoustic waves?

We studied acoustic intensities as great as 70 dB, which is about the equivalent of highway noise. This limit was due to the maximum safe amplitude that the earbud could output. By generating multiple barriers in the path of an acoustic wave, complete suppression can be achieved after just four consecutive optical barriers.

In your sound-channeling experiments, how is the doughnut-shaped laser beam formed for photothermal excitation of the air with the acoustic source located within the doughnut? What is the potential range of effectiveness for this technique?

As described before, the channel in these proof-of-principle concept studies was simply generated by blocking the center of an expanded laser beam with the earbud speaker (which doubled as the sound source). Theoretically, with enough laser power this technique would allow sound to travel hundreds of kilometers. Even with our current setup (based on our 1/r^0.6 measured acoustic decay profile), a 70-dB acoustic wave should be capable of being heard at over a kilometer away.

What are the most important applications or fields of use for this technique? Are there any potential biomedical applications?

We are really excited about this because, there are many potential applications for this phenomenon, including standoff chemical sensing, secure communications, acoustic stealth, and dynamic acoustic manipulation. There are also potential biomedical applications like increased tissue penetration for photoacoustic sensing and imaging by optically guiding acoustic or ultrasonic signals to or from specific locations within tissues.

What are the next steps in your research?

There are many. Currently, we are focusing on applying this phenomenon to the numerous ambient air applications, including acoustic suppression and standoff sensing. In addition, we are also focused on further characterizing the properties of this phenomenon, including incident acoustic angle on reflection–propagation efficiency, the effect of localized thermal gradients, and many more.

Reference

1.    B.M. Cullum, E.L. Holthoff, and P.M. Pellegrino, Optics Express25(19), 18 September 2017. https://doi.org/101364/OE.25.022738