Optical methods have been widely used for imaging, light delivery and manipulation in biomedical applications, machine vision, material processing and metrology. The key advantage of using light in these applications is the non-invasive nature of light-matter interaction. 

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Confinement of light deep into the medium is not always possible using external optical methods. To address this issue, optical waveguides such as optical fibers have been used to keep light confined into the medium. Inserting such an optical device into the medium defeats the purpose of using light as a non-invasive modality. 

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Moreover, beam steering and patterning of light inside the medium is another important requirement for many applications, which cannot be fully addressed using external spatial light modulators.

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We have developed a novel technique for in-situ confinement and steering of light using ultrasound. In our method, ultrasound waves define and control the trajectory of light within the medium without having to insert a physical light guide into the medium.

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When ultrasound propagates through the medium, in the positive high pressure regions of the wave, the medium is compressed and in the negative pressure regions of the wave, the medium is rarefied. In the compressed regions, the local refractive index is slightly increased and in the rarefied regions, the refractive index is slightly reduced. When light encounters such a modulated medium, the phasefront of light gets modulated and as a result, the trajectory of light beam will be changed. By changing the spatial pattern of ultrasound, the trajectory of light can be modified dynamically without invasively inserting an optical device into the medium.

 

As a simple example, when ultrasound planewaves travel through a medium, the refractive index is spatially modulated such that the high-index regions at the peak pressure of ultrasound are flanked by low-index rarefied regions. Each of these high refractive index regions can be considered as an optical waveguide core and the low-index regions as cladding, thus forming a virtual optical waveguide. Therefore, light can be confined through these virtual waveguides as shown in the schematic below.

 

 

 

 

 

 

 

 

 

 

 

 

 

Obviously, these virtual waveguides travel over time and their locations are not stationary. The temporal dynamics of ultrasound waves are much slower than the speed of light. Therefore, if light is pulsed and synchronized with ultrasound and is turned on only when the virtual waveguide is formed at the intended location, the virtual waveguide remains stationary from the viewpoint of light.         

 

• We have demonstrated that this technique can be used to form in situ reconfigurable patterns of light in the medium, when complex ultrasound interference patterns are formed into the medium from outside.

• We have also shown that these virtual waveguides can be used for collecting light from the depth of the medium, thus enabling non-invasive virtual micro-imaging.

• Recently, we have demonstrated that this technique can be used for breaking fundamental trade-offs between confinement and focal distance in conventional lenses, when the virtual ultrasonic waveguides are used in tandem with external optical lenses. 

• In all of these demonstrations, we have shown that this technique can be used on transparent as well as scattering  and turbid media, including brain tissue.

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You can find out more about these projects here:

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