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We present a novel technique to extend the focal length of an external lens without increasing the spot size using virtual optical waveguides ultrasonically sculpted in transparent and turbid media. When used in tandem with an external lens, these non-invasive virtual waveguides can further relay the focused beam of light into the medium without losing the spatial resolution. We will also show how the optical properties of this physical-virtual cascade system can be reconfigured by changing the pattern of ultrasound. This method can find intriguing applications for manipulating the trajectory of light in transparent and turbid media, including biological tissue.

Conventional optical lenses have been used to focus light from outside without disturbing the medium. The focused spot size is proportional to the focal distance in a conventional lens, resulting in a tradeoff between depth of penetration in the target medium and spatial resolution. We have shown that virtual ultrasonically sculpted gradient-index (GRIN) optical waveguides can be formed in the target medium to guide and steer light without disturbing the medium. Here, we demonstrate that such virtual waveguides can relay an externally focused beam of light through the medium beyond the focal distance of an external physical lens to extend the penetration depth without compromising the spot size. Moreover, the spot size can be tuned by reconfiguring the virtual waveguide. We show that these virtual GRIN waveguides can be formed in transparent as well as turbid media to enhance the confinement and contrast ratio of the focused beam of light at the target location. This method can be extended to realize complex optical systems of external physical lenses and in situ virtual waveguides to extend the reach and flexibility of optical methods., Comment: 11 pages, 6 figures