Breaking the tradeoff between confinement and focal distance using virtual ultrasonic optical waveguides Matteo Giuseppe Scopelliti, Hengji Huang, Maysamreza Chamanzar 1 1 Electrical and Computer Engineering Department, Carnegie Mellon University, Pittsburgh, USA, 15213 Abstract 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 fundamental tradeoff between depth of penetration in the target medium and spatial resolution. We have shown that virtual ultrasonically sculpted graded-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 and tune an externally focused beam of light through the medium without compromising the spot size, thus breaking the tradeoff between the depth of penetration and spatial resolution in conventional physical lenses. We show that the virtual GRIN waveguides can be formed in transparent as well as in turbid media to provide an unprecedented enhancement of spot size and contrast ratio of the confined beam of light. This method can be used to realize more complex optical systems of external physical lenses and in situ virtual waveguides to extend the reach and flexibility of optical methods. Introduction: Light-matter interaction has been used in many different applications, ranging from biological imaging and manipulation to metrology, material processing, and machine vision 1–7 . Through the interaction of light with matter, a signature of the medium may affect light, which can be used for sensing, detection, or imaging. Moreover, light can affect the medium when it is concentrated to a high enough intensity at specific locations within the medium. Optical manipulation has been used in a wide range of applications such as optogenetic stimulation of biological events, photothermal therapy of cancer tumors, 3D printing, machining, and material processing. A key advantage of using light, whether for probing or manipulation, is that it can penetrate through materials non-invasively at the appropriate wavelength range. Different optical components such as lenses, spatial light modulators and waveguides have been used to shape the trajectory of light. Tunable external optical components such as optical modulators, tunable lenses and gratings have also been realized based on electro-optic or acousto-optic effects to reconfigure the pattern of light before it is launched into the target medium 8–11 . The ability to manipulate the trajectory of light within the target medium would expand the power and flexibility of optical methods. Invasive insertion of traditional optical components such as lenses or waveguides into the medium defeats the purpose of using light as a non-invasive modality for interaction with the medium, especially in the case of non-destructive testing of materials or imaging and stimulation of biological tissue. We have recently shown that ultrasound waves can be used to guide and pattern the trajectory of light by locally changing the refractive index of the medium. Using this technique, we have demonstrated the possibility of forming in situ virtual graded- index (GRIN) waveguides, virtual relay lenses, and spatial light modulators non-invasively 12–14 . Since ultrasound in the proper frequency range can propagate deep with minimal attenuation in the medium, the virtual optical components can be realized within the target medium to manipulate the trajectory of light without implanting any physical devices to disturb the medium. We have shown that these linear virtual optical components can be formed in transparent as well as scattering media such as biological tissue 12 . In this method, the virtual optical component can be reconfigured by simply changing the pattern of ultrasound waves from outside the medium. A nonlinear photoacoustic wave implementation of this idea has also been