Abstract
Manipulating and focusing light deep inside biological tissue and tissue-like complex media has been desired for long yet considered challenging. One feasible strategy is through optical wavefront engineering, where the optical scattering-induced phase distortions are time reversed or pre-compensated so that photons travel along different optical paths interfere constructively at the targeted position within a scattering medium. To define the targeted position, an internal guidestar is needed to guide or provide a feedback for wavefront engineering. It could be injected or embedded probes such as fluorescence or nonlinear microspheres, ultrasonic modulation, as well as absorption perturbation. Here we propose to use a magnetically controlled optical absorbing microsphere as the internal guidestar. Using a digital optical phase conjugation system, we obtained sharp optical focusing within scattering media through time-reversing the scattered light perturbed by the magnetic microsphere. Since the object is magnetically controlled, dynamic optical focusing is allowed with a relatively large field-of-view by scanning the magnetic field externally. Moreover, the magnetic microsphere can be packaged with an organic membrane, using biological or chemical means to serve as a carrier. Therefore, the technique may find particular applications for enhanced targeted drug delivery, and imaging and photoablation of angiogenic vessels in tumours.
Highlights
Due to the strong scattering of light in biological tissue, conventional optical manipulation methods, such as using objective lenses, can only focus visible and near infrared (NIR) light to shallow depths of up to a few hundred micrometers[1]
Ultrasonic mediation has served as an encouraging noninvasive internal guidestar in a class of techniques recently developed by researchers, such as time-reversed ultrasonically encoded (TRUE) optical focusing[6,26,27,28], time reversal of variance-encoded (TROVE) light[29], and photoacoustically guided wavefront shaping (PAWS)[10,14]
To tackle the aforementioned limitations, in this paper we propose a new approach called time-reversed magnetically controlled perturbation (TRMCP) optical focusing, using magnetically controlled optical absorbing microspheres as internal guidestar for digital optical phase conjugation (DOPC)
Summary
Due to the strong scattering of light in biological tissue, conventional optical manipulation methods, such as using objective lenses, can only focus visible and near infrared (NIR) light to shallow depths of up to a few hundred micrometers[1]. Various wavefront engineering approaches have been developed[5,6,7,8,9] to compensate for or reverse the scattering-induced phase distortions, so that diffused light travelling along different optical paths may interfere constructively again at the targeted position, forming an optical focus out of the seemingly random speckle background. To enable focusing inside a scattering medium, which is more of biomedical interests, an internal guidestar is required to modulate diffused light traversing the voxel of interest or to produce an effective light source (real or virtual) within the medium It could be embedded probes, such as fluorescent[16,17,24] or nonlinear beads[25]. Such focusing may potentially benefit a wide range of biomedical applications in vessel-like aquatic environment, such as blood vessels and lymph vessels
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