Implantable integrated nanophotonic probes with light sheet illumination for deep imaging
Doctor of Philosophy
Optical techniques to record and stimulate neural activity are becoming increasingly powerful and effective, but delivering light to specific regions deep within the brain is limited by the scattering of neural tissue. By focusing light to a single illumination plane, researchers can perform high-speed volumetric imaging or optically stimulate cells within specific layers of tissue. This single plane illumination, which is called light sheet illumination, also reduces the background from out-of-plane fluorescence creating images with higher contrast. However, conventional light sheet microscopy based on discrete optical components is incompatible with large living animals. To overcome this limitation, I have designed and fabricated two different kind of miniature, implantable silicon-based photonic probes that act as light sheet illumination devices. The first probe consists of a properly designed planar metallic microlens integrated onto aluminum nitride (AlN) waveguide gratings on a thermal oxide silicon substrate. Light diffraction by the metallic slit microlens can be considered as the interference between quasi-cylindrical waves and the finite-difference time-domain (FDTD) simulations match the experimental measurements. These results have verified that a single metallic slit is an efficient nanophotonic element for light sheet illumination. Furthermore, I have integrated the microlens onto the aluminum nitride resonant waveguide gratings on a silicon-based probe and the properly designed waveguide gratings can radiate the guided visible light vertically. Subsequently, the diffraction field radiated from the gratings will be focused by the microlens to achieve light sheet illumination that can be used for deep brain imaging. The second probe is as thin as 20 μm that can also produce a thin layer of illumination from the properly designed aluminum nitride waveguide gratings without using the microlens. I have theoretically and experimentally verified that the aluminum nitride resonant waveguide gratings are effective for producing light sheet illumination. I also show that three dimensional imaging, is possible by scanning the light sheet illumination plane. Compared to single photon imaging techniques like epi-fluorescence microscopy and confocal microscopy, the nanophotonic integrated light sheet microscopy can image more than twice the depth in a brain tissue phantom, which is confirmed experimentally. Compared to table-top systems and multi-photon microscopy, this implantable photonic probe enables a low-cost, small-form solution for light sheet illumination and deep brain imaging.
silicon photonic probe; light sheet imaging