Metasurface integrated optics for fluorescence imaging endoscopy.

Metasurfaces are shown to offer off-axis optical aberration correction for reflectance imaging in confocal scanning fibre endoscopy. However, in fluorescence imaging, chromatic aberration is a crucial factor that limits image quality. Typically, color correction is achieved by implementing multiple refractive lenses that complicate lens assembly and alignment. In this work, we report the effectiveness of a single metasurface with a gradient refractive index (GRIN) lens for confocal fluorescence imaging, showing significant improvement in image quality. We will show that it is not necessary to correct explicitly for chromatic aberration, as the system is designed such that the excitation and emission wavelengths focus at different planes. This relaxes the need for conventional achromatic correction, as each wavelength is optimised for its respective function. Although two photon endoscopy imaging offers better optical sectioning and imaging performance, higher cost of the source and limited portability of the detection system are some factors that restrict its usage. We implemented a metasurface-integrated aberration-corrected lens system for confocal endoscopy imaging with a compact and portable detection unit and compared the hybrid meta-GRIN lens system to one without a meta-element. The results of reflectance imaging on resolution targets show a 3.5-fold contrast improvement, and fluorescence imaging on biological tissue (mammalian brain) shows a 2-fold increase in the contrast-to-noise ratio at the intermediate to extreme fields (without compromising on-axis performance). The hybrid meta-GRIN lens system achieved a focal spot with a 1/e2 beamwidth of 3.1 µm (on-axis) and 3.3 µm (extreme field), over a field of view of 175 µm × 40 µm.