Double-wavelength metasurface objective lens for two-photon microscopy (Conference Presentation)

Two-photon microscopy is a key imaging technique in biological sciences because of its superior deep tissue imaging capabilities in addition to high transverse and axial resolution. In recent years, development of low-weight miniature two-photon microscopes has been of great interest for in vivo imaging of brain activity. Limited by these mechanical constraints, most of the developed miniature two-photon microscopes utilize graded index objective lenses that usually have inferior optical characteristics compared to conventional refractive objective lenses. Dielectric metasurfaces, a recent category of diffractive optical elements with enhanced capabilities, have proven versatile in various applications ranging from lensing to holography and polarization control. Their ultrathin form factor and potentially extremely low-weight make them very attractive for applications with stringent size and weight constraints. However, despite their success in various types of microscopy and imaging applications, they have not been previously utilized for multi-photon fluorescence microscopy. The main barrier for using metasurface lenses in multi-photon microscopy arises from their large chromatic dispersion that effectively makes them single-wavelength. Here we will present a double-wavelength metasurface lens especially designed to have the same focal length at 820 and 605 nm, corresponding to the excitation and emission wavelengths of a certain fluorophore. After characterizing the poly-silicon metasurface lens at both wavelengths, we used it in a two-photon microscopy setup and demonstrated its capability to capture two-photon images qualitatively similar to images taken with a conventional objective lens. We will also discuss the effects of chromatic dispersion of the metasurface lens on its two-photon imaging performance.