Ultra High Synthetic Numerical Aperture in Fourier Ptychographic Microscopy Using an Oil-Filled Hemispherical Illuminator

The past decade has witnessed numerous innovations in Fourier Ptychographic Microscopy (FPM). This study presents a record high-resolution FPM using a hemispherical LED illuminator. We then further expand the synthetic numerical aperture (NA) by filling the illuminator with oil. Therefore, the wave vector is artificially stretched within the higher refractive index optical medium. Comparing several experiments with different objective lenses and operating wavelengths, a high synthetic NA of 1.82 was achieved for 470 nm light and 20x 0.6NA objective lens. This is particularly noteworthy because the illumination NA can exceed unity by this technique, something which is impossible in free space. We demonstrate that the oil-immersion FPM system significantly improved the experimental half-pitch resolution down to 137 nm, which is close to the theoretically predicted value of 130 nm. Experimental reconstruction with blue and green LEDs, and for USAF target and biological sample show the performance of our work.