High numerical aperture hybrid optics for two-photon polymerization.

We report on an immersion hybrid optics specially designed for focusing ultrashort laser pulses into a polymer for direct laser writing via two-photon polymerization. The hybrid optics allows for well-corrected focusing over a large working distance range of 577 μm with a numerical aperture (NA) of 1.33 and low internal dispersion. We combine the concepts of an aplanatic solid immersion lens (ASIL) for achieving a high NA with a diffractive optical element (DOE) for correction of aberrations. To demonstrate the improvements for volume structuring of the polymer, we compare the achievable structure sizes of our optics with a commercially available oil-immersion objective (100x, NA=1.4).