High dioptric power micro-lenses fabricated by two-photon polymerization

Specimen-induced aberrations limit the penetration depth of standard optical imaging techniques in vivo, mainly due to the propagation of high NA beams in a non-homogenous medium. Overcoming these limitations requires complex optical imaging systems and techniques. Implantable high NA micro-optics can be a solution to tissue induced spherical aberrations but in order to be implanted they need to have reduced complexity, offering reduced surface to the host immune reaction. Here, we design, fabricate and test a single micro-optical element with high dioptric power and high NA (up to 1.4 in air). The sag function is inspired to the classical metalens phase and improved to reduce the spherical aberrations arising from the refractive origin of the phase delay at the lens periphery. We successfully fabricated these high-NA quasi-parabolic aspheric microlenses with varying focal lengths by two-photon polymerization in biocompatible photoresist SZ2080. The entire process is optimized to minimize fabrication time while maintaining the structures' robustness: the smoothness reaches optical (λ/20) quality. The dioptric power and magnification of the microlenses were quantified over a 200×200 μm aberration-free field of view. Our results indicate that these microlenses can be used for wide-field imaging under linear excitation and have the optical quality to be utilized for nonlinear excitation imaging. Moreover, being made of biocompatible photoresist, they can be implanted close to the observation volume and help to reduce the spherical aberration of laser beams penetrating in living tissues.