A focusing optical phased array for tissue interrogation with side-lobe suppression and simplified beam steering
We implement an integrated multi-electrode array on a silicon-nitride-based photonic integrated circuit for ex-vivo retinal characterization via optical stimulation. The interrogation beam formers, based on curved grating emitters and optical phased arrays, are designed to achieve transverse focusing with spot sizes in the 1 - 2 $\mu$m range to target single cells. The experimentally realized focusing optical phased arrays show suppressed side-lobes, with approximately 11.5% of the power in each side-lobe and ~60% in the main lobe, reducing unintentional cellular excitation. Additional design refinement enables further suppression of the side-lobes to a few percent of the total power. Additionally, we demonstrate a compact design of meandered thermal phase shifters implemented across the array that allow push-pull steering in the transverse direction as well as focusing and defocusing of the beam, with a total of only four control signals. Transverse angular steering by $\pm$5.1{\deg} and axial translation of the focal spot by 204 $\mu$m are demonstrated with tuning currents below 50 mA, together with longitudinal angular steering by 4.26{\deg} obtained by means of wavelength tuning in a $\pm$15 nm range centered on 525 nm.