Simulation, fabrication and morphological characterization of a PDMS microlens for light collimation on optrodes

Optrodes are neural probes for delivering light at neurons, at least with an irradiance of 1 mW/mm². MicroLED (microlight-emitting diode) optrodes with a microlens to collimate the light can improve the irradiance without increasing the microLED power consumption, minimizing the brain overheating. This paper presents FEM (finite element method) simulations of a PDMS (polydimethylsiloxane) microlens capable of collimating the light from a microLED, ensuring an average irradiance of 310 mW/mm². Based on the simulated microlens, a microlens array was fabricated, using microfabrication processes. One of the PDMS microlens from the fabricated array was experimentally characterized, using AFM (atomic force microscopy) and SEM (scanning electron microscopy). The average surface roughness, base diameter and height of the microlens is 2.3 nm, 80 μm and 55 μm, respectively. It was also possible to determine the microlens focal length (96 μm), numerical aperture (0.42), f-number (1.2) and depth of focus (2.66 μm), using Gaussian optics. The fabrication process was validated, even using PDMS/PDMS demolding without surface treatment of the negative mold.