Flexible $\mu\text{LED}$-Based Optogenetic Tool with Integrated $\mu$-Lens Array and Conical Concentrators Providing Light Extraction Improvements above 80%

Abstract

This paper reports on the fabrication and characterization of spherical micro-lenses ( $\mu$ -lenses) made from polydimethylsiloxane (PDMS) and their integration into optogenetic implants based on micro light-emitting diodes ( $\mu \text{LEDs}$ ). The $\mu$ -lenses are fabricated using a reusable silicon mold realized by wet chemical HNA (hydrofluoric, nitric and acetic acid) etching. It applies a masking layer with openings as small as $5 \mu \mathrm{m}$ and was optimized for hemispherical $\mu$ -lens cavities of low surface roughness ( $R_{\mathrm{a}} ). The $\mu$ -lenses with diameters down to $10 \mu \mathrm{m}$ are realized by PDMS molding using a polymeric release layer (PRL) improving the $\mu$ -lens release and enabling $R_{\mathrm{a}}$ values below 5 nm. Linear $\mu$ -lens arrays are transferred onto the emission side of $\mu \text{LEDs}$ comprising integrated conical concentrators (CCs). The PDMS-filled CCs improve the $\mu$ -lens emission behavior by light reflection at the interface between the polymeric probe substrate and PDMS, without increasing the overall probe dimensions. Using $\mu \text{LEDs}$ with integrated $\mu$ -lens arrays in combination with CCs, the overall light extraction of the optical implants is increased by 115% and 83% in air and water, with the peak intensity raised by 145% and 95%, respectively.