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

This paper reports on the fabrication and characterization of spherical micro-lenses ( μ-lenses) made from polydimethylsiloxane (PDMS) and their integration into optogenetic implants based on micro light-emitting diodes ( μLEDs). The μ-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 μm and was optimized for hemispherical μ-lens cavities of low surface roughness ( ). The μ-lenses with diameters down to 10 μm are realized by PDMS molding using a polymeric release layer (PRL) improving the μ-lens release and enabling Ra values below 5 nm. Linear μ-lens arrays are transferred onto the emission side of μLEDs comprising integrated conical concentrators (CCs). The PDMS-filled CCs improve the μ-lens emission behavior by light reflection at the interface between the polymeric probe substrate and PDMS, without increasing the overall probe dimensions. Using μLEDs with integrated μ-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.