Hybrid polymer waveguide characterization for microoptical tools with integrated laser diode chips for optogenetic applications at 430 nm and 650 nm

Appropriate micro-optical tools are required to exploit the key advantages of optogenetics in neuroscience, i.e. optical stimulation and inhibition of neural tissue at high spatial as well as temporal resolutions, providing cell specificity and the opportunity to simultaneously record electrophysiological signals. Besides the need for minimally invasive probes mandatory for a reduced tissue damage, highly flexible or wireless interfaces are demanded for experiments with freely behaving animals. Both these technical system requirements are achieved by integrating miniaturized waveguides for light transmission combined with bare laser diode (LD) chips integrated directly into neural probes. This paper describes a system concept using integrated, side emitting LD chips directly coupled to miniaturized waveguides implemented on silicon (Si) substrates. It details the fabrication, assembly, and optical as well as electrical characterization of waveguides (WG) made from the hybrid polymer Ormorcere. The WGs were photolithographically patterned to have a cross-section of 20x15 μm 2 . Bare LD chips are flip-chip bonded to electroplated gold (Au) pads with ±5 μm accuracy relative to the WG facets. Transmitted radiant fluxes for blue (430 nm, (Al,In)GaN) and red (650 nm, AlGaInP) LDs are measured to be 150 μW (ID = 35 mA, 5% duty cycle) and 4.35 μW (ID = 225 mA, 0.5% duty cycle), respectively. This corresponds to an efficiency of the coupled and transmitted light of 44% for the red LDs. Long term measurements for 24 h using these systems with red LDs showed a decrease of the radiant flux of about 4% caused by LD aging at stable WG transmission properties. WGs immersed into Ringer’s solution showed no significant change of their optical transmission properties after four weeks of exposure to the ionic solution.