Original citationCharash, R., Maaskant, P. P., Lewis, L., McAleese, C., Kappers, M. J.,

Implantable photonic probes are of increasing interest to the field of biophotonics and in particular, optogenetic neural stimulation. Active probes with onboard light emissive elements allow for electronic multiplexing and can be manufactured through existing microelectronics methods. However, as the optogenetics field moves towards clinical practice, an important question arises as to whether such probes will cause excessive thermal heating of the surrounding tissue. Light emitting diodes typically produce more heat than light. The resultant temperature rise of the probe surface therefore needs to be maintained under the regulatory limit of 2°C. This work combines optical and thermal modelling, which have been experimental verified. Analysis has been performed on the effect of probe/emitter geometries, emitter, and radiance requirements. Finally, the effective illumination volume has been calculated within thermal limits for different probe emitter types and required thresholds.

Optical neuron stimulation arrays are important for both in-vitro biology and retinal prosthetic biomedical applications. Hence, in this work, we present an 8100 pixel high radiance photonic stimulator. The chip module vertically combines custom made gallium nitride μ LEDs with a CMOS application specific integrated circuit. This is designed with active pixels to ensure random access and to allow continuous illumination of all required pixels. The μLEDs have been assembled on the chip using a solder ball flip-chip bonding technique which has allowed for reliable and repeatable manufacture. We have evaluated the performance of the matrix by measuring the different factors including the static, dynamic power consumption, the illumination, and the current consumption by each LED. We show that the power consumption is within a range suitable for portable use. Finally, the thermal behavior of the matrix is monitored and the matrix proved to be thermally stable.

We have systematically investigated the doping of (1122) with Si and Mg by metal-organic vapour phase epitaxy for light emitting diodes (LEDs). By Si doping of GaN we reached electron concentrations close to 10 20 cm −3 , but the topography degrades above mid 10 19 cm −3. By Mg doping we reached hole concentrations close to 5×10 17 cm −3 , using Mg partial pressures about 3× higher than those for (0001). Exceeding the maximum Mg partial pressure led to a quick degradation of the sample. Low resistivities as well as high hole concentrations required a growth temperature of 900 • C or higher. At optimised conditions the electrical properties as well as the photoluminescence of (1122) p-GaN were similar to (0001) p-GaN. The best ohmic p-contacts were achieved by NiAg metallisation. A single quantum well LED emitting at 465 nm was realised on (0001) and (1122). Droop (sub-linear increase of the light output power) occurred at much higher current densities on (1122). However, the light output of the (0001) LED was higher than that of (1122) until deep in the droop regime. Our LEDs as well as those in the literature indicate a reduction in efficiency from (0001) over semi-polar to non-polar orientations. We propose that reduced fields open a loss channel for carriers.

InGaN-based micro-light-emitting diodes (µ-LEDs) emitting at 470 nm and composed of micropixels each with controlled shaping achieves directed light output with an angular full width at half maximum of 48°. The reflected light from the mesa sidewalls is azimuthally polarized. The small signal bandwidth of an individual µ-LED is >500 MHz. A cluster of 14 µ-LEDs is used to achieve a large signal data transfer rate of 500 Mbps in a form which is compatible with communication over plastic optical fibre.