Physical and chemical technologies have been continuously progressing advances in neuroscience research. The development of research tools for closed-loop control and monitoring neural activities in behaving animals is highly desirable. In this paper, we introduce a wirelessly operated, miniaturized microprobe system for optical interrogation and neurochemical sensing in the deep brain. Via epitaxial liftoff and transfer printing, microscale light-emitting diodes (micro-LEDs) as light sources and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-coated diamond films as electrochemical sensors are vertically assembled to form implantable optoelectrochemical probes for real-time optogenetic stimulation and dopamine detection capabilities. A customized, lightweight circuit module is employed for untethered, remote signal control, and data acquisition. After the probe is injected into the ventral tegmental area (VTA) of freely behaving mice, in vivo experiments clearly demonstrate the utilities of the multifunctional optoelectrochemical microprobe system for optogenetic interference of place preferences and detection of dopamine release. The presented options for material and device integrations provide a practical route to simultaneous optical control and electrochemical sensing of complex nervous systems.

A preliminary on-chip integration of GaN-based laser, modulator, and photodetector grown on Si is reported. The modulator is integrated into the laser and shares the same InGaN quantum well active region with the laser and the photodetector. By varying the applied voltage to the modulator, the absorption of the modulator can be adjusted due to the changed band bending of the InGaN quantum well active region, and hence the threshold current and the light output power of the laser can be tuned. The photodetector can effectively detect the output power of the laser tuned by the applied voltage to the modulator, which opens up a new way for GaN-based on-chip photonic integration on Si.

Direct bandgap III-V semiconductor lasers grown on silicon (Si) are highly desired for monolithic integration with Si photonics. Fabrication of semiconductor lasers with a Fabry–Pérot cavity usually includes facet cleavage, however, that is not compatible with on-chip photonic integration. Etching as an alternative approach holds a great advantage in preparing cavity mirrors with no need of breaking wafer into bars. However, gallium nitride (GaN) sidewalls prepared by dry etching often have a large roughness and etching damages, which would cause mirror loss due to optical scattering and carrier injection loss because of surface non-radiative recombination. A wet chemical polishing process of GaN sidewall facets formed by dry etching was studied in detail to remove the etching damages and smooth the vertical sidewalls. The wet chemical polishing technique combined with dry etching was successfully applied to the on-wafer fabrication of cavity mirrors, which enabled the realization of room temperature electrically injected InGaN-based laser diodes grown on Si.

GaN-based Micro-LED has become a research hotspot as a novel display technology due to its numerous unique advantages. Especially in augmented reality and smart glasses applications, Micro-LED display chip with high transparency possesses unparalleled opportunities. In this work, GaN-based transparent single-color Micro-LED display chip based on double-side polished sapphire substrate was designed with a single pixel size of $20\,\,\mu \text{m}\times 20\,\,\mu \text{m}$ , and a resolution of 254 pixel per inch (PPI). The transparency reaches 80% among the whole display area ~ 0.18 inch, and the luminance of display chip can reach up to 25000 nits. It demonstrates a broad application prospects in augmented reality, smart glasses, and etc., providing promising vista for the development of “Metaverse” in the future.