We have developed a novel conductivity based selective electrochemical etching to introduce nanometer sized pores into GaN. By controlling the doping and electrochemical etching bias, we are able to control the pore morphology. The nanoporous (NP) GaN can be considered a new form of GaN with an unprecedented tunability in optical index We show the potential of this NPGaN to overcome the optical and epitaxial limitations of AlGaN, which has been the bottleneck for GaN-based laser diodes for decades. The advantages of NP-GaN for both vertical surfaceemitting laser diodes (VCSEL) and edge-emitting laser diodes are exhibited in subsequent sections. We first demonstrate the record high reflectances (R > 99.5 %) from epitaxial NP-GaN mirrors, which are used in a low threshold optically pumped VCSEL. We then show a two-fold increase of modal gain in an edge-emitting waveguide geometry with optical confinement Γ engineering. The increase of the Γ also leads to a two-fold reduction in the threshold power density under optical pumping with a threshold material gain of 400 cm<sup>-1</sup>, which is more than two times lower than previously reported (> 1,000 cm<sup>-1</sup>).
As the GaN material research is reaching maturity with the phenomenal success in LED industry, there is now need to
look beyond conventional epitaxy. In this paper we will summarize a few novel directions that we are pursuing. In the
first part of this paper, we highlighted our effort to grow single crystal GaN on amorphous substrate. With the successive
applications of a phenomenon called evolutionary selection along two perpendicular axes, we remove the degree of
freedom in grain orientations from 3 to 0 and successfully prepared single-crystalline GaN on amorphous oxide template.
We dedicated the second part of this paper to our recent findings in GaN nanomembrane. Via conductivity selective
electrochemical etching, we have fabricated GaN nanomembrane as thin as 90 nm. The thin and “soft” GaN
nanomembrane is proven to maintain its as-grown crystal quality. We have also demonstrated a 300 nm thick
InGaN/GaN nanomembrane LED.