Optical Engineering of Modal Gain in a III-Nitride Laser with Nanoporous GaN

Abstract

By using a novel conductivity-based selective electrochemical etching, we have introduced nanometer-sized pores into GaN to significantly decrease its refractive index while maintaining its crystallinity. Such advantages of nanoporous (NP) GaN can thus overcome the epitaxial (strain) and optical (index) limitations of AlGaN, which has been used as the cladding layers in InGaN laser diodes for two decades. Compared to Al0.1Ga0.9GaN cladding, which has a refractive index contrast (Δn) of 0.04 to GaN, the Δn of NP-GaN to GaN can be engineered to be over 0.4 without inducing any tensile strain. The high Δn not only increases the optical confinement factor (Γ) from lower than 3% in the state-of-the-art (SOTA) InGaN laser diodes to 9% but also offers a broad tunability of the Γ and the modal gain as well. We have observed a clear correlation between the Γ and the modal gain by using the variable stripe length method and have demonstrated an over 100% increase of the modal gain by engineering the Γ of the waveguides. The increase of the Γ also leads to a more than 2-fold reduction of the lasing threshold under optical pumping. A threshold material gain (gth) of 401.1 cm–1, which is more than 50% lower than the threshold of the SOTA LDs (gth > 1000 cm–1), suggests opportunities in lowering the lasing threshold and increasing the efficiencies of future III-nitride laser diodes.