InGaN microtube optical resonator with sub-wavelength wall thickness and its application to refractive index sensing

Subwavelength freestanding InGaN/GaN quantum well microtubes with a wall thickness of about 50 nm are formed by selective release of a coherently strained bilayer heterostructure from a hosting substrate. Highly polarized spontaneous and stimulated emissions due to whispering gallery mode oscillation were observed in photoluminescence spectra at room temperature. The quality factor was increased by a factor of 3 by elevating the microtube from the substrate and reducing light leakage before threshold. A lasing threshold was characterized as low as 3 mJ/cm². Microtubes were immersed in different liquids with refractive index variation as small as 0.01 and tested below and above threshold. The peak positions shift obviously while exhibiting excellent repeatability. Using a simple optical probe-detect method, a 5-μm diameter microtube cavity achieves a sensitivity of 40 nm/refractive index unit (RIU) and a detection limit of 6 × 10^-3 RIU.