Characterization of very small aperture GaN lasers

We have fabricated GaN blue VSALs (wavelength, lambda = 405 nm), and investigated their emission in terms of slope efficiency through far-field measurements. The slope efficiency of power emission from a 100 nm x 200 nm rectangular aperture as collected in the far field by an objective lens (NA = 0.65) was 1.8 +/- 10.6 mW/A for the blue VSAL's as compared with 3.6 mW/A for red VSAL's (658 nm). Both of these numbers have background transmission through the Al coating subtracted. Prior to apertures formation, slope efficiencies of 1.4 mW/A (blue) and 0.3 mW/A (red) were measured as light was transmitted through the Al coating. Finite-Difference Frequency-Domain (FDFD) simulation was used to estimate the collection efficiency of the optical measurement setup and showed the lens to collect 45% of the light emitted by the blue VSAL and 38% of the light emitted by the red. The FDFD simulations were also used to compute the efficiency of power transmission through the aperture and into the objective lens (94 ppm for blue and 65 ppm for red). Using the expected transmission coefficient for the 50 nm Al layers (220 ppm for blue and 95 ppm for red), the predicted SE values of the apertured structures are 0.6 mW/A for blue and 0.21 mW/A for red. Measured values are significantly higher, suggesting that non-idealities in aperture shape result in higher transmission efficiency, and/or that the Al coating thickness and its associated transmission efficiency have been overestimated. A thickness error of 10 nm out of 50 is enough to explain the discrepancy. Extending the analysis to 30 nm x 60 nm rectangular aperture (in cut-off for both wavelengths), the calculated output power slope efficiency for the VSAL is 7.3 muW/A for the blue and 0.06 muW/A for the red, a factor of over 100 x greater output power slope efficiency for the blue light through these smaller structures.