Influence of surface roughness on the lasing characteristics of optically pumped thin-film GaN microdisks.

Optically pumped whispering-gallery mode (WGM) lasing is observed from a thin-film GaN microdisk processed from GaN-on-Si InGaN/GaN multi-quantum well wafers by selective wet-etch removal of the substrate. Compared with thin-film microdisks processed from GaN-on-sapphire wafers through laser lift-off of the sapphire substrate, the exposed surface is significantly smoother as laser-induced damage is avoided, with a root-mean-square roughness of 1.3 nm compared with 5.8 nm of the latter wafer. The ∼8-µm diameter microdisks, fabricated by pattern transfer from a silica microsphere and dry etching, benefit from the surface smoothness to offer superior optical confinement within the cavity. WGM lasing thresholds of ∼2.9 mJ/cm2 and ∼3.5 mJ/cm2 with quality (Q)-factors of ∼3100 and ∼1700 are observed at the peak lasing wavelengths of ∼453 nm and ∼532 nm, respectively, which are significantly better than thin-film microdisks processed from GaN-on-sapphire wafers despite lower internal quantum efficiency, highlighting the importance of surface smoothness in such optical cavities.

Monolithic integration of ultraviolet microdisk lasers into photonic circuits in a III-nitride-on-silicon platform.

Ultraviolet microdisk lasers are integrated monolithically into photonic circuits using a III-nitride-on-silicon platform with gallium nitride (GaN) as the main waveguide layer. The photonic circuits consist of a microdisk and a pulley waveguide, terminated by out-coupling gratings. In this Letter, we measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 to 399 nm under pulsed excitation, achieving low-threshold energies of 0.14mJ/cm2 per pulse (threshold peak powers of 35kW/cm2). A large peak-to-background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and the waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.

Demonstration of critical coupling in an active III-nitride microdisk photonic circuit on silicon.

On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Efficient second harmonic generation in low-loss planar GaN waveguides.

We demonstrate low-loss GaN/AlGaN planar waveguides grown by molecular beam epitaxy on sapphire substrates. By using a proper AlGaN cladding layer and reducing surface roughness we reach <1dB/cm propagation losses at 633nm. These low propagation losses allow an efficient second harmonic generation using modal phase matching between a TM0 pump at 1260nm and a TM2 second harmonic at 630nm. A maximal power conversion of 2% is realized with an efficiency of 0.15%·W-1cm-2. We provide a modelling that demonstrates broadband features of GaN/AlGaN platform by showing second harmonic wavelengths tunability from the visible up to the near-infrared spectral region. We discuss drawbacks of modal phase matching and propose a novel solution which allows a drastic improvement of modal overlaps with the help of a planar polarity inversion. This new approach is compatible with low propagation losses and may allow as high as 100%·W-1cm-2 conversion efficiencies in the future.