Snake-shaped waveguides for quasi-phase-matched second harmonic generation

Phase-matched three-wave mixing is possible in optically isotropic zinc-blende materials, thanks to quasi-phase matching occurring when the interacting fields propagate along a curved path about a principal dielectric axis... [ view full abstract ]

Phase-matched three-wave mixing is possible in optically isotropic zinc-blende materials, thanks to quasi-phase matching occurring when the interacting fields propagate along a curved path about a principal dielectric axis [1]. Following this scheme, efficient second harmonic generation (SHG) was recently demonstrated in GaAs [2] and AlGaAs microdisks [3], and it was predicted in curved waveguides [4].
In order to obtain this type of SHG with monolithic low-loss photonic wires in the optical telecom range, we designed and fabricated snake-shaped semiconductor waveguides of two different kinds: 1) air-suspended waveguides; and 2) waveguides on a thick aluminium-oxide (AlOx) substrate. In the first case, a 123nm Al0.18Ga0.82As layer was MBE grown over an Al0.80Ga0.20As sacrificial layer, while in the second case a 100nm Al0.18Ga0.82As layer was grown on an Al0.98Ga0.02As that was wet-oxidized after lithography and dry etching.
Losses in ideal bent waveguides are of two distinct types: radiation losses at the bent semiconductor-air interface, and overlap mismatch losses stemming from the incomplete mode coupling at the waveguide sections with a sudden change of curvature radius. Finite-element simulations predict them to be < 1dB/cm both for 0.75µm wide air-suspended waveguides with 20µm radius of curvature, and for 1µm wide waveguides on AlOx with 50µm radius of curvature. In order to assess the additional guided-mode losses due to scattering at the non-ideal interfaces of the fabricated nanowires, systematic transmission measurements are under way, the light injection and collection being made with micro-lensed-fiber end-fire coupling into inverted tapers.
Our preliminary results show the good potential of such snake-shaped waveguides for SHG.

REFERENCES
[1] Y. Dumeige and P. Feron, “Whispering-gallery-mode analysis of phase-matched doubly resonant second-harmonic generation”, Phys. Rev. A 74, 063804 (2006).
[2] P. S. Kuo, J. Bravo-Abad, and G. S. Solomon, “Second-harmonic generation using 4bar-quasi-phasematching in a GaAs whispering-gallery-mode microcavity”, Nat. Commun. 5, 3109 (2014).
[3] S. Mariani et al., “Second-harmonic generation in AlGaAs microdisks in the telecom range”, Optics Letters 39, 3062 (2014).
[4] R. T. Horn and G. Weihs, “Directional Quasi-Phase Matching in Curved Waveguides”, arXiv: 1008.2190 (2010).