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We proposed a nitride semiconductor transverse quasi-phase-matched wavelength conversion device with a polarity inverted structure along the vertical direction formed by surface activated bonding and etching processes. Inductively coupled plasma etching of a GaN layer with maintaining a root-mean square roughness of less than 2 nm in a 100 µm square area was achieved even after deep etching of 1 µm using Cl2/Ar mixture gas and optimizing the antenna and bias powers. This smooth etching enabled surface activated bonding of the ultrathin GaN layer with designed thickness. The fabrication process of the GaN polarity inverted channel waveguide was established.

We designed and fabricated ring/Fabry–Perot composite cavity semiconductor lasers. Single-mode lasing under CW operation was accomplished. The threshold current was 140 mA, and an output power of 12 mW was obtained at an injection current of 250 mA. A side-mode suppression ratio (SMSR) higher than 25 dB was also obtained. The temperature coefficient of the lasing wavelength of 0.23 nm/°C was explained by considering the temperature coefficient of the bandgap energy of the quantum well.

Watt-class wavelength conversion is expected by enlarging the mode size of the channel waveguides to dozens of micrometers to avoid optical damage with high-power pump lasers. In this work, uniform periodically-poled (PP) structures in MgO doped stoichiometric LiTaO3 were fabricated by applying voltage with an SiO2 insulation layer. Annealed proton-exchanged waveguides with a full width at 1/e 2 maximum in a width of 43 μm and that in depth of 28 μm were obtained by proton exchanging for 4 h and thermal diffusion for 75 h. This waveguide can be excited by pump waves with a power of 8.6 W considering the optical damage threshold. Second harmonic generation with a wavelength of 515 nm was conducted in the low power region of about 35 mW to investigate the basic characteristics. Normalized wavelength conversion efficiency in the 15-mm-long PP structures was estimated to be 4.3%/W.

A wavelength conversion device with an input grating coupler that enables easy alignment was fabricated using a vertically polarity inverted GaN layer. The device was excited with a femtosecond laser, and a second harmonic wave with a peak wavelength of 438.4 nm was obtained. A normalized wavelength conversion efficiency of 4.7  10−4%W−1 and a spectral bandwidth of the second harmonic wave of 2.9 nm were comparable to the theoretical estimations taking into account serious walk-off and waveguide losses. High efficiency of 33-34%W−1 can be expected under continuous-wave excitation.

A phase-shifted grating coupler (GC) is proposed and designed to obtain a two-dimensional spot array. A GaInP red distributed Bragg reflector laser integrated with the phase-shifted GC for multispot focusing is designed and fabricated. A 3 Â 3 spot array output from the fabricated laser was demonstrated.