| In fiber optics systems there is an increasing need for fast, compact switches. Currently employed opto-electro-optical converters are relatively slow and power demanding. Nonlinear optical devices, such as lithium niobate (LiNbO3) waveguides, can provide fast, all-optical wavelength conversion. However, they require centimeter-long interaction length and integration with active devices is difficult, since they are not semiconductors.; GaAs has a nonlinear coefficient four times larger than LiNbO3 and a well-established fabrication technology. Moreover, passive and active devices could be potentially integrated to realize on-chip photonic circuits. Gallium arsenide quasi-phasematched (QPM) waveguides have also been demonstrated and although the required interaction length is smaller (millimeters), they require a complex fabrication process.; In this work we present the design, fabrication and characterization of tightly-confining aluminum gallium arsenide (AlGaAs)/aluminum oxide (AlOx) nonlinear waveguide devices. Because of the high index contrast of the AlGaAs/AlOx material system, phasematching can be achieved by artificial birefringence. This approach greatly simplifies the fabrication process.; Artificial birefringence in weakly-confining AlGaAs/AlOx waveguides has been already demonstrated; however, the conversion efficiency remained low, due to poor lateral confinement.; We employed the AlGaAs/AlOx material system to achieve both birefringent phasematching and sub-micron confinement. The normalized conversion efficiency is larger than 20,000 %/W/cm2. This value is 10x higher than previously reported works. We developed a new fabrication process to realize high aspect-ratio, extremely smooth waveguides, which are characterized by very low propagation loss (∼2 dB/mm) at the fundamental wavelength.; Moreover, we showed that nonlinear effects can be further enhanced using a cavity embedded in the waveguide, resonant at the fundamental wavelength. For this purpose a novel dichroic mirror, highly reflective at the fundamental wavelength but with high transmission at the second harmonic, was designed and fabricated. Using these mirrors, we demonstrated for the first time enhanced second harmonic generation in resonant, collinear waveguide devices.; In conclusion, we demonstrated tightly-confining, birefringently phasematched waveguides and resonant cavities with the highest normalized conversion efficiency ever reported. Furthermore, we designed and fabricated the first dichroic mirror for tightly-confining waveguides. Cavities realized with these mirrors achieved large resonant enhancement of second harmonic generation. |
