Waveguide Mach-Zehnder interferometers (MZIs) composed of two directional couplers (DCs) are widely used as basic building blocks in the optical communications systems and photonic quantum circuits. Since nitride semiconductors have a strong electro-optic effect, they are suitable for implementation of the fast electric-field driven phase shifters in MZIs. In addition, based on its strong optical nonlinearity, the nitride semiconductors can be applied to the quantum light source of 800 nm band via an optical parametric down conversion process. By integration of the nitride-based waveguide MZI devices and the InGaN laser diodes pumped high-efficiency quantum light sources, novel quantum information processing systems made of single material can be expected.
In this work, we report on the design, fabrication and evaluation of two types of waveguide DCs: rib waveguide and strip waveguide.
As for the design the DC which splits an incident wave at a wavelength of 810 nm into two waves with equal power, 3D beam propagation method simulations were performed. The DCs with different coupled waveguide lengths were fabricated by RIE using Ni hard masks. After a SiO2 cladding layer deposition, the devices were diced and end facets were polished.
The splitting characteristics of the rib waveguide DCs were evaluated using a DFB laser, and almost 1:1 splitting ratio was successfully obtained. Measured data shows that we can control the splitting ratio by changing coupled waveguide length.
In addition to the above results on rib waveguide DC, we will report on the fabrication and evaluation of the strip waveguide DCs.
Nitride semiconductors with large optical nonlinearity and high optical damage tolerance have potential for high-efficiency wavelength conversion. However, it is difficult to form the quasi-phase matched (QPM) periodic polarity-inverted structure, and a long interaction length of the order of cm is usually required for achieving high SHG efficiency in QPM devices. Then we propose a GaN monolithic microcavity SHG device with high-reflectivity Bragg reflectors located on both sides of the GaN resonator to enhance the excitation light intensity within the resonator significantly. By appropriate phase control at reflection, phase mismatch can be compensated even in the absence of the polarity-inverted structure. Due to a very short cavity length of ~ 10 μm, it is necessary to secure a laser beam path by etching the substrate outside of the cavity. In this work, the monolithic microcavity on the Si trapezoidal structure was fabricated using a thick GaN film on a Si substrate. The Cr/Ni masks with microcavity pattern were formed by EB drawing, EB evaporation and lift-off process. After the dry etching by ICP-RIE using Cl2 gas, KOH wet etching was performed in order to improve the sidewall verticality and flatness. The structure was covered by a Ni mask again. After removing the remaining GaN film and the underlying buffer layer, the Si substrate outside of the microcavity was deeply etched by ICP-RIE using SF6 gas. Finally, the Ni mask was removed by wet etching. We will report on the optical characteristics of the fabricated device in the presentation.