Aluminum nitride as nonlinear optical material for on-chip frequency comb generation and frequency conversion

A number of dielectric materials have been employed for on-chip frequency comb generation. Silicon based dielectrics such as silicon dioxide (SiO2) and silicon nitride (SiN) are particularly attractive comb materials due to their low optical loss and maturity in nanofabrication. They offer third-order Kerr nonlinearity (chi((3))), but little second-order Pockels (chi((2))) effect. Materials possessing both strong chi((2)) and chi((3)) are desired to enable selfreferenced frequency combs and active control of comb generation. In this review, we introduce another CMOScompatible comb material, aluminum nitride (AlN), which offers both second and third order nonlinearities. A review of the advantages of AlN as linear and nonlinear optical material will be provided, and fabrication techniques of low loss AlN waveguides from the visible to infrared (IR) region will be discussed. We will then show the frequency comb generation including IR, red, and green combs in high-Q AlN micro-rings from single CW IR laser input via combination of Kerr and Pockels nonlinearity. Finally, the fast speed on-off switching of frequency comb using the Pockels effect of AlN will be shown, which further enriches the applications of the frequency comb.