![]() ![]() 5 Bulk acoustic waves are launched vertically, which modulate the Si 3N 4 optical waveguide through the stress-optic effect. Our team was able to break Lorentz reciprocity and time-reversal symmetry via spatio-temporal modulation 4 of an integrated Si 3N 4 microring resonator with three monolithically integrated aluminium nitride (AlN) piezoelectric modulators. These approaches, however, are limited by dynamic reciprocity, high required optical pump power or narrow bandwidth. Magnetic-free operation is thus highly desired, and has been extensively studied based on optical nonlinearities, stimulated Brillouin scattering and optomechanically induced transparency. 3 Such a system, which requires a bulky external magnet, is still challenging for chip-scale integration. Traditionally, optical nonreciprocity is based on the Faraday effect of magneto-optic materials under an external magnetic field. 2ĭespite these advanced functionalities, nonreciprocal integrated optical devices have posed challenges. Meanwhile, high-quality-factor Si 3N 4 microresonators have recently been used in integrated semiconductor lasers with linewidths comparable to those of commercial fiber lasers. 1 Si 3N 4 has been widely used for Kerr frequency combs, microwave photonics and photonic computing networks. Over the past decade, Si 3N 4 has become a leading material platform for integrated photonics, benefiting from record low loss, large Kerr nonlinearity and compatibility with foundry processes. ![]() In recent work, we have developed a key building block for future compact systems: an integrated silicon nitride (Si 3N 4) optical isolator free of the need for external magnetic fields. Moreover, tight confinement of light in optical waveguides allows the exploration of optical nonlinearities, including optical-frequency-comb, second-harmonic and supercontinuum generation. Advances in nanofabrication technology have enabled the integration of important building blocks, from integrated laser sources to optical modulators to photodetectors. The compactness, robustness and low power consumption of integrated photonics offer an elegant way to bring tabletop optical systems from the research lab to daily life. Strong acousto-optic modulation allows indirect interband transition between a pair of strongly coupled (TE and TM) optical modes in only one direction. A Si 3N 4 microring resonator with three monolithically integrated AlN piezoelectric modulators breaks optical transmission reciprocity without an external magnetic field. ![]()
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