Broadband 200-nm second-harmonic generation in silicon in the telecom band

Silicon is well known for its strong third-order optical nonlinearity, exhibiting efficient supercontinuum and four-wave mixing processes. A strong second-order effect that is naturally inhibited in silicon can also be observed, for example, by electrically breaking the inversion symmetry and quasi-phase matching the pump and the signal. To generate an efficient broadband second-harmonic signal, however, the most promising technique requires matching the group velocities of the pump and the signal. In this work, we utilize dispersion engineering of a silicon waveguide to achieve group velocity matching between the pump and the signal, along with an additional degree of freedom to broaden the second harmonic through the strong third-order nonlinearity. We demonstrate that the strong self-phase modulation and cross-phase modulation in silicon help broaden the second harmonic by 200 nm in the O-band. Furthermore, we show a waveguide design that can be used to generate a second-harmonic signal in the entire near-infrared region. Our work paves the way for various applications, such as efficient and broadband complementary-metal oxide semiconductor based on—chip frequency synthesizers, entangled photon pair generators, and optical parametric oscillators. Advances in silicon engineering now allow waveguides to deliver frequency doubled light across the entire near-infrared spectrum, an important range for biological imaging and optical communication. Second-harmonic generation, which is normally restricted in silicon because of its underlying crystal symmetry, can be observed by applying strong electric field in silicon, however, the spectral response remains quite narrow to a few nanometers. Neetesh Singh from the Massachusetts Institute of Technology, in Cambridge, United States, and colleagues report a very broadband response of 100s of nanometers using silicon waveguides containing rows of diode junctions. When an electric field is applied to the waveguide, the diodes disrupt the typical silicon symmetry and enable second-harmonic generation from a pump laser. Modifying the cross section of the waveguide to ensure that pump and signal pulses travel though the waveguide at similar velocities enabled generation of frequency doubled light with a broader bandwidth than seen with current silicon or any other material based microstructures.