Abstract
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.
Highlights
Modern nonlinear optics is considered to have originated in the demonstration of second-harmonic generation (SHG) soon after the invention of the laser by Franken et al in the 1960s
To break the centrosymmetry of silicon, a constant electric field was applied across the waveguide through the p-i-n junction using direct current (DC)
The weak selfphase modulation (SPM) of the pump will be of limited use for applications, such as self-referencing, where an octavespanning supercontinuum is required
Summary
Modern nonlinear optics is considered to have originated in the demonstration of second-harmonic generation (SHG) soon after the invention of the laser by Franken et al in the 1960s (ref. 1). Bulk SHG, is limited to materials that do not possess inversion symmetry. Such materials are currently emerging in integrated platforms[10,11,12,13,14,15,16], for mass production, a complementary-metal oxide semiconductor (CMOS) material is desirable. Most CMOS materials are either centrosymmetric (semiconductors) or isotropic (oxides) and do not exhibit a bulk second-order susceptibility (χ(2)), except at interfaces where the inversion symmetry is broken—a fact that is used for sensitive interface and surface characterizations[17,18,19]. The SHG bandwidth in this work, as in most demonstrations with an integrated platform to date, was limited to a few nanometers due to the walk-off between the pump and signal, resulting from the lack of group velocity matching between the pump and the signal
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