Nonlinear optics has not stopped evolving, offering opportunities to develop novel functionalities in photonics. Supercontinuum generation, a nonlinear optical phenomenon responsible for extreme spectral broadening, attracts the interest of researchers due to its high potential in many applications, including sensing, imaging, or optical communications. In particular, with the emergence of silicon photonics, integrated supercontinuum sources in silicon platforms have seen tremendous progress during the past decades. This article aims at giving an overview of supercontinuum generation in three main silicon-compatible photonics platforms, namely, silicon, silicon germanium, and silicon nitride, as well as the essential theoretical elements to understand this fascinating phenomenon.

Dispersion engineering of optical waveguides is among the most important steps in enabling the realization of Kerr optical frequency combs. A recurring problem is the limited bandwidth in which the nonlinear phase matching condition is satisfied, due to the dispersion of the waveguide. This limitation is particularly stringent in high-index-contrast technologies such as silicon-on-insulator. We propose a general approach to stretch the bandwidth of Kerr frequency combs based on subwavelength engineering of single-mode waveguides with self-adaptive boundaries. The wideband flattened dispersion operation comes from the particular property of the waveguide optical mode that automatically self-adapts its spatial profile at different wavelengths to slightly different effective spatial spans determined by its effective index values. This flattened dispersion relies on the squeezing of small normal-dispersion regions between two anomalous spectral zones, which enables it to achieve two Cherenkov radiation points and substantially broaden the comb, achieving a bandwidth between 2.2 and 3.4 μm wavelength. This strategy opens up a design space for trimming the spectra of Kerr frequency combs using high-index-contrast platforms and can provide benefits to various nonlinear applications in which the manipulation of energy spacing and phase matching are pivotal.

We report supercontinuum generation in nitrogen-rich (N-rich) silicon nitride waveguides fabricated through back-end complementary-metal-oxide-semiconductor (CMOS)-compatible processes on a 300 mm platform. By pumping in the anomalous dispersion regime at a wavelength of 1200 nm, two-octave spanning spectra covering the visible and near-infrared ranges, including the O band, were obtained. Numerical calculations showed that the nonlinear index of N-rich silicon nitride is within the same order of magnitude as that of stoichiometric silicon nitride, despite the lower silicon content. N-rich silicon nitride then appears to be a promising candidate for nonlinear devices compatible with back-end CMOS processes.

Nonlinear all-optical technology is an ultimate route for next-generation ultrafast signal processing of optical communication systems. New nonlinear functionalities need to be implemented in photonics, and complex oxides are considered as promising candidates due to their wide panel of attributes. In this context, yttria-stabilized zirconia (YSZ) stands out, thanks to its ability to be epitaxially grown on silicon, adapting the lattice for the crystalline oxide family of materials. We report, for the first time to the best of our knowledge, a detailed theoretical and experimental study about the third-order nonlinear susceptibility in crystalline YSZ. Via self-phase modulation-induced broadening and considering the in-plane orientation of YSZ, we experimentally obtained an effective Kerr coefficient of n^2YSZ=4.0±2×10?19m2·W?1 in an 8% (mole fraction) YSZ waveguide. In agreement with the theoretically predicted n^2YSZ=1.3×10?19m2·W?1, the third-order nonlinear coefficient of YSZ is comparable with the one of silicon nitride, which is already being used in nonlinear optics. These promising results are a new step toward the implementation of functional oxides for nonlinear optical applications.

Near-infrared germanium (Ge) photodetectors monolithically integrated on top of silicon-on-insulator substrates are universally regarded as key enablers towards chip-scale nanophotonics, with applications ranging from sensing and health monitoring to object recognition and optical communications. In this work, we report on the high-data-rate performance pin waveguide photodetectors made of a lateral hetero-structured silicon-Ge-silicon (Si-Ge-Si) junction operating under low reverse bias at 1.55 μm. The pin photodetector integration scheme considerably eases device manufacturing and is fully compatible with complementary metal-oxide-semiconductor technology. In particular, the hetero-structured Si-Ge-Si photodetectors show efficiency-bandwidth products of ～9 GHz at ?1 V and ～30 GHz at ?3 V, with a leakage dark current as low as ～150 nA, allowing superior signal detection of high-speed data traffic. A bit-error rate of 10?9 is achieved for conventional 10 Gbps, 20 Gbps, and 25 Gbps data rates, yielding optical power sensitivities of ?13.85 dBm, ?12.70 dBm, and ?11.25 dBm, respectively. This demonstration opens up new horizons towards cost-effective Ge pin waveguide photodetectors that combine fast device operation at low voltages with standard semiconductor fabrication processes, as desired for reliable on-chip architectures in next-generation nanophotonics integrated circuits.

In this paper, we report the experimental characterization of highly nonlinear GeSbS chalcogenide glass waveguides. We used a single-beam characterization protocol that accounts for the magnitude and sign of the real and imaginary parts of the third-order nonlinear susceptibility of integrated Ge23Sb7S70 (GeSbS) chalcogenide glass waveguides in the near-infrared wavelength range at λ=1580 nm. We measured a waveguide nonlinear parameter of 7.0±0.7 W 1·m 1, which corresponds to a nonlinear refractive index of n2=(0.93±0.08)×10 18 m2/W, comparable to that of silicon, but with an 80 times lower two-photon absorption coefficient βTPA=(0.010±0.003) cm/GW, accompanied with linear propagation losses as low as 0.5 dB/cm. The outstanding linear and nonlinear properties of GeSbS, with a measured nonlinear figure of merit FOMTPA=6.0±1.4 at λ=1580 nm, ultimately make it one of the most promising integrated platforms for the realization of nonlinear functionalities.

We introduce a family of slot photonic crystal waveguides (SPhCWs) for the hybrid integration of low-index active materials in silicon photonics with energy-confinement factors of ～30% in low-index regions. The proposed approach, which is based on a periodic indentation of the etched slot in the middle of the SPhCW, makes it possible to reconcile a simultaneously narrow and wide slot for exploiting the two modes of even symmetry of a SPhCW. The resulting mode-selection mechanism allows a flexible choice of the modes to be used. Furthermore, the proposed structure offers tremendous flexibility for adjusting the dispersive properties of the slot-confined modes, in particular of their slow-light effects. Flat band slow light in a bandwidth of about 60 nm with a group velocity dispersion factor |β2| below 1 ps2/mm is numerically demonstrated by this approach, corresponding to a normalized delay bandwidth product of around 0.4. These results, obtained from hollow-core periodic waveguides that are directly designed in view of hybrid integration of active materials in mechanically robust structures (not based on free-standing membranes) could pave the way for the realization of on-chip slow-light bio-sensing, active hybrid-silicon optoelectronic devices, or all-optical hybrid-silicon nonlinear functionalities.

We propose an efficient and low-power second harmonic generation (SHG) process in a silicon-compatible hybrid plasmonic microring resonator. By making the microring resonator doubly resonant at the fundamental wavelength of 3.1 μm and second harmonic wavelength of 1.55 μm, the SHG efficiency is enhanced by almost two orders of magnitude when compared to the previous result induced in a straight plasmonic waveguide. A SHG efficiency of 13.71% is predicted for a low pump power of 20 mW in a ring with radius of 2.325 μm. This device provides a potential route for realizing efficient frequency conversion between mid-infrared and near-infrared wavebands on a chip.