Abstract-The high index contrast silicon-on-insulator platform is the dominant CMOS 1 compatible platform for photonic integration. The successful use of silicon photonic chips in optical communication applications has now paved the way for new areas where photonic chips can be applied. It is already emerging as a competing technology for sensing and spectroscopic applications. This increasing range of applications for silicon photonics instigates an interest in exploring new materials, as silicon-oninsulator has some drawbacks for these emerging applications, e.g. silicon is not transparent in the visible wavelength range. Silicon nitride is an alternate material platform. It has moderately high index contrast, and like silicon-on-insulator, it uses CMOS processes to manufacture photonic integrated circuits. In this paper, the advantages and challenges associated with these two material platforms are discussed. The case of dispersive spectrometers, which are widely used in various silicon photonic applications, is presented for these two material platforms.

Optical links are moving to higher and higher transmission speeds while shrinking to shorter and shorter ranges where optical links are envisaged even at the chip scale. The scaling in data speed and span of the optical links demands modulators to be concurrently performant and cost-effective. Silicon photonics (SiPh), a photonic integrated circuit technology that leverages the fabrication sophistication of complementary metal-oxide-semiconductor technology, is well-positioned to deliver the performance, price, and manufacturing volume for the high-speed modulators of future optical communication links. SiPh has relied on the plasma dispersion effect, either in injection, depletion, or accumulation mode, to demonstrate efficient high-speed modulators. The high-speed plasma dispersion silicon modulators have been commercially deployed and have demonstrated excellent performance. Recent years have seen a paradigm shift where the integration of various electro-refractive and electro-absorptive materials has opened up additional routes toward performant SiPh modulators. These modulators are in the early years of their development. They promise to extend the performance beyond the limits set by the physical properties of silicon. The focus of our study is to provide a comprehensive review of contemporary (i.e., plasma dispersion modulators) and new modulator implementations that involve the integration of novel materials with SiPh.

We report the fabrication of artificial unidimensional crystals exhibiting an effective bulk second-order nonlinearity. The crystals are created by cycling atomic layer deposition of three dielectric materials such that the resulting metamaterial is noncentrosymmetric in the direction of the deposition. Characterization of the structures by second-harmonic generation Maker-fringe measurements shows that the main component of their nonlinear susceptibility tensor is about 5 pm/V, which is comparable to well-established materials and more than an order of magnitude greater than reported for a similar crystal [Appl. Phys. Lett.107, 121903 (2015)APPLAB0003-695110.1063/1.4931492]. Our demonstration opens new possibilities for second-order nonlinear effects on CMOS-compatible nanophotonic platforms.

We present a comprehensive tensorial characterization of second-harmonic generation from silicon nitride films with varying compositions. The samples were fabricated using plasma-enhanced chemical vapor deposition, and the material composition was varied by the reactive gas mixture in the process. We found a six-fold enhancement between the lowest and highest second-order susceptibility, with the highest value of approximately 5 pm/V from the most silicon-rich sample. Moreover, the optical losses were found to be sufficiently small (below 6 dB/cm) for applications. The tensorial results show that all samples retain in-plane isotropy independent of the silicon content, highlighting the controllability of the fabrication process. High-performance complementary metal oxide semiconductor (CMOS) compatible materials are essential elements for advanced on-chip photonic devices to realize the future progress in all-optical processing. The ultra-fast speed and high bandwidth of integrated photonic networks continuously require new materials possessing excellent linear and nonlinear optical properties [1,2]. Although silicon (Si) is still the most commonly used CMOS material, the intrinsic drawbacks of Si, such as its narrow bandgap and centrosymmetric structure, highly limit its future applications especially in the visible and ultraviolet spectral regimes [2,3]. Thus, exploring novel CMOScompatible materials with wide bandgap and strong optical nonlinearities is very important for future integrated devices.Many photonic applications rely on nonlinear optical effects. One of the limitations of many nonlinear materials for CMOS-compatible platforms is the lack of second-order nonlinearity due to centrosymmetry. The problem can be overcome by poling [4,5], straining the material [3] or by using multilayer composites [6][7][8]. Unexpectedly, CMOS-compatible amorphous silicon nitride films (SiN) have been shown to possess a bulk second-order nonlinearity by measuring strong second-harmonic generation (SHG) from thin films [9][10][11]. Although the exact reason for this strong SHG response remains unclear, it is believed that the complicated composition, crystalline phase, and defects in the film during the deposition may be responsible [10,[12][13][14][15][16].In this Letter, we show that the strong second-harmonic signal from SiN films can be further enhanced by varying the composition of the films prepared with plasma-enhanced chemical vapor deposition (PECVD). Furthermore, we demonstrate that such composition tuning does not compromise the linear optical properties or optical losses of the material for applications. Our results are crucial for the comprehensive understanding of the linear and nonlinear optical properties in SiN films with different structures, opening the path for further optimization of SiN for on-chip devices.We recognize that there have been previous studies yielding different values for the SHG susceptibility of SiN [9,10,11,17,18]. Samples prepared by sputtering can yield very high values of the su...