Enhancement of bulk second-harmonic generation from silicon nitride films by material composition

Abstract

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...

High-confinement high-Q silicon-rich silicon nitride nonlinear microresonators

“…Figure 2(b) shows an example of a typical absorption resonance in another resonator. The loaded (intrinsic) Q is 0.8x10 6 (1x10 6 ) and the mean value of the intrinsic Q measured across the C band is 0.9x10 6 . These values are still below what can be attained with high-confinement microresonators based on stoichiometric Si3N4 [10][11][12], but the fabrication process involves much less steps.…”

“…Interestingly, the chemical composition in SiN can be precisely controlled at the deposition process [3]. Although the influence of the film's stoichiometry on the linear refractive index has been widely studied before, its impact on the nonlinear refractive index has only been investigated recently [4][5][6]. In particular, our group performed a systematic study of the change in refractive index with the low-pressure chemical vapor deposition (LPCVD) parameters.…”

Second‐Harmonic Generation in Integrated Photonics on Silicon

“…However, the performance of SiN waveguides for SHG has been limited by the low nonlinearity itself. It is important to note that several recent reports suggest that the χ (2) response of SiN is influenced not only by strain and interface effects, but also defects and the overall material composition …”

Optically reconfigurable quasi-phase-matching in silicon nitride microresonators

“…An initial weak SH wave (e.g. arising from intrinsic nonlinearity at interface and defects 23,33,35 ), is resonantly enhanced together with the pump, and seeds the photogalvanic process. Similar to AOP in optical fibers [36][37][38][39] , the interference of intracavity pump and SH gives rise to a self-organized space charge grating along the light propagation direction.…”