In recent times, the integration of nonlinear optical functions into the world of integrated optics has generated considerable excitement. These innovations have showcased the potential of integrated photonic platforms. Moreover, the ability to manufacture these devices at scale while keeping costs affordable has spurred the development of fully integrated nonlinear optical devices for a wide range of applications, including on-chip spectroscopy, quantum computations, data communications, metrology, bio-sensing, and LIDAR systems.
A paper published in Light: Advanced Manufacturing, authored by a team of scientists led by Professors Vincent Pelgrin and Zhipei Sun, delves into the landscape of hybrid photonic integration structures.
The integration of various active functions into silicon photonics has garnered significant interest. Its compatibility with CMOS processes and cost-effectiveness make it an appealing choice for the industry, providing the means to create dense optical circuits directly.
The robust integration capabilities of this silicon-compatible platform make it a fascinating playground for experimentation. However, when it comes to harnessing nonlinear optical processes, silicon photonics encounters some challenges. These mainly revolve around the weak nonlinear responses of commonly used materials in clean room processes or the presence of free carriers.
Silicon exhibits high nonlinearity in the C-band range but is hindered by two-photon absorption (TPA) due to its low bandgap. Unfortunately, many classical materials compatible with silicon integration lack the necessary optical nonlinearities. For instance, the nonlinear refractive index (n2) of stoichiometric SiN is nearly two orders of magnitude lower than that of silicon.
Impressive demonstrations have employed various silicon-based materials to create integrated optical functions, including supercontinuum sources, frequency combs, and photon pair sources through spontaneous four-wave mixing. In some areas, efforts have been focused on developing more efficient devices that deliver high performance with low power consumption.
Researchers have demonstrated annealed SiN waveguides with loss levels of just a few dB/m, although substantial pump power was still required. Achieving sufficient nonlinear processes necessitated long waveguides, making the creation of fully integrated devices a demanding task.
Alternatively, materials like Si-rich waveguides or the use of p-i-n junctions to manipulate carrier density have shown promise. However, these approaches come with their own set of challenges. Si-rich waveguides still exhibit TPA, and the utilization of multiple p-i-n junctions adds complexity to devices and circuits. Consequently, addressing these limitations remains a priority.
Source: Chinese Academy of Sciences