The monolithic integration of Fabry-Perot cavities has many applications, such as label-free sensing, high-finesse filters, semiconductor lasers, and frequency comb generation. However, the excess loss of integrated reflectors makes it challenging to realize integrated Fabry-Perot cavities working in the ultrahigh-Q regime (>106). Here, we propose and experimentally demonstrate what we believe is the first silicon integrated million-Q Fabry-Perot cavity. Inspired by free-space optics, a novel monolithically integrated retroreflector is utilized to obtain near-unity reflectance and negligible reflection losses. The corner scattering in the retroreflector is prevented by the use of the TE1 mode, taking advantage of its zero central field intensity. Losses incurred by other mechanisms are also meticulously engineered. The measurement results show resonances with an ultrahigh intrinsic Q factor of ≈3.4×106 spanning an 80-nm bandwidth. The measured loaded Q factor is ≈2.1×106. Ultralow reflection losses (≈0.05dB) and propagation losses (≈0.18dB/cm) are experimentally realized.

In recent years, optical modulators, photodetectors, (de)multiplexers, and heterogeneously integrated lasers based on silicon optical platforms have been verified. The performance of some devices even surpasses the traditional III-V and photonic integrated circuit (PIC) platforms, laying the foundation for large-scale photonic integration. Silicon photonic technology can overcome the limitations of traditional transceiver technology in high-speed transmission networks to support faster interconnection between data centers. In this article, we will review recent progress for silicon PICs. The first part gives an overview of recent achievements in silicon PICs. The second part introduces the silicon photonic building blocks, including low-loss waveguides, passive devices, modulators, photodetectors, heterogeneously integrated lasers, and so on. In the third part, the recent progress on high-capacity silicon photonic transceivers is discussed. In the fourth part, we give a review of high-capacity silicon photonic networks on chip.

Polarizers have been widely used in various optical systems to reduce polarization cross talk. The polarizers based on the silicon nanowire waveguide can provide chip-scale device size and a high polarization extinction ratio. However, the working bandwidth for the on-chip silicon polarizers is always limited (<～100nm) by the strong waveguide dispersion. In this paper, an on-chip all-silicon polarizer with an extremely broad working bandwidth is proposed and demonstrated. The device is based on a 180° sharp waveguide bend, assisted with anisotropic subwavelength grating (SWG) metamaterial cladding to enhance the polarization selectivity. For TE polarization, the effective refractive index for SWG is extraordinary, so the incident TE mode can propagate through the sharp waveguide bend. For TM polarization, the effective refractive index for SWG is ordinary, so the incident TM mode will be coupled into the radiation mode regardless of the wavelength. The fabricated polarizer shows low loss (<1dB) and high polarization extinction ratio (>20dB) over a >415nm bandwidth from 1.26 to 1.675 μm, which is at least fourfold better than what has been demonstrated in all previous works. To the best of our knowledge, such a device is the first all-silicon polarizer that covers O-, E-, S-, C-, L-, and U-bands.