Second-order topological photonic crystals support localized corner modes that deviate from the conventional bulk-edge correspondence. However, the frequency shift of corner modes spanning the photonic band gap has not been experimentally reported. Here, we observe the gapless corner modes of photonic crystal slabs within a parameter space by considering translation as an additional synthetic dimension. These corner modes, protected by topological pumping in synthetic translation dimensions, are found to exist independently of the specific corner configuration. The gapless corner modes are experimentally imaged via the near-field scanning measurement and validated numerically by full-wave simulations. We propose a topological rainbow with gradient translation, demonstrating the ability to extract and separate specific frequency components of light into different spatial locations. Our work contributes to the advancement of topological photonics and provides valuable insights into the exploration of gapless corner modes in synthetic dimensions.

氮化硅材料提供了一种与CMOS兼容的集成光子平台，具有丰富的光学特性。通过调节相关制备参数，可以获得特定折射率的薄膜材料，且消光系数和非线性系数具有较大可调控范围。因此，氮化硅材料在薄膜光学、微纳平面光学、非线性集成光学等诸多领域具有广泛的应用前景。本文主要介绍氮化硅材料的光学特性及其在光学薄膜、微纳超构材料和硅光集成器件方面的研究现状，综述氮化硅材料在太阳能薄膜、可见光超构表面、光栅耦合器和非线性光波导等应用领域的研究概况。

In silicon photonics, the cavity mode is a fundamental mechanism to design integrated passive devices for on-chip optical information processing. Recently, the corner state in a second-order topological photonic crystal (PC) rendered a global method to achieve an intrinsic cavity mode. It is crucial to explore such a topological corner state in silicon photonic integrated circuits (PICs) under in-plane excitation. Here, we study both theoretically and experimentally the topological nanophotonic corner state in a silicon-on-insulator PC cavity at a telecommunications wavelength. In theory, the expectation values of a mirror-flip operation for the Bloch modes of a PC slab are used to characterize the topological phase. Derived from topologically distinct bulk polarizations of two types of dielectric-vein PCs, the corner state is induced in a 90-deg-bend interface, localizing at the corner point of real space and the Brillouin zone boundary of reciprocal space. To implement in-plane excitation in an experiment, we fabricate a cross-coupled PC cavity based on the bend interface and directly image the corner state near 1383 nm using a far-field microscope. Finally, by means of the temporal coupled-mode theory, the intrinsic Q factor of a cross-coupled cavity (about 8000) is retrieved from the measured transmission spectra. This work gives deterministic guidance and potential applications for cavity-mode-based passive devices in silicon PICs, such as optical filters, routers, and multiplexers.

Microendoscopes are vital for disease detection and clinical diagnosis. The essential issue for microendoscopes is to achieve minimally invasive and high-resolution observations of soft tissue structures inside deep body cavities. Obviously, the microscope objective is a must with the capabilities of both high lateral resolution in a wide field of view (FOV) and miniaturization in size. Here, we propose a meta-objective, i.e., microscope objective based on cascaded metalenses. The two metalenses, with the optical diameters of 400 μm and 180 μm, respectively, are mounted on both sides of a 500-μm-thick silica film. Sub-micrometer lateral resolution reaches as high as 775 nm in such a naked meta-objective, with monochromatic aberration correction in a 125 μm full FOV and near diffraction limit imaging. Combined with a fiber bundle microscope system, the single cell contour of biological tissue (e.g., water lily leaf) can be clearly observed, compared to the indistinguishable features in other conventional lens-based fiber bundle systems, such as plano–convex and gradient refractive index (GRIN) cases.