Broadband and fabrication-tolerant 3-dB couplers with topological valley edge modes

3-dB couplers,which are commonly used in photonic integrated circuits for on-chip information processing,precision measurement,and quantum computing,face challenges in achieving robust performance due to their limited 3-dB bandwidths and sensitivity to fabrication errors.To address this,we introduce topological physics to nanophotonics,developing a framework for topological 3-dB couplers.These couplers exhibit broad working wavelength range and robustness against fabrication dimensional errors.By leveraging valley-Hall topology and mirror symmetry,the photonic-crystal-slab couplers achieve ideal 3-dB splitting characterized by a wavelength-insensitive scattering matrix.Tolerance analysis confirms the superiority on broad bandwidth of 48 nm and robust splitting against dimensional errors of 20 nm.We further propose a topological interferometer for on-chip distance measurement,which also exhibits robustness against dimensional errors.This extension of topological principles to the fields of interferometers,may open up new possibilities for constructing robust wavelength division multiplexing,temperature-drift-insensitive sensing,and optical coherence tomography applications.

Meta-objective with sub-micrometer resolution for microendoscopes

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.

In-plane excitation of a topological nanophotonic corner state at telecom wavelengths in a cross-coupled cavity

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.

Dual-polarization two-dimensional valley photonic crystals

The recent realization of valley physics in photonic systems has enriched the topological phases of light with protected edge modes and shown applications in designing high-performance photonic devices. However, the widely reported valley Hall effect of light in two-dimensional systems is limited to one single polarization. Here, we present dual-polarization two-dimensional valley photonic crystals by simultaneously opening two frequency accidental degenerate Dirac cones. Two band gaps with different polarizations are characterized by opposite-valley Chern numbers, which are confirmed by the opposite-phase vortex distributions of bulk modes and opposite Berry curvatures. This situation results in the polarization-dependent refraction of bulk and edge modes, which locate in opposite valleys. The polarization-independent topological valley transport is also demonstrated. Our work shows the flexible control of light in topological photonic systems with a polarization degree of freedom and has applications in polarization multiplexing photonic devices.