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C points, characterized by circular polarization in momentum space, play crucial roles in chiral wave manipulations. However, conventional approaches of achieving intrinsic C points using photonic crystals with broken symmetries suffer from low Q factor and are highly sensitive to structural geometry, rendering them fragile and susceptible to perturbations and disorders. In this letter, we report the realization of magneto-optical (MO) bound states in the continuum (BICs) using a symmetry-preserved planar photonic crystal, achieving intrinsic at-{\Gamma} C points that are robust against variation in structural geometry and external magnetic field. MO coupling between two dipole modes induces Zeeman splitting of the eigenfrequencies, leading to MO BICs and quasi-BICs with circular eigenstates for high-Q chiral responses. Furthermore, switchable C point handedness and circular dichroism are enabled by reversing the magnetic field. These findings unveil a new type of BICs with circular eigenstates and on-demand control of C points, paving the way for advanced chiral wave manipulation with enhanced light-matter interaction., Comment: 13 pages, 4 figures

Optical tweezers employing forces produced by light underpin important manipulation tools in many areas of applied and biological physics. Conventional optical tweezers are based on refractive optics, and they require excessive auxiliary optical elements to reshape both amplitude and phase, as well as wavevector and angular momentum of light, and thus impose limitations to the overall cost and integration of optical systems. Metamaterials provide both electric and optically induced magnetic response in subwavelength optical structures, and they are highly beneficial to achieve unprecedented control of light required for many applications, also opening new opportunities for optical manipulation. Here, we review the recent advances in the field of optical tweezers employing the physics and concepts of metamaterials (the so-called meta-tweezers) and demonstrate that metamaterial structures could not only advance classical operations with particles, such as trapping, transporting, and sorting, but they uncover exotic optical forces such as pulling and lateral forces. Remarkably, apart from manipulation of particles, metastructures can be powered dynamically by light to realize ingenious meta-robots. We provide an outlook for future opportunities in this area ranging from enhanced particle manipulation to meta-robot actuation., Comment: 49 pages, 5 figures

We experimentally demonstrate a very large dynamic optical reflection modulation from a simple unpatterned layered stack of phase-change materials ultrathin films. Specifically, we theoretically and experimentally demonstrate that properly designed deeply subwavelength GeSbTe (GST) films on a metallic mirror produce a dynamic modulation of light in the near-infrared from very strong reflection (R>80%) to perfect absorption (A > 99,97%) by simply switching the crystalline state of the phase-change material. While the amplitude of modulation can lead to an optical contrast up to 10^6, we can also actively "write" intermediate levels of reflection in between extreme values, corresponding to partial crystallization of the GST layer. We further explore several layered system designs and provide guidelines to tailor the wavelength efficiency range, the angle of operation and the degree of crystallization leading to perfect absorption.

An optical metasurface consists of a dense and usually non-uniform layer of scattering nanostructures behaving as a continuous and extremely thin optical component, with predefined phase and intensity transmission/reflection profiles. To date, various sorts of metasurfaces (metallic, dielectric, Huygens-like, Pancharatman-Berry, etc.) have been introduced to design ultrathin lenses, beam deflectors, holograms, or polarizing interfaces. Their actual efficiencies depend on the ability to predict their optical properties and to fabricate non-uniform assemblies of billions of nanoscale structures on macroscopic surfaces. To further help improve the design of metasurfaces, precise and versatile post-characterization techniques need to be developed. Today, most of the techniques used to characterize metasurfaces rely on light intensity measurements. Here, we demonstrate how quadriwave lateral shearing interferometry (QLSI), a quantitative phase microscopy technique, can easily achieve full optical characterization of metasurfaces of any kind, as it can probe the local phase imparted by a metasurface with high sensitivity and spatial resolution. As a means to illustrate the versatility of this technique, we present measurements on two types of metasurfaces, namely Pancharatnam-Berry and effective-refractive-index metasurfaces, and present results on uniform metasurfaces, metalenses and deflectors.