Ultra-thin, planar, Babinet-inverted plasmonic metalenses

We experimentally demonstrate the focusing of visible light with ultra-thin,planar metasurfaces made of concentrically perforated,30-nm-thick gold films.The perforated nano-voids—Babinet-inverted(complementary)nano-antennas—create discrete phase shifts and form a desired wavefront of cross-polarized,scattered light.The signal-to-noise ratio in our complementary nano-antenna design is at least one order of magnitude higher than in previous metallic nano-antenna designs.We first study our proof-of-concept‘metalens’with extremely strong focusing ability:focusing at a distance of only 2.5 mm is achieved experimentally with a 4-mm-diameter lens for light at a wavelength of 676 nm.We then extend our work with one of these‘metalenses’and achieve a wavelength-controllable focal length.Optical characterization of the lens confirms that switching the incident wavelength from 676 to 476 nm changes the focal length from 7 to 10 mm,which opens up new opportunities for tuning and spatially separating light at different wavelengths within small,micrometer-scale areas.All the proposed designs can be embedded on-chip or at the end of an optical fiber.The designs also all work for two orthogonal,linear polarizations of incident light.

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip.The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits,giving rise to numerous metawaveguides with unprecedented strength in controlling guided electromagnetic waves.Here,we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms,such as dielectric or plasmonic waveguides and optical fibers.Foundational results and representative applications are comprehensively summarized.Brief physical models with explicit design tutorials,either physical intuition-based design methods or computer algorithms-based inverse designs,are cataloged as well.We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems,by enhancing light-matter interaction strength to drastically boost device performance,or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities.We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits,biomedical sensing,artificial intelligence and beyond.

Citric Acid-Based Intrinsic Band-Shifting Photoluminescent Materials

Citric acid,an important metabolite with abundant reactive groups,has been demonstrated as a promising starting material to synthesize diverse photoluminescent materials including small molecules,polymers,and carbon dots.The unique citrate chemistry enables the development of a series of citric acid-based molecules and nanomaterials with intriguing intrinsic band-shifting behavior,where the emission wavelength shifts as the excitation wavelength increases,ideal for chromatic imaging and many other applications.In this review,we discuss the concept of“intrinsic band-shifting photoluminescent materials”,introduce the recent advances in citric acid-based intrinsic band-shifting materials,and discuss their potential applications such as chromatic imaging and multimodal sensing.It is our hope that the insightful and forward-thinking discussion in this review will spur the innovation and applications of the unique band-shifting photoluminescent materials.

Nonreciprocal metasurface with space-time phase modulation

Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations on wave propagation.However,it is challenging to realize efficient and ultrafast temporal modulation in a photonic system.Here,leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface,we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm.The metasurface,with travelling-wave modulation upon nonlinear Kerr building blocks,creates spatial phase gradient and multi-terahertz temporal phase wobbling,which leads to unidirectional photonic transitions in both the momentum and energy spaces.We observed completely asymmetric reflections in forward and backward light propagations over a large bandwidth around 5.77 THz within a sub-wavelength interaction length of 150 nm.Our approach highlights a potential means for creating miniaturized and integratable nonreciprocal optical components.

Single-cavity bi-color laser enabled by optical anti-parity-time symmetry

The exploration of quantum inspired symmetries in optical systems has spawned promising physics and provided fertile ground for developing devices exhibiting exotic functionalities. Founded on the anti-parity–time (APT)symmetry that is enabled by both spatial and temporal interplay between gain and loss,we demonstrate theoretically and numerically bi-color lasing in a single micro-ring resonator with spatiotemporal modulation along its azimuthal direction. In contrast to conventional multi-mode lasers that have mixed-frequency output,our laser exhibits stable,demultiplexed,tunable bi-color emission at different output ports. Our APT-symmetry-based laser may point out a new route for realizing compact on-chip coherent multi-color light sources.