Off-axis metasurfaces for folded flat optics.

The overall size of an optical system is limited by the volume of the components and the internal optical path length. To reach the limits of miniaturization, it is possible to reduce both component volume and path length by combining the concepts of metasurface flat optics and folded optics. In addition to their subwavelength component thickness, metasurfaces enable bending conventional folded geometries off axis beyond the law of reflection. However, designing metasurfaces for highly off-axis illumination with visible light in combination with a high numerical aperture is non-trivial. In this case, traditional designs with gradient metasurfaces exhibit low diffraction efficiencies and require the use of deep-subwavelength, high-index, and high-aspect-ratio semiconductor nanostructures that preclude inexpensive, large-area nanofabrication. Here, we describe a design approach that enables the use of low-index (n ≈ 1.5), low-aspect ratio structures for off-axis metagratings that can redirect and focus visible light (λ = 532 nm) with near-unity efficiency. We show that fabricated optical elements offer a very large angle-of-view (110°) and lend themselves to scalable fabrication by nano-imprint lithography.

Ultrafast Plasmonic Nucleic Acid Amplification and Real-Time Quantification for Decentralized Molecular Diagnostics.

Point-of-care real-time reverse-transcription polymerase chain reaction (RT-PCR) facilitates the widespread use of rapid, accurate, and cost-effective near-patient testing that is available to the public. Here, we report ultrafast plasmonic nucleic acid amplification and real-time quantification for decentralized molecular diagnostics. The plasmonic real-time RT-PCR system features an ultrafast plasmonic thermocycler (PTC), a disposable plastic-on-metal (PoM) cartridge, and an ultrathin microlens array fluorescence (MAF) microscope. The PTC provides ultrafast photothermal cycling under white-light-emitting diode illumination and precise temperature monitoring with an integrated resistance temperature detector. The PoM thin film cartridge allows rapid heat transfer as well as complete light blocking from the photothermal excitation source, resulting in real-time and highly efficient PCR quantification. Besides, the MAF microscope exhibits close-up and high-contrast fluorescence microscopic imaging. All of the systems were fully packaged in a palm size for point-of-care testing. The real-time RT-PCR system demonstrates the rapid diagnosis of coronavirus disease-19 RNA virus within 10 min and yields 95.6% of amplification efficiency, 96.6% of classification accuracy for preoperational test, and 91% of total percent agreement for clinical diagnostic test. The ultrafast and compact PCR system can decentralize point-of-care molecular diagnostic testing in primary care and developing countries.

Subwavelength pixelated CMOS color sensors based on anti-Hermitian metasurface.

The demand for essential pixel components with ever-decreasing size and enhanced performance is central to current optoelectronic applications, including imaging, sensing, photovoltaics and communications. The size of the pixels, however, are severely limited by the fundamental constraints of lightwave diffraction. Current development using transmissive filters and planar absorbing layers can shrink the pixel size, yet there are two major issues, optical and electrical crosstalk, that need to be addressed when the pixel dimension approaches wavelength scale. All these fundamental constraints preclude the continual reduction of pixel dimensions and enhanced performance. Here we demonstrate subwavelength scale color pixels in a CMOS compatible platform based on anti-Hermitian metasurfaces. In stark contrast to conventional pixels, spectral filtering is achieved through structural color rather than transmissive filters leading to simultaneously high color purity and quantum efficiency. As a result, this subwavelength anti-Hermitian metasurface sensor, over 28,000 pixels, is able to sort three colors over a 100 nm bandwidth in the visible regime, independently of the polarization of normally-incident light. Furthermore, the quantum yield approaches that of commercial silicon photodiodes, with a responsivity exceeding 0.25 A/W for each channel. Our demonstration opens a new door to sub-wavelength pixelated CMOS sensors and promises future high-performance optoelectronic systems.

Effect of Geometric and Chemical Anisotropy of Janus Ellipsoids on Janus Boundary Mismatch at the Fluid-Fluid Interface.

We investigated the geometric and chemical factors of nonspherical Janus particles (i.e., Janus ellipsoids) with regard to the pinning and unpinning behaviors of the Janus boundary at the oil-water interface using attachment energy numerical calculations. The geometric factors were characterized by aspect ratio (AR) and location of the Janus boundary (α) separating the polar and apolar regions of the particle. The chemical factor indicated the supplementary wettability (ß) of the two sides of the particle with identical deviations of apolarity and polarity from neutral wetting. These two factors competed with each other to determine particle configurations at the interface. In general, the critical value of ß (ßc) required to preserve the pinned configuration was inversely proportional to the values of α and AR. From the numerical calculations, the empirical relationship of the parameter values of Janus ellipsoids was found; that is, λ = Δ ß c / Δ α ≈ 0.61 A R - 1.61 . Particularly for the Janus ellipsoids with AR > 1, the ßc value is consistent with the boundary between the tilted only and the tilted equilibrium/upright metastable region in their configuration phase diagram. We believe that this work performed at the single particle level offers a fundamental understanding of the manipulation of interparticle interactions and control of the rheological properties of particle-laden interfaces when particles are used as solid surfactants.