Flexible broadband metamaterial absorber in long-wave infrared with visible transparency fabricated by laser direct writing.

Absorption of the long-wave infrared from human beings and the surroundings is a key step to infrared imaging and sensing. Here we demonstrate a flexible and transparent broadband infrared absorber using the photoresist-assisted metamaterials fabricated by one-step laser direct writing. The photoresist is patterned by the laser as an insulator layer as well as a mask to build the complementary bilayer metamaterials without lithography. The average absorptivity is 94.5% from 8 to 14 µm in experiment due to the broadband destructive interference of the reflected beam explained by the Fabry-Perot cavity model. The proposed absorber is applicable to various substrates with additional merits of polarization insensitivity and large angle tolerance, which offers a promising solution for thermal detection and management.

Perovskite nickelates as electric-field sensors in salt water.

Designing materials to function in harsh environments, such as conductive aqueous media, is a problem of broad interest to a range of technologies, including energy, ocean monitoring and biological applications. The main challenge is to retain the stability and morphology of the material as it interacts dynamically with the surrounding environment. Materials that respond to mild stimuli through collective phase transitions and amplify signals could open up new avenues for sensing. Here we present the discovery of an electric-field-driven, water-mediated reversible phase change in a perovskite-structured nickelate, SmNiO3. This prototypical strongly correlated quantum material is stable in salt water, does not corrode, and allows exchange of protons with the surrounding water at ambient temperature, with the concurrent modification in electrical resistance and optical properties being capable of multi-modal readout. Besides operating both as thermistors and pH sensors, devices made of this material can detect sub-volt electric potentials in salt water. We postulate that such devices could be used in oceanic environments for monitoring electrical signals from various maritime vessels and sea creatures.

Precisely Encoded Barcodes through the Structure-Fluorescence Combinational Strategy: A Flexible, Robust, and Versatile Multiplexed Biodetection Platform with Ultrahigh Encoding Capacities.

With the rapid development of suspension array technology, microbeads-based barcodes as the core element with sufficient encoding capacity are urgently required for high-throughput multiplexed detection. Here, a novel structure-fluorescence combinational encoding strategy is proposed for the first time to establish a barcode library with ultrahigh encoding capacities. Based on the never revealed transformability of the structural parameters (e.g., porosity and matrix component) of mesoporous microbeads into scattering signals in flow cytometry, the enlargement of codes number has been successfully realized in combination with two other fluorescent elements of fluorescein isothiocyanate isomer I (FITC) and quantum dots (QDs). The barcodes are constructed with precise architectures including FITC encapsulated within mesopores and magnetic nanoparticles as well as QDs immobilized on the outer surface to achieve the ultrahigh encoding level of 300 accompanied with superparamagnetism. To the best of knowledge, it is the highest record of single excitation laser-based encoding capacity up to now. Moreover, a ten-plexed tumor markers bioassay based on the tailored-designed barcodes has been evaluated to confirm their feasibility and effectiveness, and the results indicate that the barcodes platform is a promising and robust tool for practical multiplexed biodetection.

Predictions of resonant mode characteristics for terahertz quantum cascade lasers with distributed feedback utilizing machine learning.

Terahertz quantum cascade lasers (THz QCLs) are the most powerful solid-state THz sources so far and THz QCLs with various distributed feedback (DFB) gratings have demonstrated single-mode emission, collimated beam, frequency tunability and high output power. Resonant mode characteristics of THz QCLs with DFB, including frequency, loss and electric-field distributions, are important for waveguide analysis, fabrication and indication of THz QCLs' radiative performance. Typically, predictions of these characteristics rely on numerical simulations. However, traditional numerical simulations demand a large amount of running time and computing resources, and have to deal with the trade-off between accuracy and efficiency. In this work, machine learning models are designed to predict resonant mode characteristics of THz QCLs with first-order, second-order, third-order DFB and antenna-feedback waveguides according to the four input structural parameters, i.e. grating period, total length of waveguide, duty cycle of grating and length of highly-doped contact layer. The machine learning models are composed of a multi-layer perceptron for predictions of frequency and loss, and an up-sampling convolutional neural network for predictions of electric-field distribution of the lowest-loss mode, respectively. A detailed study on more than 1000 samples shows high accuracy and efficiency of the proposed models, with Pearson correlation coefficients over 0.99 for predictions of lasing frequency and loss, median peak signal-to-noise ratios over 33.74dB for predictions of electric-field distribution, and the required time of prediction is within several seconds. Moreover, the designed models are widely applicable to various DFB structures for THz QCLs. Resonators with graded photonic heterostructures and novel phase-locked arrays are accurately predicted as examples.

Proposal for a broadband THz refractive-index sensor based on quantum-cascade laser arrays.

Many molecules have strong and characteristic rotational and vibrational transitions at terahertz (THz) frequencies, which makes this frequency range unique for applications in spectroscopic sensing of chemical and biological species. Here, we propose a broadband THz sensor based on arrays of single-mode QCLs, which could be utilized for sensing of the refractive-index of solids or liquids in reflection geometry. The proposed scheme does not require expensive THz detectors and consists of no movable parts. A recently developed antenna-feedback geometry is utilized to enhance optical coupling between two single-mode QCLs, which facilitates optical downconversion of the THz frequency signal to microwave regime. Arrays of THz QCLs emitting at discrete frequencies could be utilized to provide more than 2 THz of spectral coverage to realize a broadband, low-cost, and portable THz sensor.

A Multi-foci Sparse-Aperture Metalens.

Multi-foci lenses are essential components for optical communications, virtual reality display and microscopy, yet the bulkiness of conventional counterparts has significantly hindered their widespread applications. Benefiting from the unprecedented capability of metasurfaces in light modulation, metalenses are able to provide multi-foci functionality with a more compact footprint. However, achieving imaging quality comparable to that of corresponding single-foci metalenses at each focal point poses a challenge for existing multi-foci metalenses. Here, a polarization-independent all-dielectric multi-foci metalens is proposed and experimentally demonstrated by spatially integrating single-foci optical sparse-aperture sub-metalenses. Such design enables the metalens to generate multiple focal points, while maintaining the ability to capture target information comparable to that of a single-foci metalens. The proposed multi-foci metalens is composed of square-nanohole units array fabricated by two-photon polymerization. The focusing characteristic and imaging capability are demonstrated upon the illumination of an unpolarized light beam. This work finds a novel route toward multi-foci metalenses and may open a new avenue for dealing with the trade-off between multi-foci functionality and high-quality imaging performance.

Label-free investigation of infected acute pyelonephritis tissue by FTIR microspectroscopy with unsupervised and supervised analytical methods.

Acute pyelonephritis (AP) is a severe urinary tract infection (UTI) syndrome with a large population of patients worldwide. Current approaches to confirming AP are limited to urinalysis, radiological imaging methods and histological assessment. Fourier transform infrared (FTIR) microspectroscopy is a promising label-free modality that can offer information about both morphological and molecular pathologic alterations from biological tissues. Here, FTIR microspectroscopy serves to investigate renal biological histology of a rat model with AP and classify normal cortex, normal medulla and infected acute pyelonephritis tissues. The spectra were experimentally collected by FTIR with an infrared Globar source through raster scanning procedure. Unsupervised analysis methods, including integrating, clustering and principal component analysis (PCA) were performed on such spectra data to form infrared histological maps of entire kidney section. In comparison to Hematoxylin & Eosin-stained results of the adjacent tissue sections, these infrared maps were proved to enable the differentiation of the renal tissue types. The results of both integration and clustering indicated that the concentration of amide II decreases in the infected acute pyelonephritis tissues, with an increased presence of nucleic acids and lipids. By means of PCA, the infected tissue was linearly separated from normal ones by plotting confident ellipses with the score values of the first and second principal components. Moreover, supervised analysis was performed based on the supported vector machines (SVM). Normal cortex, normal medulla and infected acute pyelonephritis tissues were classified by SVM models with the best accuracy of 96.11% in testing dataset. In addition, these analytical methods were further employed on synchrotron-based FTIR spectra data and successfully form high-resolution infrared histological maps of glomerulus and necrotic cell mass. This work demonstrates that FTIR microspectroscopy will be a powerful manner to investigate AP tissue and differentiate infected tissue from normal tissue in a renal infected model system.

Publisher Correction: High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings.

The original PDF version of this Article contained an error in Equation 1. The 'Λ' was missing from the denominator. This has been corrected in the PDF version of the Article. The HTML version was correct from the time of publication.

High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings.

A surface-emitting distributed feedback (DFB) laser with second-order gratings typically excites an antisymmetric mode that has low radiative efficiency and a double-lobed far-field beam. The radiative efficiency could be increased by using curved and chirped gratings for infrared diode lasers, plasmon-assisted mode selection for mid-infrared quantum cascade lasers (QCLs), and graded photonic structures for terahertz QCLs. Here, we demonstrate a new hybrid grating scheme that uses a superposition of second and fourth-order Bragg gratings that excite a symmetric mode with much greater radiative efficiency. The scheme is implemented for terahertz QCLs with metallic waveguides. Peak power output of 170 mW with a slope-efficiency of 993 mW A-1 is detected with robust single-mode single-lobed emission for a 3.4 THz QCL operating at 62 K. The hybrid grating scheme is arguably simpler to implement than aforementioned DFB schemes and could be used to increase power output for surface-emitting DFB lasers at any wavelength.