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  1. Miniaturized spectrometers in the mid-infrared (MIR) are critical in developing next-generation portable electronics for advanced sensing and analysis. The bulky gratings or detector/filter arrays in conventional micro-spectrometers set a physical limitation to their miniaturization. In this work, we demonstrate a single-pixel MIR micro-spectrometer that reconstructs the sample transmission spectrum by a spectrally dispersed light source instead of spatially grated light beams. The spectrally tunable MIR light source is realized based on the thermal emissivity engineered via the metal-insulator phase transition of vanadium dioxide (VO2). We validate the performance by showing that the transmission spectrum of a magnesium fluoride (MgF2) sample can be computationally reconstructed from sensor responses at varied light source temperatures. With potentially minimum footprint due to the array-free design, our work opens the possibility where compact MIR spectrometers are integrated into portable electronic systems for versatile applications.

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  2. Merging the properties of VO2 and van der Waals (vdW) materials has given rise to novel tunable photonic devices. Despite recent studies on the effect of the phase change of VO2 on tuning near-field optical response of phonon polaritons in the infrared range, active tuning of optical phonons (OPhs) using far-field techniques has been scarce. Here, we investigate the tunability of OPhs of α-MoO3 in a multilayer structure with VO2. Our experiments show the frequency and intensity tuning of 2 cm–1 and 11% for OPhs in the [100] direction and 2 cm–1 and 28% for OPhs in the [010] crystal direction of α-MoO3. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We then use loss tangent analysis and remove the effect of the substrate to understand the origin of these spectral characteristics. We expect that these findings will assist in intelligently designing tunable photonic devices for infrared applications, such as tunable camouflage and radiative cooling devices. 
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  3. Abstract

    Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400 K.

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  4. Abstract

    Thermal radiation from a black body increases with the fourth power of absolute temperature (T4), an effect known as the Stefan–Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (εint), is insensitive to temperature (|dεint/dT| ≈ 10−4°C–1). The resultant radiance bound by theT4law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which εintcan be engineered to decrease in an arbitrary manner near room temperature (|dεint/dT| ≈ 8 × 10−3°C–1), enabling unprecedented manipulation of infrared radiation. As an example, εintis programmed to vary with temperature as the inverse ofT4, precisely counteracting theT4dependence; hence, thermal radiance from the surface becomes temperature‐independent, allowing the fabrication of flexible and power‐free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten‐doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.

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  5. Abstract

    Reactive ion etching (RIE) used to fabricate high‐aspect‐ratio (HAR) nano/microstructures is known to damage semiconductor surfaces which enhances surface recombination and limits the conversion efficiency of nanostructured solar cells. Here, defect passivation of ultrathin Al2O3‐coated Si micropillars (MPs) using different surface pretreatment steps is reported. Effects on interface state density are quantified by means of electrochemical impedance spectroscopy which is used to extract quantitative capacitance–voltage and conductance–voltage characteristics from HAR dielectric–semiconductor structures which would otherwise suffer from high gate leakage currents if tested using solid‐state metal–insulator–semiconductor structures. High‐temperature thermal oxidation to form a sacrificial oxide on RIE‐fabricated Si MPs, followed by atomic layer deposition of 4 nm thick Al2O3after removal of the sacrificial layer produces an interface trap density (Dit) as low as 1.5 × 1011cm−2eV−1at the mid‐gap energy of silicon. However, a greatly reduced mid‐gapDit(2 × 1011cm−2eV−1) is possible even with a simple air annealing procedure having a maximum temperature of 400 °C.

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