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A simple prediction of the well-known unification model of active galactic nuclei is that a sample of sources should exhibit an anticorrelation between the solid angle of the dusty torus and of the ionization cone (as the sum of them shall equal 4π); however, this has never been detected. In this work, we analyse the correlation between the [O  iii ] 5007 narrow emission line equivalent width and L IR(λ)/ L bol for a large sample of luminous quasars. For the first time, we detect a clear intrinsic anticorrelation between them, which immediately verifies the torus/ionization-cone geometry in luminous quasars. More interestingly, the anticorrelation significantly weakens with increasing wavelength from ∼2 to 12 |$\mu$| m, and disappears at ∼12 |$\mu$| m. Simulations show that a cool dust component (in addition to the equatorial torus) with its strength positively correlating with the solid angle of the ionization cone is required to explain the observations. This shows that the polar dust seen in nearby active galaxies also exists in luminous quasars, with its contribution to total dust emission increasing with λ (from ∼2 to 12 |$\mu$| m) and reaching between 39 per cent and 62 per cent (model dependent) at rest frame 12 |$\mu$| m. Our findings provide a unique approach to map the otherwise spatially unresolvable inner structure of quasars. [ABSTRACT FROM AUTHOR]

Deep-ultraviolet (DUV) phototransistors have shown great potential applications in UV imaging, artificial intelligence, and wearable optoelectronics. Among a large number of wide bandgap semiconductors, the quasi-two-dimensional β-Ga2O3 is considered as an ideal candidate for DUV photodetector applications. Herein, we report a high responsivity (R) and fully flexible Ta-doped β-Ga2O3 DUV phototransistor which exhibits outstanding optoelectrical properties with a high R of 1.32 × 106 A/W, a large detectivity of 5.68 × 1014 Jones, a great photo-to-dark current ratio of 1.10 × 1010%, a high external quantum efficiency of 6.60 × 108%, and an ultra-fast response time of ~3.50 ms. Besides, the flexible Ta-doped β-Ga2O3 device also displays high reliability and mechanical flexibility that can sustain well after over 1 × 104 bending cycles. Moreover, high-contrast imaging of UV light was obtained on the flexible DUV detector arrays, which can be efficiently trained and recognized by an artificial neural network. Our findings offer a perspective to develop wearable optoelectronics and UV imaging based on high-performance flexible β-Ga2O3 DUV phototransistors, providing an inspiration for the future work in artificial intelligence and bionic robot fields. [ABSTRACT FROM AUTHOR]

Two‐dimensional material (2D) that possesses atomic thin geometry and remarkable properties is a star material for the fundamental researches and advanced applications. Defects in 2D materials are critical and fundamental to understand the chemical, physical, and optical properties. Photoluminescence arises in 2D materials owing to various physical phenomena including activator/dopant‐induced luminescence and defect‐related emissions, and so forth. With the advanced transmission electron microscopy (TEM) technologies, such as aberration correction and low voltage technologies, the morphology, chemical compositions and electronic structures of defects in 2D material could be directly characterized at the atomic scale. In this review, we introduce the applications of state‐of‐the‐art TEM technologies on the studies of the role of atomic defects in the photoluminescence characteristics in 2D material. The challenges in spatial and time resolution are also discussed. It is proved that TEM is a powerful tool to pinpoint the relationship between the defects and the photoluminescence characteristics. [ABSTRACT FROM AUTHOR]

Abstract: Single‐crystalline diamond wafers are synthesized, and all‐carbon solar‐blind photodetectors are constructed based on the diamond with interdigitated graphite as electrodes. The graphite electrodes are made by laser‐induced graphitization of the diamond. The peak responsivity of the photodetector is 21.8 A W−1, and detectivity is 1.39 × 1012 cm Hz1/2 W−1 when the bias voltage is 50 V, it is noted that both of these are among the best values ever reported for diamond‐based photodetectors. The photodetectors are employed as the sensing pixels in an imaging system, and a clear image is obtained, which is the first report on solar‐blind imaging of diamond photodetectors. The results reported in this paper provide a facile approach to high‐performance diamond solar‐blind photodetectors and their applications in solar‐blind imaging, thus may address a significant step toward the preparation and application of diamond photodetectors. [ABSTRACT FROM AUTHOR]

We present a computational study of the plasmon resonances in a periodic square coaxial hole array in a graphene sheet, which consists of a square hole array and a square strip array. According to charge oscillation picture, we find that a new plasmon mode, which locates on the edges of square hole along the polarization direction of incident light, emerges in our proposed structure. Two hybridized plasmon resonance modes (i.e., symmetric and asymmetric plasmon resonance modes) are formed due to two different manners of coupling between the new plasmon mode of square hole and the plasmon mode of square strip. The two plasmon resonance modes can be tuned over a wide wavelength range by a small change in the chemical potential of graphene. Furthermore, the two plasmon resonances can also be controlled by changing L and L (which are the strip offsets from the hole center perpendicular and parallel to the polarization direction of incident light, respectively), originating from the change in the strength of electromagnetic coupling between square hole and square strip. Our study gives an insight into the physical mechanism of plasmon resonances in square graphene coaxial hole array, and our findings will be useful for designing graphene-based plasmonic devices and metamaterials. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the plasmon resonances in a stacked pair of periodic graphene hole arrays by using charge oscillation picture. It is found that the plasmon resonances can be controlled by changing the longitudinal interval G between two arrays, and the lateral displacements L and L, which are parallel and perpendicular to the polarization direction of the incident light, respectively. The plasmon resonance can be tuned over a wide wavelength range by a small change of G due to its influence on the strength of electromagnetic coupling between two arrays. The lateral displacement L causes new plasmon resonance modes, originating from the appearance of new electromagnetic coupling manners, while the lateral displacement L leads to the splitting of plasmon resonance mode, which results from two different electromagnetic coupling manners. Our study gives an insight into the physical mechanism of plasmon resonances in two graphene hole arrays, and our findings may hold great promise in designing graphene-based plasmonic devices and metamaterials. [ABSTRACT FROM AUTHOR]

HgCdTe electron avalanche photodiodes (e-APDs) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we performed accurate numerical simulations of the current–voltage characteristics and multiplication factor in planar homojunction (p-i-n) HgCdTe APDs. Based on the Okuto–Crowell avalanche model, an efficient physical model has been obtained by concerning the major generation-recombination processes, such as trap-assisted tunneling and band-to-band tunneling (BBT) recombination. Simulated current–voltage characteristics were in good agreement with available data in the literature, which demonstrates the validity of the proposed model. The origins of dark current in high reverse voltages are jointly dominated by BBT and the avalanche mechanism. It is proved to be effective for reducing BBT by improving the uniformity of the electric field distribution across the junction. The electric performance of p-i-n e-APD can be improved by optimizing the APD structure, such as eliminating the sharp corners of junctions, light doping, and the appropriate thickness in multiplication region. Our works provide a good deal of insight into the fundamental carrier transport processes involved in HgCdTe e-APDs. [ABSTRACT FROM AUTHOR]

The optoelectronic performance of the mid-wavelength HgCdTe infrared focal plane array (IRFPA) with refractive microlenses integrated on its CdZnTe substrate has been numerically simulated. A reduced light-distribution model based on scalar Kirchhoff diffraction theory was adopted to reveal the true behavior of IRFPAs operating in an optical system under imaging conditions. The pixel crosstalk obtained and the energy-gathering characteristics demonstrated that the microlenses can delay the rise in crosstalk when the image point shifts toward pixel boundaries, and can restrict the major optical absorption process in any case within a narrow region around the pixel center. The dependence of the microlenses' effects on the system's properties was also analyzed; this showed that intermediate relative aperture and small microlens radius are required for optimized device performance. Simulation results also indicated that for detectors farther from the center of the field of view, the efficacy of microlenses in crosstalk suppression and energy gathering is still maintained, except for a negligible difference in the lateral magnification from an ordinary array without microlenses. [ABSTRACT FROM AUTHOR]

We have investigated terahertz (THz) resonant absorption spectra in grating-gate GaN high electron mobility transistors. The results indicate that both the symmetrical plasmon mode and the asymmetrical plasmon mode play an important role in the strong absorption of THz waves. The excitation process and dynamic response of these plasmons are investigated in detail. Our results also indicate that the asymmetrical plasmon is induced by the surface polarization field of the electrodes and the resonant strength of this plasmon is reduced significantly by the decay of the polarization field. Variations in the resonant strength of the plasmonic peaks are consistent with the surface resonant layer model showing that the method we used can be utilized for the study of coupling between THz radiation and plasmons in the channel. In order to achieve wider tunability, more advanced device structures can be explored such as a device that contains double channel layers, in which complicated plasmons can be excited. [ABSTRACT FROM PUBLISHER]

The current study investigates GaN-based light-emitting diodes (LEDs) with InGaN/GaN/InGaN multi-layer barrier (MLB). Simulation results show that GaN-based LEDs with MLB have better performance than conventional GaN-based LEDs with only one GaN barrier because of the enhancement in hole injection into the quantum well and decrease in electron leakage current. [ABSTRACT FROM AUTHOR]

The direct current characteristics of AlGaN/GaN double-channel HEMTs (DC-HEMTs) are investigated by using 2-D numerical simulations. The output characteristics have been predicted with the drift-diffusion, thermodynamic, hydrodynamic, and hot-electron models, respectively. The prediction by the hydrodynamic model is in good agreement with the experiment. It is demonstrated that the hot-electron effect makes a negligible contribution to the negative differential conductance (NDC) of an AlGaN/GaN DC-HEMT; instead, the NDC effect is caused by the self-heating effect. The transfer and transconductance characteristics of an AlGaN/GaN DC-HEMT are also discussed in detail. Finally, a new \In0.18\Al0.82\N /GaN/AlGaN/GaN DC-HEMT structure is proposed for optimizing AlGaN/GaN DC-HEMTs. [ABSTRACT FROM PUBLISHER]

Blue InGaN/GaN multiple quantum well light-emitting diodes (LEDs) with the conventional AlGaN and AlGaN-GaN-AlGaN (AGA) electron blocking layer (EBL) are investigated numerically. The simulation results show that the LEDs with the AGA EBL exhibit much higher output power and smaller efficiency droop at high current as compared to those with the conventional EBL due to the enhancement of the electron confinement and improvement of the hole injection from p-type region, which are induced by the strong electrostatic fields and tunneling effect in the AGA EBL. [ABSTRACT FROM AUTHOR]