One of the most important issues in the design of photodetectors is absorption efficiency. An innovative structure of a graphene-based photodetector consisting of an array of plasmonic nanostructures is proposed. Surface plasmons resonance occurs near the plasmonic nanostructures, enhancing the interaction between light and graphene; this gives rise to a strong light absorption. The numerical analysis results show that the proposed photodetector has a total absorption efficiency of 87.7% and a photocurrent of 1000  μA. Also, this structure exhibits the responsivity of 2053  mA  /  W and the bandwidth of 25.17 GHz at the wavelength of 1.55  μm. The other outstanding features of this design are low power consumption and being CMOS compatible. The results reveal the superiority of the proposed photodetector performance in terms of responsivity and optical bandwidth in comparison with the other existing designs.

Al-doped porous ZnO hollow micro-spheres were prepared using a sol-gel method with a template of agglomerates of polystyrene (PS) nanospheres, and the composite of these ZnO hollow micro-spheres and Ag nanoparticles was prepared with further photo-deposition. The effects of Al doped on the structure and the optical properties of the specimen were investigated by means of x-ray diffractometer, SEM, TEM, an UV-Vis diffuse reflectance analyzer, and photoluminescence (PL) spectroscopy. Different doping concentrations of Al affect not only the existing form of Al atoms in the crystal but also the concentration of the intrinsic defects of ZnO, thus influencing the morphology of the hollow micro-spheres. The appropriate amount of Al doping provides the hollow spheres with a porous, loose morphology, showing the largest specific surface area, which significantly improves the absorbance of light and the number of reaction sites in photocatalysis. For the composite specimen, the introduction of Ag nanoparticles improves the absorbance of light in the visible region because of the surface plasmon resonance: in the PL measurements, because many electrons in the ZnO crystal flow to the surface of Ag nanoparticles due to the difference of energy levels between Ag and ZnO crystals, the intensity of the intrinsic and defect-related emissions of ZnO is reduced. The photocatalytic properties of the specimens were evaluated with the degradation of a methyl orange dye solution under simulated solar irradiation at room temperature. The specimen of porous ZnO hollow micro-spheres with 3% Al dopant concentration demonstrates excellent photocatalytic performance due to its large specific surface area. However, with the designed energy structure, the composite of porous ZnO hollow micro-spheres and Ag nanoparticles exhibits the best photocatalytic performance among the specimens tested by separating electrons and holes in the ZnO crystal.

Based on the effective mass approximation and envelope function theory, the exciton binding energy in a single InAs  /  In_xAl_{1  −  x}As quantum well is calculated by the variational method under the applied electric field and laser field. The effects of material composition, well width, laser parameters, and electric field intensity on the exciton binding energy are calculated. The results show that the confinement potential is distorted by the laser field and the barrier is tilted by the electric field. The exciton binding energy is reduced by the presence of laser field or electric field. It is also shown that the exciton binding energy increases with the decrease of the indium component within a certain range. In addition, the effect of the well width on the exciton binding energy is not linear, and the exciton binding energy increases to a maximum first and then decreases with the decrease of the well width. The binding energy of excitons depends on the interaction of several parameters.

We have theoretically modeled the “giant” colloidal core–shell quantum dot with strain adapting alloyed interfacial layer between core and shell materials for optoelectronic applications. The intersubband optical properties, such as absorption coefficients (ACs), refractive index changes (RICs), oscillator strength, and transition lifetime of giant CdSe  /  CdSe_xS_{1  −  x}  /  CdS core–interfacial–shell quantum dot (g-CISQD) have been investigated for different alloying “x” of the interfacial layer with varying core radius. The results have been plotted considering the significant impact of strain between the heterojunctions and compared for different structural parameters. The results show that the oscillator strength and transition lifetime strongly depend on alloying x and core radius. The energy eigenvalue in g-CISQD decreases with increasing x and saturates beyond the value x  =  0.5. The resultant redshift of the resonant peak in ACs and RICs is concurrence with the obtained energy eigenvalues. Further, the ACs and RICs have been investigated for different intensities of incident light   (  I  )   and core radius variation. The results provide the tuning possibility of the optical properties using appropriate alloying x and core radius for application in optoelectronic devices.

Quantum dots are becoming more recognized as a tool to use in varying photonic and sensing technologies. When quantum dots are illuminated, they can emit light; by altering the quantum dot characteristics, the wavelength of the emitted light can be finely tuned. The emitted light of quantum dots may gradually increase in intensity when continuously illuminated, a behavior of quantum dots called photobrightening. We focus on this brightening behavior and explore different factors that contribute to quantum dot photobrightening. Increased excitation of the quantum dots results in increased rate of photobrightening, shown in this work by exciting quantum dot samples with different laser intensities. By adding plasmonic nanostructures, the excitation light to the quantum dots is enhanced, which also increases the rate of photobrightening. By combining gold nanoparticles with cadmium selenide quantum dots, we determined that gold nanoparticles enhance QD photobrightening by a significant factor, potentially leading to more efficient quantum dot technologies.

We propose the design of nanotapered bus-waveguide to enhance the waveguide-to-cavity coupling. By enlarging the mode-field diameters of the high-index integrated waveguide with a nanotaper, the coupling strength can be doubled comparing to the conventional coupling of a uniform bus-waveguide. In addition, by utilizing the discretization of transfer matrix, the coupling between tapered bus-waveguide shows a good agreement with the finite-difference time-domain method. This nanotaper design provides the flexibility for coupling optimization, especially the ideality in the overcoupled regime.

We report a full-wave analysis of a graphene-based plasmonic nanoantenna with frequency and pattern reconfigurable characteristics. The proposed antenna consists of a circular graphene patch and four graphene fan-shaped parasitic elements. Ten different operational states of radiation patterns can be obtained by controlling the conductivity of parasitic elements. The main beam is rotated from −45  deg to +45  deg with a step increment of 90 deg in the azimuth plane. The resonance frequency for different operational states slightly varies from 2.43 to 2.47 THz with almost stable radiation performance. Simulated gain has been achieved up to 4.68 dBi with a total efficiency of 62%. The proposed antenna supports the surface plasmon waves; hence it can be successfully miniaturized at the THz frequencies. The simulated results for return loss, gain, and radiation patterns have been reported for 10 different operational states.

Cesium-doped tungsten bronze Cs_xWO₃ (CWO) has potential applications in the field of energy-saving products due to its high-near-infrared (NIR) shielding ability. We for the first time demonstrate sensitive detection of small polaron transitions in the NIR for CWO nanoarchitectures on the basis of magnetic circular dichroism (MCD) spectroscopy. Two different CWO nanostructures are prepared by organic colloidal and solvothermal methods. Cube-like CWO nanoparticles obtained by the colloidal method exhibit a broad absorption peak at 1500 nm and a weak shoulder at 950 nm, which can be attributed to the localized surface plasmon resonance (LSPR) and small polaron transition, respectively. On the other hand, CWO nanoparticles synthesized by the solvothermal protocol show only a single LSPR peak. Simultaneous deconvolution of both absorption and MCD spectra allows us to discuss the detailed nature of the electronic excitations, and consequently, MCD distinctly unveils the small-polaron transition unresolved in the absorption. Hence, the MCD spectroscopic technique can be a significant optical-modulation method allowing for design of magneto-optically active materials in nanophotonics.

We present the investigation of energy wavefunctions, dispersion profile, dipole momentum behavior, and optical gain for a nanoscale heterostructure formed with material layers of InAlAs/InGaAs/GaAsSb/InGaAs/InAlAs on a GaAs substrate. The designed structure is simulated with a 6 band k p formalism for the investigation. At room temperature, the 1548-nm wavelength has an optical gain of 2352/cm, making it suitable for NIR applications. The response of the designed heterostructure is also investigated with external temperature, strain, charge density, and electric field variations to report the tunability of the optical gain and wavelength. Due to its tunability in the NIR wavelength region, this structure can be used for different NIR applications without changing the material system.

A broadband and scalable mode multiplexer (MUX) using an adiabatic coupler (AC) assisted with subwavelength grating (SWG) is proposed and analyzed in detail. The adiabatic coupler consists of a strip bus waveguide and a tapered waveguide whose width decreases along the propagation direction so that the fundamental TE mode cannot be supported well and will be coupled into the high-order mode in the bus waveguide via mode evolution due to the phase matching. On the contrary, the fundamental TE mode injected into the bus waveguide will keep propagating in the same waveguide with negligible coupling. The SWG is incorporated into the gap between the bus and tapered waveguides for effectively enhancing the mode coupling, leading to a reduction in length of the device. By carefully choosing the structural parameters, the proposed TE₀  /  TE₁ mode MUX shows a low insertion loss (IL) of 0.07 dB (0.01 dB) and crosstalk (CT) of −24.01  dB (−21.25  dB) for the coupled TE₁ mode (the through TE₀ mode) at wavelength 1550 nm while the coupling length is compact as ∼8.8  μm. The device achieves a broad bandwidth of 165 nm for CT  <    −  19  dB and IL  <  0.11  dB over a wavelength range of 1460 to 1625 nm, covering the whole S, C, and L bands. Moreover, the device also shows a good fabrication tolerance to the deviations of the waveguide width, the total height of the device and the grating duty cycle. In addition, the TE₂ and TE₃ multiplexing are also presented, showing the scalability of the proposed scheme.

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5  μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25  μm and high transmission efficiencies of the first three TE modes (TE₀  −  TE₂) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes  TE₀, TE₁, and TE₂ are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58  dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5  μm, which will be helpful for large scale integration when a great number of bends are cascaded.

In this research, the electronic and chemical properties of a series of   (  C₅N_)n nanoribbons have been investigated. The effect of nanoribbon length on the polarizability and hyperpolarizability of the molecules has been considered. It has been demonstrated that odd or even number of constructive units impresses on the polarizability of this type of nanoribbons. Clearly,   (  C₅N_)2n nanoribbons show better NLO properties. To survey the chemical properties of considered nanoribbons, the frontier molecular orbitals have been studied. Aromaticity of involving rings in considered nanoribbons has been investigated, too.

We design and numerically investigate a high-quality (Q)-factor, high modulation depth, and multiple Fano resonances in an all-dielectric metamaterial consisting of an asymmetric nanocylinder dimer. It is demonstrated that, due to excitation of subradiant modes, eight sharp Fano resonances are achieved by removing parts of the nanocylinder from the edge, and the corresponding Q-factors and modulation depth are flexibly tuned by geometric parameters. Further, we demonstrate that the maximal figure of merit exceeds 2352 with a RI sensitivity of 400 nm/refractive index unit (RIU), and even the minimal figure of merit exceeds 277 with a RI sensitivity of 120 nm/RIU. The metamaterial shows the potential applications in designing multiple switching, filters, and sensors.