Dual-controlled tunable dual-band and ultra-broadband coherent perfect absorber in the THz range.

This paper proposes a vanadium dioxide metamaterial-based tunable, polarization-independent coherent perfect absorber (CPA) in the terahertz frequency range. The designed CPA demonstrates intelligent reconfigurable switch modulation from an ultra-broadband absorber mode to a dual-band absorber mode via the thermally controlled of VO2. The mode of ultra-broadband absorber is realized when the conductivity of VO2 reaches 11850 S/m via controlling its temperature around T = 328 K. In this mode, the CPA demonstrates more than 90% absorption efficiency within the ultra-wide frequency band that extends from 0.1 THz to 10.8 THz. As the conductivity of VO2 reaches 2×105 S/m (T = 340 K), the CPA switches to a dual-band absorber mode where a relatively high absorption efficiency of 98% and 99.7% is detected at frequencies of 4.5 THz and 9.8 THz, respectively. Additionally, using phase modulation of the incident light, the proposed CPA can regulate the absorption efficiency, which can be intelligently controlled from perfect absorption to high pass-through transmission. Owing to the ability of the proposed CPA to intelligently control the performance of light, this study can contribute towards enhancing the performance of stealth devices, all-optical switches and coherent photodetectors.

Low-voltage and fast-response SnO2nanotubes/perovskite heterostructure photodetector.

One-dimensional metal-oxides (1D-MO) nanostructure has been regarded as one of the most promising candidates for high-performance photodetectors due to their outstanding electronic properties, low-cost and environmental stability. However, the current bottlenecks are high energy consumption and relatively low sensitivity. Here, Schottky junctions between nanotubes (NTs) and FTO were fabricated by electrospinning SnO2NTs on FTO glass substrate, and the bias voltage of SnO2NTs photodetectors was as low as ∼1.76 V, which can effectively reduce energy consumption. Additionally, for improving the response and recovery speed of SnO2NTs photodetectors, the NTs were covered with organic/inorganic hybrid perovskite. SnO2NTs/perovskite heterostructure photodetectors exhibit fast response/recovery speed (∼0.075/0.04 s), and a wide optical response range (∼220-800 nm). At the same time, the bias voltage of heterostructure photodetectors was further reduced to 0.42 V. The outstanding performance is mainly attributed to the formation of type-II heterojunctions between SnO2NTs and perovskite, which can facilitate the separation of photogenerated carriers, as well as Schottky junction between SnO2NTs and FTO, which reduce the bias voltage. All the results indicate that the rational design of 1D-MO/perovskite heterostructure is a facile and efficient way to achieve high-performance photodetectors.

Ultrabroadband metamaterial absorbers from ultraviolet to near-infrared based on multiple resonances for harvesting solar energy.

In this paper, a metal-dielectric metamaterial absorber is proposed to achieve ultrabroadband absorption at frequencies from ultraviolet to near-infrared. Based on finite element method solutions, the average absorption of the absorber is 97.75% from 382 nm to 1100 nm, with a maximum of 99.92%, resulting from multiple resonance coupling. The influences of geometric parameters and incident conditions on absorption are investigated. Broadband and narrowband absorption changes are realized by changing incident light polarization. Polarization-independent properties can be realized by changing the dielectric structure to centrosymmetric. The average absorption of the polarization-independent structure is 97.11% from 250 nm to 1115 nm, with a maximum of 99.98%. The proposed absorber structure has wide optical applications including solar energy harvesting and light-emitting devices.

Tunable polarization-independent and angle-insensitive broadband terahertz absorber with graphene metamaterials.

In this paper, we design a polarization-independent and angle-insensitive broadband THz graphene metamaterial absorber based on the surface plasmon-polaritons resonance. Full-wave simulation is conducted, and the results show that the designed metamaterial absorber has an absorption above 99% in the frequency range from 1.23 THz to 1.68 THz, which refers to a very high standard. Furthermore, the absorber has the properties of tunability, and the absorption can be nearly adjusted from 1% to 99% by varying the Fermi energy level of the graphene from 0 eV to 0.7 eV. In the simulation, when the incident angles of TE and TM waves change from 0° to 60°, the average absorption keeps greater than 80%. The proposed absorber shows promising performance, which has potential applications in developing graphene-based terahertz energy harvesting and thermal emission.

Tunable dual plasmon-induced transparency based on a monolayer graphene metamaterial and its terahertz sensing performance.

In this paper, tunable dual plasmon-induced transparency (PIT) is achieved by using a monolayer graphene metamaterial in the terahertz region, which consists of two graphene strips of different sizes and a graphene ring. As the dual PIT effect is induced by the destructive interference between the two quasi-dark modes and the bright mode, we propose a four-level plasmonic system based on the linearly coupled Lorentzian oscillators to explain the mechanism behind the dual PIT. It is proved that the theoretical results agree well with the simulation results. Most importantly, the sensing properties of the designed device have been investigated in detail and we found that it can exhibit high sensitivities and figure of merit (FOM). Furthermore, the dual PIT windows can be effectively modulated by changing the Fermi energy of the graphene layer and the angle of incidence. Thus, the proposed graphene-based metamaterial can hold wide applications for switches, modulators, and multi-band refractive index sensors in the terahertz region.

Graphene-based dual-band independently tunable infrared absorber.

In this paper, we theoretically demonstrate a dual-band independently tunable absorber consisting of a stacked graphene nanodisk and graphene layer with nanohole structure, and a metal reflector spaced by insulator layers. This structure exhibits a dipole resonance mode in graphene nanodisks and a quadrupole resonance mode in the graphene layer with nanoholes, which results in the enhancement of absorption over a wide range of incident angles for both TE and TM polarizations. The peak absorption wavelength is analyzed in detail for different geometrical parameters and the Fermi energy levels of graphene. The results show that both peaks of the absorber can be tuned dynamically and simultaneously by varying the Fermi energy level of graphene nanodisks and graphene layer with nanoholes structure. In addition, one can also independently tune each resonant frequency by only changing the Fermi energy level of one graphene layer. Such a device could be used as a chemical sensor, detector or multi-band absorber.

Ag/alginate nanofiber membrane for flexible electronic skin.

Flexible electronic skin has stimulated significant interest due to its widespread applications in the fields of human-machine interactivity, smart robots and health monitoring. As typical elements of electrical skin, the fabrication process of most pressure sensors combined nanomaterials and PDMS films are redundant, expensive and complicated, and their unknown biological toxicity could not be widely used in electronic skin. Hence, we report a novel, cost-effective and antibacterial approach to immobilizing silver nanoparticles into-electrospun Na-alginate nanofibers. Due to the unique role of carboxyl and hydroxyl groups in Na-alginate, the silver nanopaticles with 30 nm size in diameter were uniformly distributed inside and outside the alginate nanofibers, which obtained pressure sensor shows stable response, including an ultralow detection limited (1 pa) and high durability (>1000 cycles). Notably, the pressure sensor fabricated by these Ag/alginate nanofibers could not only follow human respiration but also accurately distinguish words like 'Nano' and 'Perfect' spoke by a tester. Interestingly, the pixelated sensor arrays based on these Ag/alginate nanofibers could monitor distribution of objects and reflect their weight by measuring the different current values. Moreover, these Ag/alginate nanofibers exhibit great antibacterial activity, implying the great potential application in artificial electronic skin.

Wideband slow light with ultralow dispersion in a W1 photonic crystal waveguide.

A dispersion tailoring scheme for obtaining slow light in a silicon-on-insulator W1-type photonic crystal waveguide, novel to our knowledge, is proposed in this paper. It is shown that, by simply shifting the first two rows of air holes adjacent to the waveguide to specific directions, slow light with large group-index, wideband, and low group-velocity dispersion can be realized. Defining a criterion of restricting the group-index variation within a ±0.8% range as a flattened region, we obtain the ultraflat slow light with bandwidths over 5.0, 4.0, 2.5, and 1.0 nm when keeping the group index at 38.0, 48.8, 65.2, and 100.4, respectively. Numerical simulations are performed utilizing the three-dimensional (3D) plane-wave expansion method and the 3D finite-difference time-domain method.

[New method for wave-plate orientation determination based on the shape of the modulated spectrum].

A new method for the determination of fast axis of wave-plate is introduced. Based on the principle of electro-optic phase compensation in the wave-plate phase measuring method, the relation between the polarization extinction system out-put signal and the modulating signal can be acquired through the composition of frequency fixed alternating voltage modulation signal. Through the analysis of this process with Jones matrix, the out-put signal was usually found to be the composition of fundamental frequency and second harmonic frequency, the composition signal reflects the axis information as well as the phase information of the wave-plate, and this phenomenon has never been noticed before. Through the analysis of this phenomenon, the transformation trends of the composed wave-form by rotating of wave-plates with different orientations were deduced, and through the observation of the transformation trend, one can easily distinguish the fast and slow axes of wave-plates with phase retardation > pi as well as those with phase retardation < pi, and their phenomena are reverse to each other.

Precise analysis of combination homogeneous-inhomogeneous-material superresolution filters with double-tunable modes.

By using a Jones matrix, the precise expression for the pupil function of a two-adjustable-mode superresolving filter, a combination of a radial birefringent plate and a glass annular plate, is obtained. This filter can provide superresolution in both radial and axial adjustment operations, which can supplement each other in setting accuracy and superresolution range in practical use. As an adjustable filter, it is less dependent on wavelength. With the relative radius of the inner plate set to be epsilon=0.52 and the rotating angle set to be 45 degrees , this type of filter can achieve better superresolution performance than the continuous-phase filters reported in Opt. Lett.28, 607 (2003).

Mutual alignment errors due to wave-front aberrations in intersatellite laser communications.

We analyze mutual alignment errors due to wave-front aberrations. To solve the central obscured problem, we introduce modified Zernike polynomials, which are a set of complete orthogonal polynomials. It is found that different aberrations have different effects on mutual alignment errors. Some aberrations influence only the line of sight, while some aberrations influence both the line of sight and the intensity distributions.

Three-dimensional superresolution by three-zone complex pupil filters.

Complex pupil filters are introduced to improve the three-dimensional resolving power of an optical imaging system. Through the design of the essential parameters of such filters, the transmittance and radius of the first zone, three-dimensional superresolution is realized. The Strehl ratio and the transverse and axial gains of such filters are analyzed in detail. A series of simulation examples of such filters are also presented that prove that three-dimensional superresolution can be realized. The advantage of such filters is that it is easy to realize three-dimensional superresolution, and the disadvantage is that the sidelobes of the axial intensity distribution are too high. But this can be overcome by the application of a confocal system.

Transverse or axial superresolution with radial birefringent filter.

The superresolution technique is well known for its ability to compress the central diffraction spot to a size that is smaller than the Airy diffraction spot. The radial birefringent filter, which consists of two parallel polarizers and a rotationally symmetric birefringent element, is introduced into the superresolution technology, and the pupil function of it is deduced. It is shown that such a filter can be adapted either for transverse superresolution or for axial superresolution simply by changing the angle between either of the two polarizers and the radial birefringent element. At the same time the superresolution parameters are discussed. The filter is relatively simple in construction as it requires no phase changes, and low-cost replication is possible.