Narrow Results

Quantum well infrared photodetectors (QWIPs) have been widely investigated for the 3–5 μm mid-wavelength infrared (MWIR) and 8–12 μm long-wavelength infrared (LWIR) atmospheric spectral windows as well as very long wavelength infrared (VLWIR: λ_c > 14 μm) imaging array applications in the past decade. The mature III-V compound semiconductor growth technology and the design flexibility of device structures have led to the rapid development of various QWIP structures for infrared focal plane arrays (FPAs) applications. In addition to the single-color QWIP with narrow bandwidth, multi-color or broadband QWIPs required for advanced IR sensing and imaging applications have also emerged in recent years. Using band gap engineering approach, the multi-color (2, 3, and 4-color) QWIPs with multi-stack quantum wells and voltage-tunable asymmetrical coupled quantum well structures for detection in the MWIR, LWIR, and VLWIR bands have been demonstrated recently. The triple-coupled (TC-) QWIP employs the quantum confined Stark effect to tune the peak detection wavelength by the applied bias voltage, A typical single-color QWIP exhibits a rather narrow spectral bandwidth of 1 to 2 μm. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. Using the stacked quantum wells with different well width and depth, the digital-graded superlattice barrier (DGSLB) or the linear-graded barrier (LGB) structures, broadband (BB-) QWIPs covering the 8–14 μm atmospheric spectral window have been reported recently.

In this chapter, the basic operation principles of a QWIP, and the design, fabrication, and characterization of multi-color and broadband QWIPs based on the GaAs/AlGaAs and InGaAs/AlGaAs material systems for the MW/LW/VLWIR applications are depicted.

Serial IOs are widely used to expand the system bandwidth in communication systems. This paper provides an overview of serial IO design trade-offs with regard to power, cost, and performance. Circuit techniques are discussed to achieve low jitter and high bandwidth.

In this paper, a two-dimensional acoustic ground cloak with alternating layered structure composed of mercury and water is designed on the basis of transformation acoustics and effective medium theory. The cloak exhibits excellent cloaking performance to hide an object from the detection of acoustic waves. Cosine similarity is proposed to precisely quantize and evaluate the cloaking performance, which turns out to be succinct and effective. Numerical simulations confirm that the cloak could work well in a broad frequency band in which the cloaking performance displays an oscillatory decrease with increasing frequency. In addition, the omnidirectional property, larger incident angle of the acoustic beam has the better cloaking performance, is analyzed. This multilayered structure of cloak may offer an access to fabrication simplicity and experimental demonstration. The concept of cosine similarity may be an enrichment of the assessment system for acoustic cloaks.

Low-frequency and broadband are the critical challenges in real-life applications. Here, we try to tackle the challenges by proposing a reconfigurable acoustic metasurface (AM) composed of the membrane-type metamaterial (MAM) structure of deep sub-wavelength scale. By employing the external air pumping system into each individual unit cell of the AM, the tension of the membrane can be readily tailored by the system with little interference from other unit cells. Two strategies of the constant pressure method (CPM) and constant volume method (CVM) are reported to design the MAM. And the CVM is adopted as the ultimate design strategy by comparing both methods from aspects of the dimension, operating frequency, and structure complexity. In order to validate the low-frequency and broadband performances of the AM, the Airy-like beams and the acoustic converging based on two identical Airy-like beams are introduced and proof-of-concept simulations are performed with the finite element method. The simulated results agree well with the theoretical predictions. Our design provides the little-interference active design method for the low-frequency and broadband AM to manipulate the wave front, and may have practical engineering applications in areas of the aerospace, high-speed train, marine vessel, and power transmission and transformation project.

Narrow bandwidth and specific incident angle are the main drawbacks in real-life applications for the existed carpet cloaking based on the acoustic metasurface (AM). Here, we tackle to get over the problems by proposing a reprogrammable AM. The unit cell is composed of water sink and filling nozzle. By incorporating an external water pumping system into each individual unit cell, the reflected phase can be readily regulated. Since the pumping process is reversible, the AM is reprogrammable under the control of the water pumping system in the frequency range of 3430–6860Hz. Both the acoustic cloaking and disguising are designed based on the proposed AM. The double security for the target object can be ensured to avoid being detected by combining the two designs. Simulated results with the finite element method indicate that the acoustic cloaking and disguising can work in the broad bandwidth of 66.7% of the central frequency with full-range incident angles from $-90^{\circ }$ to $90^{\circ }$. Our design shows promise for applications in realizing the practical skin cloaking and disguising one step closer.

In this research, a new design of spiral nanoantenna for solar energy harvesting was proposed and analyzed by using three-dimensional finite difference time-domain method. The structure of the proposed nanoantenna consisted of two Archimedean spiral arms integrated on a supporting substrate layer. The analysis included the radiation efficiencies, polarization, near-field characteristics and far-field patterns. The new nanoantenna considers the polarized characteristics of sunlight and has high total radiation efficiency of 74.49% within the broad wavelength range from 400 to 1600 nm, which are both superior to those of previously proposed linear antennas. Moreover, the same order of electric enhancement factor at the spiral gap was also obtained for different linearly polarized incident waves. Furthermore, the far-field characteristics of the proposed nanoantenna were studied for wide-angle reception.

Since the growth of single layer of Si has emerged, silicene became a potential candidate material to make up the disadvantage of graphene. In this paper, the complex surface conductivity is applied to characterize the properties of silicene and we investigate the optical characterization of silicene-dielectric interfaces from IR to far UV range. The silicene-Si and silicene-Ge interfaces along both parallel and perpendicular polarization directions of electromagnetic field with normal incidence are considered in this work. The optical properties of the silicene-dielectric systems proposed in this paper lay a foundation for the performance of complex silicene-based optoelectronic devices such as sensors, detectors, filters, UV absorbers and so on.

A new method, namely the nonlinear conjugate-gradient (NCG) method, is proposed to design nonlinear domains with a disordered distribution, in which an efficient broadband second harmonic generation can be achieved simultaneously with high conversion efficiency. It is demonstrated by numerical simulation that the NCG method has obvious advantages in realizing the optimal quasi-phase-matching, in comparison with the traditional simulated annealing method.

A broadband pulsed mid-infrared difference frequency generation (DFG) laser source based on MgO-doped congruent LiNbO₃ bulk is experimentally demonstrated, which employs a homemade pulsed ytterbium-doped ring fiber laser and a continuous wave erbium-doped ring fiber laser to act as seed sources. The experimental results indicate that the perfect phase match crystal temperature is about 74.5${}^{\circ }$C. The maximum spectrum bandwidth of idler is about 60 nm with suitable polarization states of fundamental lights. The central wavelength of idlers varies from 3293 nm to 3333 nm over the crystal temperature ranges of 70.4–76${}^{\circ }$C. A jump of central wavelength exists around crystal temperature of 72${}^{\circ }$C with variation of about 30 nm. The conversion efficiency of DFG can be tuned with the crystal temperature and polarization states of fundamental lights.

A broadband MoS₂-based absorber composed of Ag rod/MoS₂/dielectric/Ag is proposed in the visible band. The relative bandwidth is 65% for the absorption above 80%. The absorber also has the properties of polarization-independence and wide-angle absorption. Impedance matching theory is used to analyze the physical mechanism of the broadband absorption. By investigating the absorption property of each part of the absorber, it is found that the absorption is enhanced by introducing the two-dimensional material MoS₂. The broadband absorber can be changed to be multiband absorber by changing the thickness of dielectric substrate. This structure provides a new perspective to enhance absorption in the visible band and has promising applications in solar cells.

A broadband absorber composed of silicon rods and nickel ground is proposed in the visible band. The absorption above 98% can be obtained in the frequency range of $478\sim 1044$ THz with strong polarization independence and angle independence. The impedance matching theory and field distributions of eigenmodes are used to analyze the physical mechanism of the broadband absorption. The absorber has a simple structure with only two layers, which is composed of silicon and nickel. Nickel is a non-precious metal, which is cheaper than the precious metal materials commonly used in metamaterial absorber. The proposed cost-effective absorber with simple structure has great potential in the application of solar cells.

As an artificial periodic material, phononic crystals (PnCs) are suitable for energy harvesting from vibration and noise environments of equipments. A two-dimensional (2D) PnC with a point defect is presented to design a piezoelectric energy-harvesting (PEH) device. Using finite element (FE) simulations, bandwidth and electric power of the initial supercell using the PnC are obtained but do not attain the optimal performance. Therefore, five geometric parameters are considered to perform the numerical experiment using a five-factor and seven-level experimental design. The two relationships between the five geometric parameters and PnCs bandwidth, the five geometric parameters, and output power are finally obtained. The two functional relationships are regarded as a unified objective function to further optimize five geometric parameters. The optimized PnC-based PEH device will achieve better output power within broadband. It is expected to be used to design PEH devices to achieve better output power in wider bandwidth.

In this paper, an innovative approach to achieve broadband linear power amplifier (PA) with a continuous broadband is presented. The proposed method mainly depends on the theory of control of intermodulation products and harmonics. This method results in a continuous sweet spots over a wide bandwidth. Based on the continuous sweet spots, a suboptimal solution is derived for improving the efficiency and linearity of broadband PAs. To verify the effectiveness of this method, one broadband PA operating over 4.5–5.5GHz is developed and measured, it indicates that the peak output power of the PA is 35–57dBm for a small signal input with a gain of 12dB, and the peak drain efficiency (DE) of the PA is larger than 57% over the whole working band. However, when the PA is stimulated by a 5MHz two-tone signal input, we observe that the DE keeps above 38% under the condition of the measured third-order intermodulation distortion (IMD3) is not larger than $-30$dBc.

In this paper, the design of a wideband monolithic microwave integrated circuit (MMIC) low-noise amplifier (LNA) fabricated in 0.13-$\mu$m GaAs pHEMT process is presented. A simple T-type input matching network (IMN) and a source feedback structure are employed to achieve low noise figure (NF). The MMIC LNA, which operates across 12–18GHz, can be used for satellite applications. Experimental results show an NF around 1.5dB in 12–17.5GHz and a minimum NF of 1.21dB at 16.5GHz. In addition, a flat small-signal gain of $22\pm 0.5$dB is achieved at 13.5–17.5GHz. The input return loss is lower than $-10$ dB at 12–14.5GHz and the output return loss is lower than $-10$ dB at 12–17GHz. The power consumed is lower than 0.3W and the $P_{1\mathrm{\text{dB}}}$ (1-dB compression point) output power is around 13dBm.

It has been widely validated that continuous working modes are powerful theories for designing broadband power amplifier (BPA). Theoretically, the conduction angle of all continuous-mode power amplifiers (PAs) is 180^∘ (class-B-biased). However, in practice, these PAs are always biased in class-AB condition. Thus, continuous-mode PAs biased in class-AB condition should be researched. This paper generalizes the theory of hybrid continuous mode (HCM) for implementing broadband power amplifiers. The intrinsic drain current waveform of HCM biased above the pinch-off point (conduction angle is larger than 180^∘) is first derived. Then, the impedance space of the generalized HCM (class-AB-biased) is explored and analyzed. The conclusion is that the generalized HCM possesses a shifted fundamental impedance space along with the enlargement of conduction angle. For validating the proposed theory, a broadband PA working over 1.6–3.0GHz is implemented. Experimental results indicate that the designed BPA achieves a saturation power of 40.3–42.7dBm and a drain efficiency of 64.3–74.4%.

In this work, a broadband high gain frequency selective surface (FSS)-based microstrip patch antenna is proposed. The dimensions of the microstrip antenna and proposed FSS are $22\mathrm{\text{mm}}\times 22\mathrm{\text{mm}}\times 1.6\mathrm{\text{mm}}$ and $60.8\mathrm{\text{mm}}\times 48.6\mathrm{\text{mm}}\times 1.6\mathrm{\text{mm}}$. A broadband high gain reference antenna has been selected to improve antenna performance. The reference antenna offers 1.2GHz bandwidth with 6.03dBi peak gain. Some modifications have been done on the patch and ground plane to enhance the bandwidth and gain. The impedance bandwidth of 7.70GHz (3.42–11.12GHz) with 4.9 dBi peak gain is achieved by the microstrip antenna without FSS. The antenna performance is improved by using FSS beneath the antenna structure. The maximum impedance bandwidth of 7.70GHz (3.32–11.02GHz) and peak gain of 8.6dBi are achieved by the proposed antenna with FSS. Maximum co- and cross-polarization differences are 21dB. The simulation and measurement have been done using Ansoft Designer software and vector network analyzer. The measured results are in good parity with the simulated one.

In this paper, a highly efficient tree-shaped fractal antenna with multi-broadband resonance characteristics is proposed. The proposed antenna exhibits broad operating bandwidth, high gain, and high efficiency characteristics due to the suggested modifications in the antenna geometry. The suggested circular patch is modified by introducing slots in the form of a tree-shaped fractal structure, and the partial ground plane is modified by incorporating a narrow rectangular slot. The proposed antenna is designed on an FR4 substrate of $40\times 25.05\times 1.6$mm³. The prototype of the proposed antenna has been fabricated and tested to justify the simulation results. The measurement results are in good agreement with the simulation that validates the multi-broadband design approach of the proposed fractal antenna. As per the measurement results, the proposed antenna operates at four distinct bands with $-$10dB impedance bandwidths of 600MHz (2.2–2.8) GHz, 1070MHz (3.3–4.37) GHz, 2550MHz (4.75–7.3) GHz, and 2200MHz (9.7–11.9) GHz. Furthermore, a high peak gain of 10.23dB and a peak radiation efficiency of 96.65% are recorded for the suggested fractal antenna. The suggested compact bandwidth enhanced multi-band antenna can be useful for several wireless communication systems such as wireless fidelity (Wi-Fi), wireless local area network (WLAN), Bluetooth, long-term evolution (LTE), C, S and X bands.

Broadband and narrowband time-domain asymptotics are proposed for pulse propagation in range-independent ocean environments. The broadband approximation results by applying the stationary-phase method to the Fourier transform of the Green's function, expressed in terms of normal modes. The narrowband approximation is obtained by incorporating the shape function of the emitted signal — assumed Gaussian — into the phase term and applying the steepest-descent method. The roots of the frequency-derivative of the phase are located in the complex plane by using a second-order expansion of the eigenvalues. The performance of the two approximations is studied numerically. While the broadband approximation improves with increasing bandwidth, the narrowband approximation improves when the bandwidth decreases. Both approximations improve with increasing range, and they can be used for delivering time-domain results more efficiently than with standard Fourier synthesis.

The prediction of the spatial mean-square pressure distribution within enclosed high-frequency broadband sound fields is computationally intensive if determined on a frequency-by-frequency basis. Recently an energy-intensity boundary element method (EIBEM) has been formally developed. This method employs uncorrelated broadband directional energy sources to expeditiously predict such pressure distributions. The source directivity accounts for local correlation effects and specular reflection. The method is applicable to high modal density fields, but not restricted to the usual low-absorption, diffuse, and quasi-uniform assumptions. The approach can accommodate fully specular reflection, or any combination of diffuse and specular reflection. This boundary element method differs from the classical version in that element size is large compared to an acoustic wavelength and equations are not solved on a frequency-by-frequency basis. In the earlier EIBEM, the source strength and directivity associated with the energy sources, distributed over enclosure boundaries, were determined in an iterative manner and the directivity was limited to three terms of a Fourier expansion. Here, the original method is improved by eliminating the iteration and allowing for an unlimited number of terms in the Fourier expansion of the directivity function. For verification, the improved EIBEM is compared to experimental measurements and exact analytical solutions; excellent agreement is obtained.

In many long-range ocean propagation experiments the source and receiver are placed close to the depth of the waveguide axis. In this case, rays emerging from the source at sufficiently small angles intersect the sound-channel axis many times and form in its vicinity a large number of caustics with caustics cusps located repeatedly along the axis. In neighborhoods of cusped caustics there exists a very complicated interference pattern. Neighborhoods of interference grow with range and at long ranges they overlap. As a result, a complex interference wave (axial wave), that propagates along the waveguide axis, appears. The goal of this paper is to obtain the representation for the axial wave in the time domain and calculate its magnitude for a realistic model of a three-dimensional range-independent medium. Numerical computations are done for the average profile from the Acoustic Engineering Test (AET) experiment. The pulse center frequency of 75 Hz with 30-Hz bandwidth is used that corresponds to broadband acoustic signals which were transmitted during November 1994 in the eastern North Pacific Ocean as a part of the AET. The propagation range is 3250 km. The sound source is located on the waveguide axis, and the receiver is placed close to the depth of the axis. Through numerical simulation the dependencies of the magnitude of the axial wave on depth of the receiver and propagation range are studied.