Search Results (129)

We propose and experimentally demonstrate an approach for generating a wideband optical frequency comb (OFC) featuring multiple comb lines and wavelength tunability based on a gain-switched weak-resonant-cavity Fabry–Perot laser diode (WRC-FPLD) under multi-wavelength optical injection. The longitudinal mode interval of the utilized WRC-FPLD is about 0.28 nm (35.0 GHz), and its relaxation oscillation frequency is about 2.0 GHz at 1.15 times the threshold current. Under current modulation with a power of 20.00 dBm and a frequency of 2.0 GHz, the WRC-FPLD is driven into the gain-switched state. By further introducing multi-wavelength injection light (MWIL) containing four power equalization comb lines with an interval of 0.56 nm, a wideband OFC featuring multiple comb lines and wavelength tenability can be obtained. The experimental results demonstrate that by gradually increasing the injection’s optical power, the number of produced OFC lines initially increases and then decreases. By meticulously adjusting the wavelengths of the MWIL and carefully selecting the matched injection power, the broadband OFC can be tuned across an extensive spectral range. Under optimized operation parameters, an OFC with 147 lines, and a bandwidth of approximately 292 GHz within a 10 dB amplitude, variation is achieved. In this case, the measured single-sideband phase noise at the fundamental frequency is about −115 dBc/Hz @ 10 kHz, indicating that the comb lines possess good stability and strong coherence. Full article

Relaxation oscillators are preferred in low-frequency applications due to their lower power consumption and superior temperature stability. However, frequency errors arise from variations in the comparator’s offset voltage and delay time due to PVT changes. To address these issues, this paper proposes the low-power delay time cancellation (DTC) technique and several enhancement methods, including a novel offset trimming approach, an error state detection and recovery (ESDAR) circuit, and a specialized frequency-trimming method. Simulation results for an 8 MHz relaxation oscillator in a 40 nm CMOS process show that the proposed DTC technique and enhancements improve frequency variation due to power supply fluctuations to ±0.05% and reduce temperature-induced frequency variation to ±0.4%. Full article

We study the nonlinear absorptive and dispersive optical properties of molecular systems immersed in a thermal reservoir interacting with a four-wave mixing (FWM) signal. Residual spin-orbit Hamiltonians are considered in order to take into account the internal structure of the molecule. As system parameters in the dissipation processes, transverse and longitudinal relaxation times are considered for stochastic solute–solvent interaction processes. The intramolecular coupling effects on the optical responses are studied using a molecule model consisting of two coupled harmonic curves of electronic energies with displaced minima in nuclear energies and positions. In this study, the complete frequency space is considered through the pump–probe detuning, without restricting the derivations to only maximums of population oscillations. This approach opens the possibility of studying the behavior of optical responses, which is very useful in experimental design. Our results indicate the sensitivity of the optical responses to parameters of the molecular structure as well as to those derived from the photonic process of FWM signal generation. Full article

A high resolution, acceptable accuracy and low power consumption time-domain temperature sensor is proposed and simulated in this paper based on a 180 nm standard CMOS technology. A diode stacking structure is introduced to enhance the accuracy of the temperature sensing core. To improve the resolution of the sensor, a dual-input capacitor multiplexing voltage-to-time converter (VTC) is implemented. Additionally, a low-temperature drift voltage-mode relaxation oscillator (ROSC) is proposed, effectively reducing the large oscillation frequency drift caused by significant temperature impacts on delay errors. The simulated results show that the resolution is as high as 0.0071 °C over 0∼120 °C with +0.43 °C/−0.38 °C inaccuracy and 1.9 pJ · K² resolution FoM, consuming only 1.48 $\mathsf{\mu }$W at a 1.2 V supply voltage. Full article

In this paper, we present a CMOS rail-to-rail second-generation voltage conveyor (VCII) suitable for low power applications, implemented in 180 nm CMOS technology with a supply voltage of ± 0.9 V. The proposed VCII consists of a current and voltage buffer operating in class AB. At the input of the voltage buffer, there is a bulk-driven differential amplifier, which provides a rail-to-rail input common-mode voltage. A common source output stage in class AB provides rail-to-rail at the output of the voltage buffer. The transistors are designed to operate in moderate inversion, achieving a relatively large current and voltage buffer bandwidth of 298.3 MHz and 173.2 MHz, respectively, with a power consumption of 157 μW. A sine wave with an amplitude of 1.5 V_pp and a frequency of 1 MHz on the output buffer has a total harmonic distortion of only 0.29%. The application of VCII in a relaxation oscillator with a frequency of up to 10 MHz is demonstrated, as well as its comparative characteristics with reference to other relevant square-wave generators published in the literature. Full article

The narrowband light from a scannable, single-mode dye laser influences the electrical properties of gas discharges. The variation in these properties as the laser wavelength λ is scanned yields the optogalvanic spectrum of the discharge (i.e., electrical conductivity vs. frequency). By connecting a neon lamp, capacitor, and power supply in parallel, an undriven relaxation oscillator is formed whose natural frequency f₀ is affected by neon-resonant laser light and this λ-dependence of the relaxation oscillator frequency f₀ yields a variant optogalvanic spectrum (i.e., f₀ vs. frequency). In this paper, a driving force is effectively applied to an otherwise undriven oscillator when the incident light is chopped periodically at f_d. For f_d ≈ f₀ and a sufficiently large driving force amplitude (laser intensity and the degree of neon resonance), the relaxation oscillator can be entrained so that f₀ is locked on f_d and is independent of λ. For the new chopped-light technique described here, f_d is adjusted to be the subthreshold of the entrainment range, where the λ-dependence of f₀ is advantageously exaggerated by periodic pulling, and the beat frequency |f_d − f₀| vs. λ provides an optogalvanic spectrum with appealingly amplified signal-to-noise qualities. Beat frequency neon spectra are reported for the cases f_d < f₀ and f_d > f₀ and are compared with spectra obtained using the unchopped-light (i.e., undriven) method. Full article

An oscillating flow past a structure represents a complex, high-dimensional, and nonlinear flow phenomenon, which can lead to the failure of structures due to material fatigue or constraint relaxation. In order to better understand flow-induced vibration (FIV) and coupled flow fields, a numerical simulation of a two-degrees-of-freedom FIV in a cylinder was conducted. Based on the Lagrangian-based dynamic mode decomposition (L-DMD) method, the vorticity field and motion characteristics of a cylinder were decomposed, reconstructed, and predicted. A comparison was made to the traditional Eulerian-based dynamic mode decomposition (E-DMD) method. The research results show that the first-order mode in the stable phase represents the mean flow field, showcasing the slander tail vortex structure during the vortex-shedding period and the average displacement in the in-line direction. The second mode predominantly captures the crossflow displacement, with a frequency of approximately 0.43 Hz, closely matching the corresponding frequency observed in the CFD results. The higher dominant modes mainly capture outward-spreading, smaller-scale vortex structures with detail displacement characteristics. The motion of the cylinder in the in-line direction was accompanied by symmetric vortex structures, while the motion of the cylinder in the crossflow direction was associated with anti-symmetric vortex structures. Additionally, crossflow displacement will cause a symmetrical vortex structure that spreads laterally along the axis behind the cylinder. Finally, when compared with E-DMD, the L-DMD method demonstrates a notable advantage in analyzing the nonlinear characteristics of FIV. Full article

A theoretical investigation of electronic properties of self-assembled InAs/InP quantum dots (QDs) is presented, utilizing a novel two-step modeling approach derived from a double-capping procedure following QD growth processes, a method pioneered in this study. The electronic band structure of the QD is calculated by the newly established accurate two-step method, i.e., the improved strain-dependent, eight-band k p method. The impact of various QD structural parameters (e.g., height, diameter, material composition, sublayer, and inter-layer spacer) on electronic states’ distribution and transition energies is investigated. Analysis of carrier dynamics within QDs includes intraband and interband transitions. The calculation of the carrier transitions between two atomic states, providing insights into optical gain or loss within QDs, is in terms of dipole matrix element, momentum matrix element, and oscillation strength, etc. In addition, the time-domain, traveling-wave method (i.e., rate equations coupled with traveling-wave equations) is used to investigate the optical properties of QD-based lasers. Several optical properties of the QD-based lasers are investigated, such as polarization, gain bandwidth, two-state lasing, etc. Based on the aforementioned method, our key findings include the optimization of carrier non-radiative intraband relaxation through sublayer manipulation, wavelength control through emission blue-shifting and gain bandwidth via variation of sublayer, polarization control of QDs photoluminescence via excited states’ transitions, and the enhancement of two-state lasing in InAs/InP QD lasers by thin inter-layer spacers. This review offers comprehensive insights into QDs electronic band structures and carrier dynamics, providing valuable guidance for optimizing QD-based lasers and their potential designs. Full article

Reduced-order models (ROMs) have achieved a lot of success in reducing the computational cost of traditional numerical methods across many disciplines. In fluid dynamics, ROMs have been successful in providing efficient and relatively accurate solutions for the numerical simulation of laminar flows. For convection-dominated (e.g., turbulent) flows, however, standard ROMs generally yield inaccurate results, usually affected by spurious oscillations. Thus, ROMs are usually equipped with numerical stabilization or closure models in order to account for the effect of the discarded modes. The literature on ROM closures and stabilizations is large and growing fast. In this paper, instead of reviewing all the ROM closures and stabilizations, we took a more modest step and focused on one particular type of ROM closure and stabilization that is inspired by large eddy simulation (LES), a classical strategy in computational fluid dynamics (CFD). These ROMs, which we call LES-ROMs, are extremely easy to implement, very efficient, and accurate. Indeed, LES-ROMs are modular and generally require minimal modifications to standard (“legacy”) ROM formulations. Furthermore, the computational overhead of these modifications is minimal. Finally, carefully tuned LES-ROMs can accurately capture the average physical quantities of interest in challenging convection-dominated flows in science and engineering applications. LES-ROMs are constructed by leveraging spatial filtering, which is the same principle used to build classical LES models. This ensures a modeling consistency between LES-ROMs and the approaches that generated the data used to train them. It also “bridges” two distinct research fields (LES and ROMs) that have been disconnected until now. This paper is a review of LES-ROMs, with a particular focus on the LES concepts and models that enable the construction of LES-inspired ROMs and the bridging of LES and reduced-order modeling. This paper starts with a description of a versatile LES strategy called evolve–filter–relax (EFR) that has been successfully used as a full-order method for both incompressible and compressible convection-dominated flows. We present evidence of this success. We then show how the EFR strategy, and spatial filtering in general, can be leveraged to construct LES-ROMs (e.g., EFR-ROM). Several applications of LES-ROMs to the numerical simulation of incompressible and compressible convection-dominated flows are presented. Finally, we draw conclusions and outline several research directions and open questions in LES-ROM development. While we do not claim this review to be comprehensive, we certainly hope it serves as a brief and friendly introduction to this exciting research area, which we believe has a lot of potential in the practical numerical simulation of convection-dominated flows in science, engineering, and medicine. Full article

An efficient numerical approach based on weighted-average finite differences is used to solve the Newtonian plane Couette flow with wall slip, obeying a dynamic slip law that generalizes the Navier slip law with the inclusion of a relaxation term. Slip is exhibited only along the fixed lower plate, and the motion is triggered by the motion of the upper plate. Three different cases are considered for the motion of the moving plate, i.e., constant speed, oscillating speed, and a single-period sinusoidal speed. The velocity and the volumetric flow rate are calculated in all cases and comparisons are made with the results of other methods and available results in the literature. The numerical outcomes confirm the damping with time and the lagging effects arising from the Navier and dynamic wall slip conditions and demonstrate the hysteretic behavior of the slip velocity in following the harmonic boundary motion. Full article

Currently, new functional materials are being created with a strong emphasis on their ecological aspect. Materials and devices based on DNA biopolymers, being environmentally friendly, are therefore very interesting from the point of view of applications. In this paper, we present the results of research on complexes in the powder form based on plasmid DNA (pDNA) and three surfactants with aliphatic chains containing 16 carbon atoms (cetyltrimethylammonium chloride, benzyldimethylhexadecylammonium chloride and hexadecylpyridinium chloride). The X-ray diffraction results indicate a local hexagonal packing of DNA helices in plasmid DNA complexes, resembling the packing for corresponding complexes based on linear DNA. Based on the Fourier-transform infrared spectroscopy results, the DNA conformation in all three complexes was determined as predominantly of A-type. The two relaxation processes revealed by dielectric spectroscopy for all the studied complexes are connected with two different contributions to total conductivity (crystallite part and grain boundaries). The crystallite part (grain interior) was interpreted as an oscillation of the polar surfactant head groups and is dependent on the conformation of the surfactant chain. The influence of the DNA type on the properties of the complexes is discussed, taking into account our previous results for complexes based on linear DNA. We showed that the type of DNA has an impact on the properties of the complexes, which has not been demonstrated so far. It was also found that the layer of pDNA–surfactant complexes can be used as a layer with variable specific electric conductivity by selecting the frequency, which is interesting from an application point of view. Full article

In response to challenges such as voltage limit violations, excessive currents, and power imbalances caused by the integration of distributed photovoltaic (distributed PV) systems into the distribution network, this study proposes at two-level optimization configuration method. This method effectively balances the grid capacity and reduces the active power losses, thereby decreasing the operating costs. The upper-level optimization enhances the distribution network’s capacity by determining the siting and sizing of distributed PV devices. The lower-level aims to reduce the active power losses, improve the voltage stability margins, and minimize the voltage deviations. The upper-level planning results, which include the siting and sizing of the distributed PV, are used as initial conditions for the lower level. Subsequently, the lower level feeds back its optimization results to further refine the configuration. The model is solved using an improved second-order oscillating chaotic map particle swarm optimization algorithm (SCMPSO) combined with a second-order relaxation method. The simulation experiments on an improved IEEE 33-bus test system show that the SCMPSO algorithm can effectively reduce the voltage deviations, decrease the voltage fluctuations, lower the active power losses in the distribution network, and significantly enhance the power quality. Full article

Relaxing the temporal constancy constraint on coupling constants in an expanding universe results in Friedmann equations containing terms that may be interpreted as dark energy and dark matter. When tired light (TL) was considered to complement the redshift due to the expanding universe, the resulting covarying coupling constants (CCC+TL) model not only fit the Type Ia supernovae data as precisely as the ΛCDM model, but also resolved concerns about the angular size of cosmic dawn galaxies observed by the James Webb Space Telescope. The model was recently shown to be compliant with the baryon acoustic oscillation features in the galaxy distribution and the cosmic microwave background (CMB). This paper demonstrates that dark energy and dark matter of the standard ΛCDM model are not arbitrary but can be derived from the CCC approach based on Dirac’s 1937 hypothesis. The energy densities associated with dark matter and dark energy turn out to be about the same in the ΛCDM and the CCC+TL models. However, the critical density in the new model can only account for the baryonic matter in the universe, raising concerns about how to account for observations requiring dark matter. We therefore analyze some key parameters of structure formation and show how they are affected in the absence of dark matter in the CCC+TL scenario. It requires reconsidering alternatives to dark matter to explain observations on gravitationally bound structures. Incidentally, since the CCC models inherently have no dark energy, it has no coincidence problem. The model’s consistency with the CMB power spectrum, BBN element abundances, and other critical observations is yet to be established. Full article

The distinct spin, optical, and coherence characteristics of solid-state spin defects in semiconductors have positioned them as potential qubits for quantum technologies. Both bulk and two-dimensional materials, with varying structural properties, can serve as crystalline hosts for color centers. In this study, we conduct a comparative analysis of the spin–optical, electron–nuclear, and relaxation properties of nitrogen-bound vacancy defects using electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) techniques. We examine key parameters of the spin Hamiltonian for the nitrogen vacancy (${NV}^{-}$) center in 4H-SiC: D = 1.3 GHz, A_zz = 1.1 MHz, and C_Q = 2.53 MHz, as well as for the boron vacancy ($V_B^{-}$) in hBN: D = 3.6 GHz, A_zz = 85 MHz, and C_Q = 2.11 MHz, and their dependence on the material matrix. The spin–spin relaxation times T₂ (${NV}^{-}$ center: 50 µs and $V_B^{-}$: 15 µs) are influenced by the local nuclear environment and spin diffusion while Rabi oscillation damping times depend on crystal size and the spatial distribution of microwave excitation. The ENDOR absorption width varies significantly among color centers due to differences in crystal structures. These findings underscore the importance of selecting an appropriate material platform for developing quantum registers based on high-spin color centers in quantum information systems. Full article

Hyperfine structure constants have many applications, but are often hard to calculate accurately due to large and canceling contributions from different terms of the hyperfine interaction operator, and also from different closed and spherically symmetric core subshells that break up due to electron correlation effects. In multiconfiguration calculations, the wave functions are expanded in terms of configuration state functions (CSFs) built from sets of one-electron orbitals. The orbital sets are typically enlarged within the layer-by-layer approach. The calculations are energy-driven, and orbitals in each new layer of correlation orbitals are spatially localized in regions where the weighted total energy decreases the most, overlapping and breaking up different closed core subshells in an irregular pattern. As a result, hyperfine structure constants, computed as expectation values of the hyperfine operators, often show irregular or oscillating convergence patterns. Large orbital sets, and associated large CSF expansions, are needed to obtain converged values of the hyperfine structure constants. We analyze the situation for the states of the $\{2s^{2}2p^3,2s^{2}2p^{2}3p,2s^{2}2p^{2}4p\}$ odd and $\{2s^{2}2p^{2}3s,2s2p^4,2s^{2}2p^{2}4s,2s^{2}2p^{2}3d\}$ even configurations in N I, and show that the convergence with respect to the increasing sets of orbitals is radically improved by introducing separately optimized orbital sets targeted for describing the spin- and orbital-polarization effects of the $1s$ and $2s$ core subshells that are merged with, and orthogonalized against, the ordinary energy-optimized orbitals. In the layer-by-layer approach, the spectroscopic orbitals are kept frozen from the initial calculation and are not allowed to relax in response to the introduced layers of correlation orbitals. To compensate for this lack of variational freedom, the orbitals are transformed to natural orbitals prior to the final calculation based on single and double substitutions from an increased multireference set. The use of natural orbitals has an important impact on the states of the $2s^{2}2p^{2}3s$ configuration, bringing the corresponding hyperfine interaction constants in closer agreement with experiment. Relying on recent progress in methodology, the multiconfiguration calculations are based on configuration state function generators, cutting down the time for spin-angular integration by factors of up to 50, compared to ordinary calculations. Full article