Smooth Tuning Research Articles

Predefined-time event-triggered adaptive tracking control for strict-feedback nonlinear systems with full-state constraints

This paper investigates the problem of predefined-time event-triggered adaptive tracking control for strict-feedback nonlinear systems (SFNSs) with full-state constraints. To handle asymmetric full-state constraints, a nonlinear state-dependent function (NSDF) that purely depends on the constraint state is introduced. Then, a switching threshold event-triggered mechanism (ETM) is designed to enhance usage efficiency of communication resources. Moreover, a predefined-time event-triggered adaptive tracking controller is constructed by incorporating a smooth tuning function into each step of the backstepping design based on dynamic surface technology. Under such a control scheme, the output tracking error can be steered to the small neighborhood of the origin within the user setting time. Compared to various previous control schemes developed for full-state constrained systems, our proposed method can circumvent the demanding feasibility conditions in controller designs. Finally, a simulation result is given to illustrate the effectiveness of the control scheme.

Regulating Polaron Transport Regime via Heterojunction Engineering in Cove‐Type Graphene Nanoribbons

AbstractGraphene nanoribbons (GNRs) are emerging materials inheriting excellent properties from graphene while potentially exhibiting semiconducting behavior. All these features sparked numerous efforts to insert GNRs in nanoelectronics. As a result, synthesis routes with atomic resolution are now a reality. Recently, the rise of heterojunction (HJ) engineering pushed even further the prospects, allowing the blending of different GNRs as building blocks. However, much of the potential behind it remains untouched for some junctions. In this work, the consequences of forming a cove‐type GNR (CGNR) HJ by assembling specimens with borders of different zig‐zag/armchair ratios are explored. The nanoribbons are simulated using the extended two‐dimensional Su–Schrieffer–Heeger model with electron–phonon coupling. The findings show that manipulating the junction creates multiple routes for smooth monotonic gap tuning. Moreover, the changes in the hopping mechanism, mobility, and effective mass are reported leading to variations up to 10 000 cm2 V−1 s−1 and 0.425 me. This work reveals a pathway to expand the modularity of CGNRs through smooth control of the charge carrier's properties. Future applications can explore this feature to design devices with highly specific charge transport characteristics. The study also serves as theoretical background, potentially inspiring new tuning strategies in other GNRs.

Experimental and numerical Comparison of traditional spring DVA and shape memory alloy actuated DVA for fixed beam vibration control

A desirable smart material that might be employed in dynamic vibration absorbers (DVA) as a stiffness adjustment element is shape memory alloy (SMA). This paper offers a numerical and experimental platform that examines the vibration of a stationary beam that has been operated by a SMA. In this study, a DVA is created using springs made of shape-memory alloy to eliminate vibration in a fixed beam. The system has a fixed beam secured by a C clamp. Real-time vibration was provided by the shaker with the help of a function generator. Results for the vibration's amplitude were recorded after the conventional spring with mass was mounted at a distance of 305.1 mm from the left end of the fixed beam. The SMA spring with mass was used in place of the conventional spring, and experiments were run at various frequencies. To record the results, the FFT and accelerometer were interfaced using Pulse software. The fixed beam with spring mass system is modelled in ANSYS Workbench R15.0 in accordance with the experimental setup. Over a range of frequencies, the results of a harmonic response analysis were compared. The experimental & analytical results are in good agreement when one SMA spring or two SMA springs connected in parallel or series are used to dampen the system effectively. The outcomes of this work can be utilised as a reference for designing DVA with a Nitinol spring as a stiffness tuning component in circumstances where smooth and continuous tuning of the absorber is required.

Tuning landscapes of the ventral stream

A goal in visual neuroscience is to explain how neurons respond to natural scenes. However, neurons are generally tested using simpler stimuli, often because they can be transformed smoothly, allowing the measurement of tuning functions (i.e., response peaks and slopes). Here, we test the idea that all classic tuning curves can be viewed as slices of a higher-dimensional tuning landscape. We use activation-maximizing stimuli ("prototypes") as landmarks in a generative image space and map tuning functions around these peaks. We find that neurons show smooth bell-shaped tuning consistent with radial basis functions, spanning a vast image transformation range, with systematic differences in landscape geometry from V1 to inferotemporal cortex. By modeling these trends, we infer that neurons in the higher visual cortex have higher intrinsic feature dimensionality. Overall, these results suggest that visual neurons are better viewed as signaling distances to prototypes on an image manifold.

Radiation source based on an optical parametric oscillator with MgO : PPLN crystal and volume Bragg grating, tunable in ranges of 2050 – 2117 and 2140 – 2208 nm

A tunable radiation source based on an optical parametric oscillator (OPO) with a periodically poled lithium niobate (PPLN), generating light with a wavelength of ∼2.1 μm in a near-degenerate regime, is developed. The use of a volume Bragg grating (VBG) as a selective element makes it possible to reduce significantly (by a factor of more than 180) the radiation linewidth. Wavelength tuning from 2050 to 2117 nm for the signal wave and from 2140 to 2208 nm for the idler wave is demonstrated. The maximum average OPO output power is found to be 617 mW (123 μJ), which corresponds to a conversion efficiency of 10 %. The use of a VBG provides a fundamental possibility of designing a pump source with a small linewidth (of no more than 1 nm) and smooth wavelength tuning in a desired range for frequency converters based on non-oxide nonlinear crystals generating mid-IR light (5 – 15 μm).

Generation of Short Microwave Pulses by Compression of Chirped Signals Obtained by Raman Backscattering on Electron Beam With Variable Energy

We suggest method of short microwave pulse generation by means of compression of chirped pulses obtained in the process of Stimulate Raman Backscattering (SRB) on electron beam with variable energy. We assume that in frequency modulator the pump wave, co-propagating with the electrons is scattered into counter propagating signal wave with frequency downshift. The absolute instability, developing in this system, allows for the smooth tuning of the signal frequency by varying the energy of electrons. The SRB modulator can produce a frequency-tunable signal with a bandwidth up to 10% and conversion efficiency up to 70%, when using a 50 kW, 35 GHz gyrotron as a source of pump wave. Simulation of subsequent compression with multi-fold helically corrugated waveguide (HCW) as dispersive line showed that optimization of energy temporal dependence in modulator together with compressor parameters provides the possibility to produce pulses with the peak power up to 3 MW which exceeds the power of driving gyrotron by the factor of 60.

Continuous Resonance Tuning without Blindness by Applying Nonlinear Properties of PIN Diodes

Metamaterial antennas consisting of periodical units are suitable for achieving tunable properties by employing active elements to each unit. However, for compact metamaterials with a very limited number of periodical units, resonance blindness exists. In this paper, we introduce a method to achieve continuous tuning without resonance blindness by exploring hence, taking advantage of nonlinear properties of PIN diodes. First, we obtain the equivalent impedance of the PIN diode through measurements, then fit these nonlinear curves with mathematical expressions. Afterwards, we build the PIN diode model with these mathematical equations, making it compatible with implementing co-simulation between the passive electromagnetic model and the active element of PIN diodes and, particularly, the nonlinear effects can be considered. Next, we design a compact two-unit metamaterial antenna as an example to illustrate the electromagnetic co-simulation. Finally, we implement the experiments with a micro-control unit to validate this method. In addition, the nonlinear stability and the supplying voltage tolerance of nonlinear states for both two kinds of PIN diodes are investigated as well. This method of obtaining smooth tuning with nonlinear properties of PIN diodes can be applied to other active devices, if only PIN diodes are utilized.

Frequency Tuning Characteristics of a High-Power Sub-THz Gyrotron with Quasi-Optical Cavity

Motivated by some emerging high-frequency applications, a high-power frequency-tunable sub-THz quasi-optical gyrotron cavity based on a confocal waveguide is designed in this paper. The frequency tuning characteristics of different approaches, including magnetic field tuning, mirror separation adjustment, and hybrid tuning, have been investigated by particle-in-cell (PIC) simulation. Results predict that it is possible to realize a smooth continuous frequency tuning band with an extraordinarily broad bandwidth of 41.55 GHz, corresponding to a relative bandwidth of 18.7% to the center frequency of 0.22 THz. The frequency tunability is provided by varying the separation distance between two mirrors and correspondingly adjusting the external magnetic field. During the frequency tuning, the output power remains higher than 20 kW, which corresponds to an interaction efficiency of 10%. Providing great advantages in terms of broad bandwidth, smooth tuning, and high power, this research may be conducive to the development of high-power frequency-tunable THz gyrotron oscillators.

Tunable near infrared laser

The article introduces a tunable NIR laser. The tuning process realizes by the dispersion elements placed inside the resonator. Energy parameters depending on the laser operating mode are presented. The possibility of smooth tuning of the fundamental lasing wavelength depending on the rotation parameters of the dispersion element is shown.

Liquid‐Metal‐Induced Memristor Behavior in Polymer Insulators

Resistive switching devices are enabling neuromorphic functionalities, which are exceptionally appealing in electronic gates and memories as active components. Polymer materials have been proven to be promising for resistive memory devices due to low cost, easy processability, mechanical flexibility, and smooth electronic performance tuning through innovative molecular design. However, conventional rigid metals utilized for such devices are not easy to induce resistive memory behavior. In addition, only some specific materials as active layers have been used to date. Herein, it is demonstrated that liquid metals (LMs) can be destined for memristors with various insulators including poly dimethyl siloxane (PDMS), poly (vinylidene fluoride) (PVDF), and poly methyl methacrylate (PMMA). In contrast to other metals that show memristor behaviors only for specific active materials or device architectures, LMs are found to easily form switchable conductive filaments in all the organic materials. All devices show uniform and bipolar resistive memory behavior with a low switching voltage, high ON/OFF ratio, longer endurance, and retention characteristics, possessing capability for next‐generation wearable and soft brain‐like neuromorphic resistive memory devices.

Theoretical and Experimental Investigations of Terahertz-Range Gyrotrons with Frequency and Spectrum Control

We present the results of recent IAP RAS investigations, which were aimed at controlling the radiation frequency and spectrum of THz-range medium-power gyrotrons. Problems of extending the smooth frequency tuning band and providing high-frequency stability in CW regimes are discussed. The development of gyrotrons with stated parameters are of interest for many modern applications, including DNP/NMR and RAD spectroscopy, direct measurements of positronium hyperfine structure, and diagnostic of various media.

Smooth gap tuning strategy for cove-type graphene nanoribbons.

Graphene is a carbon-based material with an extensive range of promising properties. Since it does not present a bandgap, graphene is not suitable for optoelectronic applications. One possible way to open a gap is achieved by reducing graphene to its nanoribbon (GNR) form. Recently, a GNR with well defined cove-type periphery proper for large-scale production was synthesized showing an energy bandgap of 1.88 eV. In this work, we propose an edge termination strategy that allows for smoothly tuning the energy bandgap of cove-type GNRs by systematically changing the periodicity with which armchair-like and zigzag-like edges alternate. Using an extended two-dimensional Su–Schrieffer–Heeger tight-binding model we compare the effects of this edge termination process on lattice deformation with those arising from changes in nanoribbon width. Results show that modifications to the edges of cove-type GNRs are able to smoothly reduce energy bandgaps at the expense of losses in conjugation and increased morphological spreading. Energy band gap values starting from ≈3 eV to almost 0 eV were obtained. The flexibility provided by this gap tuning procedure places the cove-type GNR as an interesting candidate material for optoelectronic applications.

Widely Tunable, 25-mW Power, Ti:sapphire Crystal-Fiber Laser

Using a glass-clad Ti:sapphire crystal fiber (CF) as the gain medium, we demonstrate for the first time a diode-pumped, 25-mW tunable laser with a 180-nm tuning range. Active medium is a 2.4-cm long, a-cut Ti:sapphire CF we have drawn with an elliptical core of 13- $\mu \text{m}$ diameter in the [0001] axis and 15.5- $\mu \text{m}$ diameter in the [ $1\bar {1}00$ ] axis. The CF is cladded with silica and has high efficiency (88%) of coupling to the pump, a diode laser emitting 1-W at 520-nm. A blazed grating provides a continuous smooth tuning on the wavelength range from 700 to 880 nm.

Role of Spectral Line Profile in Laser IR Analysis of Multicomponent Gas Mixtures

Topical issues of the quantitative analysis of multicomponent gas mixtures with use of laser absorption infrared technologies are addressed. It is shown that a number of questions exist regarding the detection of pollutants using these technologies, and these concern not only the position of the center of the analytical line of the detected substance but also the shape of its profile, especially in case of strong intermolecular interactions under conditions of open atmosphere. A number of examples show various aspects of the influence of the error of the spectroscopic model embedded in the solution of the inverse problem (the width and shape of the laser spectral line and gas absorption lines) on the determination of gas concentrations. The parameters of the sensitivity and selectivity of laser gas analysis that support making the optimal choice of laser radiation frequencies for studying specific gas mixtures are discussed. The importance of the high monochromaticity of laser lines, the smooth tuning of the radiation frequency over a wide spectral range, and the adequate simulation of the IR spectra of multicomponent gas mixtures are demonstrated.

Adiabatic and high-fidelity quantum gates with hybrid Rydberg-Rydberg interactions.

Rydberg blockaded gate is a fundamental ingredient for scalable quantum computation with neutral Rydberg atoms. However the fidelity of such a gate is intrinsically limited by a blockade error coming from a Rydberg level shift that forbids its extensive use. Based on a dark-state adiabatic passage, we develop a novel protocol for realizing a two-atom blockade-error-free quantum gate in a hybrid system with simultaneous van der Waals (vdWsI) and resonant dipole-dipole interactions (DDI). The basic idea relies on converting the roles of two interactions, which is, the DDI serves as one time-dependent tunable pulse and the vdWsI acts as a negligible middle level shift, as long as the adiabatic condition is preserved. We adopt an optimized super-Gaussian optical pulse with kπ(k ≫ 1) area accompanied by a smooth tuning for the DDI, composing a circular stimulated Raman adiabatic passage, which can robustly ensure a faster operation time ∼ 80ns as well as a highly-efficient gate fidelity ∼ 0.9996. This theoretical protocol offers a flexible treatment for hybrid interactions in complex Rydberg systems, enabling on-demand design of new types of effective Rydberg quantum gate devices.

Experimental Demonstration of the Possibility to Expand the Band of Smooth Tuning of Frequency Generation in Short-Cavity Gyrotrons

We show experimentally the possibility to expand significantly the band of smooth tuning of the generation frequency in gyrotrons using cavities that have shorter lengths. Due to a decrease in the sensitivity of the electron-wave interaction process to the spread in the electron beam velocities, one can increase the power of generation at higher longitudinal modes up to a level comparable with the radiation power in the case of excitation of a mode with one longitudinal variation. In this case, overlapping of generation bands at the neighboring longitudinal modes is achieved by increasing the current of the electron beam. In the experiment performed in a gyrotron having an operating frequency of about 12 GHz, we demonstrated a frequency tuning band which exceeded 4% at the kilowatt level of the output radiation power in its greater part. The obtained results open up the possibility of developing tunable high-frequency moderate-power gyrotrons.

Injection-Locked Frequency Divider With a Resistively Distributed Resonator for Wide-Locking-Range Performance

Distributed LC resonator and resistively distributed resonator belong to the same technique used to extend the locking range of injection-locked frequency divider (ILFD). ILFD using the former resonator often has one locking range, and extension of locking range is attributed to oscillation frequency increment. This paper measures and analyzes the input sensitivity of a CMOS ILFD with a resistively distributed resonator and with the divide-by-3 and divide-by-2 functions, and the input sensitivity shows a wide single-band locking range. The fabricated 0.18-μm CMOS ILFD is made of a pair of cross-coupled n-type transistors, two direct-injection MOSFETs, and a resistively dualresonance resonator. The wide single-band locking range of the designed ILFD is owing to the overlapped locking ranges, and it is verified by the smooth tuning range without forbidden region and nonoverlapped locking ranges measured on the same chip with the unbalanced injection structure. The frequency tuning is obtained with large tank resistance, which also reduces the frequency tuning hysteresis effect.

Analysis of the Methods of Discrete and Smooth Frequency Tuning in Gyrotrons for Spectroscopy, on the Example of a Generator Operated in the 0.20–0.27 THz Frequency Range

We consider the main features of a low-power frequency-tunable gyrotron with an oversized cavity, which is designed for the purposes of nuclear magnetic resonance spectroscopy and other applications and operates in the 0.20–0.27 frequency range producing an output power of 200 W. We study the possibilities of wideband output frequency tuning by exciting a sequence of modes with similar caustics using magnetic-field variations and smooth tuning due to the excitation of modes with a great number of longitudinal variations. Aiming at widening the frequency tuning range, we also analyzed the possibility of smooth frequency tuning determined by controlled variations of the cavity temperature. Specific features of the electron-optical system of such a gyrotron is discussed, along with the possibility of increasing its efficiency by means of single-stage recovery of the residual energy of the electron beam.

Method for smooth tuning of the pulse energy and width of a passively Q-switched laser

A layout for constructing the cavity of a passively Q-switched laser is considered, which allows smooth tuning of the energy and width of the lasing pulses. The cavity consists of three arms with polarization couplings between them that are controlled by the rotation of a phase plate. By varying the coupling factor, it is possible to change the lasing threshold as well as the energy and width of the lasing pulse.

Frequency Tunable sub-THz Gyrotron for Direct Measurements of Positronium Hyperfine Structure

The feasibility of a high-power sub-THz gyrotron with smooth wideband frequency tuning suitable for direct measurement of the positronium hyperfine structure is demonstrated numerically using both averaged equations and PIC-code simulations. Analytical estimates show that the frequency-tunable powerful sub-THz radiation can be generated through the excitation of high-order axial modes in a gyrotron with short cavity driven by an electron beam with high current. Simulations show that an output power of 0.5–1 kW can be obtained at a frequency of about 0.2 THz within a 10-GHz band which are the parameters needed for testing of quantum electrodynamics predictions through the spectroscopy of positronium.