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High-power terahertz emission from a plasma penetrated by counterstreaming different-size electron beams
摘要： It is found that multi-cycle pulses of high-power coherent terahertz radiation can be efficiently generated in a plasma by counterstreaming long-pulse electron beams driving potential plasma waves via the two-stream instability. Instead of the well-known three-wave interaction processes between oblique beam-driven modes, we propose to generate electromagnetic radiation near the doubled plasma frequency due to the novel and much more efficient mechanism based on the head-on collision of longitudinal plasma waves with mismatching potential profiles. It is shown that this radiation mechanism can be implemented experimentally either by the collision of low-density electron beams with different transverse sizes or by the counter injection of denser equal-size beams unstable against filamentation perturbations. Particle-in-cell simulations for kiloampere electron beams capable of focusing into millimeter-scale sizes demonstrate the possibility to reach the high efficiency of beams-to-THz power conversion (3%–7%), which opens the path to gigawatt-class THz sources with a narrow spectral line.
关键词： electromagnetic radiation,two-stream instability,plasma,electron beams,terahertz emission
更新于2025-09-10 09:29:36
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B7 liquid crystal filament growth in the presence of carbon nanotubes
摘要： Liquid crystal phases formed from bent-core mesogens have attracted much interest of the liquid crystal research community, due to the manifestation of chirality effects from achiral molecules. One of the most elusive of the bent-core phases is the B7 phase, which at its early stage often forms in a helical filament fashion. We investigate the growth of such filaments in the presence of single-walled nanotubes to elucidate possible effects on the growth dynamics and helicity of B7 helical filaments. It is found that the filament width slightly decreases in comparison to the neat B7 material, suggesting a more tightly bound structure around the nanotubes, with the nanotubes likely acting as the core of the helical filament. No effects on pitch or periodicity of the helical superstructure is observed. The filament growth velocity quickly decreases as nanotubes are added to the B7 phase, indicating that a more tightly bound structure needs a longer time of formation. An observed buckling instability is of interest as a microscopic example for the study of nonlinear dynamics theories of filaments. The present investigation is thus of general importance for nanoparticle directed growth of filaments, which has applications in biomolecular growth and high tensile strength fibres.
关键词： bent-core mesogens,nanotubes,buckling instability,B7 phase,liquid crystal,filament growth
更新于2025-09-09 09:28:46
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Pinning Transition of Bose-Einstein Condensates in Optical Ring Resonators
摘要： We experimentally investigate the dynamic instability of Bose-Einstein condensates in an optical ring resonator that is asymmetrically pumped in both directions. We find that, beyond a critical resonator-pump detuning, the system becomes stable regardless of the pump strength. Phase diagrams and quenching curves are presented and described by numerical simulations. We discuss a physical explanation based on a geometric interpretation of the underlying nonlinear equations of motion.
关键词： dynamic instability,pinning transition,optical ring resonators,Bose-Einstein condensates
更新于2025-09-09 09:28:46
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A Feasibility Study of Controllable Gas Foil Bearings With Piezoelectric Materials Via Rotordynamic Model Predictions
摘要： This study presents a new concept of controllable gas foil bearings (C-GFBs) with piezoelectric actuators. The C-GFB consists of a laminated top foil, bump foil, and piezo stacks and can simply change the bearing shape or film thickness locally and globally by varying the thickness of the piezo stacks with input voltages. The control schemes are (1) clearance control: the bearing clearance adjusted by changing overall piezo stack thickness, and (2) preload control: the mechanical preload modulated by the thickness expansion of several piezo stacks. Bearing lubrication performance is predicted for four cases of C-GFBs with different bearing clearances and preloads. The piezo stack control generates meaningful differences in the fluid-film thickness and pressure. Clearance control has a great effect on the dynamic force coefficients, but preload control slightly increases. Furthermore, the rotordynamic prediction of a rotor supported on two journal C-GFBs is conducted. As a result, both control modes for C-GFB are found to have a positive effect on rotordynamic amplitudes. Finally, using the orbit simulations, the C-GFB is controlled to have a small bearing clearance and large preload at critical speeds to make it possible to stably pass through the critical speeds. Consequently, it turns out that the C-GFB can improve bearing lubrication and rotordynamic performances by controlling only the input voltage of the piezo stacks. In addition, the C-GFB can be used to form various shapes to meet the operation conditions of an applied system.
关键词： active gas bearing,rotordynamic instability,piezoelectric actuator,vibration control,gas foil bearing
更新于2025-09-09 09:28:46
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Coherent surface plasmon amplification through the dissipative instability of 2D direct current
摘要： We propose an original concept for on-chip excitation and amplification of surface plasmon polaritons. Our approach, named nanoresotron, utilizes the collective effect of dissipative instability of a 2D direct current flowing in vicinity of a metal surface. The instability arises through the excitation of self-consistent plasma oscillations and results in the creation of a pair of collective surface electromagnetic modes in addition to conventional plasmon resonances. We derive the dispersion equations for these modes using self-consistent solutions of Maxwell’s and 2D hydrodynamics equations. We find that the phase velocities of these new collective modes are close to the drift velocity of 2D electrons. We demonstrate that the slow mode is amplified while the fast mode exhibits absorption. Estimates indicate that very high gain are attainable, which makes the nanoresotron a promising scheme to electrically excite and regenerate surface plasmon polaritons.
关键词： dissipative instability of DC current,electrical excitation of nanoantenna,self-consistent 2D plasma oscillation
更新于2025-09-09 09:28:46
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Current-Driven Dyakonov-Shur Instability in Ballistic Nanostructures with a Stub
摘要： We develop a compact model for the THz plasmonic unstable structures with tunable narrow-channel regions of an increased width (called plasmonic “stubs”) using the transmission line analogy and derive the dispersion equations describing unstable plasmons. The solutions of the dispersion equations in the plasmonic systems with the electron drift, which are derived using the hydrodynamic model and generalized to account for the stubs, illustrate the device physics and can be used for design and characterization of THz plasmonic-electronic sources. Our results show that adding stubs allows one to control the Dyakonov-Shur instability in plasmonic ?eld-e?ect transistors by optimizing the boundary conditions, controlling the plasma velocity, and making it possible to drive periodic plasmonic structures wirelessly, thus avoiding the contact problems.
关键词： THz plasmonic,hydrodynamic model,plasmonic stubs,Dyakonov-Shur instability,ballistic nanostructures
更新于2025-09-09 09:28:46
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Approximate and explicit expression of optical forces and pull-in instability of a silicon nano-optomechanical device
摘要： Nano-optomechanical systems actuated by optical forces enable many interesting scientific and technological applications. They are vulnerable to the effects of surface stress and Casimir forces. Therefore, calculation of optical forces is essential for the reliability applications of these advanced devices. In this paper, an approximate and explicit expression is developed for the evaluation of the optical force existing between a waveguide and a substrate through the effective refractive index. The influences of surface stress and Casimir forces on the pull-in instability of a silicon nano-optomechanical device actuated by optical forces are investigated. It is found that if neglecting the effect of surface stress, the maximum size, which indicates the device can be safely fabricated, will be over-predicted. The surface stress reduces the critical optical power and its effect is more significant for a slender waveguide.
关键词： surface stress,pull-in instability,Casimir forces,optical force,Nano-optomechanical device
更新于2025-09-09 09:28:46
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Effect of crosslinks, entanglements, and chain extensibilities on dynamic electromechanical instability of dielectric elastomers
摘要： Dynamic electromechanical instability (EMI) can affect the actuation performance of dielectric elastomers (DEs). This letter develops a theoretical framework to investigate the dynamic EMI of DEs with diverse crosslinks, entanglements, and chain extensibilities under a step voltage and equal biaxial pre-stresses. Based on the work-energy principle combining the EMI condition, the dynamic instability parameters at transition state between the stable and instable states are determined. At the small pre-stress, weak crosslinks or strong entanglements contribute to achieving a high actuation stretch ratio. When the pre-stress further increases, the DE with strong entanglements is prone to the dynamic EMI, and the variation in the dynamic instability actuation stretch ratio with the level of crosslinks is non-monotonic, which forms a peak. Moreover, as the level of chain extensibilities reduces, the dynamic instability actuation stretch ratio increases, and the enhancement effect can be significant at the large pre-stress. These results can help optimize the dynamic DE to obtain a stable oscillation without the dynamic EMI or utilize dynamic EMI to achieve a dramatic deformation.
关键词： Entanglements,Chain extensibilities,Dielectric elastomers,Crosslinks,Dynamic electromechanical instability
更新于2025-09-04 15:30:14
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Pulse duration effect during pulsed gate-bias stress in a-InGaZnO thin film transistors
摘要： We investigated how the zero-voltage duration (0Vd) affects the tendency of degradation during pulsed gate bias stress in a-InGaZnO thin film transistors (TFTs). DC or pulsed negative bias illumination stress (NBIS) or positive bias stress (PBS) was applied to the TFTs for effective stress time of 4,000 s. While pulsed bias stress was being applied, stress-voltage duration (SVd) was set as either 10 s or 1 s per cycle, and 0Vd was varied from 100% to 1% of the SVd. During NBIS, degradation in both threshold voltage and sub-threshold slope became increasingly severe as 0Vd was shortened. However, during pulsed PBS, these trends were almost absent. These different tendencies may occur because the cause of each stress-induced degradation is fundamentally dissimilar; NBIS involves ionization of oxygen vacancies, whereas PBS involves electron trapping. The proposed mechanism was supported by additional bias stress tests on TFTs that had been immersed in H2O, where hydrogen became dominant factor causing the degradation.
关键词： Hydrogen,IGZO,Dynamic stress,Instability,Oxygen vacancy,AC stress
更新于2025-09-04 15:30:14
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Impact of active geomagnetic conditions on stimulated radiation during ionospheric second electron gyroharmonic heating
摘要： Recently, narrowband emissions ordered near the H+ (proton) gyrofrequency (fcH) were reported in the stimulated electromagnetic emission (SEE) spectrum during active geomagnetic conditions. This work presents new observations and theoretical analysis of these recently discovered emissions. These emission lines are observed in the stimulated electromagnetic emission (SEE) spectrum when the transmitter is tuned near the second electron gyroharmonic frequency (2fce) during recent ionospheric modification experiments at the High Frequency Active Auroral Research (HAARP) facility near Gakona, Alaska. The spectral lines are typically shifted below and above the pump wave frequency by harmonics of a frequency roughly 10% less than fcH (≈ 800 Hz) with a narrow emission bandwidth less than the O+ gyrofrequency (≈ 50 Hz). However, new observations and analysis of emission lines ordered by a frequency approximately 10% greater than fcH are presented here for the first time as well. The interaction altitude for the heating for all the observations is in the range of 160 km up to 200 km. As described previously, proton precipitation due to active geomagnetic conditions is considered as the reason for the presence of H+ ions known to be a minor background constituent in this altitude region. DMSP satellite observations over HAARP during the heating experiments and ground-based magnetometer and riometer data validate active geomagnetic conditions. The theory of parametric decay instability in multi-ion component plasma including H+ ions as a minority species described in previous work is expanded in light of simultaneously observed preexisting SEE features to interpret the newly reported observations. Impact of active geomagnetic conditions on the SEE spectrum as a diagnostic tool for proton precipitation event characterization is discussed.
关键词： stimulated electromagnetic emission,parametric decay instability,proton precipitation,HAARP,ionospheric heating
更新于2025-09-04 15:30:14