arXiv subjects

Lixin Ge

Publications and source records attributed to Lixin Ge.

15 recordsLinked to original sources

arXiv2024

Integrated simulation of cavity design and radiation transport codes (ACE3P + Geant4)

A simulation workflow has been developed to study dark current (DC) radiation effects using ACE3P and Geant4. The integrated workflow interfaces particle data transfer and geometry between the electromagnetic (EM) cavity simulation code ACE3P and the radiation code Geant4, targeting large-scale problems using high-performance computing. The process begins by calculating the operating mode in the vacuum region of an accelerator structure and tracking field-emitted electrons influenced by the EM fields of the mode calculated by ACE3P. It then transfers particle data at the vacuum-wall interface for subsequent radiation calculations within the wall enclosure materials through Geant4 calculation. The whole integrated simulation workflow will be demonstrated through large-scale dark current radiation calculations for the KEK 56-cell traveling-wave structure, and the efficiency of performing these simulations on the NERSC supercomputer Perlmutter will be presented.

physics.comp-ph↗

arXiv2024

Fabry-P\'{e}rot nanocavities controlled by Casimir forces in electrolyte solutions

We propose a design for tuning the resonant spectra of Fabry-P\'{e}rot nanocavities mediated by the Casimir force. The system involves a suspended gold nanoplate approaching to a dielectric-coated gold substrate in a univalent electrolyte solution. The gold nanoplate can be stably suspended due to the delicate balance between repulsive and attractive components of the Casimir forces. In an electrolyte solution, the presence of ionic-charge fluctuations can partially or totally screen the thermal $n$=0 Matsubara term, resulting in strongly modified interactions. As a result, the separation between the gold nanoplate and the substrate experiences a significant modulation in response to variations in salt concentration. Under proper conditions, we find that the modulation of the Casimir force would strongly shift the resonances of Fabry-P\'{e}rot nanocavities at the optical frequencies, when the Debye length of the electrolyte decreases from 1000 nm to 10 nm. Finally, the temperature dependence of the thermal Casimir force would provide an additional modulation of Fabry-P\'{e}rot nanocavity resonances for their eventual fine tuning. These results open up a promising venue for general tuning of the optical resonances with potential applications in re-configurable microfluidic nanophotonics.

quant-ph↗

arXiv2023

Electrical and thermal control of Fabry-P\'{e}rot cavities mediated by Casimir forces

Dynamic tuning of optical cavities is highly desired in many photonic systems. Here, we show that Fabry-P\'{e}rot(FP) cavities can be actively controlled by the Casimir force. The optical FP cavities consist of a gold nanoplate confronted to an electrical-connecting multi-layer substrate in a liquid environment. The gold nanoplate can be stably suspended due to the balance of repulsive and attractive Casimir forces. Moreover, the suspension distance are modulated strongly by the electric gating or temperature of the system. As a result, we could shift the resonant wavelengthes of the cavities with tens of nanometers at optical frequencies. Finally, we analyze the influence of Brownian motion on the equilibrium distances. Due to the high Q-factor of the FP cavities, our proposed system offers a remarkable platform to experimentally investigate the thermal Casimir effect at sub-micrometer separations

quant-ph↗

arXiv2023

An Integrated Simulation Tool for Dark Current Radiation Effects Using ACE3P and Geant4

A simulation workflow is under development to interface particle data transfer and matching of geometry between the electromagnetic (EM) cavity simulation code ACE3P and radiation code Geant4. The target is to simulate dark current (DC) radiation effects for the KEK 56-cell S-band accelerating structure using ACE3P and Geant4, and benchmark against KEK experiment data. As a first step, ACE3P DC simulations using a 7-cell structure have been performed by first calculating the operating mode in the structure and then tracking field-emitted electrons under the influence of the EM fields of the mode. The ACE3P simulation results agree well with the EM software CST for an accelerating gradient of 21.8 MV/m. The reader/writer I/O in ACE3P and the transfer of particle data from Track3P to Geant4 for DC radiation effects studies have been implemented. The simulation workflow between the two codes will be demonstrated with the goal of performing large-scale simulations for the KEK 56-cell structure. In addition to modeling DC effects in linacs, the integrated simulation workflow will be applicable to studying positron source and capture structure for future lepton colliders.

hep-ex↗

arXiv2023

Fluctuation-induced dispersion forces on thin DNA films

In this work, the calculation of Casimir forces across thin DNA films is carried out based on the Lifshitz theory. The variations of Casimir forces due to the DNA thicknesses, volume fractions of containing water, covering media and substrates are investigated. For a DNA film suspended in the air or water, the Casimir force is attractive, and its magnitude increases with decreasing the thickness of DNA films and the water volume fraction. For DNA films deposited on a dielectric(silica) substrate, the Casimir force is attractive for the air environment. However, the Casimir force shows unusual features in a water environment. Under specific conditions, switching signs of the Casimir force from attractive to repulsive can be achieved by increasing the DNA-film thickness. Finally, the Casimir force for DNA films deposited on a metallic substrate are investigated. The Casimir force is dominant by the repulsive interactions at a small DNA-film thickness for both the air and water environment. In a water environment, the Casimir force turns out to be attractive at a large DNA-film thickness, and a stable Casimir equilibrium can be found. In addition to the adhesion stability, our finding could be applicable to the problems of condensation and de-condensation of DNA, due to the fluctuation-induced dispersion forces.

physics.bio-ph↗

arXiv2020

Tunable Casimir equilibria with phase change materials: from quantum trapping to its release

A stable suspension of nanoscale particles due to the Casimir force is of great interest for many applications such as sensing, non-contract nano-machines. However, the suspension properties are difficult to change once the devices are fabricated. Vanadium dioxide (VO$_2$) is a phase change material, which undergoes a transition from a low-temperature insulating phase to a high-temperature metallic phase around a temperature of 340 K. In this work, we study Casimir forces between a nanoplate (gold or Teflon) and a layered structure containing a VO$_2$ film. It is found that stable Casimir suspensions of nanoplates can be realized in a liquid environment, and the equilibrium distances are determined, not only by the layer thicknesses but also by the matter phases of VO$_2$. Under proper designs, a switch from quantum trapping of the gold nanoplate ("on" state) to its release ("off" state) as a result of the metal-to-insulator transition of VO$_2$, is revealed. On the other hand, the quantum trapping and release of a Teflon nanoplate is found under the insulator-to-metal transition of VO$_2 $. Our findings offer the possibility of designing switchable devices for applications in micro-and nano-electromechanical systems.

physics.app-ph↗

arXiv2018

Magnetically tunable multi-band near-field radiative heat transfer between two graphene sheets

Near-field radiative heat transfer (NFRHT) is strongly related with many applications such as near-field imaging, thermos-photovoltaics and thermal circuit devices. The active control of NFRHT is of great interest since it provides a degree of tunability by external means. In this work, a magnetically tunable multi-band NFRHT is revealed in a system of two suspended graphene sheets at room temperature. It is found that the single-band spectra for B=0 split into multi-band spectra under an external magnetic field. Dual-band spectra can be realized for a modest magnetic field (e.g., B=4 T). One band is determined by intra-band transitions in the classical regime, which undergoes a blue shift as the chemical potential increases. Meanwhile, the other band is contributed by inter-Landau-level transitions in the quantum regime, which is robust against the change of chemical potentials. For a strong magnetic field (e.g., B=15 T), there is an additional band with the resonant peak appearing at near-zero frequency (microwave regime), stemming from the magneto-plasmon zero modes. The great enhancement of NFRHT at such low frequency has not been found in any previous systems yet. This work may pave a way for multi-band thermal information transfer based on atomically thin graphene sheets.

cond-mat.mes-hall↗

arXiv2018

Control of near-field radiative heat transfer based on anisotropic 2D materials

In this work, we study the near-field radiative heat transfer between two suspended sheets of anisotropic 2D materials. It is found that the radiative heat transfer can be enhanced with orders-of-magnitude over the blackbody limit for nanoscale separation. The enhancement is attributed to the excitation of anisotropic and hyperbolic plasmonic modes. Meanwhile, a large thermal modulation effect, depending on the twisted angle of principal axes between the upper and bottom sheets of anisotropic 2D materials, is revealed. The near-field radiative heat transfer for different concentrations of electron is demonstrated and the role of hyperbolic plasmonic modes is analyzed. Our finding of radiative heat transfer between anisotropic 2D materials may find promising applications in thermal nano-devices, such as non-contact thermal modulators, thermal lithography, thermos-photovoltaics, etc.

physics.optics↗

arXiv2018

Operation of normal-conducting RF cavities in multi-tesla magnetic fields for muon ionization cooling: a feasibility demonstration

Ionization cooling is the preferred method for producing bright muon beams. This cooling technique requires the operation of normal conducting, radio-frequency (RF) accelerating cavities within the multi-tesla fields of DC solenoid magnets. Under these conditions, cavities exhibit increased susceptibility to RF breakdown, which can damage channel components and imposes limits on channel length and transmission efficiency. We present a solution to the problem of breakdown in strong magnetic fields. We report, for the first time, stable high-vacuum, copper cavity operation at gradients above 50 MV/m and in an external magnetic field of three tesla. This eliminates a significant technical risk that has previously been inherent in ionization cooling channel designs.

physics.acc-ph↗

arXiv2018

Designing topological interface states in phononic crystals based on the full phase diagrams

The topological invariants of a periodic system can be used to define the topological phase of each band and determine the existence of topological interface states within a certain bandgap. Here, we propose a scheme based on the full phase diagrams, and design the topological interface states within any specified bandgaps. As an example, here we propose a kind of one-dimensional phononic crystals. By connecting two semi-infinite structures with different topological phases, the interface states within any specific bandgap or their combinations can be achieved in a rational manner. The existence of interface states in a single bandgap, in all odd bandgaps, in all even bandgaps, or in all bandgaps, are verified in simulations and experiments. The scheme of full phase diagrams we introduce here can be extended to other kinds of periodic systems, such as photonic crystals and designer plasmonic crystals.

physics.app-ph↗

arXiv2017

Topological phase transition and interface states in hybrid plasmonic-photonic systems

The geometric phase and topological property for one-dimensional hybrid plasmonic-photonic crystals consisting of a simple lattice of graphene sheets are investigated systematically. For transverse magnetic waves, both plasmonic and photonic modes exist in the momentum space. The accidental degeneracy point of these two kinds of modes is identified to be a diabolic point accompanied with a topological phase transition. For a closed loop around this degeneracy point, the Berry phase is Pi as a consequence of the discontinuous jump of the geometric Zak phase. The wave impedance is calculated analytically for the semi-infinite system, and the corresponding topological interface states either start from or terminate at the degeneracy point. This type of localized interface states may find potential applications in photonics and plasmonics.

physics.optics↗

arXiv2016

Unidirectional scattering induced by the toroidal dipole moment in the system of plasmonic nanoparticles

Unidirectional backward and forward scattering of electromagnetic waves by nanoparticles are usually interpreted as the interference of conventional multipole moments (i.e., electric and magnetic dipole, electric quadrupole, etc.). The role of toroidal dipole moments in unidirectional scattering is generally overlooked. In this work, we investigate the unidirectional scattering for the system of three plasmonic nanospheres. It is found that the unidirectional backward scattering is caused by the interference between the toroidal dipole moment and other conventional multipole moments. Tunable primary backward and forward scattering can be achieved under some specific configurations. Our results can find applications in the design of nanoantennas.

physics.optics↗

arXiv2016

A Generic Minimal Discrete Model for Toroidal Moments and Its Experimental Realization

It is well known that a closed loop of magnetic dipoles can give rise to the rather elusive toroidal moment. However, artificial structures required to generate the necessary magnetic moments are typically optically large, complex to make and easily compromised by the kinetic inductance at high frequencies. Instead of using magnetic dipoles, we propose a minimal model based on just three aligned discrete electric dipoles in which the occurrence of resonant toroidal modes is guaranteed by symmetry. The advantage of this model is its simplicity and the same model supports toroidal moments from the microwave regime up to optical frequencies as exemplified by a three-antenna array and a system consisting of three nano-sized plasmonic particles. Both the microwave and high-frequency configurations exhibit non-radiating "anapoles". Experiments in the microwave regime confirm the theoretical predictions.

physics.optics↗

arXiv2015

Electromagnetic Scattering by Spheres of Topological Insulators

The electromagnetic scattering properties of topological insulator (TI) spheres are systematically studied in this paper. Unconventional backward scattering caused by the topological magneto-electric (TME) effect of TIs are found in both Rayleigh and Mie scattering regimes. This enhanced backward scattering can be achieved by introducing an impedance-matched background which can suppress the bulk scattering. For the cross-polarized scattering coefficients, interesting antiresonances are found in the Mie scattering regime, wherein the cross-polarized electromagnetic fields induced by the TME effect are trapped inside TI spheres. In the Rayleigh limit, the quantized TME effect of TIs can be determined by measuring the electric-field components of scattered waves in the far field.

physics.optics↗

arXiv2014

Determination of the quantized topological magneto-electric effect in topological insulators from Rayleigh scattering

Topological insulators (TIs) exhibit many exotic properties. In particular, a topological magneto-electric (TME) effect, quantized in units of the fine structure constant, exists in TIs. In this Letter, we study theoretically the scattering properties of electromagnetic waves by TI circular cylinders particularly in the Rayleigh scattering limit. Compared with ordinary dielectric cylinders, the scattering by TI cylinders shows many unusual features due to the TME effect. Two proposals are suggested to determine the TME effect of TIs simply based on measuring the electric-field components of scattered waves in the far field at one or two scattering angles. Our results could also offer a way to measure the fine structure constant.

cond-mat.mtrl-sci↗