Wake field in dielectric acceleration structures.

Received 18 November 2003DOI:https://doi.org/10.1103/PhysRevE.69.049901©2004 American Physical Society

Electron accelerators have a wide range of applications in basic research, medicine and industry. In order to reduce costs and size of future devices and facilities, advanced compact accelerator concepts are subject to extensive research and development efforts in accelerator physics. A promising approach is the dielectric laser accelerator (DLA). The electromagnetic near-fields of an ultra-short laser pulse scattered by a dielectric structure are used to accelerate electrons. Dielectrics increases the achievable accelerating gradient by a factor of at least 10 in comparison to metallic structures. Recent progress in the development of femtosecond lasers and lithographic techniques in semiconductor fabrication has paved the way to experimental demonstration of DLAs in 2013. Possible applications of DLAs are in attosecond science, in ultrafast electron-microscopy and -diffraction, and in providing relativistic electrons for lithography and radiotherapy. The use of near-fields in structures with period lengths in the order of micrometers leads to complicated particle dynamics during acceleration. Theoretical descriptions of the electromagnetic fields and the resulting particle dynamics are presented in this thesis. In order to confine the particles in the accelerating structures, a focusing scheme is introduced which is adapted from ion accelerators. This allows to transport particles through a DLA structure of arbitrary length. Due to non-linear dynamics of the accelerated electrons, the particle trajectories are calculated numerically. A simulation tool based on the analytical description of the electromagnetic fields in the structures is implemented. Since DLA structures are quasiperiodic, the description simplifies to one complex number per structure period, i.e., the spatial harmonic of the synchronous accelerating mode. The required computational effort is thus significantly reduced as compared to the full electromagnetic fields. Many applications of DLAs require particle distributions of high charge. The beam dynamics are additionally affected by wake fields, which are generated by the interaction of the charge distribution ensemble with the dielectric structures. Detailed studies of wake fields in DLA structures are discussed in this thesis. Moreover, first measurements of energy spectrum modulations caused by wake fields are presented. The wake field effects can lead to instabilities. For the analysis of possible instabilities, wake fields are integrated in the aforementioned numerical simulation tool. This extended tool is applied to compare particle dynamics with simplified analytical models of longitudinal and transverse wake effects and instabilities. These studies provide estimates for the maximum attainable charges of an accelerated particle bunch. For higher bunch charges, the effects from the wake fields exceed the accelerating and focusing laser fields and thus lead to beam loss. Finally, a possible damping mechanism is presented in order to counteract the instabilities and increase the bunch charge limits.

To study the beam instability caused by wake field effect, a dielectric wall accelerator module model consisting of symmetric pulse forming line, micro-stacked insulator and conductor layer was established. The finite integral method was used to simulate the wake field in the accelerating structure of dielectric wall accelerator (DWA) for high-current electron beam, wake potential and wake field impedance were calculated for the single and two accelerating units. Structural parameters of distance between two modules, thickness of metal and dielectric constant of insulator in micro-stacked insulator were changed to study the influence of different parameters on wake field. The results shows the wake potential and wake field impedance was small in x, y direction and different structural parameters can influence the wake field effect. Also it proved the advantage of accelerating cavity length and continuity in the DWA structure.

Electromagnetic metamaterials/metasurfaces have been widely concerned by researchers because of their rich resonance properties. Metamaterial absorbers with single resonance mode and narrow bandwidth are often used in sensing fields. However, the development of such metamaterial absorbers by optimizing the dielectric layer structure has not received much attention. In this paper, a metamaterial absorber containing dielectric layers of the same thickness (PMMA and STO dielectric layers) are proposed and confirmed. At room temperature, an individual absorption peak is excited, and the single resonance mode and narrow bandwidth characteristic is obtained (resonant frequency is 266.14THz, peak value is 0.902, the FWHM is 1.1THz). The amplitude and resonance frequency of the metamaterial sample can be controlled by changing the thickness of PMMA or STO dielectric layers. An equivalent LC resonant circuit is constructed and used to interpret the relationship between the resonant frequency and the thickness of the dielectric layer. In further experimental measurements, the metamaterial sample was placed in an environment with a gradual change in temperature. Two metamaterial units with the same dielectric layer structure strategy are also proposed and simulated, and similar resonance behaviors can also be excited by these two new metamaterial units. The results show that the absorptive properties of the metamaterial sample show remarkable repeatability in the process of temperature increase and temperature decrease. Therefore, based on the properties of single resonance mode, narrow bandwidth, and reversibility, the metamaterial sample has the potential to be used in smart packaging sensing.

Currently, the IC industry has been steadily advancing towards 7 nm and 5 nm nodes with further reductions projected in the near future to progressively create large number of inputs and outputs (IOs) at finer pitch. Today the high-density interconnect (HDI) organic redistribution layer (RDL) can only achieve an IO density of about 40 IOs per mm per layer with line and space of 6 μm and microvia diameter of 20 μm at 50 μm pitch. However, to achieve further increases in IO density, RDL with 1 μm routing lines and spaces together with 1 to 2 μm diameter microvias are required. Such advances in the RDL technology are of great importance to accomplish IO densities of 500 IOs/mm/layer to enable high bandwidths of 500 Gb/s at low cost. In this paper we present the latest progress at the Packaging Research Center, Georgia Institute of Technology in the following 4 key areas.1. Fine line photolithography: Various methods that can achieve 1 μm critical dimension (CD) are discussed and recent results on 1 μm L/S using both dry film and liquid photoresists together with advanced lithographic tools are presented.2. Small microvia creation: Microvia is the most important barrier limiting the RDL to achieve high IO density and fine IO pitch. In this paper, microvia diameter scaling down to 2 μm along with the feasibility to achieve 1 μm and via pitch of 4 to 8 μm using both photo and picosecond pulsed UV laser will be presented.3. Low Dk and Low Df dielectric materials: Dielectric material layers are an important part of RDL. For achieving multi-functional high speed and/or low loss systems and modules, dielectric layers with low Dk and/or low Df materials are critical. The material requirements, availability and process challenges will be addressed in this paper.4. Process methodology: The semi-additive process (SAP) has been the process of record for RDL fabrication. In this paper, the conventional SAP and its modifications such as modified-SAP (m-SAP) and advanced SAP (a-SAP) together with alternative organic damascene process (ODP) along with back-end- of-line (BEOL) will be reviewed and compared.Finally, considerations for future trends are presented.

Electromagnetic Band-Gap (EBG) structures are popular and efficient techniques for microwave applications; they are periodic metallic or dielectric materials. EBG may be combined with microstrip patch antenna to increase the directivity, the radiation efficiency and/or to suppress surface waves, to reduce the side lobes of the radiation pattern and to increase the bandwidth. In this paper, we study the influence of Unidimensionnel Electromagnetic Band Gap structure (1D-EBG) on the performances of an aperture coupled rectangular patch antenna. This 1D-EBG is chosen as two similar dielectric layers disposed above the patch. The return loss, radiation pattern, and directivity are studied using HFSS software. The simulation results show that the gain, directivity and S11 parameter of the antenna with dielectric superstrate layers are increased at X band (8–12GHz). Compared with the patch feed with the same aperture size but without dielectric layers, the performance of the proposed antenna is improved obviously.

Electric field assisted dissolution of metal films on coated soda-lime glass

Transverse mode coupling instability of a bunch with space charge and wake field is considered within the frameworks of the boxcar model. Eigenfunctions of the bunch without wake are used as a basis for the solution of the equations with the wake field included. A dispersion equation for constant wake is presented in the form of an infinite continued fraction and also as the recursive relation with an arbitrary number of the basis functions. Realistic wake fields are considered as well including resistive wall, square, and oscillating wakes. It is shown that the TMCI threshold of the negative wake grows up in absolute value when the SC tune shift increases. Threshold of positive wake goes down at the increasing SC tune shift. The explanation is developed by an analysis of the bunch spectrum.

A new derivation for the radiation reaction on a point charge is presented. The field of the charge is written as a superposition of plane waves. The plane wave spectrum of the field consists of homogeneous plane waves which propagate away from the charge at the speed of light, and inhomogeneous plane waves which constitute the Coulomb field of the point charge. The radiation field is finite at the orbit of the point charge. The force acting on the charge due to this field is the well known Abraham-Lorentz radiation reaction.

In this paper, the ground surface is equivalent to a multi-layer structure of two lossy media (such as dry soil and wet soil). Four different periodic dielectric layers are investigated, and their radio wave shielding effectiveness is analyzed and evaluated by the propagation matrix method. In view of the main structure factors such as the thickness of different dielectric layers and the arrangement of different dielectric layers, the shielding effectiveness of different dielectric layer structures in the frequency band of 300MHz is explored. The results show that the layered structure represented by structure c exhibits better shielding effectiveness in different situations, indicating that the more layers and thickness of the strong lossy medium, the better the shielding effectiveness. At the same time, this arrangement of the strong lossy dielectric layer and the weak lossy dielectric layer has the best increase effect on the shielding effectiveness compared to structures a, b, and d.

Surface plasmon resonance (SPR) is the collective oscillation of free electrons at the interface between metal and dielectric layers. Since the resonant condition of SPR is dominated by refractive indices of metal and dielectric layer, it is possible to distinguish the refractive index change of dielectric layer through the shifts of resonant condition. Considering the system resolution of SPR system nowadays, the sensing resolution up to 10-6 RIU (refractive index change) is available for a general SPR system. Accordingly, SPR provides a label-free platform to detect bio-interactions. However, for pursuing trace measurement, rapid detection, precise diagnosis, versatile functions, or lower cost, researchers devote themselves to develop different interrogations for diverse applications. For the most general four interrogations: angle, wavelength, intensity, and phase are well applied in current literatures. In this study, we focus on the sensitivity enhancement and potential applications of SPR interrogated by different routes, in order to expend its applying fields. For the non-complicated and efficient sensing platform, intensity interrogation, we discussed the relation between sensitivity and thickness of metal sensing layer, and further derived a generalized sensitivity model for intensity interrogation SPR system. In addition, we found that the optimized metal thickness in intensity interrogation is different from the thickness for best coupling efficiency, which is the most commonly used in research. This difference can be explained through two damping factors in SPR. For the sophisticated and ultra-sensitive sensing platform, phase interrogation, we applied such interrogation on LSPR. LSPR is characterized by coupler-free excitation, nano-scale, and diverse resonant modes; nevertheless, its sensitivity is over 10 folds lower than SPR. In contrast with other researchers who modified the resonant modes, we used phase interrogation instead of extinction spectra to enhance the sensitivity. Our results show that the sensitivity was enhanced over 80 folds at the same LSPR structures under the near-field excitation. This study not only confirms the feasibility of exploiting LSPR by optical phase, but also complements its insufficiency on sensitivity. For the low-cost and large linear detection range sensing platform, wavelength interrogation, we expect to have the potential application on point-of-care test. We proposed a color SPR system based on wavelength interrogation, in which we directly observed the color change of reflection rather than identified the resonant wavelength through spectrometer. Moreover, by using Ag/Au bi-metallic film to replace the general used Au film, we are able to enhance the sensitivity and color contrast without losing linear detection range. In short, we hope to promote SPR sensing system through the studies on intensity, phase, and wavelength interrogations.

The dielectric constants and loss factors of twenty-seven liquid organic halides have been measured at 1.27 and 3.2.2 cm. wavelength and used, together with certain additional measurements at 10 cm. and with measurements of viscosities, densities and refractive indices, to examine the accuracy of the equations for dielectric behavior and the relations between dielectric relaxation and molecular structure. The method of measurement employs the variation in the reflection coefficient of a uniform dielectric layer as the depth of the layer is varied, A short-circuiting plunger terminates the dielectric layer and a directional coupler is used to measure the power reflected from this plunger. The calculations involve only the ratio of the amplitudes of the reflected waves for two different lengths of the dielectric-filled guide, the free-space wavelength, the wavelength in the air-filled guide, and the wavelength in the dielectric medium.

In this paper we aim to give a further contribution concerning the analysis of the electromagnetic field of a charge in a gravitational field. This long-standing issue has given rise to a number of paradoxes, solutions to which have been proposed by several Authors, sometimes contrasting each other. Here we focus on the case of a point charge, at rest in the reference frame of a freely falling observer in the Schwarzschild geometry. We discuss the electromagnetic field of such charge as measured by the comoving observer. Working in the Lemaître coordinates, we show that the electric field of the charge does not obey the usual Coulomb law. Instead, a small correction (usually out of the experimental range) gives rise to a distortion in the lines of the electric field, suggesting that the latter lags behind the falling charge, due to tidal effects. We discuss in some detail the results, also in connection with the issue concerning the electromagnetic radiation from the charge and the equivalence principle.

Transistors based on two-dimensional (2D) materials have been considered one of the most promising techniques after the silicon-based technique. However, unlike the diversity and popularity of the study on the channel materials for 2D transistors, the research on dielectric materials received insufficient attention in the past decade. So far, the main candidates used as dielectric materials for 2D transistors are limited to native oxides such as SiO2, deposited high-κ oxides such as HfO2 by atomic layer deposition (ALD), and van der Waals crystals such as hexagonal boron nitride (h-BN). However, these materials have their respective problems. The native oxides, SiO2 as the representative, have relatively low dielectric constant and are limited by their parent materials, which can hardly be used for other semiconductors. The mechanism of deposition requires nucleation on the surface of the semiconductors, where the dangling-bond-free high-quality surface of 2D materials does not favor the uniform growth and leads to low-quality film and interface. The van der Waals crystals like h-BN are restricted by their crystal size and the dielectric constants are relatively low. Therefore, new candidates and techniques that can possibly satisfy the ultimate standards for dielectric materials in 2D transistors, i.e., scalable ultrathin high-κ materials with high quality, are urgently needed. It was found that the native oxides of liquid metals, such as gallium oxide (Ga2O3) from liquid gallium, could be prepared as ultrathin free-standing materials for applications. These ultrathin free-standing oxides might have potentials properties that meet the requirements as dielectric materials in 2D transistors. In this thesis, the potential properties of the free-standing Ga2O3 and its integration for high-performance 2D transistors are demonstrated. First, the morphology and dielectric properties, such as dielectric constant, breakdown field, optical band gap, etc. are characterized. Ultrathin thickness of less than 3 nm and an ultra-flat surface are measured. A high dielectric constant about 30 and high breakdown field about 11 MV/cm are extracted. A low equivalent oxide thickness (EOT) about 0.4 nm is calculated. These properties therefore indicate the free-standing Ga2O3 as an ultrathin and uniform high-κ dielectric material, which meets the requirements for high-performance 2D transistors. A surface-tension-driven method of wafer-scale preparation is introduced, providing the scalability for device fabrication. Second, the free-standing Ga2O3 is integrated in 2D MoS2 field-effect transistors (FETs) as dielectric layer. Subthreshold swing down to 60 mV/dec is obtained, close to the thermal limit at room temperature. Low leakage current density about 4×10–7 A cm–2 and low density of interfacial state about 4×1011 eV–1 cm–2 are extracted. Immunity to the short-channel effect is revealed with short-channel FETs with channel lengths down to about 28 nm. Scalable device fabrication is demonstrated with FET arrays and logic gates, including NOT, NAND, NOR, AND, XOR. The high performance is attributed to the outstanding dielectric properties of the Ga2O3 and the van der Waals integration. Third, the passivation ability of the ultrathin Ga2O3 as an encapsulation layer is evaluated with black phosphorus (BP) and 1T' MoTe2, which are representatives of air-sensitive 2D materials. Dramatic increases in retention time of both optical and electrical properties are obtained, due to the self-limiting oxidation that forms dense native oxide against O2, H2O, etc. in an ambient environment. Owing to the facile selective etching of the ultrathin Ga2O3, direct device fabrication without removal of the encapsulation layer. Dual-gate FETs can be directly fabricated with encapsulated BP, using the Ga2O3 simultaneously as encapsulation and dielectric material. This dissertation, therefore, exhibits that the free-standing native oxide from liquid metal has desired properties as dielectric material and encapsulation for the fabrication of high-performance 2D transistors.

The reliability of through silicon via (TSV) in a 3-dimensional integrated circuit (3D IC) determines whether the electronic system can operate continuously and steadily. The finite element method was adopted to study the response of TSV to thermoelectric coupling, the effect of different dielectric layer materials and the filling degree of carbon nanotubes (CNT) on TSV reliability were investigated, respectively. Through orthogonal simulation experiments, the high reliability of a type of TSV with benzocyclobutene (BCB) dielectric layer and filling by a coaxial hybrid CNT bundle was verified, and its lifetime was predicted by the failure of the physical model. The simulation results show that the most important influence factor to TSV reliability is the depth-to-width ratio, the second important factor is the choice of dielectric material, and the least important factor is the filling degree of CNT. The parameter values with the optimum reliability are as follows: the depth-to-width ratio of copper column is 20, the dielectric material is BCB, and the CNT filling degree is 4–8%.