File Name: diagram of scatter energy and beam canceling fields .zip
Metrics details. Linac output as a function of field sizes has a phantom and a head scatter component.
- We apologize for the inconvenience...
- Segmented Terahertz Electron Accelerator and Manipulator (STEAM)
- Looking for other ways to read this?
Artifacts are commonly encountered in clinical CT and may obscure or simulate pathology. There are many different types of CT artifacts, including noise, beam hardening, scatter, pseudoenhancement, motion, cone-beam, helical, ring and metal artifacts.
We apologize for the inconvenience...
Controlling spin electromagnetic waves by ultra-thin Pancharatnam-Berry PB metasurfaces show promising prospects in the optical and wireless communications.
One of the major challenge is to precisely control over the complex wavefronts and spatial power intensity characteristics without relying on massive algorithm optimizations, which requires independent amplitude and phase tuning.
However, traditional PB phase can only provide phase control. Here, by introducing the interference of dual geometric phases, we propose a metasurface that can provide arbitrary amplitude and phase manipulations on meta-atom level for spin waves, achieving direct routing of multi-beams with desired intensity distribution.
As the experimental demonstration, we design two microwave metasurfaces for respectively controlling the far-field and near-field multi-beam generations with desired spatial scatterings and power allocations, achieving full control of both sophisticated wavefronts and their energy distribution.
This approach to directly generate editable spatial beam intensity with tailored wavefront may pave a way to design advanced meta-devices that can be potentially used in many real-world applications, such as multifunctional, multiple-input multiple-output and high-quality imaging devices. As the two-dimensional 2D equivalence of metamaterials, metasurfaces have shown unprecedented abilities to manipulate the electromagnetic EM waves within sub-wavelength thickness that is far beyond what can be achieved by naturally occurring materials.
Benefiting from the burgeoning capabilities for fabricating materials with accuracy geometries, metasurfaces consisting of subwavelength meta-atoms can elaborately tailor the amplitude, phase and polarization state of the interacting EM wavefront with the advantages of low profile, low loss, and simple fabrication, ranging from microwave to terahertz and optical regions [ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ].
Therefore, a plenty of applications have been implemented with metasurfaces, such as ultrathin metalens [ 3 ], [ 4 ], [ 7 ], [ 8 ], [ 9 ], [ 10 ], invisibility cloak [ 11 ], [ 12 ], polarization converter [ 13 ], [ 14 ], and vortex beam generator [ 15 ], [ 16 ], [ 17 ], [ 18 ], etc. The explosive development of modern communication technology has drawn more attention to the design of miniaturized and integrated multifunctional devices that are compatible with nowadays compact systems.
Due to the complexity and variety of application requirements, it is often necessary to implement integrated design with independent, arbitrary, and simultaneous control of multiple EM functionalities and the allocation of their radiation power intensities. Though conventional phase-only metasurfaces using addition theorem can realize superposition of the multiple independent EM beams [ 19 ], which have potential applications in multiple-input multiple-output MIMO communications, the power of each independent beam cannot be freely designed, inherently limiting their use in some applications.
By employing optimization algorithms, this limitation can be somehow resolved so that the objective of simultaneously control over the beam propagation direction and power intensity allocation is consequently achieved [ 20 ]. Nevertheless, this method consumes large computing resource and time. Moreover, side-lobes are inevitably increased in the multi-beam forming, which is difficult to overcome by such optimization algorithms.
Recently, by applying complex-amplitude addition theorem and introducing meta-atoms with independent amplitude and phase modulations, the propagation of EM wave can be completely tailored by amplitude-phase metasurface. Compared with phase-only metasurfaces, the major advantage of such metasurface is that better wavefront-shaping performances, e. Due to these merits, the amplitude-phase modulation metasurfaces insure a wide range of applications such as high quality holography, synthesis of complex wave fields, and so on [ 21 ], [ 22 ].
The metasurface composing of C-shaped meta-atoms can arbitrarily control the power of each diffraction order or realize high-resolution holograms for linearly-polarized excitations [ 23 ], [ 24 ], [ 25 ]. By introducing lossy components, e. Moreover, while controlling circularly-polarized CP wave is essential for such as satellite communication, spin optics, etc.
Needless to say, it is still highly desirable to find accurate analytical method to achieve nimble metasurface designs with easy fabrication and flexible scalability. The most appealing feature of PB phase is its simple design strategy that only a single meta-atom with different rotation angles can achieve full phase coverage required for complex wavefront manipulations [ 10 ], [ 15 ].
However, traditional geometric elements have identical amplitude but opposite phase responses for excitation of different spins, which cannot provide arbitrary, independent, and decoupled phase and amplitude behaviors at the meta-atom level. Here, we show that the metasurfaces composed of dual geometric phases are capable of providing direct spatial-power-editing for versatile functionalities without relying on optimization algorithm.
To give the experimental verifications, we propose two functional devices for directly routing the far-field and near-field EM behaviors in microwave region: the first metasurface can simultaneously control the propagation directions of multiple beams and the power allocation of each scattered beam; the second bi-foci metasurface can achieve independent control of the position and power of each focal spot.
The two prototypes are fabricated and measured, which validates the good performances of our proposals. As schematically shown in Figure 1 , the metasurfaces with independent control of the spatial wave functions and desired power allocation of multiple scattering wavefronts can be achieved under the illumination of CP incidence.
The first one shown in Figure 1A can realize free and independent control of scattering directions and power level denoted by A 1 , A 2 , and A 3 of multiple beams, whereas the second one shown in Figure 1B can realize desired focal spots with different powers denoted by B 1 and B 2. Here, we assume that the incident source is right-handed circularly polarized RCP wave, and the corresponding radiation property for left-handed circularly polarized LCP incidence will be discussed in the next section.
Schematic of the proposed reflective metasurface with arbitrary and independent control of spatial and powers of the wavefront under normal CP wave excitation. A The reflective beams with independent and distinct powers can be controlled to arbitrary directions.
B Two-foci with different powers generated by the metasurface. In order to simultaneously and independently manipulate the spatial power intensity allocation of the scattering fields, the amplitude and phase responses of each pixel of a metasurface should be totally decoupled and arbitrarily tailored. Therefore, designing proper realistic meta-atom is the key point to get the devices with desired spatial-intensity-editable scattering power.
Here, we consider a metasurface particle composed of dual geometric phases, interpreted by two simple independent metallic rod structures, as shown in Figure 2. We utilize the meta-atom that is a triple-layer structure consisting of top metallic layer, a bottom metallic plane and a middle dielectric layer.
Under the illumination of normal CP incidence, the abrupt phase change caused by the single rod structure is attributed to the resonant modes of the arm. For the X-shaped metasurface with dual geometric phase interference, each geometric phase component one arm of the X-shaped structure can be regarded as an independent electric dipole approximately.
Thus, the whole X-shaped structure can be modelled by the resonances of each arm. Overall, the co-polarized reflection performance of the meta-atom can be calculated by the interference of dual geometric phases two independent electric resonances , whose superposition can be written as the reflection of a single rod structure is assumed as 1 for simplicity :.
C Schematic of the proposed X-shaped meta-atom. D Top view of the proposed X-shaped meta-atom. For the multi-beam devices, each beam can be regarded as a communication channel.
Independent control of amplitude and phase responses of each meta-atom is the key step for realizing complete control of the powers in each channel. Assume a plane wave illuminating the metasurface, the scattering pattern from the reflective metasurface can be calculated as:. We first demonstrate a practical application by utilizing the proposed X-shaped meta-atoms to generate double beams with independent control of the radiation direction and power.
Thus, the final amplitude and phase profile shown in Figure 3B can be obtained from Eqs. Design process to obtain the proposed reflective metasurface for independently controlling the radiation direction and powers of the beams.
A The phase distributions for the desired beams. In order to give a detailed comparison, we also plot in Figure 4C the corresponding 2D normalized scattering patterns in xoz plane, where good agreements are observed between the simulated result and the theoretical prediction. We have also investigated the case of metasurface shined by LCP incidence. The theoretical and simulated normalized 3D far-filed scattering patterns for LCP incidence are presented in Figure 4D and E , respectively.
The corresponding 2D normalized scattering patterns in xoz plane are also compared in Figure 4F. This can be understood by the conjugate relationship between the phase responses of geometric phase element for LCP and RCP incidence [ 28 ], [ 29 ], [ 30 ], where macroscopically speaking, the beam will be deflected by angle with the same value yet opposite constant linear gradient if the spin of the incidence is changed to its orthogonal one.
So we only consider cases operated with RCP incidence in the rest part of the paper while the performance for LCP incidence can be easily obtained by the inherent feature of geometric phase metasurface. Applying complex-amplitude addition theorem, arbitrary scattering beams with independent powers can be generated from a metasurface simultaneously.
Without loss of generality, we have designed a reflective metasurface with more complex functions that has three spatial radiation beams with different powers. The design process of metasurface for generating three beams is similar to that presented above for generating two beams. The theoretical 3D scattering pattern Figure 5B is in agreement with the numerically simulated result Figure 5C , and three scattering beams can be obtained with different spatial direction and power.
The high side-lobes in the simulations are mainly caused by the distortion of the reflection characteristics attributed to imperfect unit cell boundary condition in the real metasurface. By using optimization algorithms [ 5 ], [ 32 ], [ 33 ], the high side-lobes may be well suppressed and we may even obtain more sophisticated radiation patterns, such as flat-top beam, cosecant squared beam and so on.
Besides, the spin-decoupled metasurface with independent phase tuning of the RCP and LCP by introducing propagation phases may provide more degrees of freedom in controlling the amplitude and phase responses [ 16 ], [ 34 ], [ 35 ], [ 36 ], [ 37 ]. D The photograph of the fabricated sample. Inset shows the enlarged view. E The theoretical, simulated and measured 2D normalized scattering pattern for the RCP incidence in the xoz and yoz planes at 8. To experimentally validate the design principle and the proposed metasurface capable of achieving independently editable amplitude and phase tuning, the metasurfaces with three scattering beams is fabricated by standard printed circuit board PCB technique.
The PCB technique can ensure a precise sample fabrication in microwave region. The photograph of the fabricated metasurface is shown in Figure 5D , with enlarged view of the metasurface elements shown in the inset. We have measured the scattering patterns of the sample in a standard microwave chamber and all the measured results of the fabricated sample are calibrated to a same-sized copper slab.
The measured 2D normalized scattering patterns in xoz plane and yoz plane are compared with simulated results in Figure 5E. The measured results are roughly in agreement with the simulation results, taking into account of the fabrication and measurement tolerances. As for the cross-polarized components, only a single dominating output beam is observed in the far-field region due to that the meta-atom can only manipulate the reflection phases of the co-polarized components.
In addition, the total efficiency of the reflected beams from the metasurface defined by the ration of co-polarized reflection to input energy is about Therefore, by the proposed theory and the X-shaped particle, the functions of independent scattering beams with distinct power levels can be integrated onto a single reflective metasurface for CP incidence. In order to further demonstrate the feasibility of the proposed theory in controlling the spatial scatterings and power allocation of the EM wave arbitrarily and independently, we have designed a bi-foci metalens that can transform the CP incidence into two focal spots with controllable spatial focusing performance and desirable powers, as schematically shown in Figure 1B.
The spatial phase distribution on the lens aperture to achieve a single-focus metalens with the pre-designed focal length F 0 can be obtained by [ 38 ]:. The fabricated meta-lens by the PCB technique is depicted in Figure 6B , where the inset shows the enlarged view of the meta-atoms. B The photograph of the fabricated sample. The simulated E -field power profiles on the focal plane and xoz plane at 8.
To experimentally verify the focusing performance, two orthogonal linearly-polarized E -fields reflected from the metalens are mapped by a 3D field-scanning system with dipole probes to detect pixel by pixel in a pre-designed plane, and then the two orthogonal linearly-polarized components are transformed into CP E -filed. The details of the measurement setup of the 3D field-scanning system can be obtained in Section II of the Supplemental Material. Good agreements are observed between the measured and simulated results and they are both consistent with the theoretical prediction, clearly validating that the spatial positions and powers of focal spots can be generated and controlled independently.
C The measured normalized power distribution of E -field in focal plane. In summary, we have proposed a method to design the spatial-intensity-editable metasurface supporting independent manipulation of multiple spatial wave functions and their power allocations under CP excitation.
Assisted with the interference of dual geometric phases, the reflection amplitude and phase responses can be controlled freely and independently by the X-shaped meta-atom. Two proof-of-concept designs working in the microwave region have been fabricated and experimentally demonstrated as illustrations.
The wave functions are not limited to reflection operation and what have been achieved herein, and actually, transmissive geometry and much complex spatially power allocations may be readily designed by the proposed method, which may have potential applications in high-quality imaging e. In particular, considering the dramatic progresses made by optical geometric phase metasurface [ 10 ], [ 11 ], [ 12 ], [ 13 ], [ 14 ], [ 15 ], it is also quite promising to extend the design method to higher frequency bands, such as terahertz and optical region, stimulating the advanced processing of EM waves and novel meta-devices.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission. Conflict of interest statement: The authors declare no conflicts of interest regarding this article. Yu, P. Genevet, M. Kats, et al. Search in Google Scholar. Zhao, N. Engheta, and A. Aieta, P. Sun, Q.
Segmented Terahertz Electron Accelerator and Manipulator (STEAM)
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Acceleration and manipulation of electron bunches underlie most electron and X-ray devices used for ultrafast imaging and spectroscopy. New terahertz-driven concepts offer orders-of-magnitude improvements in field strengths, field gradients, laser synchronization and compactness relative to conventional radio-frequency devices, enabling shorter electron bunches and higher resolution with less infrastructure while maintaining high charge capacities pC , repetition rates kHz and stability. We present a segmented terahertz electron accelerator and manipulator STEAM capable of performing multiple high-field operations on the 6D-phase-space of ultrashort electron bunches. With this single device, powered by few-micro-Joule, single-cycle, 0. The STEAM device demonstrates the feasibility of THz-based electron accelerators, manipulators and diagnostic tools enabling science beyond current resolution frontiers with transformative impact.
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. This chapter and the next summarize the case for development and use of high-intensity lasers for research and applications. The impact of high-intensity laser technology on science is unusually strong and broad, spanning from the most basic questions of the cosmos to potential applications in medical therapy. The primary motivation for high-intensity science is that it overturns the foundational assumption that the forces exerted by light are weak, and may therefore be treated as small perturbations to the forces that shape matter. The fields in a high intensity laser focus exert forces that are stronger than the physical systems they encounter—stronger than the chemical forces holding molecules and solids together; stronger than the coulomb fields that bind electrons in atoms; and ultimately stronger than the vacuum itself. This chapter introduces the scientific opportunities that are enabled by high intensity.
than shielded diodes for small field sizes, and can in radiotherapeutic clinical practice Silicon forms a solid semiconducting crystal structure (lattice) with energy bands in which incident beam is kept unshielded, so that mainly scattered low-energy photons the denominator are correlated and hence partly canceling.
Looking for other ways to read this?
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. The previous chapter was concerned with laser and particle beams insofar as they are used to produce HED plasmas, whereas this chapter is concerned with the physics of the beam-plasma interaction itself.
Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer.
Transmission Electron Microscopy pp Cite as. The electron is a low-mass, negatively charged particle. As such, it can easily be deflected by passing close to other electrons or the positive nucleus of an atom.
Recently, there has been much research in the field of nanostructure technology. The objective of this article is to explore the basic physics, technology, and applications of ultra-small structures and devices with dimensions in the subnm range. Nanostructure devices are now being fabricated in many laboratories to explore various effects, such as those created by downscaling existing devices, quantum effects in mesoscopic devices, tunneling effects in single electron transistors, and so on. In addition, new phenomena are being explored in an attempt to build switching devices with dimensions down to the molecular level.