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中国造世界第一个“人造黑洞”

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HWM|  楼主 | 2010-11-20 23:19 | 只看该作者 回帖奖励 |倒序浏览 |阅读模式
中国造世界第一个“人造黑洞”


2010年11月12日 08:45:02  来源: 外滩画报 【字号 大小】【留言】【打印】【关闭】  
  

  它有着“黑洞”之名,虽然尺寸“迷你”,但任何经过的电磁波或光,都不可能逃离它的引力。10月15 日,《科学》杂志(本人注:IOP Science)宣布,世界上第一个“人造黑洞”在中国东南大学实验室里诞生。不过,这个小型“黑洞”不仅不会毁灭世界,还能帮助人们更好地吸收太阳能。

  在宇宙中,黑洞吞噬万物,甚至包括光。人们乐意议论这种天体,因为它神秘、“性情”怪异:它身处宇宙最幽暗的地方,没有人能直接观测到它,而靠近它的任何物质,都会被无情地拖曳到它的深渊里,小行星、星尘、光波、时间,无一例外。

  人们对黑洞这种天体感到好奇,但绝不会希望有任何一个黑洞接近自己,或我们的星球。然而现在却有一些科学家在自己的实验室里造出了“黑洞”,一个“迷你”黑洞。10 月15 日的《科学》杂志在介绍这种“人造黑洞”时建议,人们可以把这种“黑洞”装进自己的大衣口袋里。

  制造出“人造黑洞”的是中国东南大学的一个研究组,崔铁军教授和程强教授是其中最主要的两位研究者。“实际上,我们做的黑洞不是严格意义上的黑洞。”在接受《外滩画报》采访时,程强教授对记者说。

  实验室里的“人工黑洞”,目的当然不是为了将一个吞噬一切的“恶魔”装进口袋。据程强介绍,现在存在于东南大学毫米波国家实验室的“人造黑洞”,实际上是一个模拟装置,这种模拟装置目前可以吸收微波频段的电磁波,在未来,它还可以吸收光。但是除此之外,它并不能吸收任何实质的东西“它只吸收电磁波,不吸收能量。”程强对记者说。

  这是一个不具有危险性的“黑洞”,不仅如此,这种装置还能在未来用于收集太阳能。在这方面,“人造黑洞”将比世界上任何一种太阳能电池板都更高效。一些物理爱好者甚至为这种全新的装置设计了一些新功能,比如将它装置在航天器中的太阳帆上,或者用来吸收空气中游散的电磁波——因为手机和无线网络的普及,这种看不见的电磁波据说侵害了我们的健康,成为一种新的污染。

  不过,制造“黑洞”的研究者却从来不想那么多,现在崔铁军和程强正在继续的,是如何把实验室里的装置变成样机,“实现工程化”。面对关于“人造黑洞”的各式各样的议论,程强认为, “成果公布以后,被许多国际媒体转载和评论,确实也大大出乎我们意料。从我们个人角度而言,只觉得这是一个比较有意义的工作。

  实验室里的“黑洞”

  “我觉得很惊奇,崔和程这么快就做出了‘人造黑洞’!”看到这个研究成果后,纳瑞马诺维说。

  伊维根·纳瑞马诺维(EvgeniiNarimanov) 是美国印第安纳州西拉斐特市普渡大学的一名教授。今年年初,他和合作者亚历山大·基尔迪谢维(Alexander Kildishev) 一起,发表论文,提出了一种制造小型“黑洞”的理论和设计方案。他们的想法是通过模拟黑洞的一些性质,使在“人造黑洞”附近出现的放射性物质被吸引,然后螺旋式地进入“黑洞”中心。

  “我们的确是受到他的论文的启发,但研究本身是我们独立完成的。”程强对记者说。之所以能这么快将之变成现实,是因为他们所在的实验室也一直从事着这方面的研究,在理论和实验两方面都积累了很多年的经验,实验过程中也用到了很多他们自己的独创性想法。

  不过虽然名为“黑洞”,他们受纳瑞马诺维启发而造的“黑洞”,和真正存在于宇宙中的黑洞还是有大差别的,这种差别并不仅仅体现在质量的大小上。两种“黑洞”的原理其实并不一样。宇宙间的黑洞之所以能吞噬一切,是因为它质量巨大,而实验室里的“黑洞”,实际上是根据光波在被吸进宇宙黑洞时的性质,模拟出来的仪器,可以令光波接近时产生相似的扭曲,并被吸引。

  也就是说,两种“黑洞”可以让附近的光波出现相似的“结局”,但是光波遇到的却并不是同一回事。

  不过目前东南大学实验室里的“黑洞”,还只是适用于某些微波频率,比如人们常用的通信频率, 如GSM、CDMA 和蓝牙等,吸引光波还有待进一步研究,因为光波的频率更短,需要设计的“人造黑洞”尺寸也要更小些。

  超强吸波装置

  这样的“人造黑洞”,在未来可以用于发电。

  “当电磁波遇到这台仪器,就会立刻被捕获,并且立刻被引入到仪器里,一直被吸进黑洞中心。没有电磁波可以逃离这个黑洞。”崔铁军向《科学美国人》杂志描述“人造黑洞”时说。在他们的仪器中,被吸入的电磁波在中心位置转化为热能。

  根据《科学》杂志介绍,“人工黑洞”是一个直径22 厘米的装置。它有60 个同轴环,外层由40 个同心环组成。通过特别设计,研究组令同心环的从外到内的介电常数发生连续变化,而不同的介电常数,则能让电磁波的方向发生相应改变。

  程强把这台仪器描述成“一个超强吸波装置”。可以这样联想,一台“人造黑洞”仿佛一台吸力强大的“吸尘器”,只要它所在的地方有电磁存在,那些电磁波或光波就会源源不断地被它收入囊中,不受任何其他外界条件的限制。

  用于获取能源,这样一个超强吸波装置仿佛正在打开一座看不见却内容丰厚的“宝藏”,用它来吸收太阳能,不仅可以在任何天气里正常工作,甚至将之放入黑暗的宇宙里,它也能收集到同样多的电磁波或光波,并将之转化为热能。

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沙发
HWM|  楼主 | 2010-11-20 23:29 | 只看该作者
IOP Science 上的原文:

http://iopscience.iop.org/1367-2 ... fromSearchPage=true

An omnidirectional electromagnetic absorber made of metamaterials

Qiang Cheng, Tie Jun Cui1, Wei Xiang Jiang and Ben Geng Cai

State Key Laboratory of Millimeter Waves, Department of Radio Engineering, Southeast University, Nanjing 210096, People's Republic of China

1 Author to whom any correspondence should be addressed.

E-mail: tjcui@seu.edu.cn
Received 24 January 2010
Published 3 June 2010

Abstract. In a recent theoretical work by Narimanov and Kildishev (2009 Appl. Phys. Lett. 95 041106) an optical omnidirectional light absorber based on metamaterials was proposed, in which theoretical analysis and numerical simulations showed that all optical waves hitting the absorber are trapped and absorbed. Here we report the first experimental demonstration of an omnidirectional electromagnetic absorber in the microwave frequency. The proposed device is composed of non-resonant and resonant metamaterial structures, which can trap and absorb electromagnetic waves coming from all directions spirally inwards without any reflections due to the local control of electromagnetic fields. It is shown that the absorption rate can reach 99 per cent in the microwave frequency. The all-directional full absorption property makes the device behave like an `electromagnetic black body', and the wave trapping and absorbing properties simulate, to some extent, an `electromagnetic black hole.' We expect that such a device could be used as a thermal emitting source and to harvest electromagnetic waves.  

Contents

1. Introduction
2. Analysis
3. Results and discussions
4. Summary
Acknowledgments
References

1. Introduction

In the past ten years research in metamaterials has attracted great interest in the scientific community. The experimental and theoretical advances in artificial metamaterials have offered scientists potent ways of tailoring the properties of electromagnetic waves in curvilinear space. Metamaterials have manifested several exciting effects and devices, such as negative refraction, electromagnetic invisibility cloaks, super-resolution imaging, electromagnetic concentrators and light trapping [1]–[17], in which the required constitutive parameters could be fulfilled by periodic/non-periodic arrays of electric or magnetic resonant/non-resonant particles. Current technologies for designing and fabricating metamaterials have enabled realization of such functional devices with unusual electromagnetic properties.
In this work, we realize an omnidirectional electromagnetic absorber in microwave frequencies, based on the theoretical prediction using non-magnetic metamaterials [16], which acts like an effective microwave `black body' and absorbs incident waves from all directions efficiently. We designed and fabricated the omnidirectional electromagnetic absorbing device using non-resonant and resonant metamaterial structures, and measured internal electric fields using a planar-waveguide near-field scanning apparatus. Experimental results agree well with the full-wave numerical simulations, which show obvious phenomena of microwave bending and trapping spirally into the device without coming back. The device can absorb electromagnetic waves coming from all directions efficiently with an absorption rate of 99% in the microwave frequency, which could find wide applications in thermal emitting and microwave harvesting.

2. Analysis

In analytical mechanics, the motion of any particle can be described by Hamilton equations

in which p is the generalized momentum, q is the generalized coordinate and  is the Hamiltonian. The Hamiltonian represents the energy of the system, which is usually the sum of kinetic and potential energy, denoted by T and V, respectively, as , in which m is the mass of particle. All these could be deduced from the Hamilton principle.
In geometrical optics, similar to the Hamilton principle, the Fermat principle governs the propagation of light or electromagnetic waves. Considering that the eikonal function St(r, θ) in a two-dimensional (2D) cylindrical coordinate is expanded in series St(r, θ) = S(r, θ)–ωt as a Hamilton–Jacobi equation, we could get the Hamiltonian in optics as [18]

in which pr = ∂S/∂r and pθ = ∂S/∂θ are the radial and angular momenta in the cylindrical coordinate, ω is the radian frequency, μ0 is the magnetic permeability in free space and (r) is the electric permittivity in the isotropic non-magnetic medium. Since the frequency is invariant in a time-harmonic system, the above equation shows an energy conservation in mechanics. From the Hamilton equation, the trajectory of geometrical optics looks like a unit particle in a central potential that is given by [3]: Veff(r) = ω2c2[b–(r)]/2, in which b is the background permittivity and c is the light speed. The inhomogeneous electric permittivity is chosen as

The above permittivity distribution describes a layered dielectric cylinder, which includes a lossy circular inner core and a lossless circular shell with radially varied permittivity, as shown in figure 1(a). Here, Rc and R stand for the radii of the inner core and the outer shell of the cylinder, while b and c represent the dielectric constants of the background medium and the lossy material inside the core. The radius of the core is closely related to the ratio of two dielectric constants: . Under the ray approximation, it has been proved that the light or electromagnetic waves hitting the cylinder will bend spirally in the shell region, and be trapped and absorbed by the lossy core, as illustrated in figure 1(a). It is also shown that the scattering cross-section per unit length of the cylinder is nearly zero, which is independent of the polarization state of incident waves [16]. Hence the dielectric cylinder is a perfectly omnidirectional absorber, behaving like an `electromagnetic black body' or an `electromagnetic black hole' to some extent, which could absorb nearly all light or electromagnetic waves hitting it from every direction.


Figure 1. (a) A model of an electromagnetic omnidirectional absorber composed of a gradient-index metamaterial shell and a lossy dielectric core. (b) Photograph of the fabricated artificial omnidirectional absorbing device based on metamaterials, which is composed of 60 concentric layers, with ELC structures in the core layers and I-shaped structures in the shell layers.

3. Results and discussions

In this section, we validate the electromagnetic omnidirectional absorber through numerical simulations and experiments in microwave frequency. From equation (3), the permittivity distribution of the absorber's outer shell (Rc < r < R) varies gradually from the inner core to the background medium. Hence the outer shell can be realized by gradient refraction index metamaterials, which have been used in the design of several new concept devices such as the invisibility [5] and ground-plane cloaks [7, 8].
A number of non-resonant metallic structures can be utilized as the basic element of gradient refraction index metamaterials, such as the circular ring, I-shaped structure, and Jerusalem cross. The dimensions of unit geometries can be adjusted to meet the demand of refraction indices at specific positions, to achieve the gradient distribution. In non-resonant metamaterials, the resonant frequencies of unit geometries are much higher than the operating frequency, and the dispersion curves for effective permittivity and permeability change slowly in a broad band. Considering the anisotropy of most metamaterial units, the electromagnetic constitutive parameters for the absorber are adjusted for the transverse-electric (TE) polarization in the cylindrical coordinate as z = (r), μ = 1 and μr = 1. In our design, we choose the non-resonant I-shaped structure [12] as the basic unit for the outer shell of the omnidirectional absorbing device, and the electric-field-coupled (ELC) resonator [14] as the basic unit for the inner core, which has large permittivity and large loss tangent simultaneously near the resonant frequency.
The photograph of the fabricated omnidirectional absorbing device is shown in figure 1(b), in which the I-shaped unit cell and ELC resonator are illustrated in figures 2(a) and (b), respectively. The device is placed in the air; hence the permittivity of the background medium is simply b = 1. In order to better demonstrate the absorption effect, a relatively high microwave frequency (f = 18 GHz) is selected in simulations and experiments. The sizes of both the I-shaped unit cell and the ELC resonator are set as 1.8 mm, nearly 1/10 free-space wavelength. The whole omnidirectional absorber is composed of 60 concentric layers, and each layer is a thin printed circuit board (F4B,  = 2.65 + i0.001) etched with a number of sub-wavelength unit structures. From equation (3), the permittivity changes radially in the shell of the absorber; hence the unit cells are identical in each layer but have different sizes in adjacent layers. Since the permittivity is a constant in the lossy core, the ELC resonators are identical in the whole region.


Figure 2. Effective medium parameters for unit cells of the artificial omnidirectional absorbing device. (a) The relation between the effective permittivity (real part of z) and permeability (real parts of μr and μ) and the geometry dimension m for the I-shaped unit. The inset shows the sketch of the I-shaped unit, with w = 0.15 mm and q = 1.1 m. (b) The effective permittivity (real and imaginary parts of z) and permeability (real parts of μr and μ) versus the frequency for the ELC resonator. The inset shows the sketch of the ELC unit, where t = 1.6 mm, g = 0.3 mm, p = 0.15 mm, and s = 0.65 mm.

Figure 2 demonstrates the effective medium parameters of the I-shaped and ELC units for the designed device. The full-wave numerical tool (Microwave Studio, CST2006b) is used to simulate the electromagnetic properties. Following the standard retrieval procedure [19], the effective permittivity and permeability are obtained from the scattering parameters with the change of geometry dimensions. To determine the relation between geometry and medium parameters, an interpolation algorithm has been developed to generate the final layout according to the permittivity distribution required by the omnidirectional absorber. From figure 2(a), by changing the height of the I-shaped unit, the real part of permittivity Re(z) ranges from 1.27 to 12.64 at 18 GHz, while the permeability components, Re(μr) and Re(μ), are always close to unity. The imaginary parts of permittivity and permeability for the I-shaped unit, not shown here, are small enough to be neglected in the design. From figure 2(b), the operating frequency is close to the resonant frequency of the ELC structure, which results in a very lossy permittivity z = 9.20 + i2.65 and the permeability components μr = 0.68–i0.01 and μ = 0.84–i0.14 at 18 GHz.
In our design, the space between adjacent layers is 1.8 mm; hence the radii of the absorber and the lossy core are determined immediately as R = 108 mm and Rc = 36 mm. There are in total 40 layers of I-shaped structures and 20 layers of ELC structures. In order to fix the 60 layers with different radii together, a 0.8-mm-thick styrofoam board has been used with 60 concentric circular slots carved by the LPKF milling machine (LKPF s100). Each layer has three unit cells in the vertical direction, and hence the height of the absorber is 5.4 mm. To investigate the interactions between the fabricated absorber and incident TE-polarized electromagnetic waves, a parallel-plate waveguide near-field scanning system is used to map the field distributions near the absorber at 18 GHz. A similar apparatus is discussed in [20]. The separation between two plates is set as 6.5 mm, which is larger than the height of the device to avoid the unnecessary dragging during measurements. The cutoff frequency of the waveguide system for the dominant TEM mode is 23 GHz. The bottom plate is mounted on a step motor that can translate in two dimensions. A monopole probe is fixed inside the waveguide as the feeding source, and a corner reflector is placed on the back of source to produce the desired narrow beam. Four detection probes are placed on the top plate to measure the field distributions on a plane above the absorber under test, and each probe can scan a region of 200 mm×200 mm independently. Hence the total scanning region is 400 mm×400 mm with a step resolution of 0.5 mm. All probes are connected to a microwave switch that controls the measurement sequence after each movement of the step motors below the bottom plate. The two ports of a vector network analyzer (Agilent PNA-L N5230C) are connected to the feeding probe and the microwave switch, respectively, via cables. Then the measurement data are sent to the controlling computer for post processing.
To demonstrate the performance of the omnidirectional absorbing device in the microwave frequency, we first consider the case of Gaussian-beam incidence. Figures 3(a) and (b) illustrate the distributions of simulated electric fields |Ez| at 18 GHz when a Gaussian beam is incident on the device on-center and off-center, respectively. We note that all on-center rays are directly attracted by the device without reflections, and nearly all off-center rays bend in the shell region spirally and are trapped by the core. To evaluate the absorption of the Gaussian beam by the device, we define an absorbing rate from the Poynting theorem as

in which Pabsorb is the net power entering the device surface and Pin is the incident power. Under the on-center and off-center incidences shown in figures 3(a) and (b), the absorbing rates are calculated as 99.94 and 98.72%, respectively. Clearly, nearly all incident powers are absorbed, and the device behaves like an `electromagnetic black body'.


Figure 3. Distributions of electric fields |Ez| for the designed omnidirectional absorbing device at the frequency of 18 GHz. An electric monopole is placed inside a corner reflector to produce the desired incident beam with finite width. The two circles stand for boundaries of the outer shell and the inner core. (a) The full-wave simulation result under the on-center incidence of a Gaussian beam. (b) The full-wave simulation result under the off-center incidence of a Gaussian beam. (c) The full-wave simulation result under the vertical incidence of the produced narrow beam. (d) The full-wave simulation result under the oblique incidence of the produced narrow beam. (e) The experimental result under the vertical incidence of the produced narrow beam. (f) The experimental result under the oblique incidence of the produced narrow beam.

In experiments at microwave frequencies, however, it is difficult to generate the Gaussian beam. Hence we use a monopole probe with a corner reflector to produce the narrow beam in our experiments, as demonstrated in figures 3(c)–(f). Compared to the Gaussian beam, the produced beam is divergent while propagating. Similar to figures 3(a) and (b), we measure two cases where the electromagnetic waves are incident vertically and obliquely to the omnidirectional absorber. As a comparison, the full-wave numerical simulations are also given to illustrate the absorption effect. Figures 3(c) and (e) illustrate the distributions of simulated and measured electric fields |Ez| at 18 GHz for the vertical-incidence case, which agree with one other very well. It is clear that the incident beam becomes convergent inside the shell region and then enters the lossy core of the device, instead of being divergent in the free-space radiation. The absorbing rate corresponding to figure 3(c) is 99.55%; nearly full absorption. When the beam is incident to the device at an oblique angle of 25°, the waves are bent toward the central area and travel around the shell spirally with distinct absorption, as shown in figures 3(d) and (f). Again, the simulation and experimental results agree well. From figure 3 we see that the device is a good attractor and absorber of microwaves.
Hence we realize the electromagnetic omnidirectional absorber experimentally in the microwave frequency, which could be used to collect microwaves and energies in free space. When the incident waves are not narrow beams, they can also be absorbed efficiently by the proposed device. Figures 4(a) and (b) demonstrate the field and power distributions inside and outside the absorbing device under the incidence of plane waves. Obviously, nearly all incident waves hitting the device are trapped at the center and do not emerge, with an absorption rate of 99.12%. Being absorbed by the device, the incident waves hitting it cannot go through, and the total fields are nearly zero in the front region, making a complete shadow. We also note that the omnidirectional absorbing device almost does not disturb the electromagnetic waves in other regions. When the incident waves are excited by a nearby monopole, the simulated and measured field distributions are shown in figures 4(c) and (d), respectively. As with the case of plane-wave incidence, nearly all incident waves hitting the absorber (see the black dashed lines) are absorbed, producing a shadow region. The simulated and measured results agree well.


Figure 4. (a) Distributions of electric fields for the omnidirectional absorbing device at 18 GHz when a plane wave is incident. (b) Distributions of power flows for the omnidirectional absorbing device at 18 GHz when a plane wave is incident. (c) Full-wave simulation results of electric fields under the excitation of a monopole at 18 GHz. (d) Experimental results of electric fields under the excitation of the monopole at 18 GHz.

4. Summary

In summary, we have designed, fabricated and measured an electromagnetic omnidirectional absorbing device at the microwave frequency using non-resonant I-shaped metamaterials and resonant ELC metamaterials based on theoretical study [16]. We observed that nearly all incident waves and energies hitting the designed device from every direction are attracted and absorbed. Hence the device behaves like an `electromagnetic black body' or `electromagnetic black hole' to some extent. The good agreement between theoretical and experimental results has shown the excellent capability of metamaterials as a candidate for construction of artificial omnidirectional absorbing devices. Since the lossy core can transfer electromagnetic energies into heat energies, we expect that the proposed device could find important applications in thermal emitting and electromagnetic-wave harvesting.
Acknowledgments

This work was supported in part by the National Science Foundation of China under grant nos. 60990320, 60990324, 60871016, 60496317 and 60901011, in part by the Natural Science Foundation of Jiangsu Province under grant no. BK2008031, in part by the 111 Project under grant no. 111-2-05, and in part by the National Doctoral Foundation of China under grant no. 20090092120014

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板凳
雪山飞狐D| | 2010-11-20 23:34 | 只看该作者
2012近了

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地板
baifan46| | 2010-11-21 00:24 | 只看该作者
真的假的??

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5
chunyang| | 2010-11-21 00:35 | 只看该作者
此黑洞不是彼黑洞。

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6
chunyang| | 2010-11-21 00:36 | 只看该作者
这个应该叫作“黑体”,英文中不会有歧义。

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7
maychang| | 2010-11-21 01:06 | 只看该作者
“通过特别设计,研究组令同心环的从外到内的介电常数发生连续变化,而不同的介电常数,则能让电磁波的方向发生相应改变。”
此工作原理早就应用在透镜上了。
玩照像机的都知道,镜头是加膜的,高级镜头可以加上十几层膜,各层膜的折射率不同。加膜的目的是减少反射,增加透射光。
所谓“隐身”飞机,也部分地应用了此原理,飞机表面涂料有多层,各层介电常数不同,以减少雷达波的反射。

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HWM|  楼主 | 2010-11-21 01:24 | 只看该作者
转此文目的在于别被那些“概念”给蒙了。一个电磁范畴的简单玩意儿,硬要往广义相对论上靠。也不想想,若能造成如此大的空间弯曲,得要多大的质能使然。这根本就不是“吸引”,完全不能改变那东西以外空间的电磁波走向。

另外,由于波的相干性,实际折射效果完全不是其所想的那样。其实从他们的图中已见一二。

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chunyang| | 2010-11-21 01:29 | 只看该作者
IOP Science和顶级学术刊物《科学》也不是一码事,中译版在混淆概念。

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10
HWM|  楼主 | 2010-11-21 01:44 | 只看该作者
这个社会,是“概念”泛滥的社会。无论是科学、技术还是经济(如股票),都充斥着形形色色的“概念”,有的是滥用,有的是错位,而有的则是无中生有。因此,建议多学点知识,也许这些学问并不能帮你得到多大的利益,但却能帮你辨别方向。

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参与人数 1威望 +10 收起 理由
maychang + 10
11
chunyang| | 2010-11-21 02:39 | 只看该作者
楼上说的不错。

西方有人说过一句话:问题不在于公众知道的多少,而在于他们知道的往往不是那么一回事。

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12
tianm| | 2010-11-21 02:57 | 只看该作者
外滩画报

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13
宋业科| | 2010-11-21 11:40 | 只看该作者
外国早就有了。涂飞机上的。

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14
小李志| | 2010-11-21 11:42 | 只看该作者
网络分析仪的标准负载是不是也是这个原理啊?

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15
hexenzhou| | 2010-11-21 11:48 | 只看该作者
外滩画报有路边社可靠吗?

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16
AD9851| | 2010-11-21 12:17 | 只看该作者
又一个大忽悠,中国人就知道忽悠

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17
huangqi412| | 2010-11-21 12:22 | 只看该作者
歧义都是故意制造。

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18
huangqi412| | 2010-11-21 12:24 | 只看该作者
隐形技术要进一步了?

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19
crazyleen| | 2010-11-21 12:36 | 只看该作者
:shutup:

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20
HWM|  楼主 | 2010-11-21 15:21 | 只看该作者
继续忽悠,这可不是《外滩画报》

转自:http://scitech.people.com.cn/GB/10686804.html


我国科学家验证“光学黑洞”理论

2009年12月31日09:01  来源:《科学时报》


  近日,在国家自然科学基金和“973”项目的资助下,东南大学教授崔铁军课题组在“电磁黑洞”的研究上又取得了重要进展。他们首次使用构建的实验装置模拟了微波频段的“电磁黑洞”,并在微波频段实验验证了普渡大学科学家提出的“光学黑洞”理论方案。

  所谓黑洞,很容易让人望文生义地想象成一个“大黑窟窿”,其实不然,黑洞一般被认为是引力场达到临界状态的某个区域,可吸收碰到甚至靠近它的一切物体,它的引力场是如此之强,就连光也不能逃脱出来。作为本世纪最具有挑战性、也最让人激动的天文学说之一,许多科学家正在为揭开黑洞的神秘面纱而辛勤工作着,新的理论也不断地提出。

  事实上,这种基于引力场的黑洞很难在实验室里用实验来模拟和验证,但人们可以通过类比方法来研究它的部分性质。电磁黑洞就是其中之一。据悉,崔铁军小组所构建的这个人造电磁黑洞由谐振和非谐振型的新型人工电磁材料构成,通过应用电磁波在非均匀介质中的传播轨迹来类比物质在引力场下弯曲空间中的运动轨迹,并以此模拟黑洞的部分特性。他们的实验结果表明,电磁黑洞能够全向捕捉电磁波,引导电磁波螺旋式地行进,直至被黑洞吸收。在微波频段,黑洞对电磁波的吸收率可达到99%以上。

  上述研究成果于物理在线预印网站arXiv.org公布后,立即受到了国际主流科技媒体的极大关注。英国的《自然》、《新科学家》,美国的《发现》、《科学美国人》、《MIT技术评论》、《物理科学》等刊物都对这项工作作了详细报道,并邀请专家展开评论。新型人工电磁材料学科的创始人之一、伦敦帝国学院的Pendry博士在《科学美国人》的评论中认为,“这一新研究构建了吸收电磁波的全新方法,同时又可以控制电磁波的吸收辐射”。由于对电磁波的高效吸收性,电磁黑洞可望在电磁隐身等方面获得重要应用。

  崔铁军在实验中所用到的新型人工电磁材料(Metamaterial,或称超材料),是指将具有特定几何形状的亚波长宏观基本单元周期性或非周期性地排列所构成的人工材料。它与传统材料的区别在于用宏观尺寸单元代替了原来微观尺寸的原子或分子。因此,新型人工电磁材料的特性取决于其基本单元结构。人们可以通过人为地设计单元结构来控制材料属性,构成自然界不存在的特殊结构材料,进而控制电磁波的传播。

  崔铁军研究组关于新型人工电磁材料的研究一直受到国家自然科学基金的持续资助。在今年年初,崔铁军小组就与杜克大学史密斯教授研究小组合作,在“隐身大衣”研究上迈出了新的一步,他们利用新型人工电磁材料研制出具有频带宽、损耗小的微波频段地面目标的隐身衣。这一研究成果发表在美国《科学》杂志上,崔铁军和史密斯是这篇论文的共同通信作者。他们研制的隐身大衣实际上更像一条“隐身地毯”,将它盖在某个目标上,可以实现对这个目标的宽带隐身。上述论文发表后,同样引起了国际重要科技媒体的广泛关注。(陈晨)

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