nano 2008-05-11 21:01
纳米超结构电子学：未来电子学前沿
[b][color=blue]【纳米科技世界论坛专题】A UPenn researcher is looking into a nanotech approach to let circuits operate in a new way: Discard your definition of current as "movement of electrons and holes." Think electromagnetic wave and metananocircuitry. (In other words, not conduction current that must flow along a path but displacement current which can flow in free space.) If this theory can be proven, new technologies may be able to operate at nanometer scales (like wireless nanoscale communication using light). The building blocks of this technology are dielectric nanoparticles -- some positive (to be used as optical capacitors) and some negative (for optical inductors and resistors).[/color][/b]
A University of Pennsylvania professor is exploring an approach to nanotechnology that will allow circuit theory to operate in an entirely new regime--one where "current" is no longer defined as the movement of electrons and holes, but instead as an electromagnetic wave.
If Nader Engheta's theories prove successful in practice--and researchers are already working on experiments to test this--then the work could strike the elusive balance between finding new technologies that can reliably operate at nanometer scales and ensuring that the technologies can bootstrap on decades of knowledge about more-conventional electronics.
For one thing, Engheta said he is interested the possibility of creating switches from metananocircuitry. They could lead to a new kind of optical information processing and, perhaps, a new form of nanoscale computational unit, said Engheta, the H. Nedwill Ramsey Professor of electrical and systems engineering at Penn.
[img]http://img74.imageshack.us/img74/7144/enghetanager40088aff1hg5.jpg[/img]
Universiity of Pennsylvania engineering professor Nader Engheta
He is also excited about the idea of "wireless at nanoscales using light." In other words, Engheta said, he'd like to investigate the possibility of optical communication between nanostructures or even cells that could be pressed into service in the same way that RF and microwaves are used at other scales.
George Eleftheriades, professor of electrical and computer engineering and a Canada research chair at the University of Toronto, said Engheta's work provides "a vision, consisting of building blocks, along with instructions on how to arrange them together to enable transplanting well-known passive inductor-capacitor-resistor [LCR] electrical networks to the optical domain. This includes the direct optical realization of filters, antennas, power-distribution networks, microwave transmission-line metamaterials and many more."
The building blocks in Engheta's world are dielectric nanoparticles, Eleftheriades explained. Conventional dielectric nanoparticles--those with positive permittivity--"can realize optical capacitors," he said, whereas negative plasmonic nanoparticles, which have negative permittivity, can realize optical inductors and resistors.
[img]http://img410.imageshack.us/img410/59/news1516sbengheta1pg168lf8.png[/img]
"What makes these different from conventional electronic networks," he said, "is that instead of thinking in terms of a conduction current, one should think in terms of the displacement current, which indeed can 'flow' in free space and in dielectric materials."
[b]New kind of circuit board[/b]
Engheta's theory relies on three basic ideas. The first is that nanoparticles of various materials have properties that can be matched to electronic equivalents (such as resistance, inductance and capacitance). Further, the nano- particles can be thought of as "lumped components" that can be connected together into circuits by using additional guiding structures. Finally, the concept of metamaterials--in which composite materials exhibit properties that are dictated by their nanoscale structures rather than their chemistry--is crucial for the design of efficient devices.
To understand how these three ideas work together, it helps first to think about a lone nanoparticle made of some nonmagnetic material, its diameter a small fraction of an optical wavelength. After analyzing this using Maxwell's equations and then equating the electric displacement current density with current, it turns out that if the real part of the material permittivity, Re(e), is greater than zero, then the particle acts as a capacitor for the incoming light. If Re(e) is less than zero, then it acts as an inductor. Finally, if the imaginary part of the permittivity is not equal to zero and so energy is lost (whatever the real part is), then the element can be thought of as having resistance.
Of course, even if the optical and electronic domains can be made equivalent theoretically, the two are very different in practical terms. Electronics do not tend to be leaky; the air or insulator between components prevents current loss. Unfortunately, light cannot be kept from escaping in the same way. To guide the waves, an extra layer of structure is required. Layers of material with a very low permittivity--much smaller than that of a vacuum--can act as terminals, while layers with high permittivity act to prevent propagation. Once these wires and barriers are in place, then networks of devices can be created.
[img]http://img170.imageshack.us/img170/507/news1516sbengheta2pg168xy3.png[/img]
Though all of this is theoretically sound, there is a problem: At optical wavelengths, the ideal materials to implement such circuits don't really exist in nature. The advent of metamaterials, however, may banish that concern. Scientists have already shown that by embedding nanoscale structures of one material inside another, resonances and other interactions can change the bulk properties of the material as a whole. Most famously, this has been demonstrated for negative-refractive-index materials--those that bend light in the opposite direction to conventional, optically dense materials.
[b]Making it real[/b]
Engheta and his team have done simulations of various circuits, including an optical version of a Yagi-Uda antenna structure. It's still unclear, however, whether his ideas can be implemented in practice. Some of the metamaterials that will be required to make them work well have not been invented yet, never mind fabricated. On the other hand, negative-refractive-index materials were demonstrated less than a decade after John Pendry, a professor of theoretical physics at Imperial College London, proposed (hugely controversially) that they were possible. Such precedent may bode well for Engheta's vision, some believe.
[img]http://img206.imageshack.us/img206/2694/news1516sbengheta3pg168ua9.png[/img]
Two teams are already at work trying to demonstrate the basic nanocircuit principles. With his colleagues, Rohit Prasankumar of the Center for Integrated Nanotechnologies at Los Alamos National Laboratory is working on optical nanoantennas that he says should operate as lumped nanocircuit elements at visible wavelengths. "We are currently fabricating these nanoantennas and hope to test their operation as nanocircuits using optical scattering experiments shortly afterward," Prasankumar said. "Sub- sequent experiments will include design, fabrication and testing of more-complex nanocircuits to achieve a desired functionality"--for example, nanotransmission lines.
Prasankumar sees the endeavor as "one of the most exciting developments to emerge from research into metamaterials and their applications in the last few years, particularly if we are successful in making Prof. Engheta's theoretical ideas a reality. I am excited to be working on this project, and hope to have a working optical nanocircuit in the near future."
Penn's physics department is also working on the problem. "We plan to construct specially designed grating structures with periods much less than the operating wavelengths, and then experimentally verify the performance of such nanostructures in terms of optical reflection and transmission," said Penn physicist Marija Drndic.
According to Engheta's predictions, such nanostructures should act as optical filters at nanoscales, she said--for example, bandpass or bandstop filters depending on incident polarization. If successful, Drndic said, the experiment will show "that his concept of lumped circuit elements at optical frequencies will indeed provide useful recipes for design of optical nanocircuits with various functionalities."
[b]Moving ahead[/b]
Though generally effusive about the work, Eleftheriades at the University of Toronto sees some challenges ahead for researchers. Specifically, "plasmonic materials [such as silver and gold] can be lossy when used as interconnects," he said, "and the integration of these optical LCR nanocircuits with active devices such as lasers can be challenging."
Engheta agreed with that analysis, particularly the problem of material loss, but also said he sees huge potential for metananocircuits in the future. n
Sunny Bains is a scientist and technology journalist based in London.
来源：EETime
nano 2008-05-11 21:06
超构纳米电路——电子学的下一个前沿
美国宾夕法尼亚大学教授Nader Engheta正在针对纳米技术研究一种方法，试图使电路理论能够应用在一个全新的体制中。在该体制中，电流不再解释成电子和空穴的移动，而是解释成一种电磁波。
研究者们已经在通过实验来测试Nader Engheta的理论，如果实践证明这个理论是成功的，那将意味着我们可以找到在纳米级可靠工作的新技术，同时这些技术也可获得在过去数十年发展起来的传统电子学知识的支撑。
Engheta指出，首先，他对利用超构纳米电路(metananocircuitry)创建开关很有兴趣。它们可能会产生一种新的光学信息处理器件，或许，还能产生一种新形式的纳米级计算单元。
他对“纳米级光学无线传输”的想法感到非常兴奋。换句话说，他想研究在纳米结构、甚至纳米单元之间进行光学通讯的可能性——就像现在大家常见的RF和微波那样。
加拿大多伦多大学电子和计算机工程教授George Eleftheriades认为，Engheta的工作描述了一种构想，“其中包括光学构件以及把它们组合起来、将众所周知的无源的电阻电容电感 (RLC)电子网络移植到光学领域的方法。其中包括把滤波器、功率分配网络、微波传输线和许多其它东西直接以光学实现。”
在Engheta的世界中，光学构件是电介质纳米微粒，Eleftheriades解释说。传统的电介质纳米微粒具有正介电常数，可以实现光学电容，他指出，而负的等离子纳米微粒具有负的介电常数，可以实现光学电感和电阻。
他解释道：“之所以会产生这些与传统电子网络不同的概念，原因是我们不是从传导电流的角度而是从位移电流的角度进行考虑的，位移电流确实可以在自由空间和在电介质材料中流动。”
[b]打造微观世界的电路板[/b]
Engheta的理论依赖于三个基本的想法。首先，是不同材料的纳米微粒可以匹配对应电子器件(如电阻、电容和电感)；其次，可以把这种纳米微粒看成“集总元件”，能够通过利用额外的导向结构从而被连接在一起构成电路；最后，在超构材料的概念中，复合材料所表现出的性质由其纳米级结构决定，而不是由其化学性质决定，这对设计出高效的器件是至关重要的。
为了理解这三种想法是如何联系到一起的，可以先设想一个由非磁性材料制造的孤立的纳米微粒，其直径为光波波长的若干分之一。使用麦克斯韦方程来对它进行分析，并让电位移电流密度与电流相等，我们就可以得出：如果材料介电常数Re(e)的实部大于零，该微粒对射入光表现为电容；如果Re (e)小于0，那么它表现为电感；如果介电常数的虚部不等于零，则存在能量损失(不管实部为多少)，因而，可以认为该元件具备电阻性。
当然，即使我们在理论上实现了光子域和电子域的等价，两者在实际应用方面仍有很大不同。电子没有泄露倾向；元件间的空气和绝缘体可以防止电流损失。遗憾的是，我们不能以同样的方式阻止光子逃逸。我们需要额外的结构层来引导这些波。介电常数比真空低得多的材料层可以充当端子的角色，而具有高介电常数的层可以充当阻碍传播的角色。在这些导线和屏障都就位之后，就可以创建出由这些器件构成的网络。
尽管所有这些在理论上听起来是可行的，但仍存在一个问题：在光波波长上，实现这种电路的理想材料在自然界中并不真实存在。幸运的是，超构材料的进展有望解决这个难题。科学家们所作的展示已经表明，通过把一种材料的纳米级结构嵌入到另一种材料中，利用共振和其它交互作用可以改变该材料所表现出来的总体性质。更妙的是，负折射率材料(光的折射方向与传统光密材料的反射方向相反)已经表现出这样的性质。
[b]使之变成现实[/b]
Engheta和他的小组已经对不同电路进行了仿真，其中包括Yagi-Uda天线结构的一个光学版本。然而，他的想法是否可以在实践中被实现？这在目前仍不明朗。能使这些器件良好工作所需要的一些超构材料还没有发明出来，更谈不上制造。事实上，在伦敦帝国学院(ICL)的理论物理学教授John Pendry提出了(当时存在很大争议)可能存在负折射率材料很长时间之后(在最近十年内)才出现了这样的材料。有人认为，这样的先例预示着 Engheta的构想将有光明的前景。
已经有两个小组投入研究，试图展示纳米电路的基本原理。Los Alamos国家实验室集成纳米技术中心的Rohit Prasankumar正在与其同事一起，共同研究能在可见光波上作为集总纳米电路元件来工作的光学纳米天线。“我们正在制造这些纳米天线，并希望在近期使用光散射实验来测试这种纳米元件的运行情况。”Prasankumar说，“接下来的实验将包括设计、制造和测试一些更复杂的纳米电路(如纳米传输线)，并使之达到期望的功能。”
Prasankumar把这个努力看作是“在过去的几年里，从研究转向超构材料及应用方面所取得的最激动人心的进步之一，特别是如果我们能成功地使Engheta教授的理论性想法成为现实。我对从事这项研究感到非常兴奋，并希望在不久的将来得到能够工作的光学纳米电路。”
宾州大学物理系也在研究这个问题。“我们打算构建特殊设计的、周期远低于工作波长的光栅结构，然后通过实验来验证这样的纳米结构在光学反射和传输方面的性能。”宾州大学物理学教授Marija Drndic说。
据Engheta预测，这样的纳米结构可以在纳米级扮演光学滤波器的角色，如依赖于入射光偏振特性的带通或带阻滤波器。Drndic说，如果取得成功，“该实验将表明他的光频集总电路元件的概念确实可以为设计具有多种功能的光学纳米电路提供有用的指导。”
[b]继续前行[/b]
尽管人们总体上对这项工作充满热情，多伦多大学的Eleftheriades仍认为前进的道路上研究者们还面临着一些挑战。“特别是，等离子材料(如金和银) 在用于互连时可能是有损耗的。”他说，“这些光学RLC纳米电路与有源器件(如激光器)的集成可能具有挑战性。”
Engheta同意这个分析，特别是对材料损失问题，但也表示他认为超构纳米电路在未来有巨大的潜力。
作者Sunny Bains是科学家兼技术记者，住在伦敦。
译者注：metananocircuit目前在中国还没有合适的中文翻译，由于metamaterial在自然界中不存在，且其性质取决于它特异的结构，所以我们把利用这种超构材料而制造的纳米电路翻译为“超构纳米电路”
来源：[img]http://www.eeworld.com.cn/images/logo.gif[/img]
jackiemwd 2008-10-22 10:38
有没有书已经出版的？想进一步看看！要与时具进嘛！