Super-lens brings fine features into focus

Super-lens brings fine features into focus
30 April 2005


The Photonics Group, University of Karlsruhe
Dolling's paper in Optics Letters
Photorealistic images of objects in negative refractive index materials
The race to build an exotic material with a negative refractive index for visible light has been won by a team of researchers in Germany. The demonstration could open the door to a new generation of optical devices such as superlenses able to see details finer then the wavelength of visible light.

It may also, ultimately, lead to further breakthroughs in "invisibility cloaks" which could hide objects from the human eye.

Light waves consist of alternating electric and magnetic fields that interact with materials as they travel, or propagate. This interaction determines a property of the material called its refractive index, which is a measure of the behaviour of light as it passes through the material. The refractive index describes the way the light waves bend when they enter and leave the material and the speed at which they propagate.

The refractive index of normal materials is always positive – 1.0003 in air, about 1.5 in ordinary glass, 2.1 in zircon, and 2.4 in diamond. In the mid-1990s, however, John Pendry of Imperial College London, UK, realised that it was possible to construct artificial materials in which the refractive index could be negative.



Scaling down
The trick is to assemble an array of electronic components that resonate with the electric and magnetic fields of the light waves as they pass through. These materials are unlike any conventional substance, hence the name "metamaterial". Pendry suggested that an array of coils and wires much smaller than the wavelength of light would do the trick and first demonstrated the idea for radio waves with a frequency between 15 and 20 megahertz.

Later experiments extended the technique to shorter wavelengths, first into the microwave region and later the infrared. Now Gunnar Dolling at the University of Karlsruhe in Germany, and colleagues, have demonstrated the effect at 780 nanometres – the long-wavelength end of the red spectrum – by scaling down a structure he had developed for infrared wavelengths.

Dolling's metamaterial is made by depositing a layer of silver on a glass sheet, covering this with a thin layer of nonconducting magnesium fluoride, followed by another silver layer, forming a sandwich 100 nm thick. Dolling then etched an array of square holes through the sandwich to create a grid, similar to a wire mesh.

Bending backwards
Dolling determined the refractive index of the material by measuring the "phase velocity" of light as it passed through. His measurements show the structure has a negative refractive index of -0.6 for light with a wavelength of 780 nm.

This value drops to zero at 760 nm and 800 nm, and becomes positive at longer and shorter wavelengths. Previously, the shortest wavelength at which a negative refractive index had been demonstrated was 1400 nm.

The team has not yet observed some of the other exotic effects possible with a negative refractive index, such as the ability to bend light backwards. However, simulations show that negative-index lenses should produce exotic effects over a limited range of wavelengths. For now, Dolling is concentrating on studying the new effects rather than attempting to build devices such as superlenses. These applications are still a long way off, he told New Scientist.

The Chinese version:

德国研究小组成功创造出一种对可见光具有负折射率的奇异材料
据newscientisttech.com网站2006年12月18日报道，德国的一个研究小组赢得了构造一种对可见光具有负折射率的奇异材料的竞争。这项验证可能为研制能够了解比可见光波长更多细节的新一代光学设备如超级透镜开启了一扇门。它可能还会为人眼所不能见的“隐身斗篷”的研究带来进一步的突破。
【学科相关】http://www.SciEI.com/news/news/Photoelec/Index.html


　　光波由交互的电磁场组成，当他们在行进或传播时，这种交互电磁场可以与材料相互作用。这种交互作用决定着材料被称之为它的折射率的特性，折射率是对光经过这一材料时行为的一种量度标准。折射率主要描述当他们进入和离开材料时，光波弯曲的方式以及他们传播的速度。

　　普通材料的折射率始终是正值，在空气中为1.0003，在普通玻璃中为1.5，在锆石中为2.1，在钻石中为2.4。然而，在20世纪90年代，英国伦敦帝国理工学院的约翰·彼德利认为建造折射率为负值的人造材料是可能的。

　　按比例减少
　　
　　这种决窍在于聚集一群当他们经过时可以与光波的电磁场产生共鸣的电子元件。这些材料不像任何常规的物质，因此他们的名字叫作“超颖物质”。彼德利认为，一群比光的波长小得多的镀锡卷板和金属丝可能会实现负折射率。他还首次将这种想法在频率为15兆赫和20兆赫之间的无线电波上进行了论证。随后的实验将这种技术延伸到了更短的波长，并首次进入微波范围，然后是红外线范围。现在，德国卡尔斯鲁厄大学的贡纳尔·多林及其同事已通过按比例减少一种他为红外线波长研发的构造物论证了波长为780纳米的折射效果——红光光谱的长波长末端。多林制成超颖物质的方法是：先在一块玻璃板上沉积一层银，随后在上面覆盖一层薄薄的不传导的镁氟化物，最后再覆上一层银，这样就形成了一块100纳米厚“三明治”。然后，多林在这样金属三明治上蚀刻上一群方孔，以创造出一种类似于金属丝网的栅格。

　　向后弯曲

　　当它经过时，多林通过测量光的“相位速度”来决定材料的折射率。他的测量表明这种构造物对波长为780纳米的光的折射率为-0.6。这一数值在光的波长为760纳米和800纳米时会降到零，在更长和更短的波长时则会变为正值。此前，已被论证的出现负值折射率的最短波长为1400纳米。

　　这个研究小组还没有观察到可能由负折射率带来的其他的一些奇异效果，例如向后弯曲光的能力。然而，模拟实验表明负指数透镜会对一个有限范围内的波长产生奇异的效果。目前，多林正全神贯注地研究新的效果，而不是尝试构建如超级透镜这样的设备。他告诉《新科学家》的记者称中，这些应用还有很长的路要走。
英文原文链接参见：http://www.newscientisttech.com/article/dn10816-red-light-debut-for-exotic-metamaterial.html