New superlens opens door to nanoscale optical imaging and high-density optoelectronic devices

UC Berkeley Press Release ^ | April 21, 2005 | Sarah Yang

[PeaceBeWithYou]

BERKELEY – A group of scientists at the University of California, Berkeley, is giving new relevance to the term "sharper image" by creating a superlens that can overcome a limitation in physics that has historically constrained the resolution of optical images.

Schematic drawing of nano-scale imaging using a silver superlens that achieves a resolution beyond the optical diffraction limit. The red line indicates the enhancement of "evanescent" waves as they pass through the superlens. (Images by Cheng Sun, UC Berkeley)

Print-quality image available for download

Using a thin film of silver as the lens and ultraviolet (UV) light, the researchers recorded the images of an array of nanowires and the word "NANO" onto an organic polymer at a resolution of about 60 nanometers. In comparison, current optical microscopes can only make out details down to one-tenth the diameter of a red blood cell, or about 400 nanometers.

The breakthrough, reported in the April 22 issue of the journal Science, opens the door to dramatic technological advances in nanoengineering that could eventually lead to DVDs that store the entire contents of the Library of Congress, and computer processors that can quickly search through such a huge volume of data, the researchers said.

"The field of optics is involved in much of today's technology, including imaging and photolithography, which is used to make semiconductors and integrated circuits," said Xiang Zhang, UC Berkeley associate professor of mechanical engineering and principal investigator of the study. "Our work has a far reaching impact on the development of detailed biomedical imaging, higher density electronic circuitry and ever-faster fiber optic communications."

At left (A) is an image of an array of nanowires 60 nanometers wide created with the silver superlens. The center distance between each nanowire is 120 nanometers. To the right (B) is an image of the same nanowires. In this image, created without the superlens, the individual nanowires are not distinct. The scale bar on both images is 1 micrometer.

Nicholas Fang, one of Zhang's former Ph.D. students and lead author of the paper, said a nearer term application would be the development of medical imaging devices that could reveal never-before-seen details with optical microscopy.

With current optical microscopes, scientists can only make out relatively large structures within a cell, such as its nucleus and mitochondria. With a superlens, optical microscopes could one day reveal the movements of individual proteins traveling along the microtubules that make up a cell's skeleton, the researchers said.

Scanning electron and atomic force microscopes are now used to capture detail down to a few nanometers. However, such microscopes create images by scanning objects point by point, which means they are typically limited to non-living samples, and image capture times can take up to several minutes.

"Optical microscopes can capture an entire frame with a single snapshot in a fraction of a second," said Fang, who is now an assistant professor of mechanical engineering at the University of Illinois at Urbana-Champaign. "That opens up nanoscale imaging to living materials, which can help biologists better understand cell structure and function in real time, and ultimately help in the development of new drugs to treat human diseases."

At top (A) is the higher resolution image of the word NANO created with a silver superlens. Below that (B) is an image created during a control experiment in which the superlens is replaced by spacer layer. The averaged line width is 60 nanometers in image A with the superlens, and 321 nanometer in image B without the superlens. The scale bar in both images is 2 micrometers.

The study is the latest entry in a hotly debated topic among physicists and engineers surrounding the creation of a lens that can break the so-called diffraction limit of optics through negative refraction.

Conventional lenses, whether manmade or natural, create images by capturing the propagating light waves all objects emit and then bending them. The angle of the bend is determined by the index of refraction and has always been positive.

Yet objects also emit "evanescent" waves that carry a great deal of detail but are far more elusive. Such evanescent waves decay exponentially and thus never make it to the image plane, an optics threshold known as the diffraction limit. Breaking this diffraction limit and capturing evanescent waves are critical to the creation of a 100-percent perfect representation of an object, considered the Holy Grail in optics.

In 2000, British physicist John Pendry theorized that a material capable of a negative index of refraction could capture and "refocus" evanescent waves into a perfect image. Pendry's proposed "perfect lens" theory came more than 30 years after Russian physicist Victor Veselago first conceived of a negative refraction material that could reverse known optical phenomena.

These theories are based on the fact that when electromagnetic waves of light reach the surface of a negative refraction lens, they excite a collective movement of surface waves, such as electron oscillations, also known as surface plasmons. That results in an enhancement of the evanescent waves and is different from the way light typically behaves when it reaches a conventional lens.

Various negative refraction experiments were later conducted by researchers at UC San Diego, Boeing and Northeastern University, but they were limited to microwave beams.

In 2003, Zhang's group was the first to confirm that optical evanescent waves are enhanced as they pass through a silver superlens in carefully designed conditions.

But it wasn't until this latest experiment by Zhang's group that optical imaging with a superlens was demonstrated. Zhang and his research team used UV light at a 365-nanometer wavelength in the new experiments, so the image created actually has more detail than is possible with beams in the microwave range.

The array of nanowires imaged measured 40 nanometers wide and the word NANO was about 60 nanometers wide. The objects, embedded onto a layer of chrome, were placed before the superlens, which was a layer of silver that was about 35 nanometers thick. The researchers recorded the image onto a photoresist, a polymer coating on the other side of the superlens that becomes insoluble when exposed to UV light.

"Our work provides a new imaging method that can beat the optical diffraction limit and that has tremendous potential to revolutionize a wide range of technologies," said Zhang. "The key to the superlens is its ability to significantly enhance and recover the evanescent waves that carry information at very small scales. This enables imaging well below the diffraction limit."

Notably, no lens is yet able to completely reconstitute all the evanescent waves emitted by an object, so the goal of a 100-percent perfect image is still out there. However, many scientists believe that a true perfect lens is not possible because there will always be some energy absorption loss as the waves pass through any known material.

"We did not create a perfect image in our experiment," said Fang. "But it's clear that our image is dramatically better than the one created without the silver superlens."

In the long run, this line of research could lead to even higher resolution imaging for distant objects, the researchers said. This includes more detailed views of other planets as well as of human movement through surveillance satellites.

Other authors of the paper are Hyesog Lee, a graduate student in mechanical engineering, and Cheng Sun, a research scientist in Zhang's group.

The research was supported by the Office of Naval Research, the Defense Advanced Research Projects Agency Multidisciplinary University Research Initiative, and the National Science Foundation Center for Nanoscale Science and Engineering.

[PeaceBeWithYou]

Facinating. Enjoy

[PeaceBeWithYou]

Nanotech Ping.

[bd476]

This is great, PeaceBeWithYou. It's exciting thinking about how this technology could impact future medical research applications.

[Nathan Zachary]

Good. Hopefully it can be used to futher the study of prions (protiens) whose 'folds" just can't be seen at the moment.
As far as making things smaller, I can hardly see my cell phone as it is!

[endthematrix]

"A group of scientists "

Xiang Zhang
Nicholas Fang
Dr.Cheng Sun
Yi Xiong
Dr. Hui Liu
Yongmin Liu
Junyu Mai
Sheng Wang
Hyesog Lee

See the trend here?

UC Berkeley Xlab:

http://xlab.me.berkeley.edu/ http://xlab.me.berkeley.edu/Publications/Publications/Science-superlens.pdf

[Citizen of the Savage Nation]

Interesting. Going to have to bone up on this concept of negative index of refraction. Since the index of refraction is simply the ratio of the speed of light in the medium in question vs the speed of light in a vacuum, c, and c is NOT a negative number, that means for negative refraction media, the speed of light through that material would have to be negative. Wow.

[WestVirginiaRebel]

I spy with my really little eye...

[PeaceBeWithYou]

Wrong positive/negative, it is right vs. left handed. The speed of light is the same, but the direction of refraction is different.

[expat_panama]

When lasers first came out in the '60's,  everyone was talking about ray-guns.  We still don't have ray-guns but we do have microchips, disc storage, and fiber optics.

I'm hoping these nano lenses can take us to new horizons also.

[Moonman62]

See the trend here?

Put all those together and it would make quite a name for a law firm.

[Moonman62]

It's all a scam just like the internet. Buy gold. /sarc

[Durus]

You don't think this could lead to hand held "ray" guns?

Darn it!

[July 4th]

Better here than over there.

[Graymatter]

Hooray for the ONR!

[Phsstpok]

See the trend here?

Yeah. They're all Californians.

[Joe Brower]

Nano nano.

[Jabba the Nutt]

But they're OUR Chinese, who are better than THEIR Chinese. ;^)

[PatrickHenry]

Thanks for the ping. I donno if this is for the science list or not, but I've re-configured the ping list logo and I want to use it, so I'll let 'er rip.

[PatrickHenry]

SciencePing

An elite subset of the Evolution list. See the list's description at my freeper homepage. Then FReepmail to be added or dropped.

[general_re]

Dammit, and I was just weeks away from completing my own superlens last year, when the dog knocked over my workbench and ruined everything. Set me back months. Sigh.

[doc30]

Just like the space race during the cold war. Who had the better ex-Nazi rocket scientists: the U.S. or the Soviets? Both space programs were largely successful because of foreigner involvement.

[VadeRetro]

Brain hurt. Hard article.

[furball4paws]

A group of Asian Americans. Once the ChiComs opened the doors, a flood left. We are reaping that harvest. On the down side, a good friend of mine is a Physics Professor. He says it is very rare to have an American graduate student in his department. When I was a graduate student, Chinese were rare, but there was a lot of Japanese.
1. Math/Science education in the USA stinks.
2. Math/Science is not "cool" except to geeks (that is a quote).

[null and void]

Half the Chinese here are spying on us. The other half are spying on them...

[furball4paws]

A good light microscope can resolve things at the 0.2 micrometer range. I would love to have one of these lens things adapted to my Zeiss. I could then watch live bacteria and their flagella and maybe even see pili. Plus some viruses would also become visible. I hope this thing starts a new microscope revolution. There hasn't been a significant improvement in light microscopy in over a hundred years.

[doc30]

Here is the link to the actual Science articel from Prof. Zhang's website.

http://xlab.me.berkeley.edu/Publications/Publications/Science-superlens.pdf

This is very interesting to me becasue I used similar physics in my PhD thesis, but with an entirely different application. However, the effavecent wave decays exponentially, on the nanometer scale, from the surface of the material host to the surface plasmon resonance, so I doubt if this application will be useful in the future to astronomy as the article suggests. Otherwise, it is very exciting and could have a lot of breaktrhoughs for optical microscopy. I am curious if a metal island film would give a stronger throughput than the continuous film in the paper.

[Drew68]

He says it is very rare to have an American graduate student in his department.

So many American students are too busy majoring in "Contemporary Social Activism" and "Ethnic Transgendered Studies" to be bothered with geeky subjects like math and science.

[doc30]

Most freshman science courses treat refractive index as a simple, positive number. In reality, the refractive index is a complex number. It has a real and an imaginary component which is based upon the dielectric constant of the material. Silver, and certain other metals (gold for example) have particular relationships between the real and imaginary components of their dielectric constants that, under the correct conditions, produce surface plasmons that give rise to many interesting optical phenomena.

[Alamo-Girl]

Thanks for the ping!

[BMCDA]

Wow, thanks for the ping. Having been involved with photonics at subwavelength dimensions myself this is pretty interesting stuff.

[longshadow]

Beyond diffraction limit.

Utterly speechless.

[Dad yer funny]

yeah ,...ping for my son to walk me through this

[BMCDA]

Brain hurt. Hard article.

Aww, maybe some of this can alleviate your pain ;^)

[VadeRetro]

That web page touches on how you can see through the microwave door without the microwaves being able to get out at you. The holes, perfectly visible at optical wavelengths, are enough smaller than the relatively long "microwave" length as to be totally invisible to them.

I think the wavelength limitation on resolution is still out there, lurking somewhat below the diffraction limit mentioned in the main article as being breached.

BTW, I half expected your link to point to a Guiness ad or something similar related to alcohol consumption.

[RadioAstronomer]

On the down side, a good friend of mine is a Physics Professor. He says it is very rare to have an American graduate student in his department.

Sigh. I can add to this. A good friend of mine is an engineering Professor. He told me he has not had an American grad student for more than two years. now.

[furball4paws]

That's why they used UV light and why you get such great resolution with electrons that have shorter wavelengths in an electron microscope. But I want to look at live things. I don't know how much lower than UV they can go, but I'll be watching.

As you suggest, there will still be limitations, but an order of magnitude improvement would open up whole new worlds in microscopy.

[Forgiven_Sinner]

"This is very interesting to me Becka's I used similar physics in my PhD thesis, but with an entirely different application"

Cool! Perhaps you can answer some questions I have.

1. When they speak of a negative lens, I assume that means a convex lens will diffuse and and concave one will focus. In other words, the material accelerates the light, rather than retarding it, in comparison to air.
2. Normally silver is opaque and reflective, even to UV. Is there a tunneling (quantum) or photovoltaic effect, or does the material retransmit the light off the other surface?
3. Finally, they speak of energy loss causing the negative lens to be imperfect. Could a superconducting material be a perfect lens?

[perfect stranger]

LOL

[doc30]

What I was doing was surface enhanced spectroscopy, both in the visible and in the infrared region. It's an interesting technique to get up to 1000 times the signal intensity in the experiment. Silver and gold are the materials of choice, but they are used at extreme thinness. The article here used a 30 nm thick silve film so they are in the size domain for surface plasmons to form and it has inspired me to try a few things in the lab if I can get the time.

Let me explain what a surface plasmon is in laymans terms. In metals, the outermost electrons of the atoms in the metal are free to wander around the entire material and are not bound to a particular atom. That's why metals are great conductors. The electrons can move very easily around the entire bulk of the material.

Light can be considered an electromagnetic wave. When light hits a material it can influence the electrons in that material. It can cause the electrons to oscillate in the material with the frequency of the light hitting it. The electrons get some of the energy from the light. For bulk materials, this energy exchange isn't all that exciting since the energy gets lost as heat, but when you are dealing with a metal with the correct optical properties, then interesting things can happen. The oscillations are confined to a very thin layer, smaller than the wavelength of light used so you can consider the metal 2 dimensional from the point of view of these electron oscillations. These oscillations are called plasmons. In this 2 dimensional system, they are called surface plasmons.

The frequency of the light that will cause these oscillations is dependent on the thickness and the shape of the thin metal film. For example, silver is opaque as a bulk material, but on the scale of 5 to 200 nm, it is transparent. The surface plasmons will absorb light depending on the thicknes of the film. At the thin end, it will look yellow and at the thick end it will look blue. Gold is the same way, but typically the plasmons are shifted to longer wavelengths.

The neat thing with surface plasmons is that they can, under the right circumstances, re-radiate the energy that made them oscillate. That's where all their interesting spectroscopic properties applications come from.

This article is very interesting to me because it describes a interesting phenomenon that may have other uses besides building better microscopes. I'm going over the actual physics equations to see if there is something there that can extend into a macroscopic condition. It may be that this negative refractive index exists only at very small distances from the surface, but I want to check the math to be sure.

[Forgiven_Sinner]

Thanks a lot, doc30, for a very clear explanation.

[edwin hubble]

from your reply #39
"The surface plasmons will absorb light depending on the thicknes of the film. At the thin end, it will look yellow and at the thick end it will look blue."

Yes, the thicker the film, the longer the wavelength absorbed.
(different phenomena, but results analogous to the differential Rayleigh scattering of blue light giving red sunsets and blue sky)

On Negative Index of Refraction
The simpler description of Index of Refraction is:
(speed of light in a vacuum) / (speed of light in the medium).
Which would always be positive and greater than or equal to 1.

Here is a description of experiments on materials with a negative index of refraction:

The Reality of Negative Refraction May 2003

http://physicsweb.org/articles/world/16/5/3/1

[Coleus]

New superlens opens door to nanoscale optical imaging and high-density optoelectronic devices

[SunkenCiv]

Blast from the Past.