Tag Archives: photolithography

Better photolithography and nanoscale manipulation and manufacturing (maybe) with flat lenses

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A flat spray-on lens sounded only mildly intriguing as per the May 23, 2012 University of British Columbia news release (UBC engineer helps pioneer flat spray-on optical lens) on EurekAlert. It was the May 24, 2013 news item on ScienceDaily that provided more exciting possibilities,

For the first time, scientists working at the National Institute of Standards and Technology (NIST) have demonstrated a new type of lens that bends and focuses ultraviolet (UV) light in such an unusual way that it can create ghostly, 3D images of objects that float in free space. The easy-to-build lens could lead to improved photolithography, nanoscale manipulation and manufacturing, and even high-resolution three-dimensional imaging, as well as a number of as-yet-unimagined applications in a diverse range of fields.

The May 24, 2013 NIST news release, which originated the news item, describes some of the optical principles at work,

An article published in the journal Nature* explains that the new lens is formed from a flat slab of metamaterial with special characteristics that cause light to flow backward—a counterintuitive situation in which waves and energy travel in opposite directions, creating a negative refractive index.

Naturally occurring materials such as air or water have a positive refractive index. You can see this when you put a straw into a glass of water and look at it from the side. The straw appears bent and broken as a result of the change in index of refraction between air, which has an index of 1, and water, which has an index of about 1.33. Because the refractive indices are both positive, the portion of the straw immersed in the water appears bent forward with respect to the portion in air.

The negative refractive index of metamaterials causes light entering or exiting the material to bend in a direction opposite what would occur in almost all other materials. For instance, if we looked at our straw placed in a glass filled with a negative-index material, the immersed portion would appear to bend backward, completely unlike the way we’re used to light behaving.

In 1967, Russian physicist Victor Veselago described how a material with both negative electric permittivity and negative magnetic permeability would have a negative index of refraction. (Permittivity is a measure of a material’s response to an applied electric field, while permeability is a measure of the material’s response to an applied magnetic field.)

Veselago reasoned that a material with a refractive index of -1 could be used to make a lens that is flat, as opposed to traditional refractive lenses, which are curved. A flat lens with a refractive index of -1 could be used to directly image three-dimensional objects, projecting a three-dimensional replica into free space.

A negative-index flat lens like this has also been predicted to enable the transfer of image details substantially smaller than the wavelength of light and create higher-resolution images than are possible with lenses made of positive-index materials such as glass.

It seems the metamateriels that solve the problem posed by lenses made of glass present a few problems of their own (from the NIST news release),

… For the past decade, scientists have made metamaterials that work at microwave, infrared and visible wavelengths by fabricating repeating metallic patterns on flat substrates. However, the smaller the wavelength of light scientists want to manipulate, the smaller these features need to be, which makes fabricating the structures an increasingly difficult task. Until now, making metamaterials that work in the UV has been impossible because it required making structures with features as small as 10 nanometers, or 10 billionths of a meter.

Moreover, because of limitations inherent in their design, metamaterials of this type designed for infrared and visible wavelengths have, so far, been shown to impart a negative index of refraction to light that is traveling only in a certain direction, making them hard to use for imaging and other applications that rely on refracted light.

To overcome these problems, researchers working at NIST took inspiration from a theoretical metamaterial design recently proposed by a group at the FOM Institute for Atomic and Molecular Physics in Holland. They adapted the design to work in the UV—a frequency range of particular technological interest.

According to co-authors Xu, Amit Agrawal and Henri Lezec, aside from achieving record-short wavelengths, their metamaterial lens is inherently easy to fabricate. It doesn’t rely on nanoscale patterns, but instead is a simple sandwich of alternating nanometer-thick layers of silver and titanium dioxide, the construction of which is routine. And because its unique design consists of a stack of strongly coupled waveguides sustaining backward waves, the metamaterial exhibits a negative index of refraction to incoming light regardless of its angle of travel.

This realization of a Veselago flat lens operating in the UV is the first such demonstration of a flat lens at any frequency beyond the microwave. By using other combinations of materials, it may be possible to make similarly layered metamaterials for use in other parts of the spectrum, including the visible and the infrared.

The metamaterial flat lens achieves its refractive action over a distance of about two wavelengths of UV light, about half a millionth of a meter—a focal length challenging to achieve with conventional refractive optics such as glass lenses. Furthermore, transmission through the metamaterial can be turned on and off using higher frequency light as a switch, allowing the flat lens to also act as a shutter with no moving parts.

“Our lens will offer other researchers greater flexibility for manipulating UV light at small length scales,” says Lezec. “With its high photon energies, UV light has a myriad of applications, including photochemistry, fluorescence microscopy and semiconductor manufacturing. That, and the fact that our lens is so easy to make, should encourage other researchers to explore its possibilities.”

I would have offered some information about what they are spraying onto the lens but neither the NIST nor the University of British Columbia (UBC) news releases provides any details about the ‘spray-on’ aspect of this flat lens. There is this from the UBC news release,

“The idea of a flat lens goes way back to the 1960s when a Russian physicist came up with the theory,” Chau [Kenneth Chau, an assistant professor in the School of Engineering at UBC’s Okanagan campus] says. “The challenge is that there are no naturally occurring materials to make that type of flat lens. Through trial and error, and years of research, we have come up with a fairly simple recipe for a spray-on material that can act as that flat lens.”

The research team has developed a substance that can be affixed to surfaces like a glass slide and turn them into flat lenses for ultraviolet light imaging of biological specimens.

“Curved lenses always have a limited aperture,” he explains. “With a flat lens, suddenly you can make lenses with an arbitrary aperture size – perhaps as big as a football field.”

While the spray-on, flat lens represents a significant advancement in technology, it is only an important first step, Chau says.

“This is the closest validation we have of the original flat lens theory,” he says. “The recipe, now that we’ve got it working, is simple and cost-effective.

For those who want to pursue the research paper, here’s a link to and a citation for it,

All-angle negative refraction and active flat lensing of ultraviolet light by Ting Xu, Amit Agrawal, Maxim Abashin, Kenneth J. Chau, & Henri J. Lezec. Nature 497, 470–474 (23 May 2013) doi:10.1038/nature12158  Published online 22 May 2013

The paper is behind a paywall.

Responsible science communication and magic bullets; lego and pasta analogies; sing about physics

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Cancer’s ‘magic bullet],  a term which has been around for decades, is falling into disuse and deservedly. So it’s disturbing to see it used by someone in McGill University’s (Montreal, Canada) communications department for a recent breakthrough by their researchers.

The reason ‘magic bullet for cancer’ has been falling into is disuse because it does not function well as a metaphor with what we now know about biology. (The term itself dates from the 19th century and chemist, Paul Erlich.) It continues to exist because it’s an easy (and lazy) way to get attention and headlines. Unfortunately, hyperbolic writing of this type obscures the extraordinary and exciting work that researchers are accomplishing. From the news release on the McGill website (also available on Nanowerk here),

A team of McGill Chemistry Department researchers led by Dr. Hanadi Sleiman has achieved a major breakthrough in the development of nanotubes – tiny “magic bullets” that could one day deliver drugs to specific diseased cells.

The lead researcher seems less inclined to irresponsible hyperbole,

One of the possible future applications for this discovery is cancer treatment. However, Sleiman cautions, “we are still far from being able to treat diseases using this technology; this is only a step in that direction. Researchers need to learn how to take these DNA nanostructures, such as the nanotubes here, and bring them back to biology to solve problems in nanomedicine, from drug delivery, to tissue engineering to sensors,” she said.

You’ll notice that the researcher says these ‘DNA nanotubes’ have to be brought “back to biology.” This comment brought to mind a recent post on 2020 Science (Andrew Maynard’s blog) about noted chemist and nanoscientist’s, George Whitesides, concerns/doubts about the direction for cancer and nanotechnology research. From Andrew’s post,

Cancer treatment has been a poster-child for nanotechnology for almost as long as I’ve been involved with the field. As far back as in 1999, a brochure on nanotechnology published by the US government described future “synthetic anti-body-like nanoscale drugs or devices that might seek out and destroy malignant cells wherever they might be in the body.”

…

So I was somewhat surprised to see the eminent chemist and nano-scientist George Whitesides questioning how much progress we’ve made in developing nanotechnology-based cancer treatments, in an article published in the Columbia Chronicle.

Whitesides comments are quite illuminating (from the article, Microscopic particles have huge possibilites [sic], by Ivana Susic,

George Whitesides, professor of chemistry and chemical biology at Harvard University, said that while the technology sounds impressive, he thinks the focus should be on using nanoparticles in imaging and diagnosing, not treatment.

The problem lies in being able to deliver the treatment to the right cells, and Whitesides said this has proven difficult.

“Cancer cells are abnormal cells, but they’re still us,” he said. [emphasis is mine]

The nanoparticles sent in to destroy the cancer cells may also destroy unaffected cells, because they can sometimes have cancer markers even if they’re healthy. Tumors have also been known to be “genetically flexible” and mutate around several different therapies, Whitesides explained. This keeps them from getting recognized by the therapeutic drugs.

The other problem with targeting cancer cells is the likelihood that only large tumors will be targeted, missing smaller clumps of developing tumors.

“We need something that finds isolated [cancer] clumps that’s somewhere else in the tissue … it’s not a tumor, it’s a whole bunch of tumors,” Whitesides said.

The upside to the treatment possibilities is that they buy the patient time, he said, which is very important to many cancer patients.

“It’s easy to say that one is going to have a particle that’s going to recognize the tumor once it gets there and will do something that triggers the death of the cell, it’s just that we don’t know how to do either one of these parts,” he said.

There is no simple solution. The more scientists learn about biology the more complicated it becomes, not less. [emphasis is mine] Whitesides said one effective way to deal with cancer is to reduce the risk of getting it by reducing the environmental factors that lead to cancer.

“It’s a biology problem, not a particle problem,” he said. [emphasis is mine]

If you are interested , do read Andrew’s post and the comments that follow as well as the article that includes Whitesides’ comments and quotes from Andrew in his guise as Chief Science Advisor for the Project on Emerging Nanotechnologies.

All of this discussion follows on yesterday’s (Mar.17.10) post about how confusing inaccurate science reporting can be.

Moving onwards to two analogies, lego and pasta. Researchers at the University of Glasgow have ‘built’ inorganic (not carbon-based) molecular structures which could potentionally be used as more energy efficient and environmentally friendly catalysts for industrial purposes. From the news item on Nanowerk,

Researchers within the Department of Chemistry created hollow cube-based frameworks from polyoxometalates (POMs) – complex compounds made from metal and oxygen atoms – which stick together like LEGO bricks meaning a whole range of well-defined architectures can be developed with great ease.

…

The molecular sensing aspects of this new material are related to the potassium and lithium ions, which sit loosely in cavities in the framework. These can be displaced by other positively charged ions such as transition metals or small organic molecules while at the same time leaving the framework intact.

These characteristics highlight some of the many potential uses and applications of POM frameworks, but their principle application is their use as catalysts – a molecule used to start or speed-up a chemical reaction making it more efficient, cost-effective and environmentally friendly.

Moving from lego to pasta with a short stop at the movies, we have MIT researchers describing how they and their team have found a way to ‘imprint’ computer chips by using a new electron-beam lithography process to encourage copolymers to self-assemble on the chip. (Currently, manufacturers use light lasers in a photolithographic process which is becoming less effective as chips grow ever smaller and light waves become too large to use.) From the news item on Nanowerk,

The new technique uses “copolymers” made of two different types of polymer. Berggren [Karl] compares a copolymer molecule to the characters played by Robert De Niro and Charles Grodin in the movie Midnight Run, a bounty hunter and a white-collar criminal who are handcuffed together but can’t stand each other. Ross [Caroline] prefers a homelier analogy: “You can think of it like a piece of spaghetti joined to a piece of tagliatelle,” she says. “These two chains don’t like to mix. So given the choice, all the spaghetti ends would go here, and all the tagliatelle ends would go there, but they can’t, because they’re joined together.” In their attempts to segregate themselves, the different types of polymer chain arrange themselves into predictable patterns. By varying the length of the chains, the proportions of the two polymers, and the shape and location of the silicon hitching posts, Ross, Berggren, and their colleagues were able to produce a wide range of patterns useful in circuit design.

ETA (March 18,2010): Dexter Johnson at Nanoclast continues with his his posts (maybe these will form a series?) about more accuracy in reporting, specifically the news item I’ve just highlighted. Check it out here.

To finish on a completely different note (pun intended), I have a link (courtesy of Dave Bruggeman of the Pasco Phronesis blog by way of the Science Cheerleader blog) to a website eponymously (not sure that’s the right term) named physicssongs.org. Do enjoy such titles as: I got Physics; Snel’s Law – Macarena Style!; and much, much more.

Tomorrow: I’m not sure if I’ll have time to do much more than link to it and point to some commentary but the UK’s Nanotechnologies Strategy has just been been released today.