A revolution in lens-making

Understanding of optics has changed no end since the world’s oldest known lens was ground nearly 3,000 years ago in modern-day Iraq. Yet its Assyrian maker would instantly recognise today’s lenses, which continue to be made much as they were then: by fashioning a piece of transparent material into a solid with curved surfaces. Just as invariably, the curves introduce optical aberrations whose correction requires tweaking the lens’s geometry in complicated ways. As a consequence, lenses remain bulky, especially by the standards of modern electronics.

Enter Federico Capasso, of Harvard University. He and his colleagues have created a lens that is completely flat and the width of two human hairs. It works because its features, measured in nanometres (billionths of a metre), make it a “metamaterial”, endowed with some weird and useful properties.

According to the laws of quantum mechanics, a particle of light, called a photon, can take literally any possible path between source A and point B. However, those same laws stipulate that the path of least time is the most likely. When a photon is travelling through a uniform medium, like a vacuum, that amounts to a straight line. But although its speed in a vacuum is constant, light travels at different (lower) speeds in different media. For example, it moves more slowly in glass than it does in air. So in a medium composed of both air and glass, light’s most likely path from A to B will depend on the thickness of glass it needs to traverse, as well as the total distance it needs to cover. That means that the light may sometimes prefer to bend. This is the quantum-mechanical basis of refraction.

In order to maximise the probability that photons from A will end up precisely at B, those going in a straight line need to be slowed down relative to those taking a more circuitous route, so that, in effect, all hit B the same time. This can be done by forcing the former to pass through more glass than the latter. The result is a round piece of glass that is thick in the middle, where the straight-line path crosses, and tapers off towards the edge, where the less direct routes do—in other words, a focusing lens, with its focal point at B.

Dr Capasso’s lens, described in Nano Letters, also slows photons down. But instead of using varying thickness of glass to do the job, he and his team created an array of antennae which absorb photons, hold on to them for a short time and then release them. In order for this trick to work, though, the distance between the antennae has to be smaller than the wavelength of the light being focused. In Dr Capasso’s case that means less than 1,550 nanometres, though he thinks that with tweaking it could be made to work with shorter-wavelength visible light, too.

Creating the array involved coating a standard silicon wafer, 250 microns thick, with a 60-nanometre layer of gold. Most of this layer was then stripped away using a technique called electron-beam litography, leaving behind a forest of V-shaped antennae arranged in concentric circles. By fiddling with their precise shape, after much trial and error, antennae lying on different circles could be coaxed into holding on to the photons for slightly different lengths of time, mimicking an ordinary glass lens. The whole fragile system can be sandwiched between two sheets of transparent material to make it more robust.

At present the new-fangled lens only works for monochromatic light and so is unlikely to replace the glass sort in smartphone cameras anytime soon. But it could revolutionise instruments that rely on single-colour lasers, by making further minaturisation possible while eliminating the optical aberrations inherent to glass lenses. Such devices include laser microscopes, which are used to capture high-resolution images of cells, or optical data storage, where a more accurate and smaller lens could help squeeze more information into ever less space.

First published on economist.com.

References: 

  1. Capasso et al., Aberration-Free Ultrathin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces, Nano Letters2012.
  2. Capasso et al., Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction, Science2011.

Also appeared in The Economist. Also available in audio here.

Image credit: Francesco Aieta

The evolution of venom: Poison pill

The bite of a rattlesnake can, within minutes, cause paralysis and extensive internal bleeding. If untreated it can kill. It might also hold the key to treating high blood pressure, heart diseases and stroke. In 1998 two drugs to prevent heart attacks, derived from rattlesnake and viper venoms, were approved. Since then a number of other venom components have proved effective against some varieties of cancer and brain disorders like Alzheimer’s or Parkinson’s, but gaining regulatory approval has proved tricky. Part of the reason is that it is difficult to tweak toxins such that they preserve their medicinal effects but lose their nefarious ones, like stanching blood flow or numbing the nervous system.

Now, though, Wolfgang Wüster, of Bangor University, in Britain, and his colleagues have stumbled on an evolutionary mechanism that might make such modifications easier in future. Dr Wüster was investigating venomous snakes and lizards to understand what they had in common. They differ in many respects—most venomous lizards, for instance, have fangs in the lower jaw, whereas snakes have them in the upper jaw. But in 2005 Bryan Fry of Sydney University found that snakes and lizards in fact share venom-making genes, suggesting that both share a venomous reptilian ancestor.

Whereas Dr Fry looked only at selected venom-making genes, Dr Wüster had the luxury of complete genetic data for different snake and lizard species. This allowed him to check if venom-spitting reptiles possess other shared genetic traits, too. As he and his team report in Nature Communications, they do.

These include genes to produce enzymes that perform some basic physiological functions. Intriguingly, some of these housekeeping genes were sitting among venom-producing ones. Venom genes are known to have evolved from more innocuous sorts, but it was thought that all the genes in a particular stretch of DNA assumed the venom-producing function. To find some that did not, therefore, posed a quandary. Were the innocuous genes among the insidious ones simply evolutionary relics? Were they evolved versions of the original innocuous genes that, unlike their venom-producing neighbours, remained innocuous? Or did they in fact evolve from venom genes that had lost their venom-producing prowess?

To help decide the matter, Dr Wüster ran a computer model to trace the genes’ evolutionary histories. This revealed that the third scenario was the most likely. Moreover, it seems that certain housekeeping genes turned into venom-producing ones and back again several times in reptiles’ genetic past. This means that the venom-producing genes and the housekeeping variety nested among them are genetically similar. As such, they produce proteins which are themselves alike in many respects, but not necessarily in their ability to do harm.

Practical applications of this knowledge are not an immediate prospect. But by understanding what makes a venom protein venomous researchers may get a better idea of how to remove the unwanted sting. That is one trick drugmakers would love to be able to pull of.

First published on economist.com.

Image credit: The Economist

Circumcision: Snipped in the bud

IN 1999 the American Academy of Paediatrics (AAP) declared that although circumcision carries some health benefits, these do not outweigh the risks of giving a newborn lad the snip. Since then the number of circumcisions in America has fallen from 61% to 56% of baby boys, though it remains well above the global figure of about 30%. In 2007 the AAP set up a task force to update the recommendation. After poring over 1031 peer-reviewed papers, its experts reversed it altogether.

It has been known for a while that circumcision reduces the risk of HIV infection, because the immune cells under the foreskin are vulnerable to the virus. This has led to vastly more circumcisions being performed in AIDS-ridden parts of Africa. The AAP’s report revealed that circumcision is also associated with lower rates of infection with the human papillomavirus (HPV), which has been found to collect under the foreskin, and herpes simplex. Since cervical cancer in women is caused by HPV infections, circumcision has some protective effect on men’s female partners too. Nor did the task force find any evidence that circumcision lowers sexual function or pleasure. One study of 5,000 Ugandan men found not only that intercourse was less painful for circumcised men but that, two years after the procedure, they were in fact more sexually satisfied.

These benefits accrue to sexually active adults, not newborns or children, but Colleen Cagno, a paediatrician at the University of Arizona, points out that overall risk of complications is lower when circumcision is performed soon after birth. One reason might be that procedures carried out later on in life tend to involve general anaesthesia, which brings its own risks and which newborns are spared. In any case, circumcisions rarely go wrong in rich countries, where patients can expect proper medical care. In Israel only 0.3% of circumcisions lead to any complications. There is no overall figure for America, but just 0.2% of circumcisions result in “acute” complications.

The report finds that circumcision of newborns benefits them long before they reach sexual maturity. It reduces the risk of urinary tract infections (UTIs) in boys under the age of two. (UTIs are rare among toddlers, but the protection offered may be for life; almost half the male population will suffer from a UTI at least once in life.) The report’s authors also reckon that every 909 snips (that is, 0.1% of cases) will lead to one case of penile cancer averted. A cost-benefit analysis recently published in British Medical Journal finds that an average uncircumcised man will incur an additional $450 in health-care costs over a lifetime. This may not sound much, but it is an average; for some individuals the costs may be much greater.

If removing the foreskin brings so many benefits, why was the custom adopted in the first place? Brian Morris, a physiologist at the University of Sydney who together with colleagues reviewed the literature independently and came to the same conclusion as the AAP, thinks that the extra flesh may have played an important role in thwarting infections, acting as “nature’s underpants” when man lived in caves. In the modern, hygienic world, such paleolithic protection may be redundant. Or, as in the case of HIV and HPV, positively harmful.

According to the World Health Organisation circumcision is one of the most common medical procedures in the world. The AAP’s recommendations are coming up against millennia-old religious rituals (see article in the print edition). All the more reason that its call for the snip to be carried out only by trained professionals, using painkillers, and with parents’ informed consent, deserves attention.

First published on economist.com.

The physics of sand castles: Just add water

A day out on the beach would be incomplete without a sand castle. The mightier the castle, the better. But sand is next to useless as a building material. Without water it simply spreads out as wide as possible. So in search of a good recipe Daniel Bonn, a physicist at the University of Amsterdam, and colleagues have stumbled upon a formula for making the perfect sandy redoubt.

As they reveal in a paper published this week in Scientific Reports the key is to use sand with only 1% water by volume. Wet sand has grains coated with a thin layer of water. Owing to water’s surface tension this thin coat acts like skin stretched over many grains, holding them together by creating bridges between the grains. The strength of these bridges is enough to fight Earth’s gravity and prevent the structures from buckling under their own weight.

An easy way to achieve the right amount of water, Dr Bonn suggests, is to tamp wet sand in a mould (open at the top and the bottom) with a thumper at least 70 times, as he did in his experiments.

As for the design itself, unsurprisingly, the wider the base the taller the castle. According to calculations, using ideally moist sand, a column with a three inch diameter could rise as high as two metres. At 12 metres, the current world record for the tallest sandcastle, set by Ed Jarrett in 2011, used a base of roughly 11 metres. If Dr Bonn is right, sand engineers could in principle beat that with a castle thrice the height upon the same foundation.

First published in The Economist.

Image from here.