Archive for the ‘The ant playground’ Category

Visible light metamaterials on the cheap

Tuesday, March 10th, 2009


Only a couple of years, more than a few physicists doubted that it would ever be possible to build decent metamaterials with a negative refractive index for visible light.

Metamaterials have bulk properties that depend on the structure of their components rather than the bulk properties of the materials from which they are made. The thinking is that they can make light do all kinds of things that are no possible in naturally occurring stuff such as bending light backwards and imparting it with a reverse Doppler shift.

Metamaterials that bend microwaves backwards are straightforward to make: it’s just a question of arranging components, such as conducting wires and split rings, in a periodic 3D array on a centimetre scale.

It’s easy to think that similar structures would work for visible light were they shrunk to the nanometre scale. But, as many physicists have pointed out, the electrical properties of conducting metals do not scale with wavelength in quite the same way. Instead of transmitting light, many of these designs would be opaque to visible light.

Some people said it may never be possible to make efficient negative refraction index metamaterials for visible light. Others, who were a little more optimistic, were vindicated  last August, Xiang Zhang at the University of California, Berkeley, revealed that a periodic array of parallel silver nanowires embedded in aluminium oxide worked perfectly well as metamaterial with negative refractive index for visible light.

Now Akhlesh Lakhtakia  at Pennsylvania State University and pals have worked out how to make sheets of this stuff using a vapour deposition technique that is common in the optical industry.

So in a couple of years, we’ve gone from having little prospect of a negative refractive index material for visible light to a way of making sheets of it at extremely low cost.

That’ll make negative refractive index materials available to almost anybody who wants to play with them. Expect to see some ingenious applications in the coming months.

Ref: Vapor-deposited thin Films with Negative refractive Index in the Visible Regime

Centimetre scale models could compute Casimir forces

Thursday, March 5th, 2009


The Casimir force is notoriously difficult to measure. So tricky is it, that the first accurate measurements weren’t made until 1997 and even today only a handful of labs around the world of capable of taking its measure.

Of course there are various ways of modelling what goes on theoretically but even the most powerful simulations these struggle to cope with simple shapes let alone complex geometries. Consequently, our knowledge of the Casimir  force and how to exploit it is poor.

Now John Joannopoulos and pals at MIT are suggesting a rather entertaining third way: to calculate Casimir forces using scale models that work like analogue computers.

What the team has noticed is a mathematical analogy between the Casimir force acting on microscopic bodies in a vacuum and the electromagnetic behaviour of macroscopic bodies floating in a conducting fluid.

So imagine you want to know what Casimir forces will act on a particular geometry. The idea is to build a centimetre scale metal model of this set up and place it in a conducting liquid such as saline. Then bombard it with microwaves and see what happens.

The result should give an accurate representation of the Casimir forces that would act on the microscopic scale.

The group explains:

Such a centimeter-scale model is not a Casimir “simulator,” in that one is not measuring forces, but rather a quantity that is mathematical related to the micron-scale Casimir force. In this sense, it is a kind of analog computer.

There’s no reason why those kinds of tests can’t be done now.  And that should give researchers a way of testing machines designed to reliably exploit the Casimir force for the first time.

Ref: Ingredients of a Casimir Analog Computer

Solving stiction in MEMs devices

Monday, December 15th, 2008


Microelectromechanical devices were supposed to change the world, so where are they?

A few designs have leaked out, such as the accelerometers in air bags. But most have remained stubbornly, and literally, stuck in the lab.

One of the troubling secrets about MEMs is that many designs simply don’t work because their moving parts become stuck fast and refuse to budge.

Engineers call this “stiction”: a vaguely defined force that affects small parts but not large ones (where inertia plays a greater role in overcoming these forces). Stiction is thought to be caused variously by Van der Waals forces, electrostatic forces, hydrogen bondng and even the Casimir force, perhaps in combination. That’s why it’s so hard to avoid.

Now Raul Esquivel-Sirvent and a buddy at the Universidad Nacional Autonoma de Mexico in Mexico City, have a potential solution based on the acoustic Casimir force, an acoustic analogue of the more famous quantum Casimir force which was discovered 10 years ago by Andres Larraza, then at the Naval Postrgraduate School in Monterey.

Here’s the idea: place a couple of parallel plates close together and blast them with sound waves of a specific frequency range. If the waves are larger than the gap between the plates, they will tend to push them together but if they are smaller, they will squeeze into the gap and tend to push them apart. So changing the wavelength or the distance between the plates switches the direction of the force.

That could be useful for microswitches in MEMs devices, says Esquivel-Sirvent. But more interestingly, it could also be used to separate microcomponents that have become stuck together. Perhaps the promise of MEMs will be realised at last.

Ref: Pull-in Control in Microswitches Using Acoustic Casimir Forces

How to decelerate a molecule

Wednesday, December 10th, 2008


When it comes to shuttling individual atoms about, physicists have made giant strides in cooling, trapping and even collimating them into matter wave beams. These kinds of tricks are already being used for matter-wave interferometry on chips.

But if you want to do the same kinds of things with molecules, you’re out of luck. There are two problems. First, molecules are much harder to slow down and trap in decent quantities. And second, they are much more difficult to ID. Atoms are usually identified by the light emitted by electronic transitions, which are usually in the visible part of the spectrum. In most molecules, however, these transition are in the UV and so much harder to access.

Now Samuel Meek and friends from the Fritz-Haber Institute, a Max Plank Institute in Berlin, have tackled one of these problems by building a molecular decelerator on a chip. The device consists of an array of electrodes that create an electric field with a local minimum, or well, that polar molecules tend to fall into. The well can be moved along the array.

Decelerating molecules is then a matter of matching the velocity of the well to that of the incoming molecules and then rapidly slowing it down. Meek and co say that in this way they have halved the kinetic energy of carbon monoxide molecules by slowing them from 360m/s to 240m/s.

That’s impressive and the team reckons that with a little tweaking, the chip will be able to bring the CO molecules to a standstill.

Strangely, nobody has given much thought to what you can do with stationary CO molecules. One option is to use them to store qubits for quantum computing but there seem to be few other ideas.

Which means there’s a good opportunity here for a creative thinker to make a mark.

Ref: A Stark Decelerator on a Chip

Matter wave lithography could carve single nanometre features

Monday, November 24th, 2008


Atomic matter waves have been generating a bit of interest of late. The thinking is that atom waves can be manipulated in much the same way as light waves and so could be used to directly print atoms onto microchips to create nanoscale features.

The question is: how small can these features be made and how accurately can they be put in place? An obvious answer is that these features can’t be smaller than the size of the atoms involved and placed no more accurately than the size of the wavelength of these atom ie diffraction limited. And until a few years ago, nobody would have argued with that.

Since then, numerous techniques have been been developed for manipulating light beyond the diffraction limit so it’s reasonable to assume that a similar thing can be done with matter waves.

And so it has come to pass. Jordi Mompart at the Universitat Autonoma de Barcelona in Spain and a few amigos have shown that it is possible to achieve single nanometre resolution with matter waves of rubidium. At least in theory.
All we need now is somebody to step up to the plate and do it for real. Judging by the speed of progress in this field right now, that shouldn’t take long.

And beyond that: the first microchips patterned using matter waves rather than light?

Ref: Coherent Patterning of Matter Waves with Subwavelength Localization

Silicon ribbons pave the way for graphene-like sheets

Wednesday, November 19th, 2008


Graphene is the hottest property in materials science these days. Its extraordinary electronic, thermal and physical properties make it the most heavily studied substance on the plant right now.

But there is one thing that graphene can’t do and that is to fit easily into the silicon-based electronics industry. And while graphene based chips hold much promise, it’s hard to see chip makers re-tooling to use carbon instead of silicon in the near future.

That’s why a number of groups have become to look at the possibility of making silicon versions of grahene, a material called silicene. Silicon nanowires made their first appearance in 2005. And now Christelle Leandri at the Center for Interdisciplinary Nanoscience in Marseille, France, and a few buddies have made silicene for the first time, albeit in the form of stripes or nanoribbons.

What the team has done is create parallel stripes of silicene, just one atom thick on a silver substrate. The team says the physical and chemical properties of these nanoribbons is striking.

For a start silicon nanoribbons seem to be more chemically stable than their graphene cousins. In particular, graphene is highly reactive around its edges where carbon bonds dangle freely. This can make graphene hard to handle. The edges of silicene on the other hand seems to be naturally inert.

Leandri et amis have high hopes for silicene, saying that it could be incorporated into current manufacturing processes and thereby “help prolong the life of Moore’s law.”

Tanatalising work.

Ref: Physics of Silicene Stripes

Nanodiamonds lead to sharper images

Friday, September 19th, 2008


Zap a diamond nanoparticle with laser light and it will fluoresce, emitting single photons if it is small enough.  That makes nanodiamonds extremely useful, say Aurélien Cuche at the Université Joseph Fourier in Grenoble and pals.

For a start, nanodiamonds are easily absorbed by cells, which allows them and the processes inside them to be tracked with ease.

But Cuche and co have found a more exciting use: they have attached a nanodiamond the tip of a scanning near field microscope to provide single photon illumination when needed. And this dramatically improves the resolution of these devices, says the team.

Which means that nanodiamonds, cough, are a microscopist’s best friend.

Ref: Diamond Nanoparticles as Photoluminescent Nanoprobes for Biology and Near-Field Optics

Nanotube springboard is world’s most sensitive weighing scales

Tuesday, September 16th, 2008


Vibrating springboards have long been the darlings of nanomechanics wanting to measure the mass of small things.

Their thinking goes like this: a springboard vibrates at a specific resonant frequency that depends on its stiffness and mass. So you can work out the mass of anything that becomes stuck to the springboard by measuring any change in its resonant frequency.

Various groups have used this idea to detect all kinds of organic and inorganic molecules using springboards carved out of silicon.

But improving the sensitivity even further means reducing the mass of these springboards. The question is how.

The answer is provided today by Alex Zettl and his team at the University of California, Berkeley, who have created a springboard out of a single carbon nanotube. And their machine is one helluvan elegant device.

For starters, they exploit nanotubes’ unusual ability to act as radio transmitters to determine how fast it is vibrating. They zap the nanotube with radio waves and listen out for the radio signals it emits in return. This signal tells them how fast the nanotube is vibrating.

And because nanotubes are four orders of magnitude lighter than silicon cantilevers, they are four orders of magnitude more sensitive to mass.

All that adds up to device that is able to measure the mass of individual gold atoms as they settle on to its surface. “The sensitivity of our device is 0.40 Au atoms/√Hz. This is the lowest mass noise ever recorded for a nanomechanical resonator,” say Zettl and buddies.

And if that doesn’t impress you, how about this: their measurements were made at room temperature rather than in cryogenic conditions.

The team points out that their device works as a unique kind of mass spectrometer: it is compact, does not require powerful magnets and can easily be built into a chip. Expect to see more of them.

Ref: An Atomic-Resolution Nanomechanical Mass Sensor

First printed graphene circuits

Monday, September 15th, 2008


The world of solid state electronics is in awe of graphene. This single layer of carbon chickenwire has the potential to revolutionise electronics (and much else) because it has enviable electronic, mechanical and thermal properties that no other material can match.

The news today is that Ellen Williams and buddies at the University of Maryland at College Park have worked out how to use graphene in printed circuits. The technique is conceptually straightforward: simply stamp the graphene onto a plastic circuit board in the required shape and hope it sticks. Then stamp other components on top of it. And voila! A printed graphene circuit.

Williams and her team have wasted no time demonstrating the technique, by printing a field effect graphene transistor onto a plastic substrate. “This represents the ultimate extension of printing technology to a single atomic layer,” they say.

Engineers are going to be fascinated by the properties of these transistors, not least because they are almost perfectly transparent at optical frequencies.  Anybody think why that might be useful?

Of course, there is much work to be done, not least in making graphene itself. At the moment, Williams (and almost everyone else) can only get graphene by chipping away at a block of graphite and hunting for graphene in the debris.  That’s a technique even Fred Flintstones would recognise. Any advance is going to be hugely welcomed.

Ref: Printed Graphene Circuits

The Casimir conundrum

Thursday, July 31st, 2008

When it comes to the Casimir force, physicists are in an embarrassing position.

“Weak intermolecular forces have a truly pervasive impact, from biology to chemistry, from physics to engineering. It may therefore come as a surprise to know that there still exist, in this well established field, unresolved problems of a fundamental character. This is indeed the case with respect to the problem of determining the Van der Waals-Casimir interaction between two metallic bodies at finite temperature. As of now, people simply don’t know how to compute it, and the numerous recent literature on this subject provides contradictory recipes, which give widely different predictions for its magnitude”

So writes Giuseppe Bimonte at the Istituto Nazionale di Fisica Nucleare in Naples, Italy. That doesn’t sound good but Bimonte has a way out of the conundrum.

His proposal is to measure the change in Casimir pressure between two superconducting plates as their temperature is raised through their critical value so that they no longer superconduct.

Bimonte claims that the results should unambiguously distinguish between the various competing theories.

Get to it.

Ref: The Casimir Effect in a Superconducting Cavity: a New Tool to Resolve an Old Controversy