System Bits: Oct. 29

Coupling photons with electrons
With the potential to lead to the creation of materials whose electronic properties could be “tuned” in real time simply by shining precise laser beams at them, researchers at MIT have produced and measured a coupling of photons and electrons on the surface of an unusual type of material called a topological insulator. This type of coupling had been predicted by theorists, but never observed.

The work opens up a new avenue for optical manipulation of quantum states of matter, experts said.

The method involves shooting femtosecond (millionths of a billionth of a second) pulses of mid-infrared light at a sample of material and observing the results with an electron spectrometer, a specialized high-speed camera the team developed.

The researchers demonstrated the existence of a quantum-mechanical mixture of electrons and photons, known as a Floquet-Bloch state, in a crystalline solid. As first theorized by Swiss physicist Felix Bloch, electrons move in a crystal in a regular, repeating pattern dictated by the periodic structure of the crystal lattice. Photons are electromagnetic waves that have a distinct, regular frequency; their interaction with matter leads to Floquet states, named after the French mathematician Gaston Floquet. “Entangling” electrons with photons in a coherent manner generates the Floquet-Bloch state, which is periodic both in time and space.

The researchers mixed the photons from an intense laser pulse with the exotic surface electrons on a topological insulator. Their high-speed camera captured snapshots of the exotic state, from its generation to its rapid disappearance, a process lasting only a few hundred femtoseconds. They also found there were different kinds of mixed states when the polarization of the photons changed.

Their findings suggest that it’s possible to alter the electronic properties of a material — for example, changing it from a conductor to a semiconductor — just by changing the laser beam’s polarization.

Clearing the way for chip-sized pumps
The ability to shrink laboratory-scale processes to automated chip-sized systems would revolutionize biotechnology and medicine. For example, inexpensive and highly portable devices that process blood samples to detect biological agents such as anthrax are needed by the U.S. military and for homeland security efforts. One of the challenges of “lab-on-a-chip” technology is the need for miniaturized pumps to move solutions through micro-channels. Electroosmotic pumps (EOPs) — devices in which fluids appear to magically move through porous media in the presence of an electric field — are ideal because they can be readily miniaturized. EOPs however, require bulky, external power sources, which defeats the concept of portability. But a super-thin silicon membrane developed at the University of Rochester could now make it possible to drastically shrink the power source, paving the way for diagnostic devices the size of a credit card.

Up to now, EOPs have had to operate at a very high voltage—about 10 kilovolts – but this device works in the range of one-quarter of a volt, which means it can be integrated into devices and powered with small batteries.

The researchers used porous nanocrystalline silicon (pnc-Si) membranes that are microscopically thin—it takes more than one thousand stacked on top of each other to equal the width of a human hair – which is what allows for a low-voltage system.

A porous membrane needs to be placed between two electrodes in order to create what’s known as electroosmotic flow, which occurs when an electric field interacts with ions on a charged surface, causing fluids to move through channels. The membranes previously used in EOPs have resulted in a significant voltage drop between the electrodes, forcing engineers to begin with bulky, high-voltage power sources, they said, but the thin pnc Si membranes allow the electrodes to be placed much closer to each other, creating a much stronger electric field with a much smaller drop in voltage. As a result, a smaller power source is needed.

Along with medical applications, it’s been suggested that EOPs could be used to cool electronic devices.

A microfluidic bioreactor consists of two chambers separated by a nanoporous silicon membrane. It allows for flow-based assays using minimal amounts of reagent. The ultra-thin silicon membrane provides an excellent mimic of biological barrier properties. NOTE: This image combines two exposures in order to capture the brighter and darker parts of the scene, which exceed the dynamic range of the camera sensor. The resulting composite is truer to what the eye actually sees. (Source: University of Rochester)

Another benefit to the silicon membranes: Due to scalable fabrication methods, the nanocrystalline silicon membranes are inexpensive to make and can be easily integrated on silicon or silica-based microfluid chips.