System Bits: June 14

Microlaser phase locking arrays for terahertz security scanners
Researchers at MIT and Sandia National Laboratories reminded that terahertz radiation, the band of electromagnetic radiation between microwaves and visible light, has promising applications in security and medical diagnostics, even if such devices will require the development of compact, low-power, high-quality terahertz lasers.

To this end, the researchers have described a new way to build terahertz lasers that could significantly reduce their power consumption and size, while also allowing them to emit tighter beams, which is an important requirement for most practical applications.

This work also represents a fundamentally new approach to laser design, which could have ramifications for visible-light lasers as well.

Researchers at MIT and Sandia National Laboratories have designed a device that is an array of 37 microfabricated lasers on a single chip. Its power requirements are relatively low because the radiation emitted by all of the lasers is “phase locked,” meaning that the troughs and crests of its waves are perfectly aligned. (Source: MIT, Sandia National Laboratories)

The microlaser device is an array of 37 microfabricated lasers on a single chip with power requirements so low due to the fact that the radiation emitted by all of the lasers is phase locked, meaning that the troughs and crests of its waves are perfectly aligned.

The team believes this represents a fundamentally new way to phase-lock arrays of lasers. Specifically, they have identified four previous phase-locking techniques, but point out that all have drawbacks at the microscale: Some require positioning photonic components so closely together that they’d be difficult to manufacture; others require additional off-chip photonic components that would have to be precisely positioned relative to the lasers.

These arrays, by contrast, are monolithic, meaning they’re etched entirely from a single block of material.

Qing Hu, a distinguished professor of electrical engineering and computer science at MIT, whose group led the new work said this whole work was inspired by antenna engineering technology. “We’re working on lasers, and usually people compartmentalize that as photonics. And microwave engineering is really a different community, and they have a very different mindset. We really were inspired by microwave-engineer technology in a very thoughtful way and achieved something that is totally conceptually new.”

The researchers said their laser array is based on the same principle that underlies broadcast TV and radio: An electrical current passing through a radio antenna produces an electromagnetic field, and the electromagnetic field induces a corresponding current in nearby antennas. In this array, each laser generates an electromagnetic field that induces a current in the lasers around it, which synchronizes the phase of the radiation they emit.

In security applications, for instance, terahertz radiation could be directed at a chemical sample, which would absorb some frequencies more than others, producing a characteristic absorption fingerprint. The tighter the beam, the more radiation reaches both the sample and, subsequently, a detector, yielding a clearer signal.

Electronic sensor for medicine, food safety
Purdue University researchers are reporting that a new type of electronic sensor that might be used to quickly detect and classify bacteria for medical diagnostics and food safety has passed a key hurdle by distinguishing between dead and living bacteria cells.

They explained that conventional laboratory technologies require that samples be cultured for hours or longer to grow enough of the bacteria for identification and analysis, for example, to determine which antibiotic to prescribe. The new approach might be used to create arrays of hundreds of sensors on an electronic chip, each sensor detecting a specific type of bacteria or pinpointing the effectiveness of particular antibiotics within minutes.

A new type of electronic sensor may lead to devices that detect and classify bacteria for medical diagnostics and food safety. (Source: Purdue University)

The sensor works by detecting changes in electrical conductivity in droplets containing bacteria cells. (A YouTube video about the research is available at https://youtu.be/QN019bQJCb8 ).

The technology, which was tested with low concentrations of living and dead forms of E. coli, Salmonella and S. epidermidis bacteria, is said to be label-free because it does not require that samples be treated with fluorescent dyes, making it a potentially practical tool for medicine and food safety. Much of the research was performed at the Birck Nanotechnology Center and Bindley Bioscience Center in Purdue’s Discovery Park.

One-atom-thick graphite membrane for tuning electrical modes
With implications in improving the sensitivity of small detectors of mass — very important in detecting the mass of small molecules like viruses — researchers from the Tata Institute of Fundamental Research, Mumbai, have demonstrated the ability to manipulate the vibrations of a drum of nanometre scale thickness – realizing the world’s smallest and most versatile drum. This also opens the doors to probing exciting new aspects of fundamental physics.

The work made use of graphene, a one-atom thick material, to fabricate drums that have highly tunable mechanical frequencies and coupling between various modes. Coupling between the modes was shown to be controllable which led to the creation of new, hybrid modes and, further, allowed amplification of the vibrations.

The experiment consisted of studying the mechanical vibrational modes, or ‘notes’, similar to a musical drum. The small size of the drum ( diameter 0.003 mm, or 30 times smaller than the diameter of human hair) gave rise to high vibrational frequencies in the range of 100 Mega Hertz – implying that this drum vibrates 100 million times in one second. Interestingly, the work showed that the notes of these drums could be controlled by making use of an electrical force that bends, or strains, the drum, and that the bending of the drum also caused different modes of the drum to interact with each other, leading to a sloshing of energy between two notes.

Using this interaction, it shows that energy can be transferred between the modes leading to the creation of new ‘notes’ in the drum. The rate of energy transfer could be precisely controlled by electrical signals that modulate the coupling. The work, in addition, made use of the mechanical mode coupling to manipulate the energy lost to the environment and demonstrated amplification of the vibrational motion, equivalent to an increase in sound from the drum.

At low temperatures, the high mechanical frequencies would allow studies of energy transfer of a quantum mechanical nature between the notes, whereas the coupling between various notes of the drum could also be engineered to work as mechanical logic circuits and lead to improvements in quantum information processing, the researchers explained. The ability to amplify the mechanical motion will also help improve the sensitivity of sensors based on nanoscale drums.

Artist’s impression of two coupled, vibrational modes of a graphene drum. The coupling can be tuned electrically to transfer energy between the modes and hybridize them.
(Source: Tata Institute of Fundamental Research)