RESEARCH NEWS

MARCH 2011 | VOLUME 14 | NUMBER 3 65

A silicon heat engine, about the size

of a bacterium and thought to be

the smallest ever created, has been

developed by scientists from NXP

Semiconductors in Eindhoven. It is

powered by an electrical current that

heats an expanding and contracting

silicon beam, and offers evidence

of the potential for applying heat

engines on the micro- and nano-

scale.

The study, published in Nature Physics

[Steeneken et al. Nature Physics (2011)

doi: 10.1038/nphys1871], revealed

that a tiny silicon crystal can be made

to mechanically oscillate when a D.C. current is applied

to it. When there is enough current, a 280 nm silicon

beam begins to operate as a heat engine similar to

that in cars or steam engines.

The researchers have been working on mechanical

micro-electro-mechanical system (MEMS) oscillators

whose motion is detected by the piezoresistive

effect, as they are better than quartz oscillators due

to both the mechanical resonator and the transistor

amplifier being made in/on silicon crystals. Silicon

crystals can also be made much smaller than quartz

crystals, as they can be structured using the same

lithographic tools used to structure transistors,

bringing down costs and allowing for higher

resonance frequencies.

The team discovered the new method for generating

motion using thermodynamic/thermal expansion

forces from the piezoresistive effect; the change

of electrical resistance of a material when it is

mechanically compressed, for power supply and

amplification. With no transistors necessary for

amplification, a 1 mW current can be run through a

piece of silicon crystal, making it start to vibrate at a

very stable frequency, simplifying the production of

mechanical oscillators significantly.

They also showed that the heat engines

can modify the Brownian motion of a

resonator and amplify these Brownian

vibrations. And it can also be reversed,

reducing the Brownian motion, making

it operate as a tiny refrigerator.

When the current is applied to the

crystal, it spontaneously oscillates

mechanically at a frequency of

1.3 MHz, more than a million times

each second. They measured the

high-frequency motion by using slow-

motion video under a microscope with

stroboscopic illumination.

The silicon beam, called the engine

beam, was used to drive a larger silicon structure in the

shape of a pendulum. When electricity flows through

a resistor, it generates heat that makes the resistor’s

temperature increase. With the power focused in a

silicon resistor beam of very small volume, a huge

heating power per volume is generated in the beam.

The oscillation from the heat engine could have

applications in smaller, simpler and cheaper watches

or other electronic devices, or for use in microscopic

clocks or as a sensor. The researchers now hope

to demonstrate applications for the oscillator, and

improve its performance and operation.

Laurie Donaldson

Nano fridgeELECTRONIC MATERIALS

Heat engine motion. Courtesy P. G. Steeneken, et al.

Visibly invisibleOPTICAL MATERIALS

Fabricated calcite crystals could be all that is needed

to make a so-called “invisibility cloak”, a material that

renders an otherwise opaque object transparent to visible

light. Earlier efforts have been functional only on the

microscopic scale or in the infra-red or radio wavelengths

and used nanofabricated composites and metamaterials.

Invisibility has been a perennial topic in science

fiction and science fantasy throughout the genre and

scientists have recently begun to take steps towards

fulfilling this fantasy. Metamaterials have been

developed, for instance, that allow electromagnetic

radiation to flow, like a stream around a half-

submerged boulder, past an object as if it were not

there and so give the illusion of the object being

invisible. However, making an object invisible to our

eyes rather than infra-red or radio detectors had until

now remained impossible.

Now, two independent teams, one in the UK, the other

based in the US and Singapore, have both discovered

that calcite can act as a visible-light cloaking material

because of its natural birefringent properties. Shuang

Zhang of the University of Birmingham and John

Pendry of Imperial College London have demonstrated

using red and green laser beams and incoherent

white light that a fabricated block of calcite can hide

objects dozens of millimeters high in air [Zhang et al.

Nature Commun (2011) 2, 176 online, doi:10.1038/

ncomms1176].

They show that the cloak is capable of hiding three-

dimensional objects three to four orders of magnitude

larger than optical wavelengths and does this by

avoiding major issues usually associated with cloaking

devices, such as size, bandwidth and image distortion.

The effect is seen in the transformation of a deformed

mirror into what appears to be a flat mirror as viewed

from any angle.

Baile Zhang and colleagues at the Singapore-MIT

Alliance for Research and Technology (SMART) Centre

and at the Massachusetts Institute of Technology, USA,

found they could shield a steel wedge a maximum of 2

mm high from polarized red, green or blue light under

water using two pieces of fabricated calcite [Zhang

et al. Phys Rev Lett (2011) 106, 033901, doi:10.1103/

PhysRevLett.106.033901]. The cloak works only in a

two-dimensional plane

Both approaches now pave the way for future practical

cloaking devices. The next step will be to somehow

engineer the cloaking material to be functional

simultaneously at all wavelengths and perhaps then

with normal, as opposed to polarized, visible light.

David Bradley

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