Nano technology: how low can you go?

Forget the nano scale. DCU researchers developing free electron lasers measure the world on pico, femto and even atto scales

The measurement “pico” has an onomatopoeic ring to it. It sounds small. It is small. Not as small as its numerical subordinates though – femto, atto, zepto and yocto. These scales might be insignificant physically but in nano science, small is the new big.

It is reaching and identifying the world on these scales which makes all the difference in X-ray laser technology, an exciting branch of nanophysics where significant progress is being made in DCU. A “strategically focused collaboration” between researchers in Germany, Russia, the US and Ireland has led to the full characterisation of ultra short, ultra-intense X-ray pulses at free-electron lasers (FELs).

FELs bring together the benefits of X-ray and laser technology to create highly effective pulse measuring systems that offer exciting practical applications. “X-rays revolutionised many sectors over the last 100 years – medicine, industry and manufacturing techniques”, says DCU Professor John Costello. “Likewise lasers have radically improved many aspects of our lives.”

But, according to Costello, the technology behind X-rays has barely changed in terms of its design or use since being invented a century ago. “X-rays are produced in exactly the same way they were over 100 years ago by the guy who first made and used them – Wilhelm Conrad Röntgen ,” he says. “Basically you make an electron beam and fire it at a metal plate and the metal emits X-rays.”

Lasers were once described as a “tool looking for a solution”. Now you can’t get away from them: they’re used in TV and music systems, materials manufacture, welding, medicine, photodynamic therapy, etc.

But the “holy grail” has always been to make a usable X-ray laser system. The first attempts to build them began in the 1980s in big military labs in the US, UK and Japan, and while success was achieved, it came at a cost. “In terms of wall-plug efficiency, for every one watt of laser power generated from these original X ray lasers, over ten million watts had to go in,” says Costello. “They were terribly inefficient.”

X-ray lasers offer increased intensity thereby providing new uses and applications. “FELs generate pulsed X-ray beams where the pulses are literally millions of times shorter in duration and more intense than those in traditional X-ray beams,” says Costello.

Understanding the shape of a pulse is an important diagnostic tool. If we take an electrocardiogram (ECG) as an example: there are six different waves to be measured in the human heart, known as P, Q, R, S, T and U. The shape of these six different pulses tells you something different about heart function. Your measuring device, in this case an ECG, must be fast enough to process your heart pulse in real time.

The device traditionally used to measure the shape of electrical pulses is the oscilloscope and commercial instruments predate the second World War. “For most electrical pulses, the fastest oscilloscope can deflect the recording beam or light trace across the screen in approximately a nano second, or a billionth of a second,” says Costello. “If the pulse I want to measure is much shorter than a billionth of a second, it’s hard to resolve. State of the art oscilloscopes will allow you to resolve pulses which are about a 20th of a nanosecond, or 50 picoseconds long.”

FELs can produce X-ray pulses of one millionth-billionth of a second, or one femtosecond and with unprecedented intensity. Such ultrashort timescales mean that the dynamical behaviour of many molecular systems may now be better understood. “FELs allow us to create images of biomolecules, viruses, and cells not just much more efficiently but because they are pulsed, we can freeze their motion, not unlike the strobe light effect which makes dancers appear stationary in a disco.

“To freeze the motion of something the duration of the flash has to be comparable to the characteristic time of the motion – dancers move on a timescale of a fraction of a second and so the light pulse or flash from the strobe must also be a fraction of a second – electrons atoms in molecules, viruses, etc move on a timescale of femtoseconds and so you need flashes of light of femtosecond duration to freeze their motion.

The fact that X-ray beams from FELs are coherent will in the future allow for 3D X-ray imaging on a tiny scale, literally on the scale of single atoms and molecules. “Current FELs can produce X-rays with wavelengths as low as 0.1 nanometre. In other words, we can measure with high resolution at a molecular level. We expect to soon have the ability to image proteins, viruses and cancer causing rogue molecules in 3D with femtosecond time resolution – molecular movie making.”

This is significant as knowing the structure and dynamics of a molecule at a given time will help determine whether or not it has or will become a “rogue”. “Being able to measure on a femto time scale is important because molecules change their electronic structure and molecular shape on that time scale,” says Costello. “It is the shape of the molecule and how its electrons are distributed which will determine beneficial or negative effects.”

Tiny from teeny The nano time scale

NanosecondOne-billionth of a second

PicosecondOne-trillionth of a second

FemtosecondOne-quadrillionth or a million-billionth of a second

AttosecondOne-quintillionth or a thousandth of a femto- second

ZeptosecondOne-sextillionth

YoctosecondOne-septillionth