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Lasers in the lab

For all the great strides being made in industrial applications, the research sector still provides a wealth of applications for photonics, as William Payne discovers

In the past year alone, researchers working on key scientific questions have used lasers to make major breakthroughs. These questions include: how did life originate on earth; where do cosmic rays come from; how fast are the polar ice caps melting; and is there life on Mars?

At the same time, the pace of innovation in laser instrumentation continues unabated. Researchers have uncovered new devices that can chart the chemical composition of cells with unprecedented accuracy; new lightweight compact devices that can detect environmental pollution and greenhouse gases; compact optical frequency combs that can read the full spectrum of a gas sample to 1Hz precision; and a compact synchotron, with light billions of times brighter than the sun, small and cheap enough to fit into a university basement.

One thing stands out: lasers are increasingly downscaling the most expensive, complex scientific technology and making it more widely available.

Compact synchrotron

It’s the brightest light in the solar system, shining billions of times brighter than the sun. It cost £250m, and it covers an area the size of five football pitches in the Oxfordshire countryside. Last year, Diamond, the giant synchrotron laser and the biggest science facility to be built in Britain in three decades, ramped up the power in its lasers to 10 billion times the power of the sun. The US and the EU are building their own synchrotrons, each costing hundreds of millions of pounds. Now a team from the University of Strathclyde has developed a compact synchrotron, one thousand times smaller than Diamond, and costing little more than £2m. The team, part of the international Alpha-X research group, reckon that most universities could build one in a science block basement for as little as £5m.

The only catch is that, while Diamond's synchrotron can conduct many experiments simultaneously, Strathclyde's can do only one at a  time. But, with multiple beam lines, the Strathclyde team can set up many experiments simultaneously and run them in series.

Synchrotron technology was developed in Britain at Daresbury. It has played a major role in fundamental physics research, studying killer viruses such as HIV and H5N1, and helping develop anti-virals such as HIV-protease inhibitors and Relenza.

Transforming spectroscopy

An experiment by physicists at the National Institute for Standards and Technology (NIST) in Boulder, Colorado, shows how optical frequency combs could transform spectroscopy. Billed as by far the largest number of frequency comb teeth that have been observed to date, the NIST-Boulder study covered the full spectrum of a sample of gas over a broad region, with frequency accuracy reaching 1Hz. The study is equivalent to sending 155,000 individual single frequency lasers through the sample and measuring the resulting amplitude and phase shift on each individual laser.

Optical frequency combs depend on modelocked femtosecond lasers. The frequency comb comprises a pulsed array over a spectral range. It has been hailed as a major breakthrough in science – the 2005 Nobel physics prize went to John Hall of NIST and Ted Haensch of Max Planck for its invention.

A major problem with optical frequency combs is stabilising lasers and ensuring that each ‘tooth’, or separate individual frequency, is precisely aligned. NIST has resolved this by developing selfreferencing comb technology that combines two femtosecond combs, using the second comb to synchronise and callibrate very accurately the first spectroscope comb. NIST scientists have demonstrated extremely precise synthesis of optical frequencies, with specific colours generated to a reproduceability of 19 digits.

Laser beam ‘fire hose’

The invention of a compact, inexpensive new laser cell sorting technique that can discriminate against cellular components or behaviour patterns within the cell could open new avenues of research in the life sciences and genetics. Up to 10,000 cells on a glass slide can be sorted using the new technique with a conventional low cost laser. Its inventor, professor Joel Voldman of MIT, says the laser separates sorted cells like ‘a fire hose pushing up a beach ball’, but is gentle enough to leave the sorted cells still viable for further biological testing.

Unlike conventional cell sorting, the new technique can sort on levels of fluorescence from individual cell parts such as the nucleus. It can also separate on the basis of patterns of behaviour, such as how rapidly a component fluoresces, or for how long. According to Voldman, the new technique could cost only a few thousand dollars, and could replace more expensive cell separation techniques such as optical tweezers in many settings. As a result, researchers could do advanced cell sorting in smaller labs or clinical settings, not just in big centralised testing facilities.

 

The UK Diamond synchrotron facility.

QCL nanoantenna laser developed

Researchers at Harvard University have combined quantum cascade lasers with optical antenna nanotechnology. The result is the quantum cascade laser nanoantenna, a device that can resolve the chemical composition of samples such as the interior of a cell to unprecedented detail.

The design consists of two gold rods separated by a nanometre gap built on the facet of a quantum cascade laser emitting in the mid-infrared spectrum. The antenna near field is characterised by an apertureless near field scanning optical microscope (a-NSOM). The team has achieved spatial field confinement between 100 and 70nm.

The team is led by Professor Frederico Capasso, who led the development of the first quantum cascade laser at Bell Labs. He believes they are a year away from building a microscope that will enable chemical imaging at cell and viral structure level below a micron in size.

In a parallel development, a team led by Professor Bert Hecht at Basel University has coupled a nanoscale bowtie antenna to a single quantum dot. The result is a highly efficient, tuneable superemitter with potential applications in spectroscopy, biological microscopy and microoptoelectronics.

Capasso’s team at Harvard has also developed a compact QCL wavelength on demand sensor. It can provide fast detection of a large number of chemicals through broad tuning of emission wavelength, and could have major applications in pollution monitoring and environmental sensing of greenhouse gases.

The tuneability of the laser chip can be extended up to 10-fold, and several widely spaced absorption features can be targeted with the same chip, enabling the detection in parallel of an extremely large number of trace gases in concentrations of parts per billion in volume by a single compact lowpower device.

AFM discovers origins of life on earth

How did life start on earth? And, is there life on Mars? These are two big questions facing scientists today. In the last year, using laser-based tools, scientists have arrived at what could be conclusive answers. Both teams relied on rock analysis to reach their conclusions.

The first rock is Martian. NASA researchers created a stir in 1996 when they announced they had found signs of life in a Martian meteorite, ALH84001. Organic compounds in the rock were a key part of their evidence. Now scientists from Washington's Carnegie Institute have released a new study of the rock. Using Raman microscopy and 3D extended focal light microscopy, they have concluded that the organic compounds were formed by volcanic activity in a freezing environment, not through organic processes.

The second rock belongs to planet earth. A team led by Helen Hansma from the University of alifornia has proposed a new theory for the origin of life based on the analysis of ancient mica rocks with a laser-based AFM microscope. Hansma argues that life might have begun in the protected spaces inside layers of mica floating in ancient oceans. Her analysis of ancient mica showed that it is full of fossilised precellular life. Mica is made up of 2nm thick mineral sheets, with a hexagonal array of anionic sites spread 0.5nm apart.

The chemistry and structure of submerged mica in the early seas would have been almost identical to that of today’s living cells. The presence and position of sodium and potassium in mica is very similar to cells. Mica also has chemical properties that are similar to RNA, many proteins and lipids, which are likely to have been the building blocks of the earliest forms of life.

Cosmic ray mystery solved

An international team of astronomers using optical instruments including lasers and lidar have solved one of astronomy’s key problems. Cosmic rays are the most powerful forms of radiation known, but their source has been a mystery. However, in November, astronomers working at the Pierre Auger Observatory in Argentina announced that their source has been discovered as a particular, and rare, type of extremely violent supermassive black hole called ‘active galactic nuclei’.

Cosmic rays were first observed in 1912. They are 100 million times more powerful than anything produced by the most powerful particle accelerators on earth. The most powerful ever detected had an energy of 300 billion billion (3 x 1020) volts. A single particle, if it could penetrate the atmosphere and strike someone on the head, would feel like a cricket ball bowled by Shane Warne – hard.

Luckily, cosmic rays do not penetrate the atmosphere. Instead, they produce ‘air showers’ of smaller particles as they collide with gas in the upper atmosphere. The 19-nation Auger observatory collects data on these air showers. Air shower particles striking the earth are measured through water tanks, while the air showers themselves are observed through fluorescence detectors at up to 15km from the surface.

The fluorescence detectors are responsible for the latest breakthrough, as they provide measurement of both energy and direction of incoming cosmic rays. Lasers and lidar are vital for accurate air shower measurement. Lidar provides simultaneous atmospheric conditions data used in the algorithm for calculating the energy and trajectory of the cosmic rays. A roving 355nm laser at 4km, and three fibre light sources, are used to callibrate the flourescence detectors.

Investigating nanostructures

A team led by Professor Arnaud Devos at Lille’s Institut d’Electronique de Microélectronique et de Nanotechnologie are using femtosecond lasers to investigate nanostructures deposited on metal film.

They are exploiting the intense thermal expansion – and consequent changes to the refractive index – caused when an ultrafast laser pulse hits a thin surface of just a few nanometres. They have discovered that pump-probe detection schemes are sensitive to the wavelength of the ultrafast laser. ‘In the case of opaque metal layers, the signal sensitivity is strongly dependent on its proximity to electronic transitions,’ says Devos. He cites the example of aluminium, which has a low reflectivity at 850nm because of electronic resonance. With an alternative probe method using a transparent overcoat film on the metal, the reflected probe pulse intensity is again highly dependent on wavelength, because the transparent film acts as a Fabry-Perot etalon with its usual transmission/reflection modes. ‘In both cases, studying wavelength dependence provides superior measurement precision and another set of data about the sample,’ he says.

Devos’ group initially used a Coherent Mira titanium:sapphire laser, but recently they switched to using one of the new generation of tuneable onebox ultrafast lasers (Coherent Chameleon Ultra). ‘We chose this laser for several reasons. First it offers very fast and fully automated tuning over a very wide wavelength range: 680-1080nm. This is important as we typically take data at up to 10 different wavelengths in each experiment. Plus we get femtosecond pulses from a sealed, one-box laser without the need for frequent tweaking and adjustment. So my students can concentrate on taking and analysing data rather than becoming laser experts.’

The team is studying 10nm diameter gold nanospheres. Unlike a conventional molecule, this can have vibrational modes in the 100-200GHz window. There is no other simple way to study the vibrational spectrum. They are also particularly interested in synthetic crystals and lattices created by repetitive patterns of nanostructures. With lattice  dimensions on the order of a few 100nm, these phononic crystals – crystals exhibiting a band gap in the phonon propagation – have high frequency lattice modes that cannot be probed by traditional spectroscopic methods. Possible future uses for phononic crystals include acting as RF filters for GHz mobile phones.

Older Arctic ice gives way to thinner

Finally, few scientific questions have the urgency of the global warming debate. One major concern is the effect on the polar ice caps and rising sea levels. Now researchers at the University of Colorado have established that thick ancient ice at the North Pole is being replaced by thinner, more fragile ice. The researchers based their conclusions on visible infrared radar and laser altimeter satellite data. Performed in September 2007, the study shows that Arctic ice has declined by 23 per cent in just two years. Almost all the oldest ice has gone, say the researchers.

The study is the first analysis to quantify the magnitude of the Arctic sea ice retreat using data on the age of the ice and its thickness. Laser altimetry data from satellites operated by the National Oceanic and Atmospheric Administration (NOAA), going back to 1982 was used to reconstruct past Arctic sea ice conditions. In addition to satellite sensing data, the Colorado team also uses data collected by the Nasa Airborne Visible InfraRed Imaging Spectrometer (AVIRIS).

The data is collected by a turboprop plane carrying the instrumentation, which incorporates 224 different detectors, each with a spectral bandwidth of around 10nm. The complete instrument scans from 380 to 2500nm. Each AVIRIS flight yields around 76Gb of data. The requirement for 224 detectors as well as extensive electronics and software pre-processing, reflects the challenge of remote spectrometry, with scattering and defraction generating major signal to noise problems.

Current AVIRIS developments by NASA focus on integrating lidar with imaging spectroscopy. This includes both spectral dataset integration and inflight fusion of lidar and imaging data.

Over the last year, lasers have helped advance a number of key scientific debates. At the same time, new, more compact, cheaper and more flexible scientific measuring instruments continue to be developed based around laser technology. The pace of innovation shows no signs of slowing.