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Faster and faster

David Robson explores the growth in applications for ultrafast lasers

Many technologies have made the transition from isolated use in the scientific laboratory to general acceptance in industry and the wider world.

Computing in general, and the internet specifically, are the most obvious examples, but it is also evident in more obscure laboratory equipment. A recent example is the advent of portable spectrometers, which have already found extensive applications in viticulture to test the ripeness of grapes and the rate of fermentation of wine.

It seems that ultrafast lasers are the latest piece of laboratory equipment to follow this path. These laser sources, which produce short bursts of light that last as little as four femtoseconds (10-15 seconds) in duration, were originally used by physicists and chemists to interrogate matter with highly precise time-resolved measurements, but they are now providing the same precision to industrial, medical, and defence applications.

The requirements of the factory floor and the hospital are very different from the research laboratory, and the technology must adapt to this accordingly. Whereas scientists deal with highly technical equipment on a daily basis, industrial and medical users need rough and ready solutions that are suited to quick and easy implementation in rugged environments. Computer user interfaces developed from high-level computer languages to easy-to-use operating systems, such as Windows, and ultrafast lasers have similarly grown from complex scientific instruments to intuitive tools.

As Lawrie Gloster, CEO of Laser Quantum explains, the perception of ultrafast laser sources has changed over the past 15 years: ‘It initially all began in the research environment in the 1990s. The generation of pulses was itself a very interesting process for scientific research. Then, in 2001 we anticipated the move out of the lab and into industry, with solutions had to be turnkey, straightforward equipment. The lasers have to be very efficient, without the need for water-cooling, to ease their integration into other systems. ‘It’s important that any complexity is removed, to allow completely hands-off operation. For example, the pump sources must include selfdiagnosis, with self-alignment and self-calibration. If ultrafast lasers are to become mainstream in a range of applications, the main issues are the price and the simplicity of the devices. This approach will increase the lead adoption in industry.’

Philippe Feru, senior marketing manager at Newport, agrees: ‘We must work to adapt the technology for commercial applications. The technology is pretty difficult – they are currently only used as a last resort – and the challenge is to make an ultrafast laser system that is stable enough for these applications.’

However, the developments have been more than skin-deep, with important improvements to the laser technology itself: by tweaking the design of the optical cavity and the precision of the pumping mechanism, manufacturers can now provide lasers that produce shorter and shorter pulses with broader bandwidths at greater rates per second. ‘Ten years ago, hundreds and tens of femtoseconds were the best pulse durations we could achieve,’ says Gloster. ‘Five or six femtoseconds are now within reach.’

According to Christian Warmuth, vice president of operations at Femtolasers, this is due to a steady progression in the quality of the optics used to direct the light within the laser. ‘It’s largely due to improvements in the manufacturing of the mirrors. We have reduced energy losses and improved reflection, to compress the output to get the shortest pulses.’

The advantages of these shorter pulse lengths are far reaching, in both scientific and industrial applications. In scientific research, it is possible to track the journey and absorption of a shorter pulse with a greater accuracy than longer pulses, allowing a greater time resolution of measurement in fields such as spectroscopy.

Shorter pulses also allow scientists to manipulate biological materials with high precision for genetics and cellular research. Femtolasers and JenLab recently presented a combined oscillator and microscope that can be used to cut tiny holes in cells, just 70-100nm in diameter, to extract and manipulate DNA. Normally pulses become elongated when they propagate through optics – a particular problem in microscopy – so the laser supplier had to develop a special set of mirrors to compensate for this effect and maintain the short duration. It is hoped that the device could be used for gene therapy and stem cell research and for the optical injection of molecules into cells.

Almost since their conception, there has been great interest in using ultrafast lasers for medical imaging techniques, such as optical coherence tomography, which can produce images a couple of millimetres below the surface of tissue. The pulses of light are scanned across the tissue. When these penetrate below the surface, they reflect back at places of change within the tissue. Many of the pulses are scattered in many directions but some are not and, by separating and analysing the unscattered pulses, it is possible to obtain a picture of what the tissue looks like below the surface.

However, as the technology matures the scope of the applications of femtosecond pulses in medicine seems to be expanding. More recently, scientists have been investigating the use of short laser pulses to generate monochromatic x-rays for highly targeted radiotherapy. Normally, x-ray sources produce an output over a range of wavelengths, but scientists have found that by firing the ultrafast laser source at a copper photocathode, x-rays are emitted at the specific wavelength needed for a particular treatment, reducing the amount of radiation the patient is exposed to.

The Integral ultrafast oscillator from Femtolasers.

During treatment, the patient drinks a chemical marker that has two important properties: it collects in cancerous cells but not in healthy tissue, and it readily absorbs the exact frequency of the monochromatic x-ray source. When the x-rays are aimed at the patient, they are absorbed in the tumour, killing the cancerous cells while travelling straight through the healthy tissue leaving it relatively unharmed. A broader spectrum of x-rays would expose the patient to excess radiation that was not necessary for the treatment, whereas the monochromatic source exposes the patient to the bare minimum.

The system is currently under review by the US National Institute of Health, but Fabien Ghez, the product and systems sales engineer from Thales, which has been working on the systems, believes that two problems need to be overcome before the technique can gain widespread use in oncology wards: ‘We must reduce the size of the global system. At the moment it’s quite large and difficult to implement in hospitals. In addition, we now know that it’s possible to generate x-rays with a laser, but we want to control the generation to achieve a large output of monochromatic rays.’

Until then, the biggest benefit of ultrafast lasers to medicine may be their application in the production of delicate medical devices. ‘They produce the finest detail,’ says Mark Boyle, scientific product manager from Quantronix. ‘With a continuous wave laser, the area around the laser spot [on the material being machined] gets very hot, whereas pulsed lasers can machine material without heating the surrounding area. The cut sizes can be down to the micrometer range.’

The two processes work through very different mechanisms. A continuous wave laser would heat the material (relatively) slowly, producing a line of molten debris that is then removed to reveal the cut surface. The energy of a pulse, on the other hand, is delivered over a very short time and absorbed directly into the material, causing it to sublimate from a solid to a vapour without melting first.

This reduces the amount of debris around the cut to leave a very clean surface: an advantage of ultrafast lasers that is proving useful in many highprecision industries such as semiconductor processing and solar cell production. The ability of ultrafast lasers to deliver high powers in a very short time is also driving their application in the defence industry, to protect aeroplanes from ground-launched missiles, which track the thermal trail of the plane’s fuel discharge.

The lasers fire short pulses of light energy from the ground at a slightly different trajectory to the plane. This produces a plasma in the air that creates a very similar thermal signal to the flare of the plane, fooling the missile into going off course and missing the plane.

While the application of femtosecond laser pulses in medical, industrial and defence applications now seems assured, if not yet well-established, it seems that ultrafast technology is now returning full circle to its scientific roots. But whereas 20 years ago laser scientists were creating a buzz with pulses that lasted just 10-15 seconds, they are now working on producing pulses of light energy that last in the order of 10-18 seconds – or attoseconds.

As if to confirm this iterative cycle, attosecond pulse production relies on a femtosecond laser source to shoot pulses at a cloud of neon atoms, which then release a burst of energy for time durations of less than one femtosecond.

The parallels with the initial research into femtosecond pulse production have not gone unnoticed: ‘The application of attosecond pulses is at the same stage as the application of femtosecond pulses 15 years ago,’ says Christian Warmuth of Femtolasers. Currently, the most likely applications seem to be in spectroscopy and atomic physics, where it is hoped the shorter pulses can be used to generate reactions in particles that hadn’t been possible before.

However, it can only be a matter of time before the lasers make the same transition to other industries. ‘We expect attosecond pulses to generate an initial rumour [among the laser community], which will then attract more and more research into the potential applications,’ says Warmuth.

It will be interesting to observe whether attosecond technology can achieve the same price, size, and complexity restraints that we are already seeing in femtosecond lasers.

Ultrashort pulses create 'alchemical' change in metals

It may open up a new field of applications for ultrashort pulses. Through a seemingly alchemical process, scientists have found that they can change the colour of different metals by machining the surface with femtosecond laser pulses.

By etching tiny nanoscale structures on the surface of the metal, the researchers have already achieved gold-coloured aluminium and blue titanium, and it is hoped that other colours will soon follow.

They have also created an iridescent surface that reflects different colours depending on the direction from which it is viewed.

The research, led by Chunlei Guo at the University of Rochester, USA, follows the discovery three years ago that ultrashort pulses of laser light could be used to alter the surface of gold so that it absorbed all visible wavelengths of light, giving a pitch-black finish. ‘Since then we have been trying to get control of the process, so that we could selectively absorb certain bandwidths [of the visible spectrum] and reflect different colours,’ explains Guo.

Normally, substances reflect or absorb different wavelengths depending on the way the electrons are structured within the atoms and molecules. The team found that by creating tiny nanostructures – just 10-100nm in size – on the surface of the metal, they could confine the movement of electrons to certain pathways, changing the ways they can interact with the waves of light.

 

Gold platinum, blue titanium and gold aluminium created by Guo’s team.

The team found that by changing the shape, size and density of the nanostructures on the surface they could change the wavelengths of light that were absorbed and reflected. They achieved this by altering the pulse duration, intensity and repetition rate of the femtosecond pulses.

So far the colours have been limited to black, gold and blue but, Guo believes, the physics behind the technique means that it is only a matter of time before other colours can be achieved. ‘I believe we can achieve any colour on any metal,’ he says. He thinks it may even be possible to machine other reflective surfaces in a similar way.

In addition to these solid colours, Guo has also created an iridescent surface that reflected many different colours and shimmered like a butterfly’s wing. To achieve this, the team created a period structure on the surface that repeated itself at regular intervals. The reflected light from these structures conflict to create a new wave form, which changes depending on the direction from which it is viewed, giving the appearance of varying colour.

It is hoped the technique could have multiple uses. ‘It would be good for laser marking, to produce a picture on a metallic surface. There has also been interest from the jewellery industry,’ says Guo. It could also be used to create absorbent mirrors and optics for the photonics industry.