Picosecond, femtosecond and attosecond laser pulses... Stephen Mounsey asks what's driving development in the field
Ultrafast laser technology is living up to its name; it’s developing quickly. The shortest pulses achievable are now fewer than 100 attoseconds in duration, one attosecond being 1x10-18 seconds. Numbers and sizes become hard to visualise when they are so far removed from everyday life, but it is worth noting that there are as many attoseconds in a single second as there have been whole seconds since the beginning of the universe. In practice, ultrafast usually refers to picosecond (1x10-12s) and femtosecond (1x10-15s) pulses, and as these become more commonplace in industry, the equipment to produce and use them is becoming ever more available.
But while many industries are interested in this technology, it is still relatively new. Darryl McCoy, laser and applications engineer at Photonics Solution, believes that such cutting-edge technology is currently a viable solution only to problems that can’t be tackled in any other way. ‘The step to the ultrafast regime in the first case is not one that should be taken lightly. Generally speaking, picosecond and femtosecond lasers require the use of a modelocked seed source and careful amplification procedures.’ McCoy cites reduced heat-affected zone (HAZ) and reduced plasma shielding effects as benefits that can be obtained only through use of ultrafast lasers, and which justify the means.
These two effects come into play in the field of laser micromachining, in which ultrafast pulses of high power are used to vaporise a metal, one tiny piece at a time. When cutting or drilling a metallic component with a conventional continuous wave laser beam, the heat generated by the laser will heat the surrounding material at a rate determined by diffusion. This heating can be useful in some applications such as laser welding, but when machining or shaping a component, the heat generated will affect the material properties around the target area. The mechanical properties of a metal are determined by its microstructure, which is in turn dependent upon the metallic phases present in the piece. The thermal history of a metallic component determines the relative concentrations of various different microstructural phases, and therefore any changes in the temperature of a component, and the rate of its cooling to ambient temperature, must be carefully controlled. Machining a component using ultrafast pulses, each of a very high power compared to continuous wave lasers, eliminates these heating effects; the target metal is rapidly vaporised and is not in contact with the surrounding material long enough to heat it significantly.
Similarly, the effectiveness of slower lasers is compromised by the effects of plasma shielding. When a focused laser beam hits a target, plasma is generated around the point, which typically happens in the nanosecond time period, depending on the material that is being machined and the characteristics of the laser. If the beam is longer than a nanosecond in duration, this plasma builds up and shields the target, reducing the effectiveness of the laser and the reproducibility of the machining.
McCoy explains that for machining applications, picosecond durations are sufficient, and the step up to femtosecond technology is unnecessary: ‘In general, where high power is required, picoseconds would be the first regime to consider, because pulse amplification is simpler and more cost effective [than for shorter pulses]. These pulses are short enough for many industrial applications, such as thin-film structuring, nano-processing and solar cell structuring.’ Suitable industrial picosecond systems are now available in ultra-compact models, such as High Q’s picoREGEN series. This model, for example, draws 30W and delivers 500,000 pulses per second, each of 12ps duration; the power equivalent of each pulse is therefore 5mW.
Femtosecond lasers are also used in laser machining, and in some select biological applications. A shorter pulse allows more control of the ablation produced. McCoy states that, until recently ‘femtoseconds had been the domain of scientific and microscopy communities. Applications such as biological tissue ablation, requiring the use of amplified femtosecond pulses have seen the rise of direct pumped Ytterbium-doped material hosts, such as those used in High Q’s femtoREGEN series lasers. These systems are now seeing wider application in the nano-processing world.’
Where ultrafast technology is required, faster does not always equal better. Commercialisation requires ultrafast pulses to be generated reliably, and, for many applications, with minimum user experience required. Hans Dabeesing of Newport’s Spectra Physics group describes the market for ultrafast lasers as having three main components: scientific, biotech, and industrial, each with different requirements and different areas of development.
When it comes to industrial applications, Dabeesing says that improvements in the more ancillary aspects of the laser systems are generally seen as commercially desirable; lasers in industry must be easy to use, rugged and reliable, as well as offering good value for money. At the cutting edge, ultrafast lasers depend heavily on precise tuning and precision manufacturing of components. Technical improvements usually stem from careful refinement, and some effort of commercialisation is required to make these refinements rugged and maintenance-free enough to be of use in an industrial setting.
In contrast, when addressing scientific markets, lasers and ultrafast amplifier systems for academics and researchers working in physics or photonics for example, performance is a more significant factor to the customer. Users in this bracket often wish to develop cutting-edge technologies, and so the fastest, highest-powered lasers, firing at high frequency are required. Dabeesing states that much of the increased performance is achieved through optimisation of existing technology, rather than through new techniques, resulting in steady but continual improvements in performance. He says: ‘The recent growth in fibre lasers has driven developments in diode technology, which have in turn propagated to other applications. As a result, higher-power laser diodes are now available for ultrafast applications.’
Applications of ultrafast lasers within biology are centred primarily on non-linear microscopy such as optical coherence tomography and multi-photon imaging. Multiphoton imaging involves tagging a target with a fluorescent marker compound, often within a living specimen, and then targeting it with femtosecond pulses. None of the individual photons have sufficient energy to cause the target to fluoresce if absorbed singularly, but when two or more streams of photons are focused at the target, very specific fluorescence can be achieved. Currently, the technique allows researchers to probe to a depth of approximately 0.5mm from the surface, in living tissues, before the scattering effect of the tissue limits further penetration. In this field, Dabeesing explains, researchers wish to image to greater depths within the living tissues. The property of the ultrafast laser system that is most important to them is the pulse’s peak power; the ultrafast pulses must carry sufficient energy to cause the target molecule to fluoresce without heating it and without damaging the cellular material around it. Greater peak power allows this to be achieved.
Because these systems are aimed at biologists who cannot be assumed to have any understanding of the technology involved, they must be as simple to use as possible. However, a system capable of generating two simultaneous femtosecond laser pulses is necessarily complicated, but according to Dabeesing, considerable effort on the part of Spectra Physics has gone into making the systems reliable and hands-free; no alignments should be required, they must be computer controlled, and they are tunable from 680 to 1,000nm. The combination of femtosecond pulses, large tunable range, and high level of automation make for challenging engineering. Diode-pumped solid state lasers provide a resilient and versatile platform, and mode-locking is used to produce pulses. McCoy says: ‘Advances in free space cavity designs and pumping regimes have enabled [High Q] systems to enter scalable energy and power regimes beyond the capability of commercially available fibre technologies.’ Dabeesing adds that, in a similar way, Newport achieves its stability through the precision of its optics, capable of dispersion control over a wide range of wavelengths. Furthermore, he says, laser cavities must be designed to be adaptable to wide tuning ranges and short pulses. A further consideration is that all these components must be held very stably in place physically, and Newport has developed techniques by which to achieve this.
Case study: OPOs for multi-photon imaging
APE is a German manufacturer of components for the ultrafast industry. The company’s products include a range of optical parametric oscillators (OPOs), used to alter the output wavelength of ultrafast laser pulses of the picosecond or femtosecond order. The OPOs are synchronously pumped by a mode-locked Ti:Sapphire laser, resulting in a highly efficient frequency transformation from the Ti:Sapphire range into infrared and visible wavelengths. Since the transformation process is jitterfree, OPO equipment is well suited for two colour experiments like pump-and-probe measurements.
Edlef Buttner, managing director of APE, describes the way in which the OPOs can be used in non-linear imaging techniques: ‘When using the OPO as an additional tool, wavelength shifting can lead to better properties; penetration depth is increased in the sample due to less scattering and less photo damage within the sample. Multi-photon excitation and second harmonic effects within the sample allow the different features of the sample to be resolved.’ Currently, the devices’ large size, high price and difficult handling have meant that they have only been useful in very specialist applications, but these limitations are being addressed by increased automation and more compact models. Buttner believes that biological applications represent a significant market for APE’s products in the future, and that application-driven development will allow the technology to move into more mainstream areas, and increase demand. ‘Biomedical applications are very exciting for us. Currently we are selling 25 to 30 OPOs a year as single pieces, but the biomedical market represents the first time that a much larger number of sales is expected.’
According to McCoy, markets for ultrafast lasers likely to be important in the near future include nanoprocessing, medical devices and solar energy. Photovoltaic solar cell manufacture, in particular, is likely to benefit from the precision of ultrafast machining, as the compounds used are often heat sensitive. Renewable energy is a sector that is currently the recipient of public funding and tax incentives, particularly in Germany, and this may also encourage the adoption of new machining technologies. On the research side, McCoy adds that ‘the scientific markets will continue to be important at a time when funding in the industrial sector is being reduced.’
Although it may be some time before commercial applications are found for them, attosecond-order laser pulses are already starting to produce interesting results, beyond the research effort which goes into their methods of production. Recently, researchers at the Max-Planck institute for quantum optics used a dielectric chirped mirror to concentrate pulses to 2.5fs in duration, giving them a very high peak power in the process. When these pulses hit a cloud of charged neon gas, they stimulate the emission of pulses of highly energetic extreme ultraviolet, even shorter in duration at around 80 attoseconds. These attosecond pulses were used to probe the waveform of the 2.5fs pulse as it passed through the gas, essentially capturing a snapshot of a single photon’s propagation.
Despite the flurry of research at the attosecond scale, and the consistently good results from there, in practice, it is unlikely that any commercial players will make use of attosecond pulses in the near future. If the development of nanosecond, picosecond, and femtosecond lasers is anything to go by, academic advancements in the underlying technologies will be closely followed by research into potential applications of this new generation of ultrafast lasers.