Friday, April 16, 2010

FEMTOLASERS: An Ultrafast Success Story

When lasers were discovered in 1960, it was unclear just how this new kind of light could be used. Similar uncertainty faced Ferenc Krausz and his fellow scientists in 1994 when, as a young postdoc at Vienna University of Technology in Austria, he set up a company to market femtolasers, FEMTOLASERS Produktion.

For the first time it became possible to generate light pulses with a duration of around 10 femtoseconds.

The success story of FEMTOLASERS began ....

At the end of the 1980s, a revolution was underway in the field of laser technology. In the early part of the decade, scientists had discovered titanium ions as a crystalline gain medium in lasers. Then along came a new technology to generate short pulses, "Kerr-lens mode locking," courtesy of University of St Andrews (UK) physicist Wilson Sibbet. This young era of new inventions was set to take over from femtosecond laser technology, which worked with liquid dyes as a gain medium.

By the early 1990s, the field of ultrashort laser physics was gripped by a pioneering spirit. Ferenc Krausz, his PhD recently completed at Vienna University of Technology (TU Vienna), was enthusiastic about the new possibilities promised by solid-state laser technology.

The first working group Krausz led at TU Vienna in 1994 was dedicated to the study of new types of solid-state and short-pulse lasers. This team was driven by a desire to significantly shorten the duration of light pulses, using Ti:Sa lasers. The 100-femtosecond pulse barrier had already been cracked. But the broadband laser material titanium-doped sapphire offered an opportunity to get much lower than that.

The greatest challenge to be faced was how to counteract the dispersion that became more and more noticeable as pulse duration shortened. Dispersion occurs in all materials. It delays the colour components in broadband light to varying degrees and thus leads to a lengthening of the pulse. The shorter the pulse, the wider its bandwidth, an effect which causes the pulse to disperse even faster.
Krausz' working group solved the problem by inventing "chirped mirrors" and using them to control the dispersion of femtosecond Ti:Sa oscillators, in cooperation with Robert Szipöcs and Kárpát Ferencz of Budapest.

Chirped mirrors (CM) are made from alternating nanometer-scale layers of two different transparent optical materials with different indices of refraction. They can reflect up to 99.9% of the incident light over a wide bandwidth. In addition, they let light of different wavelengths penetrate to differing depths before it is reflected.

As a result, it now became possible to control dispersion across wide bandwidths and thereby support the formation of pulses of hitherto unachieved shortness.

The journal Optics Letters (Vol. 19, No. 3) published two separate articles on the subjects of the invention of chirped mirrors (U.S. patent 5,734,503) and the concurrent development of a Ti:Sa oscillator which could generate pulses lasting just 11 femtoseconds.

Femtosecond Pioneers

Spurred on by this success, Krausz, together with Andreas Stingl and Christian Spielmann, both scientists in his team at TU Vienna in 1994, decided to set up a company, FEMTOLASERS Produktion in Vienna.

In the first two years, the team produced a small number of Ti:Sa laser systems with chirped mirrors as a kit. These were the very first chirped mirrors ever made.

In 1997, the first commercial laser with a pulse duration of less than 12 femtoseconds was ready for market. Demand for these laser systems was rising steadily, and FEMTOLASERS was the only company serving the market.

Ultrafast Atoms

Although in the beginning it was not yet clear just what the unusually short pulses could be used for, it quickly became apparent that they could open up spectacular new perspectives in biology and physics in particular.

Ever shorter pulses permitted the observation of ever faster processes, for example, in nature. Here, time-resolved femtosecond spectroscopy has done much to further the understanding of elementary processes that occur in chemical reactions. With the aid of this technique, it is possible to track-in real time-the ultrafast movement of atoms in chemical reactions.

The laser pulses are used, for example, to capture momentary snapshots of molecular arrangements. These individual snapshots can then be put together, like an animated film, to depict the time sequence of an event. In 1999, Ahmed Zewail was awarded the Nobel Prize in Chemistry for this development.

Competition to FEMTOLASERS was not long in coming, and soon the only way to survive in a tough market environment was to keep improving the technology of short pulse lasers. FEMTOLASERS developed new amplifiers which injected a million times higher energy into the light pulses.

This development opened the way to the generation of coherent radiation; in addition to pulses in the visible spectrum it was now also possible to generate femtosecond pulses in the ultraviolet and x-ray ranges.

To date, FEMTOLASERS has sold well over 500 systems and is one of the market leaders in this area.

Further development in femtosecond pulse technology since 2001 has opened the door to the attosecond time dimension, 1000 times shorter than femtoseconds. At that time, the team at TU Vienna was the first group worldwide to succeed in generating individual attosecond pulses with a length of 650 as.

Attosecond flashes arise when electrons in an inert gas are excited by femtosecond laser pulses of a few wave cycles. Today femtosecond pulse technology is capable of generating light flashes which carry over 70% of their energy in a single oscillation cycle.

Thanks to these advances, the team succeeded in 2008 in generating light pulses shorter than 100 attoseconds.

By then, the work was being continued at the Max Planck Institute of Quantum Optics (MPQ) in Garching (Germany) by a working group led by Krausz. The group had moved there from Vienna in 2004. Krausz is director of the MPQ, and he also holds a chair in Experimental Physics at the Ludwig-Maximilians-Universität (LMU) of Munich.

Attosecond flashes below 100 as lift the veil on previously invisible electron movements. In particular it becomes possible to observe interactions between individual electrons in real time, because within and between atoms, these particles generally move on an attosecond timescale.

Attosecond Flashes

For attosecond technology to emerge, it was necessary both to shorten the femtosecond laser pulses to around the ultimate limit set by the oscillation period and to precisely control the wave form of the oscillating light field. These wave-form-controlled few-cycle light pulses, which Krausz and his group demonstrated for the first time in cooperation with Nobel Prize winner Theodor Hänsch and his team in 2003 (and which FEMTOLASERS was the first to market a little later), are not just essential for being able to consistently generate and measure individual attosecond flashes. They also open up for the first time the way to controlling the movement of atoms in atomic systems.

Imaging Potential

The potential significance of this is enormous. It may become possible, for example, to control biological processes at a molecular level, or even accelerate microelectronics to its ultimate limits, which are defined by light frequencies.

In many other ways, too, short pulse laser technology will open up brand new insights into the microcosm in the future. For example, it will improve imaging techniques such as multiphoton microscopy by generating non-linear optical effects. In non-linear microscopy, a femtosecond laser excites biomolecules. The light emitted by them is used for imaging and enables a 3D representation.

Work is also proceeding on optimizing coherence tomography. Medical applications are mainly in ophthalmology and early diagnosis of skin cancer.

Terahertz imaging also has great potential and could benefit from the revolution in femtosecond laser technology. In the terahertz frequency range (100 GHz to 10 THz) materials like paper and many plastics become transparent and can be probed. Water and metals, however, have strong absorption lines at these frequencies. This offers potential applications in the examination of solid-state materials, plastics, and biomedical textiles and also in environmental monitoring, quality control, package testing, and security checks.

Future of Ultrafast Lasers

In addition to short pulse technology, Krausz and his team devote special attention to the further development of chirped mirrors. For this purpose in 2009, the team formed a new company in Garching, UltraFast Innovations (, a spinoff from the excellence cluster, Munich-Centre for Advanced Photonics ( MPQ and LMU each have a 50% share in the company, which designs and manufactures custom optics.

The optical components offered by UltraFast Innovations are suitable for almost all areas of laser technology. The scientists test the new developments in their own research work, an aspect customers very much appreciate.

We can only guess at the enormously diverse range of potential applications that will emerge from new developments in ultrashort laser pulses and improvements in optical components in laser technology. Almost every new technological advance brings with it new applications, expanding ever further the reach of our understanding.

SPIE Professional April 2010: Open access article

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