Record performance with ultrashort laser pulses
A new laser oscillator generates extremely short pulses with a previously unrivalled high output - without any loss of quality due to amplifiers. This could help with precision measurements and materials research.
A team at ETH Zurich has used a newly developed laser to generate extremely intense pulses: the 852 femtosecond flashes of light were up to 103 megawatts strong. The average output power was 550 watts; the previous record in a laser oscillator was 350 watts, as the team led by Ursula Keller explains in the scientific journal "Optica"
More than five million such pulses leave the laser every second. In contrast to continuous wave lasers, which emit photons continuously, pulse lasers emit the radiation periodically, focussing a particularly large amount of energy on the individual pulses. Such lasers can visualise extremely fast processes, down to the attosecond range. Electrons move through molecules on these time scales, so that chemical or biological processes can be observed at a fundamental level.
In practice, amplifiers are often connected downstream of the laser source. This allows high average powers to be achieved, but at a price: at the same time, the proportion of unwanted wavelengths in the emitted light increases and its power fluctuates more. This is particularly problematic for precision measurements.
Researchers are therefore working on particularly powerful oscillators. Such lasers generate and amplify the radiation directly in the device and do not require any subsequent amplification. Disc lasers, such as those used by Ursula Keller and her team, have proven to be particularly suitable. In their design, the laser material consists of a 100 micrometre thin crystal disc. Specially arranged mirrors guide the light through several times and thus amplify it.
The working group owes the record performance it has now achieved to two decisive advances in the optical components. Firstly, the team used cleverly arranged mirrors to guide the light through the amplifying disc particularly often. Secondly, they used a special mirror made from a semiconductor material. It reflects the light from the disc better when its intensity is high. This automatically brought the laser into a pulsed state. Thanks to a special surface treatment, the mirror recovered better after emitting a pulse - such high intensities occasionally destroy individual components that cannot withstand the energy.
Source: Moritz Seidel / ETH Zurich
In addition to the high average power, the researchers achieved further superlatives with this setup. For example, the pulse energy of 100 microjoules is the highest ever measured on an ultrafast oscillator; the previous record was 80 microjoules. The team hopes that the new laser will enable better precision measurements in basic research. On the other hand, the development is well suited to industrial applications, they write in their publication, as micromaterial processing of metals and semiconductors, for example, requires precisely such short, high-energy pulses.
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