View inside the record-breaking laser: You can see the round amplifier disc, which is passed through several times by the laser beam (bright dot in the middle). (Image: Moritz Seidel / ETH Zurich)
When we hear the word laser, we usually think of a strongly focused and continuous beam of light. Lasers that produce such light are actually very useful and widely used. Often, however, science and industry also need very short and strong pulses of laser light. This can be used to process materials or generate high harmonic frequencies up to X-rays, which can be used to visualize extremely fast processes in the attosecond range (billionths of a billionth of a second).
Researchers at ETH Zurich led by Ursula Keller, Professor at the Institute of Quantum Electronics, have now set a new record for such laser pulses: with up to 550 watts of average power, they surpass the previous maximum value by more than 50 percent, making them the most powerful ever generated in a laser oscillator. At the same time, they are extremely short at less than a picosecond – i.e. the millionth of a millionth of a second – and leave the laser in regular succession at a high rate of five million pulses per second. The power of the short pulses reaches peaks of 100 megawatts (which could theoretically operate 100,000 vacuum cleaners for a short time). The researchers recently published their results in the journal Optica.
Keller's research group has been working for 25 years on the continuous improvement of so-called short-pulsed disk lasers, in which the laser material consists of a disk of a crystal containing ytterbium atoms that is only 100 micrometers thick.
Again and again, Keller and her employees encountered new problems, which initially prevented the further increase in performance. It was not uncommon for spectacular incidents to occur in which different parts within the laser were destroyed. The solution to the problems has repeatedly led to new findings, which have made the short-pulsed lasers, which are also popular in industry, more reliable.
"The combination of even higher power with pulse rates of 5.5 megahertz that has now been achieved is based on two innovations," explains Moritz Seidel, a doctoral student in Keller's laboratory. On the one hand, he and his colleagues used a special arrangement of mirrors that guide the light in the laser through the disk several times before it leaves the laser through a decoupling mirror. "This arrangement allows us to amplify the light extremely without the laser becoming unstable," says Seidel.
The second innovation concerns the heart of the pulsed laser: a special mirror made of semiconductor material, which Keller invented thirty years ago, and which is known by the catchy abbreviation SESAM (Semiconductor Saturable Absorber Mirror). Unlike normal mirrors, the reflectivity of a SESAM depends on how strong the light is that hits it.
Pulse thanks to SESAM
With the SESAM, the researchers make their laser emit short pulses instead of a continuous beam. Pulses have a higher intensity because the light energy is concentrated in a very short time. In order for a laser to emit laser light at all, the light intensity inside it must exceed a certain threshold value. This is where SESAM comes into play: it reflects the light that has passed through the amplifying disk several times particularly efficiently when the light intensity is high. As a result, the laser automatically switches to a pulsed state.
"Until now, pulses with similarly high powers as those we have now achieved could only be generated by sending weaker laser pulses through several separate amplifiers outside the laser," says Seidel. However, this has the disadvantage that the amplification also leads to stronger noise, i.e. a fluctuation in performance, which is particularly problematic for precision measurements. In order to generate the high power directly with the laser oscillator, the researchers had to solve some tricky technical problems – for example, how to apply a thin window of sapphire to the semiconductor layer of the SESAM mirror, which greatly improves the properties of the mirror. "When that finally worked out and we were able to observe how the laser generated pulses – that was really cool," says Seidel happily.
Alternative to amplifiers
Ursula Keller is also enthusiastic about these results and emphasizes: "The support of ETH Zurich over the years and the reliable funding of my research by the Swiss National Science Foundation have helped my colleagues and me to achieve this great result. We now also expect that with these high powers, we will be able to shorten the pulses very efficiently into the range of a few cycles, which is very important for generating attosecond pulses."
Keller sees further applications of the short, fast and strong pulses that will be possible with the new laser in new so-called frequency combs in the ultraviolet to X-ray range, among other things, which could make clocks even more accurate. "It would be a dream if we could use such extremely precise clocks to measure that the natural constants are not constant after all," says Keller. Terahertz radiation, which is much longer wavelength than visible light or infrared light, can also be generated with the laser and used, for example, to inspect materials. "Overall, it can be said that with our pulsed laser, we show that laser oscillators are a good alternative to laser systems with amplifiers and enable new and better measurements," Keller summarizes.
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