A large portion of my work during my PhD, from 2014 to the end of 2017, was with the goal of producing high energy terahertz pulses. Terahertz radiation is named after the unique frequency band centered on 1 THz (1*10^12 Hz) that is between radio-frequency technology and optical technology. We were interested in making this terahertz radiation for a European project called AXSIS, but it has implications on basic science as well. The results of this work were obtained during my PhD, but as always the analysis and writing up for publication took some time (along with the peer review process…). With the publication of a third article my output from that time period is finally at a close, with the rest of the team of course continuing to have success after success.
The first paper was a short “proof-of-principle” paper that showed that we properly understood our new method for terahertz generation (i.e. it worked like we expected). And beyond showing that it worked, we already produced a record energy in such terahertz pulses. The paper was entitled “Narrowband terahertz generation with chirped-and-delayed laser pulses in periodically poled lithium niobate” and was published in the journal Optics Letters in June of 2017. Although at only 4 pages we managed to fit 5 figures and a lot of information.
Of course as it is with any scientific project, we immediately had the question of “what next”. For the AXSIS project we needed much higher energy pulses, and although we had an engineering solution to improve slightly (thanks to our collaborators in Japan for manufacturing the largest crystals in the world!) we needed to also better understand what was going on. This took the rest of 2017, but using some nuances of the data from the first experiments from 2016 and a simple model, we realized that a so-called “higher-order” property of our driving laser pulses was limiting the efficiency.
In early 2017 we designed and tested a method to compensate for this limitation. That method, along with the engineering solution, and pumping two crystals at once, resulted in another increase in the pulse energy by 15 times by the end of 2017! The model and method we used to increase the efficiency of the process were all important results for the community and the final result of high energy pulses was the cherry on top. We took the better part of 2018 to finish the analysis and to put the results together in to a manuscript. After a long peer-review process (and a rejection!) we were finally accepted to Nature Communications, and published in June of 2019. The paper is titled “Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation“, and was also written about in the news section of the DESY webpage.
After getting the milestone result in the Nature Communications paper we could reflect on past nuanced data from the original experiments in 2016, and also imagine the broader impact of this higher-order property of the driving laser pulses. With a detailed analysis and outlook we published a third paper, out this month in Optics Express, “On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses“. We were quite happy to put out this longer, technical paper, since it closed some gaps and added interesting details to the other two papers, so we felt that it provided a good complement to our other work while still being new and original.
My current work no longer involves terahertz radiation, but I learned so much during these experiments (and doing the analysis). I do always keep terahertz applications of my current work in the back of my mind, since it is such a dynamic field, and is just plain cool.
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Obviously the biggest thanks go to my PhD supervisors Andreas Maier and Franz Kaertner, to my two very close collaborators Frederike Ahr and Nicholas Matlis, our collaborators in Japan Takunori Taira and Hideki Ishizuki, and the many other coauthors and colleagues mentioned in the papers.