Fast Laser Produces Coherent Soft X-​Rays

The ability to generate light pulses of sub-​femtosecond duration has given rise to the field of atto­second science and technology. Table-​top laser systems have emerged that enable studies that for decades were but a distant dream – to follow, image and charac­terise electronic processes in atoms, molecules and solids on their natural, attosecond time­scales. The laser systems that make such studies possible typi­cally operate in the extreme ultra­violet spectral band. There has long been a push to achieve higher photon energies though. Of particular interest is the water window, occupied by soft x-​ray radiation with wave­lengths between 2.2 and 4.4 nm.

A high-​harmonic-generation process in the high-​pressure gas cell, with the mid-​infrared input arriving on the right and the soft x-​ray output appearing on the left. (Source: ETHZ)

That spectral window owes its name, and importance, to the fact that at those fre­quencies, photons are not absorbed by oxygen, but they are by carbon. This is ideal for studying organic molecules and bio­logical specimens in their natural aqueous environ­ment. Today, a handful of attosecond sources spanning this frequency range exist, but their applicability is limited by relatively low repetition rates of 1 kHz or below, which in turn means low count rates and poor signal-​to-noise ratios. Now, Justinas Pupeikis and colleagues in the Ultrafast Laser Physics group of Ursula Keller at the Institute for Quantum Elec­tronics in Zurich report now an essential leap to overcome the limitations of the prior sources. They present the first soft-​x-ray source that spans the full water window at 100 kHz repetition rate – a hundred­fold improve­ment compared to the state-​of-the-art sources.

The bottleneck in producing soft x-​rays at high repe­tition rates has been the lack of suitable laser systems to drive the key process underlying atto­second-​pulse generation in table-​top systems. This high-​harmonic generation involves an intense femto­second laser pulse inter­acting with a target, typically an atomic gas. The nonlinear electronic response of the target then causes the emission of attosecond pulses at an odd-​order multiple of the frequency of the driving laser field. To ensure that that response contains x-​ray photons spanning the water-​window range, the femto­second source has to operate in the mid-​infrared range. Also, it has to deliver high-​peak-power pulses. And all of that at high repe­tition rates. Such a source did not exist so far.

Pupeikis and colleagues took up the challenge and systemati­cally improved a layout they had already explored in earlier work, based on optical para­metric chirped pulse amplification (OPCPA). They had established before that the approach is promising with a view to realizing high-​power mid-​infrared sources, but substantial improve­ments were still needed to reach the per­formance required for the high-​harmonic generation of x-​ray photons in the water window. In parti­cular, they pushed the peak power from previously 6.3 GW to 14.2 GW, and they reached an average power of 25 W for pulses just a bit longer than two oscilla­tions of the underlying optical field (16.5 fs). The peak power demons­trated is comfortably the highest reported to date for any high-​repe­tition-rate system with a wavelength above 2 μm.

With this level of performance at their disposal, the team was ready for the next stage, frequency upcon­version through high-​harmonic gene­ration. For that, the output beam of the OPCPA was routed via a periscope system to another labora­tory more than 15 m away, to accommodate for local lab-​space constraints. There, the beam met a helium target, kept at a pressure of 45 bar. Such high pressure was necessary for phase-​matching between the infrared and the x-​ray radiation, and thus optimal energy-​conversion effi­ciency. All pieces carefully put in place, the system indeed delivered. It generated coherent soft x-​ray radiation extending to an energy of 620 eV, covering the full water window – a stand-​out achieve­ment relative to other high-​repe­tition-rate sources in this frequency range.

This demons­tration opens up a vast spectrum of fresh oppor­tunities. Coherent imaging in the water-​window spectral region, highly relevant for chemistry and biology, should be possible with a compact setup. At the same time, the high repetition rate available helps, for instance, addressing the limi­tations due to space-​charge formation which plague photo­emission experi­ments with pulsed sources. Moreover, the ‘water window’ comprises not only the K-​edges of carbon, nitrogen and oxygen, but also the L- and M-​edges of a range of metals, which can now be studied with higher sensi­tivity or speci­ficity.

With such bright prospects, the realiza­tion of the source now presented heralds the beginning of the next generation of atto­second tech­nology, one where experi­mentalists for the first time can make combined use of high repetition rates and high photon energies. An atto­second beamline designed to exploit these new capa­bilities is currently under construction in the Keller lab. (Source: ETHZ)

Reference: J. Pupeikis et al.: Water window soft x-ray source enabled by a 25 W few-cycle 2.2 µm OPCPA at 100 kHz, Optica 7, 168 (2020); DOI: 10.1364/OPTICA.379846

Link: Ultrafast Laser Physics, Dept. of Physics, Institute of Quantum Electronics, ETH Zurich, Switzerland

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