X-ray vision through the water window

The flexibility to generate gentle pulses of sub-​femtosecond period, first demonstrated some 20 years in the past, has given rise to a completely new discipline: attosecond science and expertise. Desk-​high laser methods have emerged that allow research that for many years have been however a distant dream — to observe, picture and characterise digital processes in atoms, molecules and solids on their pure, attosecond timescales. The laser methods that make such research doable sometimes function within the excessive ultraviolet spectral band. There has lengthy been a push to attain greater photon energies although. Of explicit curiosity is the ‘water window’, occupied by delicate x-​ray radiation with wavelengths between 2.2 and four.four nm. That spectral window owes its title, and significance, to the truth that at these frequencies, photons usually are not absorbed by oxygen (and therefore by water), however they’re by carbon. That is excellent for finding out natural molecules and organic specimens of their pure aqueous surroundings. In the present day, a handful of attosecond sources spanning this frequency vary exist, however their applicability is proscribed by comparatively low repetition charges of 1 kHz or under, which in flip means low rely charges and poor signal-​to-noise ratios. Writing in Optica, Justinas Pupeikis and colleagues within the Ultrafast Laser Physics group of Prof. Ursula Keller on the Institute for Quantum Electronics report now a vital leap to beat the restrictions of the prior sources. They current the primary soft-​x-ray supply that spans the complete water window at 100 kHz repetition charge — a hundredfold enchancment in comparison with the state-​of-the-art sources.

A lift in technological functionality

The bottleneck in producing delicate x-​rays at excessive repetition charges has been the shortage of appropriate laser methods to drive the important thing course of underlying attosecond-​pulse technology in table-​high methods. That course of is called high-​harmonic technology, and it includes an intense femtosecond laser pulse interacting with a goal, sometimes an atomic fuel. The nonlinear digital response of the goal then causes the emission of attosecond pulses at an odd-​order a number of of the frequency of the driving laser discipline. To make sure that that response accommodates x-​ray photons spanning the water-​window vary, the femtosecond supply has to function within the mid-​infrared vary. Additionally, it has to ship high-​peak-power pulses. And all of that at excessive repetition charges. Such a supply didn’t exist thus far.

Pupeikis et al. took up the problem and systematically improved a structure that they had already explored in earlier work, based mostly on optical parametric chirped pulse amplification (or OPCPA for brief). That they had established earlier than that the strategy is promising with a view to realizing high-​energy mid-​infrared sources, however substantial enhancements have been nonetheless wanted to succeed in the efficiency required for the high-​harmonic technology of x-​ray photons within the water window. Particularly, they pushed the height energy from beforehand 6.three GW to 14.2 GW, they usually reached a median energy of 25 W for pulses only a bit longer than two oscillations of the underlying optical discipline (16.5 fs). The height energy demonstrated is comfortably the best reported up to now for any high-​repetition-rate system with a wavelength above 2 μm.

Prepared for the x-​ray room

With this stage of efficiency at their disposal, the group was prepared for the subsequent stage, frequency upconversion by means of high-​harmonic technology. For that, the output beam of the OPCPA was routed by way of a periscope system to a different laboratory greater than 15 m away, to accommodate for native lab-​house constraints. There, the beam met a helium goal, stored at a stress of 45 bar. Such excessive stress was essential for phase-​matching between the infrared and the x-​ray radiation, and thus optimum energy-​conversion effectivity.

All items fastidiously put in place, the system certainly delivered. It generated coherent delicate x-​ray radiation extending to an power of 620 eV (2 nm wavelength), masking the complete water window — a stand-​out achievement relative to different high-​repetition-rate sources on this frequency vary. A window of alternative

This demonstration opens up an unlimited spectrum of recent alternatives. Coherent imaging within the water-​window spectral area, extremely related for chemistry and biology, must be doable with a compact setup. On the identical time, the excessive repetition charge out there helps, for example, addressing the restrictions as a result of space-​cost formation which plague photoemission experiments with pulsed sources. Furthermore, the ‘water window’ includes not solely the Okay-​edges of carbon, nitrogen and oxygen, but in addition the L- and M-​edges of a spread of metals, which might now be studied with greater sensitivity or specificity.

With such shiny prospects, the belief of the supply now offered heralds the start of the subsequent technology of attosecond expertise, one the place experimentalists for the primary time could make mixed use of excessive repetition charges and excessive photon energies. An attosecond beamline designed to use these new capabilities is at present underneath building within the Keller lab.