EUV–soft X-ray spectrometer - monochromator, 

and wavelength-tunable focusing mirror - all in one design

 Published in The Nature Light Journal

A Twist of Fate in Optical Research: From a Budget Quip to Cutting-Edge Innovation

Imagine you're shopping for a top-notch extreme ultraviolet (EUV) beam line grating, but the sales rep cheekily suggests you need at least €50,000 to play in the big leagues, or else, try your luck at Thorlabs or MKS. Sounds discouraging? Well, not for a team of audacious scientists from TU Wien, UCSD San Diego and CAS Beijing.! This seemingly derogatory remark didn't dampen our spirits, says Dimitar Popmintchev and Tenio Popmintchev – researchers from TU Wien and UCSD San Diego. Instead, it fueled a groundbreaking discovery, now celebrated in the Nature Light Journal.

 

Monte-Carlo ray tracing of a-b Optimized spatial beam profile and chirp of a single high-order harmonic. c-d Unoptimized spatial beam profile and chirp of a single high-order harmonic. 

In their remarkable paper, the researchers present a groundbreaking EUV–soft X-ray spectrometer - monochromator, and wavelength-tunable focusing mirror - all integrated into one design. This isn't your ordinary spectrometer; it's a marvel of optical engineering that shatters efficiency records by design, boasting a whopping 60% throughput efficiency and showing promise to overcome 80%. The secret sauce? – Neural network optimization. And it achieves this feat without relying on the crutch of variable line spacing gratings.

 

TP and DP with the set up at TU Wien. Former lab of the 2023 Nobel Laureate in physics Ferenc Krausz.

The team didn't stop there. They harnessed the power of conical diffraction geometry, meticulously optimizing the optical system in a multidimensional parameter space. The result? Optimal imaging performance across a broad spectrum, with the bonus of preserving circular and elliptical polarization states in the first to third diffraction orders. But wait, there's more! The spectrometer is a timekeeper's dream, limiting pulse broadening to a mere 10 fs tail-to-tail and a standard deviation of 2 fs. This precision allows for ultrafast spectroscopic and pump-probe studies with femtosecond accuracy. What's even more exciting is its transformation capabilities. With a single grating, it morphs into a monochromator, extending its prowess to the soft X-ray region with minimal photon loss.

 

Experimental setup showing the torroidal mirror with flat grating. Simple azimuthal rotation of the torroidal mirror result in near perfect compensation of the spatio-temporal aberrations.

This is a game-changer in ultrafast spectroscopy, first and foremost, setting the stage for advanced coherent diffractive imaging of intricate nano- and bio-systems. Now, for the first time dynamic coherent diffraction imaging is feasible in the X-ray water-window, where water is more transparent that the bio- markers, promising unprecedented spatiotemporal precision at the femto-nanometer scale. Second, the design is as well a continuously wavelength-tunable X-ray mirror - a replacement for the inefficient, at best, multilayered EUV mirrors up to the water window oxygen edge and potentially the near keV magnetic L-edges with efficiency between 60%-90%.

One of the design's crowning achievements is its ability to counteract spatial aberrations in the harmonic beam caused by the grating. A simple azimuthal rotation of the toroidal mirror relative to the grating does the trick, ensuring optimal imaging performance over a wide spectrum.Their findings indicate minimal temporal dispersion, promising precision in ultrafast spectroscopic and pump-probe experiments. This level of performance is not just for show; it's a powerful tool for studying micro-scale dynamics across various scientific fields.

In summary, what started as a snide remark about budget constraints sparked a journey leading to an optical masterpiece. This spectrometer is not just another scientific instrument; it's a testament to perseverance, ingenuity, and a bit of humor in the face of adversity. The paper, available at Nature Light Journal, is not just a read; it's an inspiration for anyone who's ever been told they can't afford to dream big.

For more information, please visit the original papers Ref [1].

 Cite as:

[1]. Highly efficient and aberration-free off-plane grating spectrometer and monochromator for EUV—soft X-ray applications. Light Sci Appl 13, 12 (2024). DOI: https://doi.org/10.1038/s41377-023-01342-9