Nonlinear optics and quantum optics revolutionized the ability to create coherent photons in broad regions of the electromagnetic spectrum where laser light based on conventional physics is not practical. Breakthroughs in attosecond science and extreme nonlinear optics promise a similar revolution in the X-ray regime. In an earlier breakthrough, an international team demonstrated that the high harmonic generation process driven by mid-IR laser fields can be used to generate 1.6 keV photons, implementing a 5000 order nonlinear process while still maintaining the phase synchronization necessary for good conversion efficiency. This work represents the most extreme coherent upconversion for electromagnetic waves in the 60 year history of nonlinear optics. Moreover, the limits are still uknown –theoretically or experimentally - it may be possible to create coherent hard X-rays using a laboratory-scale apparatus. In another surprising breakthrough, the team showed that UV-driven high harmonic generation in multiply ionized plasma can also be highly efficient, representing a second route towards the X-ray region. Remarkably, this regime provides X-rays with contrasting spectral and temporal properties. Furthermore, by shaping the spin angular momentum of a bi-color mid-IR field, the team demonstrated robust phase synchronization where the quantum properties of the driver are transferred to the generated X-rays.
In this ERC XSTREAM, the fundamental quantum physics and phase synchronization limits of the upconversion in the X-ray regime have been explored starting with three most promising approaches: I. mid-IR driven X-rays, II. UV driven X-rays, and III. all-optical phase synchronization in frequency and angular momentum mixing. XSTREAM aims to identify the best paths forward for generating bright coherent X-rays at photon energies of 1-10 keV with unprecedented attosecond-to-zeptosecond pulse durations and tunable classical and quantum properties of the light.
This X-ray research is relevant to areas of interest covered by academic, industrial, and defense programs supporting the second quantum revolution of 21st century. In perspective, compact non-diverging hard X-ray laser light would allow for remote delivery of penetrating radiation as a standoff detection where X-ray absorption and fluorescence spectroscopy determines elemental composition. Research on soft - hard X-ray light tools will also support the development of cutting edge nanoelectronics, atomtronics, new generation of magnetic storage key to society, efficient photovoltaics for green energy generation, and advanced bio-sensing and medical imaging. Our ability to directly observe function on the nanoscale with femtosecond or higher temporal resolution, in buried layers of quantum materials or next gen semiconductor chips, is still severely limited and can only be done at large scale facilities. Short and long range electron correlations, disorder, and impurities can govern the function of magnetic, superconducting nanosystems, and batteries. As a result, design rules for future nanosystems that operate at fundamental limits of quantum correlations, energy density, and speed, cannot be formulated until nanoscale structure and dynamics can be visualized and understood.
Inventing a coherent quantum version of the Roentgen X-ray tube that provides an exquisite control of all classical and quantum properties of light is a central part of the short and long term research of XSTREAM. Designer X-ray light with tunable spectral, spatial, temporal shape, and spin-orbital angular momentum, promises to be both a conceptual revolution in attosecond science, extreme nonlinear and quantum optics, as well as an essential tool for bio and nanoimaging and quantum technology in the second quantum revolution.
