Nonlinear optics revolutionized the ability to create directed, coherent beams particularly in spectral regions where lasers based on conventional population inversion are not practical. New breakthroughs in extreme nonlinear optics promise a similar revolution in the X-ray regime. In a dramatic and unanticipated breakthrough, an international team lead by the PI demonstrated that the high harmonic generation process (HHG) driven by mid-IR lasers can be used to generate keV photons, implementing a >5000 order nonlinear process, while still maintaining the full phase matching that is necessary for good conversion efficiency. This work represents the most extreme, fully coherent upconversion for electromagnetic waves in the 50 year history of nonlinear optics. Moreover, the limits of HHG are still not understood, either theoretically or experimentally. It may be possible to generate coherent hard X-rays using a tabletop-scale apparatus. In another surprising breakthrough, the PI showed that UV-driven HHG in multiply ionized plasma can be also highly efficient, representing a 2nd route towards the X-ray region. Remarkably, this regime provides X-rays with contrasting spectral and temporal properties. Furthermore, by shaping the polarization of a bi-color mid-IR driving laser the PI, the JILA team in collaboration with Technion, demonstrated robust phase matching of circularly polarized soft X-rays. In the proposed work, the fundamental atomic, phase matching plus group velocity matching limits of HHG in the multi-keV X-ray regime will be explored using the 3 most promising, complimentary approaches: 1) mid-IR driven HHG, 2) UV driven HHG, and 3) all-optical quasi phase matching. The knowledge gained as a result of this effort will identify the best path forward for generating bright coherent X-ray beams on a tabletop, at photon energies of 1-10 keV and greater with unprecedented attosecond-to-zeptosecond pulse durations, and arbitrary polarization state.
The IRDS roadmap considers two-dimensional (2D) materials a promising option for scaling electronic devices down to atomic dimensions. While there has been a lot of progress regarding 2D semiconductors, all electronic devices require suitable insulators as well. Although a major show-stopper, insulators have received far less attention and their is no clear roadmap as to which insulators can be used for ultimately scaled nanoelectronics. My group was recently first to demonstrate back-gated 2D FETs using ultrathin calcium fluoride (CaF2) as an insulator. Based on these promising results, I firmly believe that fluorides, which are ionic crystals with often very wide bandgaps, can efficiently address the major challenges: (i) Although relatively exotic materials, their growth is considerably better established than that of any 2D material. (ii) CaF2 can be epitaxially grown layer-by-layer on silicon substrates and likely also on 2D semiconductors. As their F-terminated inert surface supports van der Waals epitaxy of 2D materials, they could be the missing link between 3D substrates and 2D semiconductors. (iii) The low-defectivity of the inert CaF2 surface will significantly improve device performance and stability. Thereby, fluorides will allow novel 2D devices to make the leap from promising concepts to highly performant and stable real devices. F2GO will establish fluorides as a key enabler for 2D nanoelectronics by successfully demonstrating device architectures which were previously impossible to fabricate with sufficient performance due to inadequate insulators. I will do so by investigating selected fluoride-based devices for key technologies: (i) steep slope devices for CMOS logic (Cold Source FETs) at the ultimate scaling limit to allow sub-100 mV operation and (ii) ultra-scaled non-volatile memory devices (Flash and TRAM). Thereby, F2GO will pave the way for fluoride-based nanoelectronics at the ultimate scaling limit as required for the generations 2030+.