X-ray has become an important tool for use in diverse research fields such as physics, chemistry, biology, materials science, chemical engineering, environmental engineering, energy resources, mechanical engineering, and electronics. In recent years, the table-top high order harmonic generation (HHG) has been gaining attention, since it can delivery extremely short pulses down to attoseconds. The HHG light source has been shown to be fully coherent or laser-like with a very small divergence angle of a few milliradians. The femtosecond-to-attosecond X-ray pulse enables the dynamics of chemical reactions, nano-materials and bio-molecular systems to be studied with unprecedented temporal and spatial resolution. However, the brightness of HHG light source is still limited its applications, e.g. four-dimensional X-ray microscope.
The first HHG was demonstrated in 1987 by McPherson, using a long picosecond KrF* UV (248nm) laser. However, to pursuit a higher conversion efficiency, now most of HHG works were demonstrated by popular femtosecond Ti:Sapphire lasers with a central wavelength of 800nm. Even though the conversion efficiency was still limited to 10-6 – 10-11. Our IPT NTHU research team, collaborating with KMgroup in JILA, University of Colorado at Boulder, University of Salamanca in Spain, Cornell University, Temple University, and Lawrence Livermore National Laboratory has achieved a breakthrough of finding that using a similar driving wavelength of 270 nm like McPhreson et al. did 1987, but with a much shorter pulse duration, we are able to boost the HHG conversion efficiency up to 10-3 – 10-7. The experimental results have shown that full phase matching of HHG scales very strongly with wavelength of the driving laser, making it possible for the first time to obtain bright coherent emission ranging from 10 eV to 250 eV with a conversion efficiency >1000 times that previously reported [Science, 350, 1225 (2015)]. A bright X-ray flux makes a table-top X-ray microscope feasible to implement, for applications in bio- and nano- imaging, interferometry and holography.
Ref: Dimitar Popmintchev, Carlos Hernández-García1, Franklin Dollar, Christopher Mancuso, Jose A. Pérez-Hernández, Ming-Chang Chen, Amelia Hankla, Xiaohui Gao, Bonggu Shim, Alexander L. Gaeta, Maryam Tarazkar, Dmitri A. Romanov, Robert J. Levis, Jim A. Gaffney, Mark Foord, Stephen B. Libby, Agnieszka Jaron-Becker, Andreas Becker, Luis Plaja, Margaret M. Murnane, Henry C. Kapteyn, Tenio Popmintchev1, “Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas” , Science, 350, 1225 (2015)