Session K05: Novel Compact Ultrafast Sources and their Applications

An attosecond look at liquid water

Attosecond spectroscopy is by now well established in the gas phase and first promising results have been obtained on solids. I will present an overview of the development of several new techniques that extend attosecond time-resolved spectroscopy to the liquid phase. First, the observation of coherent extreme-ultraviolet high-harmonic emission from water and several alcohols will be described. The comparison of the experimental results with a strongly-driven few-band model establishes the sensitivity of high-harmonic spectroscopy to the electronic structure of liquids. Second, the measurement of photoemission delays between the highest occupied valence band of liquid water and isolated water molecules in the gas phase will be described. These measurements provide detailed information on the scattering dynamics of electrons with the liquid environment. Third, the development of attosecond soft-X-ray absorption spectroscopy will be presented. Isolated attosecond pulses with a duration of 43 attoseconds have been measured. Time-resolved absorption measurements at the carbon K-edge have revealed ultrafast electronic and structural dynamics following ionization. First results from liquid-phase transient absorption will be presented.   

Attosecond dispersive soft X-ray absorption fine structure spectroscopy

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes, whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window-covering (284 eV to 543 eV) attosecond soft X-ray pulses that can simultaneously access electronic and lattice parameters via dispersive X-Ray absorption fine-structure (XAFS) spectroscopy. We validate attoXAFS with an identification of the $\sigma^{\mathrm{\ast }}$ and $\pi^{\mathrm{\ast \thinspace }}$orbital contributions to the density of states in graphite simultaneously with its lattice's four characteristic bonding distances. Moreover, we will show that this method can provide a real-time view on the light-field- driven inter- and intra-band carrier dynamics of the quasi-two-dimensional transition-metal dichalcogenide TiS2, exploiting the element specificity of X---ray transitions at the L edges of the titanium atoms at 460 eV. This work demonstrates the concept of attoXAFS as a powerful real-time investigative tool which is equally applicable to gas-, liquid- and condensed phase.   

Using cavity-enhanced high harmonic generation to track electrons in solids

Time-resolved photoemission spectroscopy (tr-PES) has become a key technique for studying the non-equilibrium electronic structure of molecules and solids, vibrational dynamics in molecules as well as physics/chemistry of surfaces. By adding angle-resolved capabilities (i.e., tr-ARPES), the dynamical behavior of electronic band dispersion in solids can be observed, enabling the interplay of electrons, phonons, and spin dynamics to be disentangled. While the capabilities of electron analyzers has improved drastically over the past two decades, ultrafast photon sources for tr-ARPES have remained a major technical limitation. Addressing these shortcomings, I will discuss our recent demonstration of a new laser-based, 60-MHz femtosecond XUV source ---based on cavity-enhanced high harmonic generation--- that has enabled coverage over the full Brillouin Zone ($>$2 Angstrom^{-1} with a time (energy) resolution of 190 fs (22 meV). I will summarize our characterization measurements (on a topological insulator, Bi$_2$Se$_3$, and polycrystalline Au) and present results of measuring electron-phonon coupling in graphite at the edge of its Brilliouin Zone. I will also discuss our future plans enabled by this next generation TR-ARPES source.   

Infrared laser frequency combs: generation and spectroscopic applications

The laser spectroscopy revolution that began in the 1960's largely bypassed the infrared and terahertz regions of the electro-magnetic spectrum. This includes the so-called molecular fingerprint region (500 - 1500 cm$^{-1}$, or 6.7 - 20 $\mu$m), which is appropriately named because of the rich spectral information it holds for identifying many molecular species. And despite the steady progress of more than 50 years of laser-based research, it is remarkable that one of the most widely used spectroscopic tool in this spectral region continues to be the Fourier transform infrared spectrometer, consisting of a thermal light source and mechanically-scanned Michelson interferometer. In this talk, we present recent laser and nonlinear optics advances that bring the power and precision of laser frequency combs to the long-wave infrared. We have developed a simple and robust method for generating super-octave (4-12 $\mu$m), optical frequency combs in the fingerprint region through intra-pulse difference frequency generation in an orientation-patterned gallium phosphide crystal. This frequency comb is orders of magnitude brighter than thermal light sources and comparable to infrared beam lines at synchrotron user facilities. We demonstrate the utility of this unique, coherent light source for high-precision, dual-comb spectroscopy in methanol and ethanol vapor. These results highlight the potential of laser frequency combs for a wide range of infrared molecular sensing applications, from basic molecular spectroscopy to nanoscopic imaging.