Generation and applications of high-brightness coherent soft-Xray pulses

Abstract

Advances in X-ray science and technology have resulted in breakthrough discoveries ranging from unraveling the structure of DNA and proteins to visualizing atoms, molecules, and materials at the nanoscale level. One very exciting advance in X-ray science has been the ability to generate ultrafast (0.1–10 fs), coherent X-rays from a table-top apparatus, by using the extreme nonlinear optical process of high-order harmonic generation (HHG). The femtosecond-to-attosecond pulse duration and intrinsic jitter free have made it possible to capture the coupled motions of electrons, atoms, and molecules in real time. Broadband HHG sources enable novel elemental specificity absorption spectroscopy for probing transient dynamic of multiple elements simultaneously in complex chemical and biological samples. Moreover, the low divergence and capability to produce light with full spatial coherence have enabled advanced time-resolved imaging of high-performance materials and novel devices with nanometer resolutions. This thesis lays out an attractive route toward high-flux table-top HHG sources by use of broadband power-scalable Yb:CaF2 driver laser. This development targets the soft X-ray spectral range 100-240 eV with the wavelength in the 5-13 nm range where our system delivers an orders-of-magnitude improvement in the HHG flux compared to conventionally used Ti:sapphires driver lasers. The spectral range of our interest covers important absorption edges of many atoms: Si, Ga, Se (relevant for semiconductors); Tb, Gd N-edge (relevant for magnetic materials); P, S, C (relevant to organic molecules). Combining together with advantages of power scaling, cost-efficient, robust, compactness of Yb laser, the high brightness soft-X-ray source opens up novel oppor- tunities in many applications such as advanced EUV metrology, near edge absorption microscopy, ultrafast magnetic microscopy on a table-top scale. Two novel HHG approaches for HHG driving are developed in this thesis. First, we demonstrated HHG driven directly by an intense (0.7 TW peak-power), 20 fs, 2 kHz, Yb laser amplifier system for the first time in the fully absorption-saturated regime, which results in an improvement of orders of magnitude in the conversion efficiency and with highest flux 10^9 photons/s/1% bandwidth in the soft X-ray photon regime (150 eV -220 eV). This improvement can be explained by our single-atom and macroscopic simulation of the HHG process. The second driving approach generates intense isolated attosecond pulses with a multi-color driving waveform in the mid-IR wavelength range. A proof of concept waveform synthesizer is first demonstrated and multi-color temporal gating effect is discussed. Then, we present our development of a high peak- and average-power mid-IR OPA that serves as a blueprint for Fourier synthesis driver pulse for HHG, with which soft X-ray super-continua is generated corresponding to an isolated attosecond pulse with a flux of 107 photons per second reaching into the water window (> 300 eV) spectral region. To further extend HHG cut-off, we demonstrated post compression of the mid-IR pulses with multi-TW peak-power at the wavelength of 3900-nm which were characterized with a spatial-temporal pulse characterization technique in the single optical cycle regime. Many high-performance materials and novel devices consist of multiple components and possess a natural or intentional nanostructure that defines their optimal properties and performance. Exploiting the the intrinsic electron spin dynamics and its associated magnetic moment, the opto-magnetic control on a femtosecond timescale has become an important topic for fundamental research as well as technological application such as the sensor technology and in magnetic data storage devices. We implement the first table-top ultrafast resonant X-ray magnetic diffraction measurement at the N-edge of the rare-earth ferromagnets Tb (155 eV). A serials of snapshot imaging of the nanoscale magnetic structures using femtosecond X-ray pulses with sub-100 fs temporal and sub- 100 nm spatial resolution has also been recorded. To date, similar measurements have only been accomplished in large-scale X-ray facilities. The unique advantages of the soft X-ray microscopy for studying magnetic micro- and nano-structures are the element specificity of the X-ray magnetic circular dichroism (XMCD) contrast mechanism and its direct correlation with local spin and orbital moments. Improving the performance of the HHG sources will greatly benefit the advancement of ultrafast X-ray methodology and extend the range of applications and capabilities. The development of high energy booster and efficient nonlinear conversion approaches provide us a novel ultrafast light-matter experimental platform, using a wide range of the electromagnetic radiation spectrum which extends from the low frequencies of the THz pulses, covers the mid-IR and reaches up to the high energy of the soft x-ray pulses generated via HHG.