Ali Oguz Er

Ultrafast time resolved x-ray diffraction, extended x-ray absorption fine structure and x-ray absorption near edge structure

Ultrafast time resolved x-ray absorption and x-ray diffraction have made it possible to measure, in real time, transient phenomena structures and processes induced by optical femtosecond pulses. To illustrate the power of these experimental methods, we present several representative examples from the literature. (I) Time resolved measurements of photon/electron coupling, electron/phonon interaction, pressure wave formation, melting and recrystallization by means of time resolved x-ray diffraction. (II) Ultrafast x-ray absorption, EXAFS, for the direct measurement of the structures and their kinetics, evolved during electron transfer within molecules in liquid phase. (III) XANES experiments that measure directly pathway for the population of high spin states and the study of the operating mechanism of dye activated TiO2 solar cell devices. The construction and use of novel polycapillary x-ray lenses that focus and collimate hard x-rays efficiently are described.

Coherent acoustic wave oscillations and melting on Ag(111) surface by time resolved x-ray diffraction

The transient structural disorder on the Ag(111) crystal after excitation with fs pulses was studied by means of time resolved x-ray diffraction. The lattice disorder after UV irradiation is detected by changes in the XRD rocking curve including peak shift, broadening, and total diffraction intensity. We have observed blast and pressure wave formation and melting and mosaic crystal formation during re-crystallization. A blast force was formed within 2 ps after fs UV irradiation of the crystal, followed lattice contraction, melting phase transition, and mosaic crystal formation as the temperature decreases below melting. The experimental data are also supported by theoretical simulations.

Low temperature epitaxial growth of Ge quantum dot on Si(100)-(2×1) by femtosecond laser excitation

Low temperature epitaxy of Ge quantum dots on Si(100)-(2×1) by femtosecond pulsed laser deposition under femtosecond laser excitation was investigated. Reflection high-energy electron diffraction and atomic force microscopy were used to analyze the growth mode and morphology. Epitaxial growth was achieved at ∼70 °C by using femtosecond laser excitation of the substrate. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is proposed.

Excitation-Induced Germanium Quantum Dot Formation on Si (100)-(2×1)

The effect of nanosecond pulsed laser excitation on the self-assembly of Ge quantum dots grown by pulsed laser deposition on Si(100)-(2×1) was studied. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy were used to probe the quantum dot structure and morphology. At room temperature, applying the excitation laser decreased the surface roughness of the grown Ge film. With surface electronic excitation, crystalline Ge quantum dots were formed at 250 °C, a temperature too low for their formation without excitation. At a substrate temperature of 390 °C, electronic excitation during growth was found to improve the quantum dot crystalline quality, change their morphology, and decrease their size distribution almost by half. A purely electronic mechanism of enhanced surface hopping of the Ge adatoms is proposed.

Transient lattice distortion induced by ultrashort heat pulse propagation through thin film metal/metal interface

Phonon propagation across the Cu/Ag(111) interface and transient structural disorder in Ag(111) crystal, after excitation of 18 nm copper layer with UV femtosecond pulses has been studied by means of picosecond time resolved X-ray diffraction. The lattice disorder was measured by the changes in peak shift and broadening of the XRD rocking curve. A blast force was formed within 2 ps after fs UV irradiation. After fast initial expansion and contraction, a sharp oscillation is observed, while the subsequent oscillations were found to be broader due to out-of phase relationship between the sound waves in the copper and silver layers.

Morphological and Structural Properties of NbN Thin Films Deposited by Pulsed Laser Deposition

Niobium nitride (NbN) films were deposited on Nb using pulsed laser deposition (PLD), and the effect of substrate deposition temperature on the preferred orientation, phase, and surface properties of NbN films were explored by x-ray diffraction (XRD) and atomic force microscopy (AFM). It was found that the substrate deposition temperature has a significant influence on properties of the NbN films, leading to a pronounced change in the preferred orientation of the crystal structure and the phase. We find that substrate temperature is a critical factor in determining the phase of the NbN films. For a substrate temperature of 650 °C 850 °C, the NbN film formed in the cubic δ-NbN phase mixed with the β-Nb2N hexagonal phase. With an increase in substrate temperature, NbN layers became β-Nb2N single phase. Essentially, films with a mainly β-Nb2N hexagonal phase were obtained at deposition temperatures above 850 °C. Surface roughness and crystallite sizes of the β-Nb2N hexagonal phase increased as the deposition temperatures increased.

Time-resolved X-ray diffraction studies of laser-induced acoustic wave propagation in bilayer metallic thin crystals

Phonon propagation across the interface of a Cu/Ag(111) bilayer and transient lattice disorder, induced by a femtosecond 267 nm pulse, in Ag(111) crystal have been measured by means of time resolved X-ray diffraction. A “blast” force due to thermal stress induced by suddenly heated electrons is formed within two picoseconds after excitation and its “blast wave” propagation through the interface and Ag (111) crystal was monitored by the shift and broadening of the rocking curve, I vs. ω, as a function of time after excitation. Lattice disorder, contraction and expansion as well as thermal strain formation and wave propagation have also been measured. The experimental data and mechanism proposed are supported by theoretical simulations.

X-ray laser resonator for the kilo-electron-volt range

We have designed, constructed, and tested an x-ray laser resonator operating in the hard x-ray, keV energy region. This ring x-ray laser cavity is formed by four highly oriented pyrolytic graphite crystals. The crystals are set at the Bragg angles that allow for the complete 360° round trip of the 2.37 A, 5.23 keV Lα line of neodymium. In addition, we also present experimental data of a similar ring laser resonator that utilizes the Cr Kα, 5.41 keV, x-ray line to propagate through the four mirrors of the cavity. The specific properties of these x-ray laser resonator mirrors, including reflection losses and cavity arrangement, are presented.

Generation of Plasma Plume From Ge Target by Nanosecond Laser

Properties of the plasma plume in pulsed-laser deposition affect the quality of the deposited film. We show images of laser-produced plasma upon the ablation of a germanium target by a nanosecond laser and compare the shapes of plume at different nitrogen background pressures. Understanding the mechanism of laser-produced plasma is critical to control the quality of the deposited film.