Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, in order to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. We show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure

Cold ablation driven by localized forces in alkali halides

Laser ablation has been widely used for a variety of applications. Since the mechanisms for ablation are strongly dependent on the photoexcitation level, so called cold material processing has relied on the use of high-peak-power laser fluences for which nonthermal processes become dominant; often reaching the universal threshold for plasma formation of ∼1 J cm-2 in most solids. Here we show single-shot time-resolved femtosecond electron diffraction, femtosecond optical reflectivity and ion detection experiments to study the evolution of the ablation process that follows femtosecond 400 nm laser excitation in crystalline sodium chloride, caesium iodide and potassium iodide. The phenomenon in this class of materials occurs well below the threshold for plasma formation and even below the melting point. The results reveal fast electronic and localized structural changes that lead to the ejection of particulates and the formation of micron-deep craters, reflecting the very nature of the strong repulsive forces at play

Femtosecond X-ray diffraction measurement of a solid-solid phase transition in VO2

Femtosecond x-rays were for the first time used to probe a photoinduced solid-solid phase transition in VO2. The fast timescale observed suggests that, in this regime, the structural distortion may not be thermally initiated

Femtosecond x-ray studies of the photo-induced structural phase transition in VO2

We report on the application of femtosecond x-ray scattering to experimental studies of the photo-induced, structural phase transition in VO2. The transition between the two crystalline phases of the material occurs, for sufficiently intense excitation, within 500 fs

Characterization of a broadband multi-keV laser plasma x-ray source for femtosecond time-resolved EXAFS

Recent advances in femtosecond laser plasma x-rays sources have resulted in several experiments to explore the dynamics of physical and chemical processes on the femtosecond time scale. We present our most recent progresses on the development of an intense broadband x-ray source in the multi-keV range, for application to time-resolved EXAFS experiments. Experiments have been realized with two different CPA laser systems having different pulse durations and characteristics. X-ray emissions in the 5KeV range generated from solid targets with the MRS Nd:Glass laser (400fs, high contrast) and the UCSD Ti:Sapphire laser (20fs, 20Hz) have been characterized through high resolution and time resolved x-ray spectroscopy. The application of this source to time resolved EXAFS measurements with a subpicosecond time resolution will also be discussed

Ultrafast coherent and incoherent X-ray generation by inner-shell atomic processes induced by < 25 fs, > 1 J pulses of high power CPA lasers

A new class of keV X-ray lasers based on specific inner-shell atomic transitions is analyzed. It is shown that population inversion between inner-shell vacancy levels of medium-Z elements can be created using only electron collisional ionization, or by using fast, incoherent X-ray pumping if the decay of the lower states is mediated by Coster-Kronig or super-Coster-Kronig processes. The requirements of extremely fast energy deposition on a target in order to compete with the inherently fast (0.1-20 fs) atomic processes can be fulfilled by use of an ultrahigh peak power laser system with ultrashort pulse duration. A state-of-the-art chirped pulse amplification (CPA) laser system delivering more than 1 J of optical energy in less than 25 fs with 10 Hz repetition rate is described. The laser system has also been used to produce high-brightness, narrow-linewidth incoherent X-rays that are optimal for time dependent X-ray diffraction studies. The design of sandwiched multilayer target structure and possible experimental configurations for effective production of coherent X-ray radiation are suggested. Preliminary experimental results of ultrafast (similar to 25 fs), high intensity (>10(19) W/cm(2)) excitation of layered metal targets show anomalous enhancement of specific Ti lines in the 2-14 nm wavelength range.X11sciescopu