Ultrafast Experiments - University of Tennessee

Ultrafast Experiments - University of Tennessee

Ultrafast Experiments Hao Hu The University of Tennessee Department of Physics and Astronomy, Knoxville Course: Advanced Solid State Physics II (Spring 2009) Instructor: Elbio Dagotto (Dated: April 23, 2009) Outline

Introductions Ultrafast lasers The Linac Coherent Light Source X-rays Applications Summary http://wwwssrl.slac.stanford.edu/lcls/ssrl.slac.stanford.edu/lcls/ The first pulses shorter than a nanosecond were obtained by DeMaria in 1966 by passive mode locking of a Nd-glass laser Zhou et al. (1994) obtained

a pulse of 8-fs duration from a Ti-sapphire laser. 19746 1 ns 19666 1 ps (10-12s) 1 fs (10-15s) 19946 Shank and Ippen (1974) with tunable broad-gain dye

laser media in combination with a saturable dye absorber. Timeline of ultrafast laser From nanosecond to femtosecond science, Rev. Mod. Phys. 71, S283 (1999) (a) Laser cavity and (b) energy levels of a 3-level laser. Laser is made of a gain medium enclosed in an optical resonator. Optical resonator is a face-to-face configuration of two mirrors, one with reflectivity less than unity in order to pass some output light. If only three energy levels: the ground state (having energy of E1) and two excited states (of energies E2 < E3). Energy is supplied in order to pump the E1 E3 transition. E3 level is then subject to fast non-radiation transition to E2, providing non-equilibrium state known as inverted population (meaning there are more atoms

in E2 than E1). The E2 E1 transition is the laser transition, providing laser photon of the same frequency. http://en.wikipedia.org/wiki/Laser_construction Mode locking Mode-locking is a technique by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10 -12s) or femtoseconds (10-15s). Eigen frequencies in 1D resonator of length L are given by the condition =cn/2L, n is the number of modes=cn/2L, n is the number of modescn/2L, n is the number of modes In general, each of these modes oscillate independently with random phase. Each mode operates with a fixed phase locked to the same value---constructively interfere with each other----producing an intense pulse of light. Passive modelocking A saturable absorber is placed in the cavity. It behaves differently depending on

the intensity of the light passing through it. Ideally a saturable absorber will selectively absorb low-intensity light, and transmit light which is of sufficiently high intensity. NATURE VOL 424 14 AUGUST 2003 Advantage of ultrafast laser Ultrashort pulse duration allows fast temporal resolution. A modelocked laser can freeze the motion of fast moving objects such as mole cules or electrons and therefore can measure the relaxation processes of carrie rs in semiconductors, chemical reaction dynamics and electro-optical sampling of high-speed electronics. High pulse repetition rate Lasers with multi-gigahertz repetition rates are key components of many appli cations.

High peak intensity The high peak intensity of the pulse can be used to alter materials by cold abla tion (when a material is changed to gas directly from a solid) or to generate oth er colours/wavelengths through nonlinear frequency conversion. NATURE VOL 424 14 AUGUST 2003 Introducing the Linac Coherent Light Source Xrays X-rays have a wavelength approximately equal to the distances between atoms, it has the potential to give a complete, global picture of structural changes, without any ambiguity in the interpretation. The x-ray photons scatter from all the electrons in the sample, so the diffracted intensity depends directly on the electronic density. Since most electrons follow the nuclei, the electronic density closely reflects the atomic structure. The LCLS is dramatically different from any x-ray source ever built thanks to its laser

properties: exceptionally bright, coherent, coherent means all the x-ray photons are in phase with each other and going in the same direction, and short pulses of x-ray light. It is also different from any other laser because it will produce light at x-ray wavelengths that can probe matter on the atomic scale. LCLS will take clear pictures at these phenomenal speeds, from 230 femtoseconds down to 1 femtosecond. The LCLS, and other XFELs of its kind, will produce laser like pulses of X ssrl.slac.stanford.edu/lcls/rays of about 100 fs duration and 1012 photons per pulse. Radiation damage limits the highest resolution achievable with coherent diffractive imaging.

So people developed the flash imaging technique(single-pulse image). From Fig A and B, it demonstrates that an interpretable coherent diffraction pattern with excellent signal to noise can be collected from a small isolated object in a single FEL pulse. (A) Diffraction pattern recorded with a single FEL pulse from a test object placed in the 20-mm focus of the beam . (B) The diffraction pattern recorded with a second FEL pulse

selected with a fast shutter, showing diffraction from the hole in the sample created by the first pulse. (C) SEM image of the test object (D) The image reconstructed from the single-shot diffraction pattern shown in (A) K. J. Gaffney, et al. Science 316, 1444 (2007) Summary The progress in ultrafast lasers and X-rays imaging during the past decade has been simply amazing. T he continued development of these ultrafast techniques, and their impact on our understandin g of the basic interactions that determine function s in many areas from biology to superconductivity,

will be driven by those developments. Reference Recent developments in compact ultrafast lasers, NATURE VOL 424 1

4 AUGUST 2003 From nanosecond to femtosecond science, Rev. Mod. Phys. 71, S283 (1 999) Henry n. chapman, Nature materials VOL 8 APRIL 2009 Ultrafast electron microscopy in materials science, biology, and chemist ry, JOURNAL OF APPLIED PHYSICS 97, 111101 (2005) Http://en.wikipedia.org/wiki/Laser_construction Http://wwwssrl.slac.stanford.edu/lcls/ssrl.slac.stanford.edu/lcls/ K. J. Gaffney, et al. Science 316, 1444 (2007)

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