Attosecond Control and Measurement: Lightwave Electronics

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JUNIOR RESEARCh GROUPS 1.3.1.5 ATTOSECOND DYNAMICS Leader: Dr. R. Kienberger The main goal of the Junior Research Group is to investigate electronic processes in molecules and on solid surfaces on an attosecond timescale. Efforts have been undertaken to improve the necessary tools, i.e. phase-stabilized few-cycle laser pulses, ultrashort pulses in the ultraviolet (UV) and vacuum ultraviolet (VUV), and to develop attosecond beamlines for the envisaged experiments. IMPROVEMENT OF ThE lASER SYSTEM TO 1.5 CYClE lASER PUlSES The natural limit nature sets to the duration of a light pulse is one oscillation cycle of the carrier wavelength. The pulse duration is inversely proportional to the spectral width of the pulse, i.e. the broader the spectrum the shorter the pulse. In a chirped pulse laser amplifier, pulses are temporally stretched for the amplification and have to be compressed subsequently. During the compression, e.g. in prisms, the pulses normally lose spectral width due to parasitic effects like self-phase modulation (SPM). To overcome this problem, we developed a “hybrid” compressor consisting of prisms and additional chirped mirrors which take over the last part of the compression without the problem of SPM, leading to amplified laser pulses lasting only 18 fs. Sending theses pulses into a gas filled hollow core fiber and final compression resulted in 0.4 mJ laser pulses as short as 3.8 fs – only 1.5 cycles of the carrier wave [5, see reprint]. This is a new world record and made possible the generation of intense, ultrashort UV pulses. GENERATION OF UlTRAShORT UV AND VUV PUlSES Since the separation of quantum states which are relevant for electronic dynamics in molecules is on the order of 5 to 10 eV, ultrashort light pulses at this photon energy have to be generated. The UV/VUV generation was performed in a gas jet and a special “self diffraction FROG” (Frequency Resolved Optical Gating) device has been set up for the full characterization of the UV pulses. The spectra we were able to generate in the UV are extraordinarily broad (Figure 1) and could be compressed to 3.7 fs. ATTOSECOND TEChNOlOGY ON A SOlID SURFACE In a collaboration with the University of Bielefeld, a first attosecond experiment on a solid surface was performed 1 . 3 AT T O S E C O N D A N D H I G H - F I E L D D I V I S I O N [2, see reprint]. We were able to measure a time delay between conduction band and core level electrons in single-crystal tungsten as short as 110 as. Figure 1: Ultrabroad UV spectrum supporting 3 fs pulses REFERENCES [1] Remacle F., Kienberger R., Krausz F., Levine, R.D.; “On the feasibility of an ultrafast purely electronic reorganization in lithium hydride”, Chemical Physics 338 (2-3): 342-347 ( 2007). [2] A. L. Cavalieri, N. Müller, Th. Uphues, V. Yakovlev, A. Baltuska, B. Horvath, B. Schmidt, L. Blümel, S. Hendel, P. M. Echenique, M. Drescher, U. Kleineberg, R. Kienberger, F. Krausz, U. Heinzmann; „Attosecond time-resolved photoemission from solids”, Nature 449, 1029 (2007). [3] Kienberger R, Uiberacker M, Kling MF, Krausz F.; „Attosecond physics comes of age: from tracing to steering electrons at sub-atomic scales” (invited); J. Mod. Opt. 54 (13-15), 1985-1998 (2007). [4] E. Goulielmakis, V. Yakovlev, A. L. Cavalieri, M. Uiberacker, V. Pervak, A. Apolonsky, R. Kienberger, U. Kleineberg, F. Krausz; “Attosecond Control and Measurement: Lightwave Electronics”, Science 317, 769 (2007). [5] A. L. Cavalieri, E. Goulielmakis, B. Horvath, W. Helml, M. Schultze, M. Fieß, V. Pervak, L. Veisz, V. Yakovlev, M. Uiberacker, A. Apolonski, F. Krausz, R. Kienberger; “Intense 1.5-cycle near infrared laser waveforms and their use for the generation of ultrabroad-band soft-Xray harmonic continua”, NJP 9, 242 (2007). [6] MB. Gaarde, M. Murakami, R. Kienberger; „Spatial separation of large dynamical blueshift and harmonic generation”, Phys. Rev. A 74 (5), 053401 (2006) [7] A. Scrinzi, M.Yu. Ivanov, R. Kienberger and D. M. Villeneuve; “Attosecond Physics” (invited); J. Phys. B: At. Mol. Opt. Phys. 39, R1 – R37 (2006). 148 Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008
1.3.1.6 ATTOSECOND IMAGING Leader: Dr. Matthias Kling With waveform-controlled laser fields to control electronic motion and (single) attosecond XUV pulses to probe electronic motion in real-time two powerful tools are available to explore electron dynamics in atoms and molecules. We have proposed to extend the application of these tools to studies of correlated and collective electron motion in nanosystems. One of our aims was to utilize imaging technologies as e.g. velocity-map imaging (VMI) to monitor the ultrafast electron dynamics and its control. As a demonstration that VMI can indeed be used to monitor the waveformcontrol of electron emission we have investigated atomic systems, where the obtained results have been compared to theoretical predictions (by e.g. solving the time-dependent Schrödinger equation (TDSE)). This control is applied to molecular systems, where the light-steering of binding electrons is used to coherently control a reaction. As a highlight, the waveform control of electron localization in molecular dissociation was time-resolved by an attosecond pump-probe experiment for the first time. The full spatio-temporal characterization of nanometerscale collective electron dynamics with attosecond temporal resolution was studied theoretically within the collaboration with Prof. Mark Stockman (GSU, Atlanta, USA). The original idea for studies on isolated nanoparticles was extended to photoelectron emission microscopy (PEEM) of nanosystems on surfaces, leading to a new technique dubbed “attosecond nanoplasmonic microscope”. Prof. Ulf Kleineberg (LMU) and his coworkers are currently building a UHV experiment for time-resolved Photoelectron Emission Microscopy, which will be capable for the first time to track electron dynamics in nanostructures on surfaces on a ~100 fsec time scale and with 10 nm spacial resolution. This effort complements the development of the attosecond beamline AS 3. IMAGING OF SUB-FEMTOSECOND CONTROl OF ElECTRON DYNAMICS Control of the electric field waveform of laser light pulses has opened a new way to study and control electron dynamics in strong-field processes. We have utilized velocity-map imaging to study the angleresolved electron emission from Ar, Kr and Xe atoms with waveform-controlled few-cycle pulses. The data for Ar is compared to simulations using the strongfield approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically 1 . 3 . 1 S U M M A R Y O F S C I E N T I F I C A C T I V I T I E S depends on the carrier-envelope phase of the laser field. The generally weak asymmetry of low-energy electrons can be explained by their deflection in the oscillating laser field that cancels the initial non-linear dependence of the ionization yield on the field strength. In the plateau region of the argon data, the asymmetry is larger and the phase of its oscillation with CEP shifts not only with the electron kinetic energy but also with the emission angle. A triangular pattern suggests that the observation of the asymmetry at a constant energy but two different emission angles can serve as a way to measure the phase CEP of the laser pulse without ±π ambiguity. In the case of xenon, significant asymmetry was observed even below 2 Up in particular for high emission angles around 60 degrees. This unique dependence indicates that VMI can also be used instead of Stereo-ATI detection to determine the CEP with high accuracy. A B C Figure 1: A) and B) show raw two-dimensional photoelectron images for above-threshold ionization of argon with 5fs laser pulses for phases of ϕ = 0 and ϕ = π, measured by velocity map imaging. As seen in C), where the asymmetry of the emission is plotted (red = upwards, blue = downwards), the emission can be controlled with the laser phase. A pronounced shift in the phase of the asymmetry oscillation that is seen in the experimental data is reproduced well beyond 5 Up by both SFA and TDSE calculations. While a phase jump is visible for the SFA calculated asymmetries at around 5 Up, the difference between the TDSE calculated values and the experimental Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008 149
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