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Attosecond Physics in Strong-Field Photoemission from Metal Nanotips

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Attosecond Physics in Strong-Field Photoemission from Metal Nanotips Attosecond Physics in Strong-Field Photoemission from Metal Nanotips Michael Kruger¨ Mu¨nchen 2013 Attosecond Physics in Strong-Field Photoemission from Metal Nanotips Michael Kruger¨ Dissertation an der Fakult¨at fu¨r Physik der Ludwig{Maximilians{Universit¨at Mu¨nchen vorgelegt von Michael Kru¨ger aus Burghausen Mu¨nchen, den 30. August 2013 Erstgutachter: Prof. Dr. P. Hommelhoff Zweitgutachter: Prof. Dr. A. Scrinzi Tag der mundlichen¨ Prufung:¨ 4. Oktober 2013 Na chwa le, Boga Albowiem B´og,Ten, kt´ory rozkaza l ciemno´sciom, by zaja´snia ly´swiat lem, zab lysna, l w naszych sercach, by ol´sni´cnas jasno´scia, poznania chwa ly Bo_zej na obliczu Chrystusa. 2 Kor 4,6 Zusammenfassung Diese Arbeit befasst sich mit der Untersuchung von Rekollision und Attosekundendy- namik in Elektronenemission aus einem Metall, genauergesagt aus einer scharfen Me- tallspitze. Spektral aufgel¨oste Messungen von Photoelektronen, die durch Wenig-Zyklen- Laserpulse erzeugt werden, weisen das Auftreten von Ruckstreuung,¨ Materiewelleninter- ferenz und Elektronendynamik auf der Attosekundenzeitskala an der Spitze nach. Ruck-¨ gestreute Elektronen werden in einer ersten Anwendung der beobachteten Prozesse dazu benutzt, um nanooptische Effekte an Metallspitzen zu untersuchen. In unserem Experiment werden Laserpulse aus einem Titan:Saphir-Oszillator mit einer Pulsl¨ange von 6 fs auf eine scharfe Wolframspitze fokussiert, um Elektronen aus dem Metall auszul¨osen. Das optische Feld wird dabei am scharfen Ende der Spitze verst¨arkt und erm¨oglicht somit lokale Lichtintensit¨aten auf der Gr¨oßenordnung von 1013 W cm−2. Messungen mit spektraler Aufl¨osung zeigen, dass Mehrphotonenprozesse (im weiteren Sinn) zur Emission fuhren,¨ welche sich in Elektronenspektren in klar aufgel¨osten Ma- xima niederschlagen. Bei steigender Lichtintensit¨at beobachten wir die Unterdruckung¨ des Maximums bei der niedrigsten Energie und eine Verschiebung der spektralen Ma- xima hin zu niedriger Energie. Diese Effekte, die im dynamischen Starkeffekt begrundet¨ sind, zeigen an, dass das Starkfeldregime in unserem Photoemissionsexperiment erreicht wurde. Außerdem beobachten wir elastische Ruckstreuung:¨ Ausgel¨oste Elektronen wer- den vom Laserfeld zuruck¨ zur Oberfl¨ache der Spitze getrieben und kollidieren mit dieser; dabei werden Elektronen elastisch an der Oberfl¨ache gestreut und schließlich bei hohen kinetischen Energien detektiert. Dies fuhrt¨ zur Ausbildung einer plateauartigen Struk- tur im Elektronenspektrum. Wir beschreiben Theoriemodelle, die diese Interpretation unserer experimentellen Ergebnisse untermauern. Elektronen, die ruckgestreut¨ werden, sind sehr empfindlich auf die r¨aumliche und zeitliche Struktur des verst¨arkten Nahfelds an der Spitze. Wir wenden daher Ruckstreuung¨ dazu an, die St¨arke des Nahfelds an Spitzen verschiedener Gr¨oße und Materialen auszumessen. Ruckstreuung¨ ist von Natur aus ein Effekt, der sich auf der Zeitskala von Attosekunden abspielt. Laserpulse mit kontrollierter Tr¨ager-Einhullenden-Phase¨ werden verwendet, um die zeitliche Dynamik dieses Prozesses zu studieren. Phasenaufgel¨oste Messungen zeigen, dass der Phasenwert der Pulse die maximale kinetische Energie der Elektronen bestimmt und damit als extrem schneller Schalter fur¨ hochenergetische Elektronen fungiert. Außer- dem wird der Kontrast der Maxima im Plateaubereich des Elektronenspektrums stark von der Phase beeinflusst. Diese Beobachtung stellt das Ergebnis eines Interferenzexperi- ments mit Materiewellen dar: Ein zeitlicher Doppelspalt fuhrt¨ zu Interferenzstrukturen in der Energie, n¨amlich den erw¨ahnten Maxima, w¨ahrend ein Einfachspalt diese nicht zul¨asst und ein breites Kontinuum resultiert. Die zeitliche Form des Pulses gibt mit Attosekundenpr¨azision vor, welcher Fall vorliegt. Unsere Untersuchung zeigt, dass der Einzugsbereich der Attosekundenphysik, die ublicherweise¨ mit Atomen und Molekulen¨ in der Gasphase untersucht wird, auch auf Nanostrukturen aus Metall erweitert werden kann. Abstract This work is concerned with the first observation and investigation of electron recolli- sion and attosecond dynamics in photoemission from a metal, specifically from a sharp metallic nanotip. Spectrally resolved measurements demonstrate electron rescattering, matter-wave interference effects and attosecond control of electron motion in few-cycle laser driven photoemission. As a first application, rescattered electrons were used as a probe to investigate nano-optical field enhancement effects at tips. In our experiment, 6-fs laser pulses from a Titanium:sapphire laser oscillator are fo- cused on a sharp tungsten nanotip and induce electron emission. The optical field is strongly enhanced at the tip apex and allows for high local intensities on the order of 1013 W cm−2. Spectral measurements reveal above-threshold photoemission with clearly resolved multiphoton peaks. At increasing intensity we observe the suppression of the lowest order peak and a shift of the spectral peaks to lower energy. These effects are caused by the AC Stark shift and indicate that the strong-field photoemission regime has been reached in our system. Furthermore, we observe electron recollison and elastic rescattering. Photoelectrons are driven back to the tip surface by the laser field and re- collide there. Elastic scattering takes place and leads to high kinetic energies, resulting in a plateau structure in photoelectron spectra. We present theory models that support this notion. Rescattered electrons are sensitive to the spatial and temporal shape of the enhanced nano-optical near-field at the tip. In a first application, we use rescattering to measure the strength of the near-field at tips of different size and material. The natural time scale of the rescattering process is the attosecond time scale. Carrier- envelope phase stable laser pulses are employed to study its temporal dynamics. Phase- resolved measurements show that the phase of the pulses determines the maximum kinetic energy of photoelectrons and serves as an ultrafast switch for high-energy elec- trons. Moreover, the contrast of the peaks in the plateau is strongly modulated with the phase. This observation is the result of a matter-wave interference experiment: A temporal single slit or a double slit leads to absence or presence of spectral interference in the energy domain, respectively. In the case of a single slit, a broad spectral conti- nuum results. The temporal shape of the pulse determines with attosecond precision if rescattered electrons are generated during one or two optical cycles. Our investigation shows that attosecond science { usually performed with atomic and molecular gases { can be extended to metal nanostructures. Contents Zusammenfassung vii Abstract ix 1 Introduction3 2 Theory of strong-field photoemission from metal nanotips9 2.1 Mechanisms of strong-field photoemission from metal surfaces......9 2.1.1 Multiphoton photoemission..................... 10 2.1.2 The Keldysh theory and light-induced tunneling.......... 10 2.1.3 Above-Threshold Photoemission and strong-field effects...... 13 2.1.4 Other photoemission mechanisms.................. 16 2.2 Electron recollision and the Three-Step Model............... 18 2.3 Quantum Orbit Theory of recollision.................... 22 2.3.1 The strong-field approximation................... 22 2.3.2 Saddle-point approximation and quantum orbits.......... 26 2.3.3 Spectral formation and quantum interference............ 30 2.3.4 Extended Three-Step Model..................... 35 2.4 Numerical approaches............................ 42 2.4.1 Integration of the time-dependent Schr¨odinger equation...... 42 2.4.2 Time-dependent density functional theory............. 44 2.5 Enhanced optical near-fields at nanotips.................. 46 3 Experimental methods and setup 51 3.1 Optical setup................................. 51 3.2 Vacuum chamber and detection system................... 55 3.3 Metal nanotip preparation and characterization.............. 57 3.3.1 Nanotip fabrication.......................... 57 3.3.2 Field emission............................. 61 3.3.3 In-situ tip characterization methods................. 62 4 Strong-field above-threshold photoemission from nanotips 69 4.1 Above-threshold photoemission....................... 69 4.2 Strong-field effects: Peak suppression and peak shifting.......... 72 5 Electron rescattering at nanotips 75 5.1 Electron rescattering in strong light fields and static fields........ 75 5.1.1 Rescattering plateau and high-energy cut-off............ 75 5.1.2 Comparison with theory models................... 78 2 Contents 5.1.3 Influence of a static electric field................... 83 5.1.4 Efficiency of rescattering at a metal surface............. 84 5.2 Probing of optical near-fields by electron rescattering........... 86 6 Attosecond control of electrons with the carrier-envelope phase 91 6.1 Phase effects in rescattering......................... 92 6.1.1 Carrier-envelope phase sensing.................... 94 6.2 Attosecond double-slit experiment...................... 95 6.2.1 Carrier-envelope phase spectral interferometry........... 98 6.3 Attosecond control of electron motion.................... 99 6.4 Comparison with theory models....................... 100 7 Conclusion and outlook 105 References 109 Danksagung 125 1 Introduction The atomic nature of matter has always been a fascinating subject of scientists through- out the centuries. People generally associate with atoms the attribute\small", but\small" does not only refer to size, but also to time: In Bohr's famous model of
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