Imperial College London Generation and Application of Ultrashort Laser Pulses in Attosecond Science Felix Frank May 2011 Thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy of Imperial College London Laser Consortium Quantum Optics and Laser Science Group Physics Department Imperial College London Prince Consort Road, London SW7 2AZ United Kingdom 2 Declaration I declare that the work contained within this thesis is my own unless stated otherwise. ................... Felix Frank Abstract In this thesis, I describe the development of a sub-4 fs few-cycle laser system at Imperial College London used to generate and characterise the first single attosecond (1 as = 10−18 s) pulses in the UK. Phase-stabilised few-cycle laser pulses were generated using a hollow fi- bre system with a chirped mirror compression setup. The pulse was fully characterised using frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric field reconstruction in a spatially en- coded filter arrangement (SEA-F-SPIDER). A pulse duration of 3.5 fs was measured with an argon filled hollow fibre. These phase stabilised Infra-Red (IR) pulses were used to generate a con- tinuous spectrum of high harmonics in the Extreme Ultraviolet (XUV) origi- nating from a single half-cycle of the driving field. Using subsequent spectral filtering, a single attosecond pulse was generated. The isolated XUV pulse was characterised using an atomic streaking camera and a pulse duration of ∼ 260 as was retrieved using FROG for complete reconstruction of attosecond bursts (FROG-CRAB). In an experiment conducted at the Rutherford Appleton Laboratory, high harmonics were generated using a two-colour field with an energetic beam at 1300 nm and a weak second harmonic orthogonally polarized to the funda- mental. By changing the phase between the two fields, a deep modulation of the harmonic yield is seen and an enhancement of one order of magnitude compared to the single colour field with the same energy is observed. Acknowledgements Firstly I have to thank the whole Laser Consortium for being the best and most welcoming group. This made my PhD the great experience it was (if we forget about the Monday morning meetings). I have to thank John for giving me the opportunity to work with all the attoboys. John is a great supervisor that was always there to support me, has great patience and, at some point, will run the Hyde Park relay in under 20 minutes. I also thank Jon and Roland for giving me advice on physics and cooling chambers, respectively. As mentioned above, I couldn't have done my PhD without the attoboys. In the last three years Chris has become one of my best friends in- and outside the lab. He taught me how to dance, collect 27 packs of sweets on an airplane, introduced me to a plethora of pranks and sometimes I even wear odd socks. Tobi who has brought Matlab and a new sense of fashion to our group was always there to talk about Fourier transforms and help out with all sorts of things. Jarlath, showed me the Irish efficiency both in the lab and in personal water consumption. And William, the newest addition to the lab, for becoming a true attoboy in such a short time. I was also lucky to start with the best year of PhD students. Thank you Hugo, for just being great and for never saying no to a bottle of ros´e;Leo for good times aligning irises at RAL; Ryan for letting me wear his Gretzky jersey and setting new haircut standards. David (and his Bristol No.1) for sharing your love of real ale and Eva for telling me how to get pregnant faster. Thank you. I also thank Tom not only for getting everybody involved in social outings but also for always being helpful in the lab. Malte for reminding me of the Acknowledgements 5 existence of great things outside of laser-physics. Amelle for drinking my Tequilas. Ann for being a girl and all the rest of the new (Marco, Richard, Henry, Sid, Stefani) and really new guys (The Uzbek guy, Seb and Leon, Pricey, Nargis, Damien, Simon) that float around and make our tea breaks more interesting. And let's not forget the old bunch that welcomed me to the group, when I started my PhD. Joe introduced me to the laser and fingertip-free alignment procedures before he abandoned all of us to dig for gold out west. Phil for showing me Opal and being a great friend. Hank for loving computers. Sarah for sharing all the little tricks she had learnt in a decade at the Consortium and Delphine for her violent hugs. I also thank Peter and Andy for using every dirty scribbling we gave them as a detailed technical drawing. Thank you Bandu for always taking your time to explain and sort out our electronics. And of course I have to thank my Family -Mama, Laura, Filipp- who supported me all my life. Last but not least, Katharina. She was the one that had to wait for dinner when the laser lab was flooded or the CEP was finally working. Thank you for always being there for me and putting up with all the late nights and work related grumpiness. Don't forget to read the rest of the thesis, too. iI will think of you many times in the future Streaking is the act of running nude through a public place. It is a light-hearted form of exhibitionism and public nudity not in- tended to shock but, rather, to amuse onlookers. Wikipedia 7 Contents Abstract3 Acknowledgements4 List of Figures 13 List of Tables 15 List of Abbreviations 17 1 Introduction 21 2 Theory and Background 27 2.1 Ultrashort Optical Physics.................... 27 2.1.1 Ultrashort Pulses..................... 28 2.1.2 Dispersion......................... 31 2.1.2.1 Material Dispersion............... 32 2.1.3 Ultrashort Lasers..................... 35 2.1.3.1 Mode-locking.................. 35 2.1.3.2 SESAM..................... 38 2.1.3.3 KLM in Ti:Sa.................. 40 2.1.4 Nonlinear Optics..................... 41 2.1.4.1 Second Harmonic Generation......... 42 2.1.4.2 Self Phase Modulation............. 45 2.1.5 Hollow Fibre....................... 48 2.1.5.1 Modes...................... 48 2.1.5.2 Coupling.................... 50 8 Contents 2.1.6 Dispersion Compensation................ 52 2.1.6.1 Grating Compressors.............. 53 2.1.6.2 Prism Compressors............... 54 2.1.6.3 Chirped Mirrors................ 55 2.1.7 Carrier-Envelope Phase.................. 56 2.1.7.1 Measuring and Controlling CEP....... 57 2.1.8 Femtosecond Pulse Measurements............ 61 2.1.8.1 Intensity Autocorrelation........... 62 2.1.8.2 FROG...................... 65 2.1.8.3 SPIDER..................... 67 2.2 Strong Field Physics....................... 71 2.2.1 Ionisation in a Strong Field............... 71 2.2.1.1 Ponderomotive Energy............. 72 2.2.1.2 The Keldysh Parameter............ 72 2.2.1.3 Multi Photon and Above Threshold ionisation 73 2.2.1.4 Tunnelling Regime............... 74 2.2.1.5 Over the Barrier Ionisation.......... 76 2.2.2 High Harmonic Generation................ 76 2.2.2.1 Single Atom Response - The Three-Step Model 78 2.2.2.2 Classical Electron Trajectories......... 81 2.2.2.3 The Role of Phasematching in HHG..... 83 2.2.3 Attosecond Pulses..................... 86 2.2.3.1 RABITT.................... 90 2.2.3.2 Atomic Streak Camera............. 91 3 Waveform Stabilised Few-Cycle Drive Laser 97 3.1 Front-End............................. 97 3.1.1 Chirped Pulse Amplification (CPA) Laser....... 97 Contents 9 3.1.1.1 Oscillator and Stretcher............ 98 3.1.1.2 Amplifier.................... 99 3.1.1.3 Compressor................... 100 3.1.2 Carrier Envelope Phase Stabilisation.......... 102 3.1.2.1 Fast Loop.................... 102 3.1.2.2 Slow Loop.................... 104 3.1.3 Overview......................... 106 3.2 Hollow Fibre Pulse Compression................. 107 3.2.1 Hollow Core Fibre: Optical Setup............ 108 3.2.2 Spectral Broadening................... 109 3.2.3 Conclusion......................... 111 4 Characterisation of High-Intensity Sub-4 fs Laser Pulses 113 4.1 Characterization of Ultrashort Pulses Using FROG...... 114 4.1.1 Optical Setup For a Single-Shot FROG......... 115 4.1.1.1 FROG Calibration............... 115 4.1.2 Dispersion Management................. 117 4.1.3 FROG Phase-matching Bandwidth........... 120 4.1.4 FROG Marginals..................... 122 4.1.5 FROG Results...................... 123 4.1.6 Conclusion......................... 125 4.2 Characterisation of Sub-4fs Pulses Using SEA-F-SPIDER... 126 4.2.1 The SEA-SPIDER Setup................. 126 4.2.1.1 Calibration................... 128 4.2.2 SPIDER Trace...................... 130 4.2.3 Experimental Results................... 131 4.2.4 Conclusion......................... 133 10 Contents 5 Attosecond Streaking 135 5.1 Attosecond Source........................ 136 5.1.1 Few-cycle Laser Source.................. 136 5.1.2 High Harmonic Generation Chamber and Target.... 137 5.1.3 XUV Spectrometer.................... 139 5.1.4 High Harmonic Spectra.................. 140 5.2 Time Of Flight Measurement.................. 144 5.2.1 Time Of Flight Chamber................. 145 5.2.2 Time-Of-Flight Electron Spectrometer......... 149 5.2.2.1 TOF calibration................ 152 5.3 Attosecond Streaking....................... 154 5.3.1 Pulse Contrast Optimisation............... 154 5.3.2 Attosecond Streaking................... 157 5.4 Conclusion............................. 162 6 Multi-Colour HHG 165 6.1 Brief Introduction to Two-Colour HHG and Cut-off Extension 165 6.1.1 Two-colour HHG..................... 166 6.1.2 Extension of the Cut-Off Energy............ 167 6.2 Experimental Setup and Results................. 168 6.3 Theoretical Description...................... 172 6.4 Conclusion............................. 175 7 Summary and Conclusion 177 7.1 Chapter 3 and 4: Generation and Characterisation of Few- cycle Laser Pulses........................