Attosecond pump-probe spectroscopy

Introduction

High-energy, few-cycle pulses for applications in attosecond experiments can be generated by compressing millijoule-level, ∼30fs pulses from a Ti:sapphire chirped pulse amplifier using hollow- fibre pulse compression [1]. For a typical fibre inner- diameter of around 250µm, when the input pulse energy exceeds ∼1mJ ionisation and self-focusing become increasing obstacles to further energy scaling of the technique. The impact of these unwanted non-linear effects can be mitigated by differential pumping (DP) of the fibre [2, 3].

Stabilisation of the carrier-envelope phase (CEP) of few-cycle pulses is crucial to many attosecond experiments. Fluctuations in the energy coupling into a fibre induce a CEP instability in the output pulses [4]. However up until now, the possible existence of an additional instability caused by gas flow in differentially pumped fibres has not been examined.

Experiment

In our experiment, 30fs pulses with up to 2.5mJ energy at a 1kHz repetition rate were delivered to a 260µm inner-diameter, 1m long fibre filled with neon. The maximum output energy in Fig. 1, measured for an output spectrum with a constant sub-5fs Fourier transform limit (FTL), is significantly higher with DP (∼910µJ) than for static fill (SF) (∼460µJ). The CEP stability at the output of the fibre, shown in Fig. 2, was measured for DP below the output energy clamping, using an input energy of ~1mJ. The residual CEP fluctuations have a standard deviation of 206mrad. This demonstrates that the CEP stability of differentially pumped fibres is sufficient for experiments such as attosecond pulse generation.

Current researchers

W. A. Okell, T. Witting,  M. Bocoum¹, D. Fabris, F. Frank, A. Ricci¹, A. Jullien¹, J. P. Marangos, R. Lopez-Martens¹, and J. W. G. Tisch.

¹Laboratoire d’Optique Appliquée, Ecole Nationale Supérieure de Techniques Avancées–ParisTech, Ecole Polytechnique-CNRS, 91761 Palaiseau Cedex, France

References and relevant links

[1] M. Nisoli et al., Appl. Phys. Lett. 68, 2793 (1996).
[2] A. Suda et al., Appl. Phys. Lett. 86, 111116 (2005).
[3] J. S. Robinson et al., Appl. Phys. B 85, 525 (2006).
[4] C. Li et al., Opt. Lett. 32, 796 (2007).

Introduction

Attosecond streaking can be used to measure electron dynamics on surfaces. Previous studies have investigated time-delayed photoemission from different bands of single-crystal solids [1, 2]. A number of non-crystalline solid samples, for example plasmonic nanostructures [3], thin-films, and samples exhibiting surface chemistry, could display interesting attosecond electron dynamics. In many cases in-situ ultra-high vacuum (UHV) preparation will not be possible because of the complexity of such samples. We performed attosecond streaking measurements on polycrystalline Au films, and amorphous WO 3 films, as a first proof-of-principle that streaking can be applicable to structurally disordered samples without in-situ preparation.

Experiment

Our experiment was performed using the Imperial College attosecond beamline [4]. A chirped pulse amplification laser and hollow fibre system were used to generate 3.5 fs pulses with 400 µJ energy [5]. These pulses were used for high harmonic generation (HHG) in neon. The co-propagating few-cycle infrared (IR) pulses and high harmonics entered a UHV surface science chamber with a base pressure of 3×10 -9 mbar. A UHV compatible two-part mirror setup delivered the IR pulses, and time-delayed ~300as, 90eV extreme ultraviolet (XUV) pulses produced by selecting the HHG cut-off radiation, to the gold sample. Figure 1(a) shows a streaking trace measured on a 52nm polycrystalline gold evaporation, which was not cleaned in-situ beforehand. The clear oscillations in the trace confirm that a sub-cycle photoelectron wavepacket is emitted from the gold by the XUV. The electric field retrieved from the streaking trace is shown in figure 1(b). The rather structured nature of the electric field is a possible indication of plasmonic fields on the surface. Figure 2(a) shows a streaking trace measured on amorphous tungsten oxide. Again, clear streaking of the valence band is observed, confirming the emission of a sub-cycle photoelectron wavepacket. Streaking of the 4f band of tungsten oxide is also observed.

These measurements confirm that a ttosecond streaking can be performed on structurally disordered, non-UHV- p repared samples, opening the door to streaking measurements on highly complex solid samples. The measurements constitute the first demonstration of attosecond streaking on surfaces in the UK.

This work was financially supported by EPSRC through grants EP/I032517/1 and EP/F034601/1.

Current researchers

W. A. Okell, T. Witting, D. Fabris, J. Hengster ¹, D.Y. Lei ², M. Rahmani, Y. Sonnefraud, D. Walke, C. A. Arrell³, A. Seiler⁴, S. Ibrahimkutty⁴, S. Stankov⁴, S. A. Maier, Th. Uphues¹, J. P. Marangos, and J. W. G. Tisch

¹CFEL, Hamburg University, L uruper Chauss ee 149, 22761 Hamburg, Germany
²Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China
³Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
⁴IPS, Karlsruhe Institute of Technology, Hermann von Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen,Germany

References and relevant links

[1] A. L. Calvalieri et al., Nature 449, 1029-1032 (2007).
[2] S. Neppl et al., Nature Phys. 7, 656 (2011).
[3] E. Skopalova et al., N. J. Phys. 13, 083003 (2011).
[4] F. Frank et al., Rev. Sci. Instruments, 83 071101 (2012).
[5] W. A. Okell et al., Opt. Lett. 38, 3918 (2013).

Introduction

It has recently shown to be possible to produce isolated attosecond pulses in the XUV photon energy range (∼90 eV) [1]. These can be employed in combination with infrared (IR) pulses in IR field dressed photo-ionisation experiments to investigate attosecond dynamics in XUV-pump IR-probe schemes [2]. Specific chromophores in large biomolecules have a high photo excitation cross-section in the VUV range (10-20eV)[3]. Therefore attosecond VUV pulses can enhance the excitation fraction of a molecular sample and allow direct attosecond-pump attosecond-probe experiments. We discuss our approach for production and characterisation of HHG radiation in this energy range.

Experiment

A few cycle laser (sub-4 fs) is used to generate an XUV single attosecond pulse. The pulse is produced selecting the continuum of the HHG spectrum produced in neon. In a collinear geometry we implemented a second krypton gas target for efficient production of VUV photons. A flat field spectrometer allows us to optimise the focusing geometry and gas pressure for both spectral regions. The production of a single XUV attosecond pulse is preserved in this set-up, as demonstrated by the streaking measurement [4] showed in Fig. 1.

The VUV pulse is spectrally filtered with an indium or tin foils, to target two different pulse energies, at 15 or 20 eV respectively. To obtain a temporal characterisation of the VUV pulses we performed VUV-IR streaking in a xenon gas target, suitable due to its lower I_p= 12.1 eV. Fig. 1 shows a time delay scan between the VUV pulse and the IR pulse for indium (a), and tin (b). The trace shows a sideband rather than the oscillating behaviour because the VUV pulse duration is comparable to the IR streaking field cycle time. FROG-CRAB simulations give a VUV pulse duration of ~600 as using a tin filters, and ~1 fs using indium.

The method to estimate the photon flux of such VUV sources is based on the fluorescent properties of sodium salicylate, which has an approximately constant efficiency in the range 30-300nm. The fluorescence is registered with a photomultiplier tube, and the detection system is calibrated at higher wavelengths, such as the third harmonic of our driving laser (see Fig. 2). The measured photon flux at generation is of the order of hundreds of pJ/pulse, which is an encouraging number when considering the cross-section of transitions to be studied in the future pump-probe experiments.

Current researchers

D.Fabris, P. Matia-Hernando, T. Witting, W. Okell, Z.Abdelrahman, D. Walke, J.P. Marangos, J.W.G. Tisch.

References and relevant links

[1] Krausz, F. & Ivanov, M. Reviews of Modern Physics 81, 163-234 (2009)
[2] Drescher, M. et al.Nature 419, 803-7 (2002)
[3] Cool, T. et al. International Journal of Mass Spectrometry 247, 18-27 (2005).
[4] Witting, T. et al. Journal of Physics B: Atomic, Molecular and Optical Physics 45, 074014 (2012)

Introduction

Few-cycle laser driven high harmonic generation (HHG) is used in the production of radiation with attosecond pulse duration. Here we present work where metal plasma plumes are used as an alternative nonlinear medium to a neutral gas. The plasma plumes were generated by ablating solid metal targets with a properly timed laser pump pulse. In transition metals it was found that when driven by few-cycle laser pulses, resonant induced HHG occurs resulting in a single or small group of enhanced harmonics [1] (see Fig.1).

Experiment

Of particular interest is the resonant harmonic found in manganese. It was shown that there is a resonance at 50 eV due to a transition in the Mn+ ion. This was found to have a weak CEP dependence and a hig h conversion efficiency (≈ 10−5) [2] (see Fig.1.). Theoretical simulations of this resonant harmonic has shown that it is of attosecond duration [2]. With the development of a rotating target s y stem that has an axial displacement of less than 50 µm, a stable plasma can now be produced for the HHG process at kilohertz pulse repetition rates. This low-wobble rotating system allows for a full investigation of the resonant harmonic in Mn and other resonant and non-resonant plasma plumes. By measuring the visibility of double slit Young’s interference patterns the coherence of HHG from plasmas and gas have been shown to be comparable (see Fig.2.). This demonstrates there are no significant effects on spatial coherence, due to the increased number of free electrons in the plasmas. We intend to carry out attostreaking methods to diagnose and confirm the duration of the Mn resonant harmonic.

Current researchers

Z. Abdelrahman, R. A. Ganeev, C. Hutchison, T. Witting, D. Fabris, W.A. Okell, J. P. Marangos, J. W. G. Tisch.

References and relevant links

[1] V. Strelkov, Phys. Rev. Lett. 104, 123901 (2010)
[2] R. A. Ganeev et al., Optics Express, 23, 25239-25248 (2012)