Coincident electron-ion detection after photoionization in a “reaction microscope” is a very powerful tool to study atomic and molecular dynamics. However, the implementation of this tool in the field of attosecond science has so far been rather limited, due to the lack of high repetition rate laser sources capable of delivering few-cycle pulses with sufficient energy per pulse. In this article, the development of a Non-collinear Optical Parametric Amplifier (NOPA) capable of delivering Carrier-Envelope Phase (CEP) stable pulses with sub-6 fs duration and pulse energies in the few-µJ range is presented. The potential of combining the high repetition rate source and a reaction microscope operating at this high frequency is demonstrated in a proof-of-principle experiment on strong field ionization of Ar atoms.
© 2013 OSA
In the last decade, studies of atomic and molecular single- and multi-photon ionization processes under the influence of ultra-short laser pulses have progressed significantly. Strong field ionization experiments with infrared (IR) laser fields and pump-probe experiments combining synchronized extreme ultra-violet (XUV) and few-cycle IR laser fields have allowed first explorations of electron dynamics in atoms and molecules with both femtosecond and attosecond time resolution [1–3]. Several experiments have revealed that multi-electron correlations and the coupling of electronic and nuclear degrees of freedom (in the case of molecular systems) are of fundamental importance both during and immediately after an ultrafast photoionization process. However, the dynamics of these processes is still far from being fully understood. In fact, the considerable complexity of attosecond experiments combining XUV laser fields with large photon bandwidths (typically ≥10 eV) and non-perturbative IR laser fields, calls for the implementation of multi-dimensional detection techniques that provide access to many observables at once.
Taking as our main point of reference the development of attosecond science, we can see that in most current experiments photoelectron and/or photoion kinetic energy spectra are recorded with a time-of-flight detector, or -increasingly- using charged particle imaging techniques such as Velocity Map Imaging (VMI) [4, 5]. With VMI, both the kinetic energy of ions or electrons and their angular distributions are recorded. Even more sophisticated is the use of reaction microscopes [6, 7] that are capable of recording the 3D-momentum vectors of both ions and electrons in coincidence, thereby allowing a kinematically complete reconstruction of an ionization/dissociation event. Coincidence measurements imply that the detected charged particles originate from the same atom or molecule. This imposes therefore a strict requirement on the number of ionization events that can occur, on average, per laser shot. In order to eliminate false coincidences, typical count rates are on the order of 0.01-0.1 events per laser shot. Taking into account that at least 106 counts or more typically have to be acquired in order to obtain high quality coincident momentum maps, this implies that the required time for obtaining a single measurement for a source running at 1 kHz is on the order of at least a few hours. Moreover, when experimental parameters such as pump-probe time delay need to be scanned, a single experiment can easily extend for up to several days. This explains why the use of coincidence techniques is so far rare in attosecond science  and provides the motivation for the laser development reported in the present paper.
The impressive scientific achievements in attosecond science have gone hand-in-hand and have been enabled by the rapid development of light sources with ever improving characteristics. Nowadays, Chirped Pulse Amplification (CPA) systems based on Ti:Sapphire operating at a few kHz, delivering sub-25 fs pulses and a few mJ of energy per pulse are commercially available and standard tools in many laboratories around the world. Moreover, other techniques such as Non-Collinear Parametric Amplification (NOPA), and compression of pulses from CPA systems in gas-filled hollow core waveguides or filaments, have allowed the generation of few-cycle laser pulses with pulse energies ranging from a few hundred µJ to a few mJ [9–11]. In the few-cycle regime, controlling the Carrier-Envelope Phase (CEP) of the pulses acquires particular relevance . Among other things, CEP-stable few-cycle laser pulses have made possible the generation of isolated attosecond pulses , one of the key enabling tools for experiments in attosecond science.
In this paper, a new experimental setup combining a NOPA running at 400 kHz and a recently built reaction microscope capable of working at these high repetition rates is presented. The NOPA delivers CEP-stable few-cycle pulses with an energy per pulse in the µJ range. This light source is extremely well suited for strong field ionization experiments exploring sub-cycle dynamics and, in combination with the reaction microscope, allows a reduction in the required acquisition time by 2 to 3 orders of magnitude in comparison to standard kHz amplifiers. Moreover it is expected that further developments of the source (i.e. further amplification) to be carried out in the near future will enable attosecond XUV-IR pump-probe experiments with coincident detection of all ions and electrons formed.
2. Non-collinear optical parametric amplifier setup
The required light source to drive coincidence experiments must deliver CEP-stable, few-cycle pulses at repetition rates on the order of hundreds of kHz or higher. Additionally, to induce atomic or molecular photoionization or to be able to generate attosecond XUV pulses by high-harmonic generation, the energy per pulse has to be sufficient to reach intensity levels on the order of 1013-1014 W/cm2 in a reasonable spot size. Assuming a focused beam diameter of 20 µm and sub-10 fs pulses, the required energy per pulse is therefore at least 1 µJ. The chosen approach is to amplify CEP-stable, few-cycle laser pulses from a laser oscillator in a NOPA. In a parametric amplifier the energy from a pump wave is transferred to a seed (or signal) wave during an instantaneous non-linear interaction in a non-linear material. During the process a third wave, the idler, is generated. If no absorption is present in the spectral range of signal, pump and idler, there is no heat dissipation, which is the main limitation on the repetition rate of CPA systems. Of course, the limitation is still somehow present because the pump laser itself may involve CPA, and be subject to thermal limitations. However, there is no strict requirements on the bandwidth of the pump laser and a relatively narrowband source with good thermal characteristics, such as a diode-pumped Yb-based CPA system, can be utilized [14–19]. In a parametric amplifier the gain bandwidth is limited by phase-matching. When operated in a non-collinear geometry, the gain bandwidth can be enough to support the amplification of few-cycle pulses [20, 21]. This is in stark contrast with CPA systems, which are limited by the spectral properties of the laser transition.
Our system is schematically shown in Fig. 1. A CEP-stable, octave-spanning Ti:Sapphire oscillator (Pulse:ONE, VENTEON Femtosecond Laser Technologies GmbH) seeds both the NOPA operating at a central wavelength around 800 nm, and an Yb-doped fiber CPA system (Tangerine, Amplitude Systèmes) with a central wavelength around 1030 nm. The frequency-doubled output of the fiber amplifier provides the pump pulses for the NOPA. Seed and pump pulses for the NOPA are thus automatically synchronized. Similar schemes have been implemented before [22–27]. The pump laser delivers up to 12.5 W of power at 515 nm, in pulses with a duration <350 fs. The pulse energy depends on the repetition rate, which can be varied between 400 kHz and 2 MHz. All the results presented here were taken at 400 kHz and a maximum pump power of 12 W, corresponding to a pump pulse energy at 515 nm of 30 µJ.
The NOPA is based on type-I phase matching in β-Barium Borate (BBO) crystals. Given the short duration of the pump pulses, there is no additional stretching of the seed pulses before amplification in the NOPA, beyond that already imposed by propagation in air and a fused silica window at the output of the oscillator box. During amplification, the seed pulses are stretched mostly due to material dispersion in the BBO crystals. To optimize the ratio of seed pulse to pump pulse duration, one bounce on a pair of chirped mirrors was added in the seed beam path before the first BBO crystal. The ratio of seed to pump pulse duration at the input face of the first crystal is less than 0.25.
In the first amplification stage the pump is focused to reach an intensity of approximately 200 GW/cm2 in a 2 mm thick BBO crystal. The seed beam size is 20% bigger than the pump and the divergences have been closely matched. In this first stage, the crystal is cut such that the walk-off between signal and pump beams inherent to the non-collinear geometry is partially compensated by the birefringent walk-off of the pump. This walk-off compensation geometry minimizes detrimental spatial effects during amplification . Under this configuration however, phase-matching conditions are satisfied for second harmonic generation of both signal and idler. As a consequence some energy is lost to this parasitic effect. The internal non-collinear angle between the pump and the seed beams is set at 2.2 °. The phase-matching angle and the delay between the pump and seed pulses are optimized for a compromise between broadband amplification and energy extraction. Both beams are then collimated and re-focused on a second BBO crystal with beam sizes 30% smaller than in the first crystal. In the second stage the crystal has a thickness of 1 mm. It has been cut for a non-walk off compensation geometry to avoid further parasitic second harmonic generation of the seed. The non-collinear angle is again set close to 2.2 ° and later optimized together with the phase-matching angle and the delay for broadband amplification.
After amplification in the second BBO crystal the amplified beam is collimated and sent to a chirped mirror compressor. Eight pairs of bounces pre-compensate for propagation in air towards the vacuum chamber of the reaction microscope, and the 5 mm fused silica window at the entrance of the chamber. Subsequently the polarization is rotated in a periscope and the beam is resized in a telescope constructed with silver-coated concave spherical mirrors. The angle between the mirrors is carefully adjusted to compensate beam astigmatism. Additionally, a long-pass filter (cut-off wavelength 550 nm) is used to remove the parasitic second harmonic generated in the first BBO crystal (approximately 5% of the total output power). Furthermore, a pair of thin fused silica wedges is placed in the path of the beam towards the chamber in order to fine-tune the dispersion compensation.
3. Performance of the NOPA
The system has been optimized and carefully tested to achieve the parameters defined in the previous section: broadband amplification supporting few-cycle pulses, more than 5 μJ of energy per pulse with excellent short and long term stability, beam quality allowing excellent focusing properties and CEP stability.
3.1 Broadband amplification and pulse compression
The short duration of the pump pulses allows reaching high intensities (> 100 GW/cm2) on the crystal, not accessible otherwise due to limitations imposed by material damage threshold. Under these conditions the thickness of the crystals can be reduced and the non-collinear angle can be chosen so that a high gain with an increased gain bandwidth is obtained with respect to the so-called “magic” phase matching angle configuration , which is roughly 2.6 ° for the central wavelengths of pump and seed pulses. Figure 2 shows the output spectra of the amplified pulses after parametric amplification in the first and in the second BBO crystal, both under a non-collinear angle of approximately 2.2 °.
In the first crystal the pulses are amplified from less than 2 nJ to 1.5 µJ, while in the second crystal the energy per pulse is boosted to more than 5 µJ and the output spectrum supports sub-5 fs pulses. The strong modulation in the output spectra suggests that there is a high degree of back conversion from the signal and idler to the pump wave. The optical-to-optical efficiency is approximately 18%. Although higher extraction efficiencies could be achieved, this leads to a stronger modulation in the output spectrum, which in turn gives rise to the formation of strong satellite pulses in the temporal domain.
Pulse temporal shape measurements were carried out with a SPIDER apparatus (APE FC-SPIDER) and a home-built interferometric autocorrelator. Both measurements were done reconstructing as much as possible the conditions met by the amplified pulses in the experimental chamber of the reaction microscope. Figure 3 shows the SPIDER reconstruction of the spectral phase and the temporal shape of the pulses.
The pulse duration is very close to the transform-limited value, though small deviations from zero in the spectral phase lead to a distribution of the pulse energy over a time window of more than 100 fs. The energy contained in the main pulse amounts to slightly less than 35% of the total, while the transform-limited pulse has 78% of the energy contained in the main pulse. Nevertheless, this temporal shape with sub-6 fs pulses is very well suited for many applications. Considering a central wavelength of 754 nm (calculated as the intensity averaged central wavelength of our spectrum), the FWHM of the pulse lasts 2.1 optical cycles. Additionally, the NOPA was also tested decreasing slightly the intensity of the pump in the first stage to obtain a less modulated amplified spectrum. This was possible at the expense of a reduction in the optical-to-optical efficiency, which went down from 18% to 12%. The resultant spectrum was less modulated than the one shown in Fig. 3, however the transform limited pulse duration for this spectrum increased to 5.6 fs. The pulse compression was again very close to transform-limited, yielding a pulse duration of 5.9 fs. For this pulse, 50% of the energy is contained in the main short spike (90% for the transform-limited pulse), and therefore the intensity that can be achieved is only 10% lower for this pulse (compared with the one shown in Fig. 3). Additional improvement of the pulse compression can in future be achieved implementing pulse shaping techniques.
It is worth mentioning that the amplified pulses have a slightly different spectral phase from the unamplified seed propagating through the setup, and compression is adjusted between the two situations with a pair of thin wedges. This parametric phase depends on the wavelength dependent phase mismatch, pump depletion and input signal to pump intensity ratio . It has been observed experimentally before that when the non-collinear angle is smaller than the magic angle the leading term of the parametric phase is the GDD . Our observations are in agreement with these findings.
SPIDER measurements are quite challenging with strongly modulated spectra covering such a wide spectral range, therefore we also used interferometric autocorrelation measurements as an independent tool for pulse characterization. The results of both techniques were compared by using the measured spectrum and the reconstructed spectral phase of Fig. 3 to simulate the autocorrelation trace (measured under the same conditions as the data in Fig. 3). The results are shown in Fig. 4.
The simulation trace includes a dc-component present in the measurement that prevented the signal from reaching the ideal ratio of 1:8 between the baseline and the peak of the interferometric autocorrelation trace. The measured trace and the SPIDER-based simulation are in very good agreement. For comparison the simulated trace for a transform-limited pulse is also plotted in green. The comparison shows that the main feature of the temporal shape is very close to the transform limited pulse.
3.2 Output power stability and beam characteristics
For applications in experiments, in particular with the reaction microscope, it is very important to characterize the stability of the high repetition rate source, as well as the beam quality. The output power was measured before the resizing telescope, i.e. after pulse compression and filtering of the parasitic second harmonic. The filtered parasitic second harmonic signal amounts to approximately 5% of the total output power and is estimated by measuring the output power before and after the long-pass filter. Additionally, the contribution from amplified superfluorescence was estimated by measuring the output power when the seed beam is blocked. This power was measured to be zero, within the accuracy of the detector used (< 1 mW). Figure 5 shows a power measurement of the system over a time scale of 1 hour.
The mean value of the output power is more than 2 W (> 5 µJ at 400 kHz) and the variation of the power over a period of one hour is characterized by an RMS value of 0.4% of the mean value (stability imposed by the pump laser). Over longer time scales (> 2 hours) the RMS increases but remains on the order of 1%. The maximum pulse-to-pulse variation over a period of 100 seconds is on the order of 5%.
In the previous subsection the fraction of the total energy contained in the main pulse with sub-6 fs duration was estimated from the reconstructed pulse temporal shape. This value combined with the power measurements and the repetition rate, determine the pulse peak power. For most experiments however it is important to know the intensity that can be reached and, although assuming a Gaussian beam profile yields in many cases a very good approximation to the beam size in the desired location, beam propagation factor measurements (i.e. M2-measurements) provide a quantitative tool to estimate a more realistic value of the intensity. This is particularly important in NOPAs where the beam quality can degrade substantially during amplification . However, M2 measurements are wavelength dependent and therefore are meaningful for narrow bandwidth beams. Nevertheless, keeping this restriction in mind, M2 measurements of the amplified beam after the 1st and the 2nd crystal were taken with a commercial device (DataRay Inc.). For the results presented in Fig. 6, a wavelength of 800 nm has been assumed.
The M2 values in Fig. 6(a) show that there is a small degradation in the x-direction which is consistent with the fact that this is the direction across which the pump and seed beams cross in the 2 mm thick BBO crystal. In the second crystal (Fig. 6(b)), even though the thickness is only 1 mm, the M2 value degrades further across the x-direction. This is not totally unexpected since the beams are smaller in the second BBO crystal and additionally the crystal was cut for a non-walk off compensation geometry to avoid further parasitic second harmonic generation. The M2 value remains unchanged in the direction perpendicular to the beam crossing (y-axis). Due to the wavelength dependent nature of the M2 measurement technique, the values shown here should be considered only as a strong indication that the amplified beam is well-behaved. If the Mx2 value for the data in Fig. 6(b) is calculated again changing the wavelength between 650 nm and 1000 nm, the results vary from 1.9 to 1.2. The astigmatism observed in Fig. 6 was corrected before sending the beam to the experimental chamber as described in section 2.
3.3 Carrier-envelope phase stability
In a parametric amplifier the CEP stability of the oscillator is maintained throughout amplification as long as the intensity of the pump remains constant [30, 32]. Given the stability of our pump laser, we expect that the CEP stability is preserved during amplification, as long as the amplification conditions are kept constant. In order to test the CEP stability after amplification, a home-built f-to-2f interferometer [33, 34], was implemented after the pulse compression. The home-built setup is a common path interferometer. An octave spanning spectrum was generated by focusing the amplified pulses from the NOPA into a 2 mm thick Sapphire plate. Second harmonic was generated in a 100 µm thick BBO crystal and the frequency of the spectral fringes was adjusted by introducing a thin fused silica window between the white-light generation stage and the second harmonic generation stage. The results are shown in Fig. 7.
Figure 7(a) shows the appearance of spectral fringes when the locking system in the oscillator is turned on. When the locking system is off, no modulation is present in the spectrum. The integration time of the spectrometer for these measurements was 2 ms, and therefore each spectrum averages 800 shots. This rather long integration time is imposed by the shortest acquisition time achievable with our spectrometer. The fringe visibility reaches almost complete modulation in selected parts of the spectrum, as shown in Fig. 7(b). This is a strong indication that during the integration time the CEP stability is preserved. However this is difficult to quantify and our values for the standard deviation of the CEP could be slightly worse. Figure 7(c) and 7(d) show the spectral fringe evolution on a time scale of 25 seconds, for which the standard deviation of the CEP is 180 mrad. On shorter time scales (10-20 seconds), the standard deviation reduces to 150 mrad, which approaches the CEP stability of the oscillator. On longer time scales the CEP slowly drifts, possibly due to small changes in the amplification conditions originating in small drifts in the time delay between the pump and the seed pulses in the NOPA. This has been experimentally demonstrated by Hädrich et al., in a similar non-collinear amplifier setup in which the delay between pump and signal was stabilized . We have recently implemented the same idea into our setup and preliminary results indicate that by stabilizing the delay between pump and signal the CEP stability can be extended to tens of minutes.
4. Application to coincidence experiments with the reaction microscope at 400 kHz
4.1 Experimental setup
The basic idea of the experimental setup in the reaction microscope is shown in Fig. 8. Inside the experimental chamber the collimated laser beam (in red) crosses the interaction region and impinges on an 80 mm focal length silver-coated spherical mirror at normal incidence. The beam is focused at the center of the chamber where it interacts with a molecular beam (in light blue). The intensity in the interaction region reaches 1013-1014 W/cm2, depending on the size of the beam before the focusing mirror, which is sufficient to induce photoionization in the atoms or molecules present in the interaction region. The generated ions and electrons are driven towards two position-sensitive detectors (ID and ED in Fig. 8) by electric and magnetic extraction fields. The detectors record the time of flight of the particles and the position in a plane parallel to the molecular beam and the laser beam. From these measured quantities the momentum vectors of the charged particles can be retrieved. Each detector consists of a multi-channel plate followed by a time- and position-sensitive delay-line anode (RoentDek DLD80 -ions- and RoentDek HEX80 -electrons-) capable of recording events at repetition rates as high as 2 MHz. The setup is similar to other systems shown before in the literature (see for example ).
4.2 Coincidence measurements at 400 kHz
In this section we show results on strong field ionization of Ar atoms to illustrate the potential of our setup. Figure 9 shows a reconstructed electron momentum map corresponding to single ionization of Ar atoms. The momentum has been expressed in atomic units (a.u) and the parallel direction is defined with respect to the laser polarization. Coincidence detection allows discrimination between electrons originating from single and double ionization of Argon, Argon dimers and background gas. All these species are identifiable in the ion time-of-flight spectrum. For the momentum map of Fig. 9 the pulse duration was 6 fs (less than 3 cycles) and the peak intensity approximately 7∙1013 W/cm2. The momentum map shows a clear fan-like structure going out radially which, for electrons with near zero kinetic energy, has been associated before with the interference of classical electron trajectories leaving the atom at different times during the pulse evolution and subject to the combined effect of the laser and the Coulomb field [36, 37]. These features in the angular distribution of the electrons are quite sensitive to the central wavelength, so it is not surprising that our image seems blurry compared to results of experiments performed with narrower band sources . Additionally, the fine structure in the radial distribution arising at a.u. has been observed in numerous experiments before and identified as resonantly enhanced ionization through intermediate Rydberg states . However the high number of maxima and minima extending beyond 0.3-0.4 a.u. in the parallel momentum calls for further analysis and will be the subject of future work.
The main point that needs to be stressed here is the speed at which these data can be acquired. The data set from which Fig. 9 was extracted consisted of 107 events, acquired in less than 40 minutes, at an event rate of approximately 1% of the laser repetition rate. From the complete set, more than 1.5∙106 coincidence events corresponding to single ionization of Ar could be identified. This is, to the best of our knowledge, the fastest repetition rate used in a coincidence experiment involving few-cycle pulses to date. In the near future we expect to employ the system presented here to study strong field ionization of molecular and atomic systems with sub-fs time resolution, by exploiting the CEP sensitivity of strong field ionization processes with few-cycle pulses. Moreover, the scalability of the system to higher energies is only limited by the pump laser for the NOPA. Further increase in the energy per pulse of the pump laser to several hundred µJ can be expected with alternative technologies based on Yb-doped materials such as thin-disk, cryo-cooled thick-disk or Innoslab amplifiers [14, 15, 18]. Such an upgrade in pump energy would allow bringing the energy of the few-cycle pulse into the several tens or hundreds of µJ-range, where current sub-5 kHz systems generating isolated attosecond pulses operate. In fact, evidence for the generation of isolated attosecond pulses at 600 kHz has been recently reported using a fiber CPA pumped NOPA delivering 14 µJ of energy per pulse . Therefore it is expected that up-scaling of the pump laser in our setup will allow the generation of high harmonics, paving the way for XUV-IR pump-probe experiments with the reaction microscope.
5. Summary and perspective
A new state of the art, experimental setup combining a NOPA and a reaction microscope for photoionization experiments has been presented. The NOPA provides CEP-stable few-cycle pulses at 400 kHz with sufficient energy to successfully drive strong field ionization experiments. The output power of the NOPA experiences a variation on the order of 1% over time scales of several hours and the maximum pulse-to-pulse variation is on the order of 5%. The beam propagation characteristics and beam profile are well-suited for reaching focused intensities on the order of 1014 W/cm2 given the energy per pulse of more than 5 µJ and pulse durations shorter than 6 fs. The standard deviation of the CEP over a timescale of a few seconds is less than 200 mrad and on longer time scales is limited by slow drifts. For experiments exploiting the CEP of the pulses, a delay stabilization system is envisaged.
The potential of the setup has been demonstrated in experiments where the few-cycle pulses have induced ionization in Ar atoms. The velocity vectors of ions and electrons have been detected at 400 kHz working at an event rate around 1% of the full repetition rate, and more than 1.5∙106 events corresponding to single ionization of Ar have been identified after a recording time of less than 40 minutes. This proof-of-principle experiment shows that our setup is very well suited for performing kinematically complete strong field ionization experiments with few-cycle pulses. Moreover, it is expected that further development of the pump laser for the NOPA will allow, in the near future, up-scaling of the few-cycle pulses energy and enable generation of high harmonics and in turn, XUV-IR pump-probe experiments with the reaction microscope.
The authors would like to acknowledge the financial support of the European Union through the EU-project FLAME: Femtosecond Light Amplifiers in the Megahertz regime; research for SMEs; grant agreement no.: 315744.
References and links
1. F. Krausz and M. Ivanov, “Attosecond physics,” Rev. Mod. Phys. 81(1), 163–234 (2009). [CrossRef]
2. M. F. Kling, Ch. Siedschlag, A. J. Verhoef, J. I. Khan, M. Schultze, T. Uphues, Y. Ni, M. Uiberacker, M. Drescher, F. Krausz, and M. J. J. Vrakking, “Control of electron localization in molecular dissociation,” Science 312(5771), 246–248 (2006). [CrossRef] [PubMed]
3. G. Sansone, F. Kelkensberg, J. F. Pérez-Torres, F. Morales, M. F. Kling, W. Siu, O. Ghafur, P. Johnsson, M. Swoboda, E. Benedetti, F. Ferrari, F. Lépine, J. L. Sanz-Vicario, S. Zherebtsov, I. Znakovskaya, A. L’huillier, M. Y. Ivanov, M. Nisoli, F. Martín, and M. J. J. Vrakking, “Electron localization following attosecond molecular photoionization,” Nature 465(7299), 763–766 (2010). [CrossRef] [PubMed]
4. D. W. Chandler and P. L. Houston, “Two-dimensional imaging of state-selected photodissociation products detected by multiphoton ionization,” J. Chem. Phys. 87(2), 1445–1447 (1987). [CrossRef]
5. O. Ghafur, W. Siu, P. Johnsson, M. F. Kling, M. Drescher, and M. J. J. Vrakking, “A velocity map imaging detector with an integrated gas injection system,” Rev. Sci. Instrum. 80(3), 033110 (2009). [CrossRef] [PubMed]
6. R. Dörner, V. Mergel, O. Jagutzki, L. Spielberger, J. Ullrich, R. Moshammer, and H. Schmidt-Böcking, “Cold target recoil ion momentum spectroscopy: a 'momentum microscope' to view atomic collision dynamics,” Phys. Rep. 330(2-3), 95–192 (2000). [CrossRef]
7. J. Ullrich, R. Moshammer, A. Dorn, R. Dörner, L. P. H. Schmidt, and H. Schmidt-Böcking, “Recoil-ion and electron momentum spectroscopy: reaction-microscopes,” Rep. Prog. Phys. 66(9), 1463–1545 (2003). [CrossRef]
8. A. Fischer, A. Sperl, P. Cörlin, M. Schönwald, H. Rietz, A. Palacios, A. González-Castrillo, F. Martín, T. Pfeifer, J. Ullrich, A. Senftleben, and R. Moshammer, “Electron localization involving doubly excited states in broadband extreme ultraviolet ionization of H2.,” Phys. Rev. Lett. 110(21), 213002 (2013). [CrossRef] [PubMed]
9. S. Witte, R. T. Zinkstok, A. L. Wolf, W. Hogervorst, W. Ubachs, and K. S. E. Eikema, “A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification,” Opt. Express 14(18), 8168–8177 (2006). [CrossRef] [PubMed]
10. M. Nisoli, S. De Silvestri, O. Svelto, R. Szipöcs, K. Ferencz, C. Spielmann, S. Sartania, and F. Krausz, “Compression of high-energy laser pulses below 5 fs,” Opt. Lett. 22(8), 522–524 (1997). [CrossRef] [PubMed]
12. H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, and U. Keller, “Carrier-envelope offset phase control: a novel concept for absolute optical frequency measurement and ultrashort pulse generation,” Appl. Phys. B 69(4), 327–332 (1999). [CrossRef]
13. M. Hentschel, R. Kienberger, C. Spielmann, G. A. Reider, N. Milosevic, T. Brabec, P. Corkum, U. Heinzmann, M. Drescher, and F. Krausz, “Attosecond metrology,” Nature 414(6863), 509–513 (2001). [CrossRef] [PubMed]
14. B. A. Reagan, A. H. Curtis, K. A. Wernsing, F. J. Furch, B. M. Luther, and J. J. Rocca, “Development of high energy diode-pumped thick-disk Yb:YAG chirped-pulse-amplification lasers,” IEEE J. Quantum Electron. 48(6), 827–835 (2012). [CrossRef]
15. E. Innerhofer, T. Südmeyer, F. Brunner, R. Häring, A. Aschwanden, R. Paschotta, C. Hönninger, M. Kumkar, and U. Keller, “60-W average power in 810-fs pulses from a thin-disk Yb:YAG laser,” Opt. Lett. 28(5), 367–369 (2003). [CrossRef] [PubMed]
16. S. Klingebiel, C. Wandt, C. Skrobol, I. Ahmad, S. A. Trushin, Z. Major, F. Krausz, and S. Karsch, “High energy picosecond Yb:YAG CPA system at 10 Hz repetition rate for pumping optical parametric amplifiers,” Opt. Express 19(6), 5357–5363 (2011). [CrossRef] [PubMed]
17. J. Tümmler, R. Jung, H. Stiel, P. V. Nickles, and W. Sandner, “High-repetition-rate chirped-pulse-amplification thin-disk laser system with joule-level pulse energy,” Opt. Lett. 34(9), 1378–1380 (2009). [CrossRef] [PubMed]
19. T. Eidam, S. Hanf, E. Seise, T. V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, and A. Tünnermann, “Femtosecond fiber CPA system emitting 830 W average output power,” Opt. Lett. 35(2), 94–96 (2010). [CrossRef] [PubMed]
20. G. Cerullo, M. Nisoli, S. Stagira, and S. De Silvestri, “Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible,” Opt. Lett. 23(16), 1283–1285 (1998). [CrossRef] [PubMed]
21. A. Shirakawa, I. Sakane, M. Takasaka, and T. Kobayashi, “Sub-5-fs visible pulse generation by pulse-front-matched noncollinear optical parametric amplification,” Appl. Phys. Lett. 74(16), 2268–2270 (1999). [CrossRef]
22. M. Emons, A. Steinmann, T. Binhammer, G. Palmer, M. Schultze, and U. Morgner, “Sub-10-fs pulses from a MHz-NOPA with pulse energies of 0.4 microJ,” Opt. Express 18(2), 1191–1196 (2010). [CrossRef] [PubMed]
23. M. Schultze, T. Binhammer, G. Palmer, M. Emons, T. Lang, and U. Morgner, “Multi-μJ, CEP-stabilized, two-cycle pulses from an OPCPA system with up to 500 kHz repetition rate,” Opt. Express 18(26), 27291–27297 (2010). [CrossRef] [PubMed]
24. M. Schultze, T. Binhammer, A. Steinmann, G. Palmer, M. Emons, and U. Morgner, “Few-cycle OPCPA system at 143 kHz with more than 1 microJ of pulse energy,” Opt. Express 18(3), 2836–2841 (2010). [CrossRef] [PubMed]
25. J. Rothhardt, S. Demmler, S. Hädrich, J. Limpert, and A. Tünnermann, “Octave-spanning OPCPA system delivering CEP-stable few-cycle pulses and 22 W of average power at 1 MHz repetition rate,” Opt. Express 20(10), 10870–10878 (2012). [CrossRef] [PubMed]
26. J. Rothhardt, S. Hädrich, E. Seise, M. Krebs, F. Tavella, A. Willner, S. Düsterer, H. Schlarb, J. Feldhaus, J. Limpert, J. Rossbach, and A. Tünnermann, “High average and peak power few-cycle laser pulses delivered by fiber pumped OPCPA system,” Opt. Express 18(12), 12719–12726 (2010). [CrossRef] [PubMed]
27. J. Nillon, S. Montant, J. Boullet, R. Desmarchelier, Y. Zaouter, E. Cormier, and S. Petit, “15-fs, 1-µJ, 100-kHz pulses by direct seeding of a NOPA and its fiber pump by a CEP stabilized Ti:Sapphire oscillator,” in Advanced Solid-State Photonics, Optical Society of America 2009, paper MF8.
28. J. Bromage, C. Dorrer, and J. D. Zuegel, “Angular-dispersion-induced spatiotemporal aberrations in noncollinear optical parametric amplifiers,” Opt. Lett. 35(13), 2251–2253 (2010). [CrossRef] [PubMed]
29. D. N. Schimpf, J. Rothhardt, J. Limpert, A. Tünnermann, and D. C. Hanna, “Theoretical analysis of the gain bandwidth for noncollinear parametric amplification of ultrafast pulses,” J. Opt. Soc. Am. B 24(11), 2837–2846 (2007). [CrossRef]
30. I. N. Ross, P. Matousek, G. H. C. New, and K. Osvay, “Analysis and optimization of optical parametric chirped pulse amplification,” J. Opt. Soc. Am. B 19(12), 2945–2956 (2002). [CrossRef]
31. S. Demmler, J. Rothhardt, S. Hädrich, J. Bromage, J. Limpert, and A. Tünnermann, “Control of nonlinear spectral phase induced by ultra-broadband optical parametric amplification,” Opt. Lett. 37(19), 3933–3935 (2012). [CrossRef] [PubMed]
32. A. Baltuška, T. Fuji, and T. Kobayashi, “Controlling the carrier-envelope phase of ultrashort light pulses with optical parametric amplifiers,” Phys. Rev. Lett. 88(13), 133901 (2002). [CrossRef] [PubMed]
33. M. Mehendale, S. A. Mitchell, J. P. Likforman, D. M. Villeneuve, and P. B. Corkum, “Method for single-shot measurement of the carrier envelope phase of a few-cycle laser pulse,” Opt. Lett. 25(22), 1672–1674 (2000). [CrossRef] [PubMed]
34. M. Kakehata, H. Takada, Y. Kobayashi, K. Torizuka, Y. Fujihira, T. Homma, and H. Takahashi, “Single-shot measurement of carrier-envelope phase changes by spectral interferometry,” Opt. Lett. 26(18), 1436–1438 (2001). [CrossRef] [PubMed]
35. S. Hädrich, J. Rothhardt, M. Krebs, S. Demmler, J. Limpert, and A. Tünnermann, “Improving carrier-envelope phase stability in optical parametric chirped-pulse amplifiers by control of timing jitter,” Opt. Lett. 37(23), 4910–4912 (2012). [CrossRef] [PubMed]
36. D. G. Arbó, K. I. Dimitriou, E. Persson, and J. Burgdörfer, “Sub-Poissonian angular momentum distribution near threshold in atomic ionization by short laser pulses,” Phys. Rev. A 78(1), 013406 (2008). [CrossRef]
37. T. Marchenko, H. G. Muller, K. J. Schafer, and M. J. J. Vrakking, “Electron angular distributions in near-threshold atomic ionization,” J. Phys. B 43(9), 095601 (2010). [CrossRef]
38. M. Krebs, S. Hädrich, S. Demmler, J. Rothhardt, A. Zaïr, L. Chipperfield, J. Limpert, and A. Tünnermann, “Towards isolated attosecond pulses at megahertz repetition rates,” Nat. Photonics 7(7), 555–559 (2013). [CrossRef]