Lukas Gallmann

Semiconductor saturable-absorber mirror–assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime

Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.

Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric-field reconstruction

We demonstrate spectral phase interferometry for direct electric-field reconstruction (SPIDER) as a novel method to characterize sub-6-fs pulses with nanojoule pulse energy. SPIDER reconstructs pulse phase and amplitude from a measurement of only two optical spectra by use of a fast noniterative algorithm. SPIDER is well suited to the measurement of ultrabroadband pulses because it is quite insensitive to crystal phase-matching bandwidth and to unknown detector spectral responsivity. Moreover, it combines highly accurate pulse-shape measurement with the potential for online laser system diagnostics at video refresh rates.

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.

Spatio-temporal characterization of few-cycle pulses obtained by filamentation

Intense sub-5-fs pulses were generated by filamentation in a noble gas and subsequent chirped-mirror pulse compression. The transversal spatial dependence of the temporal pulse profile was investigated by spatial selection of parts of the output beam. Selecting the central core of the beam is required for obtaining the shortest possible pulses. Higher energy efficiency is only obtained at the expense of pulse contrast since towards the outer parts of the beam the energy is spread into satellite structures leading to a double-pulse profile on the very off-axis part of the beam. Depending on the requirements for a particular application, a trade-off between the pulse duration and the pulse energy has to be done. The energy of the sub-5-fs pulses produced was sufficient for the generation of high order harmonics in Argon. In addition, full simulation is performed in space and time on pulse propagation through filamentation that explains the double-pulse structure observed as part of a conical emission enhanced by the plasma defocusing.

Attosecond Science: Recent Highlights and Future Trends

We review the first ten years of attosecond science with a selection of recent highlights and trends and give an outlook on future directions. After introducing the main spectroscopic tools, we give recent examples of representative experiments employing them. Some of the most fundamental processes in nature have been studied with some results initiating controversial discussions. Experiments on the dynamics of single-photon ionization illustrate the importance of subtle effects on such extreme timescales and lead us to question some of the well-established assumptions in this field. Attosecond transient absorption, as the first all-optical approach to resolve attosecond dynamics, has been used to study electron wave packet interferences in helium. The attoclock, a recent method providing attosecond time resolution without the explicit need for attosecond pulses, has been used to investigate electron tunneling dynamics and geometry. Pushing the frontiers in attosecond quantum mechanics with increasing temporal and spatial resolution and often limited theoretical models results in unexpected observations. At the same time, attosecond science continues to expand into more complex solid-state and molecular systems, where it starts to have impact beyond its traditional grounds.

Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors

We report on double-chirped mirrors with custom-tailored dispersion characteristics over a bandwidth of 170 THz in the visible. The mirrors are used in a prismless compressor for a noncollinear optical parametric amplifier in the visible. The compressed pulses, characterized for the what is believed to be first time by use of the spectral phase interferometry for direct electric field reconstruction technique, display a nearly flat phase from 510 to 710 nm and have a duration of 5.7 fs.

Spatially resolved amplitude and phase characterization of femtosecond optical pulses

Ultrabroadband pulses exhibit a frequency-dependent mode size owing to the wavelength dependence of free-space diffraction. Additionally, rather complex lateral dependence of the temporal pulse shape has been reported for Kerr-lens mode-locked lasers and broadband amplifier chains and in frequency-domain pulse shapers, for example. We demonstrate an ultrashort-pulse characterization technique that reveals lateral pulse-shape variations by spatially resolved amplitude and phase measurements by use of spectral phase interferometry for direct electric-field reconstruction (SPIDER). Unlike with autocorrelation techniques, with SPIDER we can obtain spatially resolved pulse characterization even after the nonlinear process. Thus, with this method the spectral phase of the pulse can be resolved very rapidly along one lateral beam axis in a single measurement.

Ultrafast resolution of tunneling delay time

The question of how long a tunneling particle spends inside the barrier region has remained unresolved since the early days of quantum mechanics. The main theoretical contenders, such as the Buttiker–Landauer, Eisenbud–Wigner, and Larmor time, give contradictory answers. On the other hand, recent attempts at reconstructing valence electron dynamics in atoms and molecules have entered a regime where the tunneling time genuinely matters. Here, we compare the main competing theories of tunneling time against experimental measurements using the attoclock in strong laser field ionization of helium atoms. The attoclock uses a close to circularly polarized femtosecond laser pulse, mapping the angle of rotation of the laser field vector to time similar to the hand of a watch. Refined attoclock measurements reveal a real (not instantaneous) tunneling delay time over a large intensity regime, using two independent experimental apparatus. Only two theoretical predictions are compatible within our experimental error: the Larmor time and the probability distribution of tunneling times constructed using a Feynman Path Integral formulation. The latter better matches the observed qualitative change in tunneling time over a wide intensity range, and predicts a broad tunneling time distribution with a long tail. The implication of such a probability distribution of tunneling times, as opposed to a distinct tunneling time, would imply that one must account for a significant, though bounded and measurable, uncertainty as to when the hole dynamics begin to evolve. We therefore expect our results to impact the reconstruction of attosecond electron dynamics following tunnel ionization.Summary form only given. We present approach and results of an angular streaking experiment with the attoclock method [1] that suggest the existence of a real tunneling time in strong field ionization of Helium. The results are compared with competing theories of tunneling time and show that the only theories that are compatible with the experimental results are the L armor time and a distribution of tunneling times with a long tail constructed using a Feynman Path Integral formulation. We find that the latter matches the experimental data the best. Our results have strong implications on investigations of the electron dynamics in attosecond science since a significant uncertainty must be taken into account about when the electron hole dynamics begins to evolve.The attoclock method is based on the angular streaking of the photoelectron that was released from the atom by tunnel ionization. The angular distribution of the photoelectron momentum distribution contains the timing of the ionization process via an offset of the maximum of the angular distribution from the theoretically predicted value assuming instantaneous tunneling. Our results indicate the existence of a real tunneling time through this angular offset. The attoclock technique was transferred to a velocity map imaging setup (VMIS) in combination with tomographic reconstruction. The gas nozzle was integrated in the repeller plate, a configuration that allows one to achieve target gas densities that are significantly higher compared to setups employing cold atomic beams [2], leading to higher statistics and smaller error bars compared to previous measurements [1, 3]. Helium was leaked into the ultra high vacuum chamber and tunnel ionized by an elliptically polarized sub-10fs few-cycle pulse with a central wavelength of 735 nm and an ellipticity of 0.87. For the tomographic reconstruction, two-dimensional momentum space electron images are recorded in steps of two degrees covering a range of 180 degrees. The three-dimensional momentum distribution and thus the electron momentum distribution in the polarization plane is retrieved by tomographic reconstruction with a filtered backprojection algorithm [4, 5]. The results from the VMIS are confirmed with accurate measurements using a cold target recoil ion momentum spectrometer (COLTRIMS).

Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry

How quanta of energy and charge are transported on both atomic spatial and ultrafast timescales is at the heart of modern technology. Recent progress in ultrafast spectroscopy has allowed us to directly study the dynamical response of an electronic system to interaction with an electromagnetic field. Here, we present energy-dependent photoemission delays from the noble metal surfaces Ag(111) and Au(111). An interferometric technique based on attosecond pulse trains is applied simultaneously in a gas phase and a solid-state target to derive surface-specific photoemission delays. Experimental delays on the order of 100 as are in the same time range as those obtained from simulations. The strong variation of measured delays with excitation energy in Ag(111), which cannot be consistently explained invoking solely electron transport or initial state localization as supposed in previous work, indicates that final state effects play a key role in photoemission from solids.

Mid-infrared pulse generation via achromatic quasi-phase-matched OPCPA

We demonstrate a new regime for mid-infrared optical parametric chirped- pulse amplification (OPCPA) based on achromatic quasi-phase-matching. Our mid-infrared OPCPA system is based on collinear aperiodically poled lithium niobate (APPLN) pre-amplifiers and a non-collinear PPLN power amplifier which is operated in an achromatic phase-matching configuration. The idler output has a bandwidth of 800 nm centered at 3.4 µm. After compression, we obtain a pulse duration of 44.2 fs and a pulse energy of 21.8 µJ at a repetition rate of 50 kHz. We explain the wide applicability of the non-collinear QPM amplification scheme we used, including how it could enable octave-spanning OPCPA in a single device when combined with an aperiodic QPM grating.