Attosecond time-resolved transient absorption spectroscopy is performed in a dense helium target by superimposing an attosecond pulse train (APT) with a moderately strong infrared field. We observe rapid oscillations of the absorption of the individual harmonics as a function of time-delay between the APT and IR field even for harmonic energies well below the ionization threshold. The phase dependence of these modulations on atto-chirp and IR intensity yields direct evidence for the interference of transiently bound electronic wavepackets as the underlying mechanism.

Recently, the ionization probability of helium atoms with APT photon energies below the ionization threshold in presence of a time-delayed infrared (IR) field was studied and was found to modulate with twice the IR frequency. The result was explained by the interference of transiently bound electron wavepackets (EWP). We investigated this physical system with attosecond time-resolved transient absorption spectroscopy, The different observable probed by this all-optical technique compared to the experiment by Johnsson et al. allows us to verify the plausibility of the previously introduced intuitive theoretical model.

We perform attosecond time-resolved transient absorption spectroscopy around the first ionization threshold of helium and observe rapid oscillations of the absorption of the individual harmonics as a function of time delay with respect to a superimposed, moderately strong infrared laser field. The phase relation between the absorption modulation of individual harmonics gives direct evidence for the interference of transiently bound electronic wave packets as the mechanism behind the absorption modulation.

Transient absorption spectroscopy using attosecond pulse trains is introduced as a new spectroscopic tool in the attosecond domain. We discuss its application to the observation of interferences between transiently bound electron wavepackets in helium.

We demonstrate a new scheme to generate harmonic continua starting with 12 fs laser pulses based on polarization gating. The effectiveness of the gating method is confirmed by an SFA calculation.

We observe theoretically and experimentally the IR-assisted absorption of an attosecond pulse train in a helium gas target. The transmitted photon yield is modulated on an attosecond time-scale, and a spectrally localized emission occurs.

High harmonic generation (HHG) of intense infrared laser radiation (Ferray et al., J. Phys. B: At. Mol. Opt. Phys. 21:L31, 1988 ; McPherson et al., J. Opt. Soc. Am. B 4:595, 1987 ) enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low conversion efficiency and the low average power of the complex laser amplifier systems (Keller, Nature 424:831, 2003 ; Südmeyer et al., Nat. Photonics 2:599, 2008 ; Röser et al., Opt. Lett. 30:2754, 2005 ; Eidam et al., IEEE J. Sel. Top. Quantum Electron. 15:187, 2009 ) which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the average photon flux. However, the higher average power comes at the expense of lower pulse energies because the repetition rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse energies were developed: external build-up cavities (Gohle et al., Nature 436:234, 2005 ; Jones et al., Phys. Rev. Lett. 94:193, 2005 ) and resonant field enhancement in nanostructured targets (Kim et al., Nature 453:757, 2008 ). Here we present a third technique, which has advantages in terms of ease of HHG light extraction, transverse beam quality, and the possibility to substantially increase conversion efficiency by phase-matching (Paul et al., Nature 421:51, 2003 ; Ren et al., Opt. Express 16:17052, 2008 ; Serebryannikov et al., Phys. Rev. E (Stat. Nonlinear Soft Matter Phys.) 70:66611, 2004 ; Serebryannikov et al., Opt. Lett. 33:977, 2008 ; Zhang et al., Nat. Phys. 3:270, 2007 ). The interaction between the laser pulses and the gas occurs in a Kagome-type Hollow-Core Photonic Crystal Fiber (HC-PCF) (Benabid et al., Science 298:399, 2002 ), which reduces the detection threshold for HHG to only 200 nJ. This novel type of fiber guides nearly all of the light in the hollow core (Couny et al., Science 318:1118, 2007 ), preventing damage even at intensities required for HHG. Our fiber guided 30-fs pulses with a pulse energy of more than 10 μJ, which is more than five times higher than for any other photonic crystal fiber (Hensley et al., Conference on Lasers and Electro-Optics (CLEO), IEEE Press, New York, 2008 ).

Intensity dependent high harmonic generation was investigated when both short and long trajectories contribute to the emission. We have directly observed for the first time clear indication of quantum-path interference through harmonic spectrum modulations.

Synopsis: We generate continuous high harmonic spectra in Argon by a superposition of two time-delayed, crosspolarized 12 fs pulses with a small imbalance of group delay dispersion (GDD) between the pulses. Our technique improves overall harmonic flux compared to traditional polarization gating methods. Recorded spectra indicate that the continuum generation is insensitive to the carrier-envelope offset phase (CEP).

The interference between the emission originating from the short and long electron quantum paths is intrinsic to the high harmonic generation process. We investigate the universal properties of these quantum-path interferences in various generation media and discuss how ionization effects influence the observed interference structures. Our comparison of quantum-path interferences observed in xenon, argon, and neon demonstrates that our experimental tools are generally applicable and should also allow investigating more complex systems such as molecules or clusters.

High Harmonic generation can be used as a probe of the emitting medium with attosecond and Angström resolutions. We show that polarization-resolved pump-probe spectroscopy with high harmonics improves the detection sensitivity of rotationally excitedmolecules.

We present theoretical and experimental studies on quantum path interferences in high-order harmonic generation. Simulations of the single-atom response allow us to calculate the different quantum paths contributions; their relative phases and the resulting interferences can be finely controlled through the laser intensity that provides an efficient means for controlling the electron trajectories with an accuracy on the ten attoseconds time scale. Simulations of the macroscopic response demonstrate the need of spatial and spectral filtering of the harmonic beam in order to observe the interferences between the two shortest quantum paths. Our numerical results are in very good agreement with experimental data. These investigations represent a step toward the full characterization and control of the atomic harmonic dipole.

We report the first HHG in a hollow-core photonic crystal fiber. We generate the 7th-13th harmonic of ap800 nm in xenon. The extremely low threshold of 0.4 muJ would be achievable by multimegahertz solid-state lasers.

We generate the 7th–13th harmonic of ≈800 nm by propagating 30-fs pulses at >1014 W/cm2 through a xenon-filled hollow-core PCF. The extremely low HHG threshold of 0.4 µJ would be achievable by multimegahertz solid-state lasers.

Recently, it was demonstrated that the probability of ionization of He atoms with XUV photons close to the threshold can be controlled by the presence of a delayed IR field [1]. In that experiment, time-resolved photoelectronspectra were recorded showing modulations with a periodicity of twice the fundamental laser frequency. We performed a complementary experiment, detecting photons instead of electrons. We generated high harmonics in a xenon gas target with 30 fs laser pulses centered at a wavelength of 800 nm. The IR radiation was removed from the harmonics using a thin aluminum foil. The obtained harmonics (orders 13 to 19) were confirmed by RABITT to form an attosecond pulse train (data not shown). The harmonics are collinearly recombined with a time-delayed replica of the driving laser pulse using an infrared mirror with a center-hole for transmitting the harmonics. The combined beam is then focused into a second gas target by a toroidal mirror. We reach IR intensities on the order of 5·10 W/cm. The harmonic radiation transmitted through the second gas target is detected in a XUV spectrometer. Helium was chosen as target medium. Its ionization potential is lower than the photon energy of harmonic order 17. Harmonic photons with order ≥17 are absorbed by single photon ionization. The density of the jet was adjusted such that about half of these photons were absorbed. Photons of harmonic 15 and below can only be absorbed in a multiphoton process. As the intensity of the harmonics is low, this is in our arrangement only possible in the presence of an additional and suitably strong IR field. When adding the IR field and changing the delay between IR and the APT, the envelope structure is clearly visible in harmonics 13 and 15 corresponding to an IR induced absorption in helium. A typical delay scan is shown in Fig. 1. In addition to this femtosecond structure, a subcycle modulation of the transmission appears at a periodicity of twice the laser frequency. This modulation is visible on all harmonics, even for those with photon energy higher than the ionization potential of helium. We experimentally show that the relative phase of this modulation does not depend on the chirp of the attosecond pulses in the train. However, the phase relation clearly changes with the intensity of the IR present in the helium target.

We investigate the influence of microscopic and macroscopic ionization effects on the intensity-dependent quantum-path interferences in high-harmonic generation. The resulting interference structures were analyzed in different gases and in spatially resolved harmonic spectra.

We report experimental measurements of high-order harmonic spectra generated in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800 nm laser field and a fraction (less than 10%) of its second harmonic. Additional spectral peaks are observed between the harmonic peaks, which are due to interferences between multiple pulses in the train. The position of these peaks varies with the CEO and their number is directly related to the number of pulses in the train. An analytical model, as well as numerical simulations, support our interpretation.

Summary form only given. Pulse compression through filamentation to the two-optical-cycle regime has been demonstrated, including its capability of maintaining the carrier-envelope-offset (CEO) phase. Simulations reproduce these results well but also indicate significant spatial structure in the output beam calling for spatio-temporal characterization of these pulses. We used 30-fs pulses centered at 795 nm with energies of up to 1 mJ, to generate ultra-short pulses by filamentation in two subsequent cells filled with Argon at an absolute pressure of 900 mbar and 820 mbar, respectively. We investigated the spatial dependence of the temporal profile and phase of these output pulses by spatially selecting a part of the output beam.

We have investigated intensity dependent high-harmonic generation, when short and long trajectories are both visible in the generated signal. We have measured a plateau-harmonic spectral broadening and yield modulations consistent with quantum-paths interferences calculations.