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1.1 Fundamentals of Extreme Photonics
Project coordinator(s): O. Smirnova, F. Intravaia
Recent Highlights
Here you will always find the most recent results of the topics::

Nonlinear optics down to the few cycle limit

  • Carrier-envelope phase stabilization with sub-10 as residual timing jitter
    We demonstrate carrier-envelope phase (CEP) stabilization of a mode-locked Ti:sapphire oscillator with unprecedented timing jitter of eight attoseconds. The stabilization performance is obtained by a combination of two different stabilization approaches. In a first step the drift of the CEP is stabilized with a conventional feedback loop by means of controlling the oscillator pump power with an acousto-optic modulator (AOM). In a second step we utilize a recently developed feed-forward type stabilization scheme which has a much higher control bandwith. Here an acousto-optic frequency shifter (AOFS) produces the stabilized output in the first diffraction order. Moreover, we present numerical results on the optimization of the length of the photonic crystal fiber, which is used to generate
    an octave-spanning spectrum, in order to optimize the sensitivity in the f -to-2f interferometers.[BKH11].

 
  • Ultrashort highly localized wavepackets
    The recently introduced concept of radially non-oscillating, temporally stable ultrashort-pulsed Bessel-like beams we referred to as needle beams is generalized to a particular class of highly localized wavepackets (HLWs). Spatio-temporally quasi-nondiffracting pulses propagating along extended zones are shaped from Ti:sapphire oscillator radiation with a spatial light modulator and characterized with spatially resolved second order autocorrelation. Few-cycle wavepackets tailored to resemble circular disks, rings and bars of light represent the closest approximation of linear-optical light bullets known so far. By combining
    multiple HLWs, complex pulsed nondiffracting patterns are obtained [BDG12]..

 
  • Saturation of the all-optical Kerr effect in solids
    We discuss the influence of the higher-order Kerr effect (HOKE) in wide bandgap solids at extreme intensities below the onset of optically induced damage. Using different theoretical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers–Kronig transform. Within this theoretical framework we provide an estimate for the appearance of significant deviations from the standard optical Kerr effect predicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercontinuum generation in photonic crystal fibers. Furthermore, we present experimental data from a multiwave mixing experiment in BaF2, which can be explained by the appearance of the HOKE.[BBB12].

Extreme wavelengths and attosecond pulse generation

 
  • sub-20fs pulses at 160nm
    We present our latest results on the generation of ultrashort vacuum UV (VUV) pulses by nonresonant four-wave mixing of chirped broadband pulses generated by filamentation of the fundamental of a Ti:sapphire laser with relatively narrowband pulses at the third harmonic. Positive chirp at the broadband idler yields negatively chirped VUV pulses necessary to compensate for material dispersion of a MgF2 window in the VUV beam path. Pulse energies exceeding 400 nJ are available for time-resolved experiments. Pulse duration is measured by pump–probe ionization of Xe gas, providing the cross correlation between the fifth harmonic and the fundamental [BGN11].

 
  • Tailoring terahertz radiation by controlling tunnel photoionization events in gases
    Various applications ranging from nonlinear terahertz (THz) spectroscopy to remote sensing require broadband and intense THz radiation, which can be generated by focusing two-color laser pulses into a gas. In this setup, THz radiation originates from the buildup of electron density in sharp steps of attosecond duration due to tunnel ionization, and the subsequent acceleration of free electrons in the laser field. We show that the spectral shape of the THz pulses generated by this mechanism is determined by the superposition of contributions from individual ionization events. This provides a straightforward analogy to linear diffraction theory, where the ionization events play the role of slits in a grating. This analogy offers simple explanations for recent experimental observations and opens new avenues for THz pulse shaping based on temporal control of the ionization events. We illustrate this novel technique by tailoring the spectral width and position of the resulting radiation using multi-color pump pulses [BSH11].

 
  • Theory of the high harmonic generation in the vicinity of the metal nanoparticles
    We have developed a semiclassical model for plasmon-enhanced high-order harmonic generation (HHG) in the vicinity of metal nanostructures [HIH11]. We show that, besides the field enhancement, both the inhomogeneity of the enhanced local fields and electron absorption by the metal surface play an important role in the HHG process and lead to the generation of even harmonics and a significantly increased cutoff. For the examples of silver-coated nanocones and bowtie antennas, we predict that the required intensity reduces by up to three orders of magnitude due to plasmonic field enhancement. The study of the enhanced high-order harmonic generation is connected with a finite-element simulation of the electric field enhancement due to the excitation of the plasmonic modes. We have also studied the generation of a single attosecond pulse by polarization gating in a vicinity of a metal nanostructure [HKH11] as well as high harmonics generation assisted by plasmonic excitation in a vicinity of a rough surface [KHH11].